Proceedings of the European Wave and Tidal Energy Conference

Analysis of Mutriku's OWC performance

2023-06-26T13:57:21+01:00

An analysis of oscillating water columns (OWC) at the Mutriku Wave Power Plant to understand their efficiency in transforming ocean energy into electricity. The study aims to estimate the rate of convergence of wave energy flux to kilowatts hour of each OWC and determine whether it is more accurate to estimate for each OWC independently or as part of a system of equations. The dataset includes electricity production data from 7 turbines in the MWPP during 2019, along with ocean measurements such as significant wave height, wave period, wave energy flux, swell direction, and wind direction. All models in the paper have varying coefficients which change with the significant wave height. The methodology includes the use of generalized least squares and local polynomial kernel methodologies for coefficient estimation.

Open Sea Trial of a Wave-Energy Converter at Tuticorin Port – Challenges

2023-06-06T07:00:01+01:00

A major challenge of ocean wave energy converter (WEC) development is to test in the actual ocean. In this article, how a point absorber (PA) WEC was developed and tested during the mid of November 2022 in the ocean is reported. The WEC named Sindhuja-1 was developed from concept, numerical modelling, and laboratory testing. The test site was the VOC Port in Tuticorin, India. The WEC consisted of a buoy, a spar, and a power take-off mechanism. The buoy diameter was 0.75m and the spar length was 10m. The whole design was done at IIT Madras and preliminary tests were done at the wave basin at IIT Madras. The system was transported to the site and a preliminary test was conducted at the harbour area where the water is calm and has a depth of more than 10m. The purpose of the tests was to check any leakage, buoyance, and stability. After that, a hired fishing boat dragged the system over the ocean surface to a location where 20m water dept is there. The distance from the coast was 6km, and wave heights of approximately 0.5-0.8 meters. For the sea trial design, electronic items were insulated, and a cylindrical cover was also put over the electrical components to avoid water splashes or rain. It was found that the system worked vertically, and the system produced a power of about 100W. The paper will explain the challenges faced during the planning and execution of the system.

HAPiGYM

2023-06-26T14:07:45+01:00

The HAPiGYM is a collection of numerical and experimental modelling environments for testing control of wave energy converters (WECs). It has two applications: rapid prototyping of the control policies themselves, and co-design of control and other WEC subsystems. This collection will grow over time. Initially two environments (‘GYM machines’) will be offered. These will be described in this paper.

The HAPiGYM addresses several technical and resourcing challenges surrounding control prototyping and co-design. Time, money, and cross-disciplinary knowledge are common barriers. Tank time can be prohibitively expensive. Hence many control researchers rely on numerical simulations, and many WEC developers use non-representative control models when designing the hydrodynamic absorbers. The state of the art in hydrodynamic models are not suitable for rapid control prototyping: they are either too slow or insufficiently accurate, leading to a ‘Sim2Tank’ gap, where simulation and tank trial results disagree. Hardware-in-the-Loop removes those
modelling uncertainties associated with the drive train and control hardware. However, uncertainties associated with the hydrodynamic model remain, and unrepresentative artefacts associated with the rig could be added. Two notable schemes that included tank testing for comparative evaluation were WECCCOMP and the Wave Energy Prize (WEP). WECCCOMP allowed participants to compare the performance of their controllers on the same tank test model. The WEP allowed participants to test their own WECs and controllers.
Both had metrics that were proxies for levelised cost of energy. The single metric and prescribed methodology focused the efforts and objectives of participants. As a consequence, simplifications inherent in metrics (e.g., the formula for electrical power) and methodology (e.g., spectral parameters of the sea state available as inputs to the control policy) led to control approaches that could not be replicated in real seas, i.e., certain types of control were able to exploit the ‘Tank2Sea’ gap. The HAPiGYM approach acknowledges the
issues surrounding Sim2Tank and Tank2Sea gaps. Rather than attempting to eliminate these, participants will be invited to contribute to a discussion of how testing methodology interacts with control. Participants will be able to suggest methods, metrics, and even future GYM machines. The HAPiGYM will offer a selection of settings for each GYM machine, including the resource (waves), type of PTO, and metrics. Participants will be able to rate their controllers against a suite of metrics and experimental set-ups. This will allow a more nuanced comparison between controllers. It will also facilitate more basic research on co-design, e.g. how PTO operational range
impacts control and hydrodynamic performance.

Stakeholder engagement identified the need for a simple environment to get started on (a small Sim2Tank gap), and a more challenging environment that reflected the control problems of commercial devices (a small Tank2Sea gap). The first two GYM machines offered will use the same buoy with different constraints: constrained to heave only, and unconstrained (6 DoF). Participants will be able to run Processor-in-the-Loop tests using a tank-calibrated rig simulation running on Open-Hardware controllers. The most promising projects
will be given free remote-access to the HAPiGYM, running in the FloWave tank.

Development & performance enhancement of an AUV wave-charging system

2023-06-16T11:25:20+01:00

Adoption of autonomous underwater vehicle (AUV) technology has recently experienced rapid growth, fueled by possibilities enabled by technological advances. AUVs are particularly useful as unmanned survey platforms, and typically have an array of on-board sensors to collect data for a variety of commercial and military applications. AUVs are autonomous and untethered systems and require a power source, typically batteries, to be carried onboard. An increase in available energy by even a small amount can be game-changing for AUV applications with benefits including longer mission durations, higher sampling rate, more sensing capability, and improved communication capability. This can be accomplished through some self-recharging capability within the AUV, allowing the AUV to extract energy from its surrounding environment, and eliminating the need to recover the vehicle until the mission is complete.

This work presents a Wave Power Capture System (WPCS) that can be integrated into an AUV, allowing it to operate for significantly longer periods of time without the need for recovery. This concept utilizes two rotary power take-off (PTO) units that are driven by two independent tendons, located axially along the length of the body. The two tendons are connected to a retractable reaction plate that can be stowed against the body of the AUV when not in use and deployed autonomously when the AUV needs to surface and recharge. This arrangement allows both pitch and heave motion to be primary contributors to relative (power generating) motion. Additional motion in surge, sway and yaw will also result in some secondary power generation.

This work focuses on the hydrodynamics and design of the reaction plate so that power capture and quality are enhanced. The geometry of the reaction plate will be constrained to a baseline semicircular shape, allowing the AUV to operate normally when the plate is stowed. Based on previous work, which has indicated that increasing the reaction plate added mass improves WEC power capture, the current work will thus look at different modifications to the reaction plate that effectively enhance its size when deployed, for example, incorporating multiple ‘nested’ reaction plates that mate together when stowed. The light weight of the reaction plate relative to its size means that there may be a tendency for the tendons to go slack and subsequently experience snap loading in cases where the reaction plate does not fall as fast as the AUV body. This work will further investigate the incorporation of dynamically adaptable geometries that reduce the reaction plate’s hydrodynamic resistance in the downward direction, for example, a structure that contains multiple flaps that hinge open during downward travel.

A series of experiments will be presented in which the different reaction plate concepts are sinusoidally forced in a quiescent basin to characterize the translational and rotational hydrodynamic coefficients over a range of representative frequencies and amplitudes. Finally, a time-domain model developed using ProteusDS software, informed by the hydrodynamic coefficients measured experimentally, will be used to calculate AUV power performance in different sea states and evaluate the effect of the different reaction plate modifications.

Successful innovation strategies to overcome the technical challenges in the development of wave energy technologies

2023-01-20T13:53:39+00:00

Despite the considerable efforts the international research community has made over the last decades, wave energy technologies have failed to achieve the desired design convergence to support their future market growth. Many technical challenges remain unresolved, leading to high costs of energy in comparison with other renewable energy sources. It becomes apparent that incremental innovation alone cannot fill the gap between the current techno-economic estimates and the medium-term policy targets established for wave energy.

A systematic problem-solving approach must be embedded from the outset of technology development to meet the high sector expectations. This approach should support the engineering design processes, facilitate traceability of engineering analysis, and provide practical tools for understanding the wave energy context, formalising wave energy system requirements, guiding techno-economic design decisions, and overcoming technical challenges.

Systems Engineering methods have been successfully applied to developing complex commercial products in many sectors. Among the many tools developed in Systems Engineering, it is worthwhile mentioning two structured innovation techniques: Quality Function Deployment (QFD) for problem formulation and selection [1]; and the Theory of Inventive Problem Solving (TRIZ) for concept generation [2]. Unfortunately, their use in wave energy is still limited and fragmented.

Taking as a starting point the technology-agnostic assessment of wave energy capabilities performed in previous research work [3] for the problem formulation and concept selection, the authors have applied QFD to obtain the prioritisation of the technical characteristics that may offer the greatest impact to the overall design for a wave energy system. The main Functional Requirements are mapped to an equal number of Design Parameters extracted from the 39 technical parameters provided by TRIZ. The TRIZ toolkit is then employed to suggest three alternative innovation strategies to overcome wave energy cost and performance limitations.

Firstly, separation principles are used to deal with physical contradictions. Examples of potentially effective strategies involving separation in time, space, scale or condition are proposed.

Subsequently, inventive principles are employed to solve technical contradictions and trade-offs. The four most promising inventive principles that have been found in this implementation are "Local quality", "Dynamism", "Pneumatics or hydraulics", and "Physical or chemical properties". These principles prompt the user to consider a broader range of alternatives and improve creative thinking. Additional examples are given on how these inventive principles could be applied in wave energy.

Finally, a system transition strategy is needed for the most complex challenges. Bypassing contradictory demands involves more radical changes in the functional allocation of requirements to the physical embodiment. Therefore, such a significant pivot in wave energy design can only be made in the initial phases of technology development.

[1] S. Mizuno, Y. Akao, and K. Ishihara, Eds., QFD, the customer-driven approach to quality planning and deployment. Tokyo, Japan: Asian Productivity Organization, 1994.

[2] K. Gadd, TRIZ for Engineers: Enabling Inventive Problem Solving, 1st ed. Wiley, 2011. doi: 10.1002/9780470684320.

[3] P. Ruiz-Minguela, J. M. Blanco, V. Nava, and H. Jeffrey, ‘Technology-Agnostic Assessment of Wave Energy System Capabilities’, Energies, vol. 15, no. 7, p. 2624, Apr. 2022, doi: 10.3390/en15072624.

Spatial focussing of wave energy for improved power capture by an oscillating water column

2023-06-26T14:10:37+01:00

The world’s oceans represent an abundant reserve of renewable energy. Wave power is a ubiquitous resource which is both temporarily and spatially consistent. This designates wave power as a particularly suitable energy resource for exploitation. Nevertheless, the development of wave energy technologies is lagging behind other renewable energy devices. Few grid-connected wave energy converters (WECs) have been constructed or deployed. The main barriers to the commercialisation of WECs is the relatively low energy conversion efficiencies that have been achieved, both with prototype devices, and with full scale installations.

Many studies have been performed to augment the wave energy capture by WECs. Until now, investigations have predominantly focussed on modifying the device geometry (e.g. additional chambers in OWCs, varied buoy shapes for point absorbers etc.), or PTO systems to improve the energy yield. In this study, a novel approach is adopted, which demonstrates that significantly improved WEC performances can be achieved by manipulating the hydrodynamic wave field in the vicinity of the WEC to consolidate the wave energy. Few studies have been undertaken on methods to focus the linearly distributed wave energy at the WEC device. The concept of concentrating renewable energy resources for improved harvest is employed in other industries, for example, concentrated solar power plants utilize vast arrays of mirrors to consolidate the areally distributed solar energy to a central PTO tower. The present study demonstrates that wave energy can be focussed in an analogous manner.

This research investigates the effectiveness of various geometry structures to reflect and focus the incident wave energy to point locations at which WEC devices can be positioned. To achieve this energy focussing effect, we install reflecting walls within a numerical wave tank to manipulate the hydrodynamic wave field. Three diverse wall configurations are examined (see Fig. 1). Initially, a straight wall is installed at an oblique angle to the incident waves. The waves are reflected from the wall and interact with propagating incident waves, forming a chequerboard type pattern on the free surface. Where the incident waves and reflected waves interact, a positive interference manifests which yields amplified free surface displacements, signifying energy concentration localisations. The second set of numerical experiments investigate wave focussing in the concave opening of a parabolic reflecting wall. Significantly augmented free surface displacement amplifications are observed at the parabolic wall focus. Finally, the study investigates wave focussing at the convex side of a double parabola wall. In this configuration, a complex reflecting wave field pattern emerges which may be particularly suitable for the installation of a WEC array. In each of the geometrical scenarios described, an oscillating water column (OWC) WEC is installed at the wave focussing position and its performance is analysed over a range of incident wave conditions. The performance of the OWC in each wave focussing case is compared with an equivalent OWC installed in open-seas conditions. In this manner, the effectiveness of the wall geometry in terms of the energy focussing and subsequent energy capture enhancement by the WEC is determined.

Relevance of Robustness and Uncertainties Analysis in the Optimal Design of Wave Energy Converters

2023-06-26T14:16:07+01:00

The optimisation design of Wave Energy Converters (WEC) to reduce the cost of energy of the technology is a widely investigated topic. In literature classical optimisation strategies have been presented and applied to identify the optimal system parameters of WECs to optimise specific techno-economic metrics. The performance of the optimal identified devices relies on these nominal parameters and it can be strongly affected by construction and modelling uncertainties. In this context, the concept of robustness of the optimal solution plays a relevant role in the identification of a device whose performance is affected as little as possible by uncertainties of various kinds. In the first part of this paper different declinations of robustness concept are derived from other fields of application and described. The identified robustness indexes are then applied to optimal solutions obtained via classical optimisation to evaluate its importance in the design process of WECs.

Strictly related to this kind of methodology is the Sensitivity Analysis (SA) technique, it aims to investigate how the input variation (due to uncertainties or external noise or additional environmental parameters) influences the output results of a defined numerical model and highlight the relative input parameters relevance. Sensitivity Analysis, therefore, can be a valuable tool applicable in the uncertainty set estimation to identify the variables most subject to such uncertainties and their prominence.

The main objective of the work is to underline the importance of introduce the robustness evaluation of WECs during the optimisation process since classical optimisation techniques can lead to solutions that are affected by uncertainties.

Tuning Wave Energy Converters to local wave conditions

2023-06-20T15:26:57+01:00

A Wave Energy Converter (WEC) cannot operate efficiently if left alone floating to the mercy of the random sea surface. Moreover, high waves may impose destructive forces while contributing nil energy production in return. For a WEC to be efficient, its natural frequency should be close to the peak frequency of a wave spectrum. At the resonance or another lower natural frequency, dynamic responses are more regular, and thus, the power take-off system (PTO) can operate in a more efficient manner. This paper introduces a general systematic methodology for tuning the natural frequency of a resonant WEC for the desired degree of freedom and converting dynamic responses into power. A barge is used as a case study to tune its roll and heave natural period, which in general are much lower than the peak period of swells. The roll natural period is increased by hanging subsea structures to the sides of the vessel by means of mooring lines, see Fig. 1. The size of the structures and their entrapped water increase the roll mass moment of inertia to achieve the desired period. For the heave response, the natural period raises by adding skirts to the sides of the vessel, increasing the entrapped water. The resonant motions are significantly suppressed by means of the drag force acting on the subsea structures. Thus, the extreme drag forces acting on the structures act as a damper to the vessel response. These forces are transferred to the mooring lines, which can be converted into a rotary motion of a shaft of the PTO. Our application focuses on the Galapagos Islands, where narrow, low-frequency southerly swells with a typical 13 s peak period dominate the wave climate. This study allows for a systematic design of resonant-type WECs, which is necessary for the development of technical and economic feasibility studies of various novel and existing concepts.

Figure 1: Schematic representation of resonant WEC with tuned roll natural period

Enabling the Ocean Internet of Things with Renewable Marine Energy

2023-06-20T12:00:38+01:00

Marine renewable energy can play an integral role in reducing the cost and complexity of collecting data from the oceans, and enhancing its exploitation. The processing and transmission of data at sea may be modelled as an `internet of things' (IoT) application. The `thing' is any offshore device which can collect and process data in-situ, while the `internet' represents the medium for transmitting data. IoT is differentiated from other internet based communication paradigms by the constraints on the system. These constraints are typically limited energy budgets and computing power, intermittent and low-bandwidth connectivity, and limited physical access. While land based IoT applications are already well served by a wide selection of hardware and software components, the needs of offshore IoT applications are not well served. Utilizing marine energy can advance the adoption of IoT at sea by providing in-situ energy generation, whilst also benefitting from the same technology. The Sustainable Energy Authority of Ireland funded `BlueBox' project aims to overcome barriers to entry for applying IoT technologies to offshore sensing by developing Ocean IoT (OIoT) hardware and software solutions. Features of the BlueBox system include modular offshore focussed hardware, no-code configuration and control of peripherals (such as sensors and actuators), duplex transmission of data using multiple media, serverless cloud server architecture and an edge computing framework. This paper presents an overview of BlueBox, tank tests for system validation of a prototype wave-energy powered ocean-observing platform, and a discussion of future applications of the technology.

Experimental evaluation of phase and velocity control for a cyclorotor wave energy converter

2023-05-26T09:00:07+01:00

The research presented in the paper is dedicated to the analysis of the 3D experimental testing results of a 1:20 scale prototype LiftWEC cyclorotor wave energy converter (WEC). The scaled prototype was built and tested in the Hydraulic and Offshore Engineering wave Tank (HOET) by Ecole Centrale Nantes (ECN) in 2022. The analysis is conducted using the analytical control-oriented point-vortex model. The presented research covers a range of tests, with particular focus on cases where positive mechanical power generation has been recorded. The analysis of such cases is important, in highlighting the conditions needed for optimum energy conversion, for future development of cyclorotor WEC technology. The study also reviews the results of tests where the rotor rotational speed is varied within each period of monochromatic waves. This is the first experimental test of such a control strategy for cyclorotor WECs.

Wave Energy Power Take-off Validation with a Hydraulicly Actuated Rotary Dynamometer and a Bi-directional High-power DC Supply

2023-06-25T09:14:06+01:00

There are many organizations working towards the commercialization of wave energy converter technologies and are advancing their designs through the technology readiness levels (TRLs). A critical step before the field deployment of prototype wave energy converters is the validation of the subsystems and components that are contained in the wave energy converter through laboratory testing and performance characterization. In 2021, the National Renewable Energy Laboratory (NREL) developed and demonstrated a system for testing power takeoffs (PTO) with a low-speed, high-torque dynamometer and a grid-tied high-power DC power source and sink before field deployment. The hydraulic dynamometer allows for the simulation of PTO actuation from wave motion and is capable of a wide range of wave periods and heights which are represented as various speeds and torques from the dynamometer. The high-power bidirectional power supply allows for hardware in the loop and controller in the loop testing to be conducted on WEC power electronics. This presentation was made to describe the methods used by NREL research staff to test all components and sub-systems in the PTO of a novel wave energy converter before field deployment.

A Removable elevated-hinge wave generator for testing marine energy devices

2023-06-26T14:20:50+01:00

A new removable elevated-hinge wave generator has been designed and commissioned in the O.H. Hinsdale Wave Research Laboratory (HWRL), at Oregon State University. The wave maker, built by Edinburgh Designs Ltd., is comprised by six electrically actuated dry-back paddles, self-contained in a single steel box and capable of generating mid-scale regular, irregular and user defined waves in a typical range of periods from 0.5 to 4 s at a maximum depth of 4 m. The system is intended to increase the available depth range in the Large Wave Flume (LWF) and satisfy the demand of intermediate to deep water waves at a relatively large scale by the marine energy industry.

The existing Large Wave Flume at the HWRL, is currently equipped with a piston-type, dry-back wave generator with a 4.2 m maximum stroke hydraulic actuator assembly. The flume is 104.24 m long, 3.66 m wide, and the sidewalls are 4.57 m high. The existing wave maker can generate large regular, random and tsunami-like long waves for the purpose of large-scale model tests, particularly in the area of coastal hazards (storm waves and tsunamis). Currently, the maximum water depth for generation of regular or random waves is 2.7 m, with a maximum wave height of 1.7 m in a wave period range from 4 to 8 s. The maximum depth for tsunami-like waves (solitary waves) in the flume is 2.0 m, with a maximum wave height of 1.4 m. The LWF works as a standard coastal (shallow water) testing facility where the models, bathymetry and instrumentation are typically installed with the facility completely empty. This allows for a full range of testing depths (from practically 0 to the maximum depth corresponding to the wave type to be generated). Correct representation of the bathymetry is necessary in coastal studies since wave propagation, transformation and breaking is part of the intended research. However, intermediate to deep water conditions, required for medium scale marine energy studies, represented a challenge.

The maximum depth at the flume for wave generation was limited by the structural design of the wave machine. However, the flume wall height is capable of handling a maximum depth in excess of 4 m. To increase the capabilities of the facility and responding to an increasing demand of deeper experimental conditions (particularly from the wave energy industry), the procurement of a specialized wave machine able to generate high-quality waves in deeper water was deemed necessary.

The uniqueness of the system relies on its flexibility. It was conceived to be, first, removable and can be relocated anywhere along the flume, at the full range of depths (from 1 m to 4 m) and it can be reversed facing both directions along the flume. This flexibility, required by the intention to keep the existing wave machine operational, increases its functionality by making it compatible to the generation of waves and co-linear currents, as well as expanding the available testing section along the flume, with generation on one side and absorption on the opposite.

Wave energy converter power take-off characterization: comparing dynamometer and field data

2023-06-26T14:13:35+01:00

Through dynamometer testing of a wave energy converter’s (WEC’s) power take-off system (PTO), we characterize friction and establish a relationship between shaft speed and torque. This characterization is conducted through oscillation of the shaft with a velocity-controlled motor, yielding some key differences between lab tests and response of the system in the field. In this work, we develop an understanding of these differences, provide both linear and nonlinear representations of the PTO that can be used in hydrodynamic modeling, and assess how accurately these representations are reflected in data collected in the field.

The WEC used in this testing is the TigerRAY, built by CPower as part of a WEC-UUV (uncrewed underwater vehicle) system funded by the Naval Facilities Engineering Systems Command. The full device is a two-body point absorber with a subsurface heave plate that acts to hydrodynamically stabilize the system and serves as a docking and charging station for the UUV. The system is undergoing its second year of development and testing, which includes both at-sea trials and shore-side dynamometer experiments at the University of Washington Applied Physics Lab. The dynamometer system records torque at the WEC drive shaft, as well as motor shaft speed. Data acquisition on board the WEC simultaneously records key generator information, such as current, voltage, and angular shaft position.

Through comparison of lab and field data, we draw conclusions about the limits of a velocity-controlled dynamometer. Since this control scheme allows near-instantaneous changes in applied torque, it pushes the WEC through electrical system-induced torque spikes more quickly than is possible in the field. Reduction in the control gains of the dynamometer smooths the applied torque curve at the expense of added noise in the velocity profile. We weigh these tradeoffs using data obtained from field testing of the device in a real wave field. Additionally, we identify key characteristics in the dynamometer data and how they impact device performance in the field. Finally, we make recommendations about how to move forward with future dynamometer testing given these lessons learned.

Limiting the available pneumatic power in a U-OWC

2023-06-08T08:32:52+01:00

A fixed oscillating water column is a hollow structure open to the sea at its submerged part. The water column motion alternately compresses and decompresses the enclosed air above the inner free surface, which drives a self-rectifying air turbine. In 2003, Boccotti proposed the so-called U-OWC having in view the integration into breakwaters. In this configuration, the inner water is connected to the sea by a U-shaped duct whose opening faces upwards rather than sideways or downwards. This allows the OWC length to be extended (and the resonance frequency reduced) without placing the opening too deeply submerged where wave energy is attenuated by distance to the free-surface. An advantage of OWC converters is the capability to control or dissipate the excess energy available to the power take-off system in excessively energetic sea states. This is in general done by limiting the air turbine torque by controlling a by-pass air valve or a valve in series with the turbine. What is proposed here is to take advantage of the water column configuration to limit the energy absorbed from the waves in the more energetic sea states. This requires the tidal amplitude to be small as occurs in coastal areas like the Mediterranean Sea. The distance of the OWC opening to the mid-sea-level should be chosen such that, in the more energetic sea states, that opening is left uncovered by the troughs of the higher waves. This introduces a limitation to the energy absorbed in the more energetic sea states. This paper reports experiments with a 1:40^th-scale U-OWC in a wave flume. The results show that limitations to the capture width ratio occurs in the more energetic wave systems.

Experimental Investigation into the Air Compressibility Scaling Effect on OWC Performance and Wave Height

2023-06-25T09:16:47+01:00

Wave energy converter arrays have the potential to provide coastal protection in addition to generating power from incoming waves. As part of a wider experimental study to investigate dual-use applications, this paper presents the results of wave flume testing conducted to analyse the performance characteristics of a single, generic Oscillating Water Column (OWC) device, in preparation for the next phase of study that will focus on multi-device arrays. The specific focus of this flume testing was to characterise parameters such as Response Amplitude Operator (RAO), Capture Width Ratio (CWR) and Phase Response, as well as the device’s effect on the local wave field.

A potential limitation when using scaled experimental results for OWCs are the differing scaling factors that should be applied to the device’s submerged volume (λ³) and air volume (λ²) which, together with the Power Take Off (PTO) damping, can greatly affect the air-spring stiffness experienced within the OWC.

A subset of 34 tests (out of a total 134) were conducted under monochromatic wave conditions with wave heights of 20 mm or 40 mm and wave periods ranging from 1.2 s to 2.2 s. In these tests the PTO damping was varied by adjusting the orifice diameter, while the air volume was varied via an adjustable auxiliary air chamber. Results show that for the smaller orifice diameters (i.e., higher damping) tested, air-spring stiffness played a significant role and counterintuitively increased with increased air volume.

Effects of the single OWC device on the wave field within the flume were also investigated. Results revealed that while there is a marked difference when comparing the OWC to an identically-shaped blockage, there was no significant measurable difference in the wave height change observed for all the damping and air volume parameter settings that were modelled, despite a general trend when comparing to the empty flume.

Enhancing the efficiency of an axial impulse turbine with a diffuser

2023-06-29T15:39:21+01:00

In oscillating water column wave energy converters, the aerodynamic losses caused by flow misalignment between the rotor outlet and the outlet guide vane of self-rectifying impulse turbines are a significant design problem. The symmetrical positioning of guiding vanes on both sides of the rotor is the reason for it. In comparison to the equivalent Wells turbine, the efficiency of impulse turbines is lower because of these losses in the outlet guide vane. This paper presents a strategy to develop an
improved design of an axial impulse turbine to reduce the losses in the outlet guide vane along with the residual kinetic energy loss by integrating a diffuser between the rotor and guide vanes. The proposed model was investigated numerically using RANS simulations. Multiple diffuser geometries were tested against the reference turbine for comparison of the performance characteristics. The results support the hypothesis behind the proposed design. The performance was compared extensively with the reference case in terms of the dimensionless loss coefficients for a better insight into the contribution of all the turbine sectors. The current work shows a possible design path for the performance improvement of the turbine. Results also support the fact that the guide vanes need to be
redesigned for obtaining maximizing efficiency with the proposed design. The losses in the outlet guide vane were reduced by approximately 50% with an overall efficiency rise of around 2%.

Numerical performance assessment of a new wave energy conversion system

2023-06-14T17:59:59+01:00

The greater wave energy content in deeper waters has initiated substantial research in offshore wave energy converters (WECs). However, the existing WECs are hindered by their high initial costs, primarily attributed to the demanding mechanical workloads they endure. This paper describes a novel wave energy conversion system developed at IST for deep waters (around 20-50 meters) based on a floater and a ballast, suspended by a set of cables. In the novel approach, the WEC is made up essentially by membranes, without a rigid structure, thus reducing the initial cost of investment comparing to other solutions.
A simplified numerical model in the time-domain of the system based on linear theory is developed and a preliminary performance assessment under regular and irregular wave conditions (for conditions prevalent along the western coast of Portugal) is performed. Additionally, the effect of the ballast mass is considered. The results reveal the system's non-linear behavior during the floater's ascending phase and emphasize the significant influence of the ballast's mass on power production.

Basin testing of the 1-2-1 M4 WEC

2023-06-26T17:30:29+01:00

The M4 family of Wave Energy Converters consist of 3 rows of floats, with the front two rows rigidly connected and a hinged connection(s) to the back row. The various possible configurations are commonly identified by the number of floats in each row – hence 1-2-1 has 1 float in front, 2 in the middle, and 1 at the rear. A study by Stansby et al 2017 [1] showed that the 1-2-1 and 1-3-2 variants had the lowest cost of energy of all configurations studied. The 1-3-2 variant has subsequently been extensively studied as it provides greater power per machine. However, the 1-2-1 machine has received less attention. In this paper we report, for the first time, wave basin testing of a 1-2-1 variant. The design is a 1:15 scaled version of a 1-2-1 M4 suitable for deployment in the prevailing short period wind waves in King George Sound, Albany, on the south coast of Western Australia.

The tests were carried out in the Model Test Basin at the Australian Maritime College, Tasmania. This basin is 35 m x 12 m and tests were run with a water depth of 0.8 m. For modelling operational conditions, the power take-off was represented by a pneumatic ram; in extreme seas the power take-off was removed.

Decay tests, irregular operational seas and irregular extreme seas were tested. All waves were long crested. Sea states tested were taken from the scatter diagram for the deployment site in King George Sound. Body displacements, mooring loads, run-up on the middle floats and forces in the PTO (where installed) were measured. Measured motions and power were compared to a linear model. The level of agreement is compared with previous tests on other variants of M4 in the published literature, including the 1-3-2 variant tested in Carpintero-Moreno et al [2] and the 1-1-1 variant tested in Stansby et al [3]. Compared to some other configurations (such as 1-3-2) the 1-2-1 M4 is more prone to roll. Implications of the test results for deployment in King George Sound are discussed.

[1] Stansby, P., Moreno, E.C. and Stallard, T., 2017. Large capacity multi-float configurations for the wave energy converter M4 using a time-domain linear diffraction model. Applied Ocean Research, 68, pp.53-64.

[2] Carpintero-Moreno, E. and Stansby, P., 2019. The 6-float wave energy converter M4: Ocean basin tests giving capture width, response and energy yield for several sites. Renewable and Sustainable Energy Reviews, 104, pp.307-318.

[3] Stansby, P., Moreno, E.C. and Stallard, T., 2015. Capture width of the three-float multi-mode multi-resonance broadband wave energy line absorber M4 from laboratory studies with irregular waves of different spectral shape and directional spread. Journal of Ocean Engineering and Marine Energy, 1, pp.287-298.

Experimental Investigation on Performance of Counter-rotating Impulse Turbine with Middle Vanes for Wave Energy Conversion

2023-06-25T09:10:53+01:00

In an oscillating water column (OWC) based wave energy device, a water column that oscillates due to the sea wave motion generates a bi-directional airflow in an air chamber, and finally, air turbine driven by the bi-directional airflow converts the pneumatic energy into the mechanical energy. A counter-rotating impulse turbine for bi-directional airflow has been proposed by M. E. McCormick of the United States Naval Academy in 1978. In previous studies by using CFD analysis, the authors investigated the effect of the turbine geometry on the performance of the counter-rotating impulse turbine for bi-directional airflow, and it was clarified that the efficiency of the turbine is higher than that of an impulse turbine with a single rotor for bi-directional airflow in a range of high flow coefficient.
On the other hand, this impulse turbine has a disadvantage that the efficiency in a range of low flow coefficient is remarkably low due to the deterioration of the flow between the two rotors. In a previous study, the authros proposed to installed middle vanes between the two rotors in order to overcome the above drawback. And the effect of the middle vanes the turbine performance was investigated by CFD analysis. As a result, it was found that the middle vanes installed to the turbine rectify the flow between the rotors in a range of low flow coefficient and it drastically increases the efficiency.
In this study, the middle vanes was actually installed in the counter-rotating impulse turbine for bi-directional airflow, and its performance was elucidated by wind tunnel tests which has the turbine with a inner diameter of 240 mm and a centrifugal fan. It was found from the experiment result that the turbine performance at low flow coefficient is improved and the peak efficiency increases by 1.14 times, by means of the installation of middle vanes to the turbine.

Design of an integrated generator and heaving buoy

2023-06-20T12:15:45+01:00

The power take off for wave energy remains one of the key technical challenges that must be overcome before technical maturity. The low speed reciprocating nature of many devices combined with the challenging environment require the development of bespoke electrical machines in a direct drive system. This paper describes a research project that is developing a small direct drive wave energy device. The aim is to prove that electrical machines can reliably operate in the marine environment.

Heaving buoys are wave energy converters where power is extracted by applying a force to oppose the vertical motion of a floating or submerged prime mover. The force is applied by the power take off which requires provision of an inertial reference, for example a drag plate or the sea bed.

Linear direct drive power take off in heaving wave energy converters can suffer from end stop problems, where large uncontrolled oscillations and forces between the prime mover and the inertial reference damage the power take off. For example, over extending either a hydraulic or electric power take off in storm waves off can cause irreconcilable damage. One way of avoiding this is to remove the inertial reference in rough seas.

In an IPS buoy the inertial reference is provided by a piston coupled to water entrapped by an open ended cylinder. If the piston leaves the cylinder, it is no longer coupled to the water mass and its inertial reference drastically reduces. In this case the piston can simply ‘follow’ the prime mover oscillation. An IPS buoy therefore presents an excellent device to demonstrate direct drive power take off.

In the direct drive demonstrator proposed in this paper, the electrical power take off is integrated with the inertial piston and tube of an IPS buoy. The piston acts as the electrical translator and the cylinder houses the stator coils.

A number of design challenges are presented. For example, the piston/translator either needs to be neutrally buoyant, or must be held in position by an external force. Neutral buoyancy imposes constraints on the size and solidity of the translator, whereas the length and radius of the cylinder directly effects the amount of power that can be captured. It is an integrated design problem. A hydrostatic model to size the piston and a linear hydrodynamic model to size the cylinder are presented. The active area of the electric power take off is constrained by the parameters of the piston and cylinder, so design requires integrated hydrostatic, hydrodynamic and electromagnetic modelling. Oscillation results are compared to predictions from commercial software for validation and used to inform the electrical design.

The electrical generator will oscillate slowly compared to conventional generators implying the use of some form of magnetic gearing, challenging for mass constrained designs, or reliance on a large amount of rare earth magnetic materials, which has a significant cost implication. A number of electrical machine topologies are investigated in the paper (surface mounted PM machines, Halbach array, and flux concentrated).

A Novel Hybrid Floating Breakwater- Wave Energy Converter Device: Preliminary Experimental Investigations

2023-06-20T11:00:52+01:00

The marine space is increasingly gaining ground as the ideal candidate to drive the energy economic sector as it hosts a huge amount of natural resources to be exploited, such as waves, wind, and solar irradiation. Innovative devices harnessing these marine renewable resources have been thought to be assembled in a new concept of an energy hub for the Mediterranean Sea, recently proposed by the National Research Council of Italy.

The feasibility of this floating energy archipelago is strongly dependent on the creation of a protected sea area with reduced wave heights, where to safely install new types of floating devices for energy harvesting from the sea, like solar islands.

For this purpose, a floating breakwater module has been specifically designed to surround the archipelago with one or more rows. Moreover, investigating the possibility to implement a dual and alternative use of this floating module, as a traditional dissipative system and wave energy converter has resulted in an extremely challenging task. With respect to the existing hybrid floating breakwater-Wave Energy Converters in fact, the novelty of this device is the optimization of both functionalities by varying its draft.

In extremely rough seas, the hybrid module should only serve as a passive breakwater, soaking up incoming waves and safeguarding the equipment installed inside the archipelago. Otherwise, the floating module should operate as a WEC in more frequent mild sea states, assisting the archipelago's energy output. The whole produced energy can be stored and used to supply the development of new productive activities, such as aquaculture, the expensive process of seawater desalination, as well as the production of low environmental impact fuels like methanol or hydrogen.

The model's dual functionality is accomplished by different draft values. This parameter is directly influenced by the volume of water pumped into the module, which raises the floater's displacement. The module is almost completely immersed in the first scenario, which simulates breakwater functioning, increasing the device's stability and the amount of reflected and dissipated waves. On the other hand, when the module exhibits WEC behavior, a lower displacement is necessary, allowing greater device motions, able to better exploit the energy of the waves.

In this study, preliminary results obtained from an experimental campaign carried out on a 1:10 Froude-scaled model are reported. In particular, the dynamic behaviour of the hybrid device is evaluated in terms of response amplitude operators, while the attenuation performances are condensed in the transmission coefficient which indicates the reduction of the wave height inside the archipelago.

Origami-adapted clam design for wave energy conversion

2023-06-29T15:10:15+01:00

The Clam wave energy converter (WEC) is a floating device composed of two side plates connected by a hinge that closes and opens under interaction with wave crests and troughs. A linear power take-off (PTO) may be installed between the two side plates to convert the mechanical motions to electricity, or the volume change may be used to pump air between chambers and across an air turbine PTO. The basic concept has been discussed since 1978 and featured as part of the UK Wave Energy research programme [1]. Some simplified clam models have been built since then and preliminary investigations were conducted by Phillips [2] to understand the wave-structure interactions at the COAST laboratory, University of Plymouth. However, the simplified models were not enclosed and hence seawater can be trapped in the device. To further the investigation, we will design the outer shell of the clam model that is enclosed and thus suitable for use in the (adverse) marine environment.

Since no enclosed flexible polyhedral structure can change its volume without bending or stretching of facets according to the bellows conjecture, the clam model must be strained when it is in motion. A portion of the wave energy will be consumed to deform the outer shell of the clam model and the rest can be captured by the PTO. Therefore, the design of the clam model will aim at minimising the strain on its facets while achieving the largest volumetric change of the device to maximise the power extraction by the PTO.

Inspired by origami, we will construct the enclosed clam-type offshore device by connecting rigid panels and elastic membranes with rotational hinges. We model the rigid panels to rotate about the hinges without facet deformation and allow stretching on elastic membranes. The strain on the elastic material shall be minimised for better structural integrity and minimal energy loss. Satisfying all the design requirements, the best geometric design is obtained through an optimisation process. Based on the optimised geometry, a downscaled prototype will be built using rigid plywood and rubber membranes and tested under dynamic wave-induced loads to prove that the strain incurred is negligible in response to forces.

References:

[1] Peatfield, A. M., Duckers, L. J., Lockftt, F. P., Loughridge, B. W., West, M. J., & White, P. R. S. (1984). The SEA-Clam wave energy converter. In Energy Developments: New Forms, Renewables, Conservation (pp. 137-142). Pergamon.

[2] Phillips, J. W. (2017). Mathematical and Physical Modelling of a Floating Clam-type Wave Energy Converter (Doctoral dissertation, University of Plymouth).

The Geometrical Design of the L-shaped Oscillating Water Column Using Artificial Neural Network

2023-06-22T15:21:28+01:00

Among the various wave energy converters available, the oscillating water column (OWC) shows a number of advantages in terms of implementation and maintenance. In dealing with the survivability issues, incorporating OWCs into reinforced concrete constructions, like breakwaters, is more cost-effective and can endure the effects of seawater impact and erosion.

This paper focuses on optimizing the geometrical design of a novel OWC type, the L-shaped OWC, by establishing a general design procedure to achieve higher energy-capturing efficiency. The performance of the OWC is influenced significantly by the OWC’s geometry under a specified wave condition. It is found that the dimension of the air chamber and water duct is critical in determining OWC’s performance. We develop the chamber and duct design procedure based on the artificial neural network approach by establishing a collection of two-dimensional RANS simulations as the training database. In the end, the performance of the optimal design is compared with the design of a previous paper. The result shows that the capture factor of the optimized chamber geometry of the L-shaped OWC is 12% more than the former design.

Maximizing the surge amplitude of a floater through an adaptable mooring tightening technique

2023-06-19T09:32:16+01:00

Abstract

A technique to optimize the response of Wave Energy Converters (WECs) by maximizing the amplitude of motion along the surge direction is presented. This is achieved by utilizing an adaptable mooring tightening technique for a floater moored with tension legs (tendons). To gain a deeper understanding of the effect of sea states and mooring cable lengths on the surge response of the system, a series of numerical simulations were conducted for various wave conditions while varying the length of the mooring cables. WEC-Sim [1] was used to solve the multi-body dynamics of a rectangular cuboid floater by solving the equations of motion using a time-domain formulation. The dynamics of the mooring cables were simulated using the MoorDyn model with a lumped-mass formulation [2]. To validate the accuracy of the numerical methods, a series of experimental tests were performed in a small-scale wave tank. It was observed that tightening the mooring cables by decreasing their length amplifies the surge motion of the floater while the mooring forces in the heave direction rise due to the increased tension in the cables. Stretching the cables further was found to (i) decrease the surge amplitude and (ii) drastically increase the mooring forces, threatening the integrity of the cables. Therefore, there is an optimum value of the length of the cables that maximizes the surge amplitude of the floater while ensuring that the cables will not break. The impact of other mooring cable parameters such as diameter and material properties were also evaluated. More specifically, increasing the stiffness by increasing the diameter or the tensile modulus of elasticity was found to reduce the floater’s surge amplitude. For the geometry used in this study, the optimum length, diameter and properties of the cables are provided for several sea states. The current results lay the foundations for the design of new types of WECs that harness the surge motion of a floater rather than the heave which is the most common approach for floating WECs.

References

[1]: Kelley Ruehl, David Ogden, Yi-Hsiang Yu, Adam Keester, Nathan Tom, Dominic Forbush, Jorge Leon, Jeff Grasberger, and Salman Husain. (2022, September), WEC-Sim (Version v5.0.1), DOI: 10.5281/zenodo.7121186.

[2]: Hall, M., & Goupee, A. (2015). Validation of a lumped-mass mooring line model with DeepCwind semisubmersible model test data. Ocean Engineering, 104, 590–603. https://doi.org/10.1016/j.oceaneng.2015.05.035

Reliability and Cost Assessment of Critical Components: Electrical generator failure of IDOM wave energy converter

2023-06-26T17:21:22+01:00

It is believed that current testing procedures in the wave energy sector are not well-balanced. Most laboratory testing has been focused on functional tests (e.g. of proof of concept and performance assessment for example) disregarding other key performance measures such as reliability and survivability. A new testing procedure that reduces development time and cost, while enabling better understanding of reliability and survivability of critical components at early Technology Readiness Levels (TRLs) has been proposed within the VALID project.

In the context of VALID, the work in this paper relates reliability of the critical component to the cost of the whole system, i.e. the wave energy converter. There is usually a trade-off between a very reliable component (low maintenance and high unit cost) against a component with a lower lifetime, lower unit cost and higher maintenance. The loss of revenue of a component failing more time during the project lifetime shall be evaluated against different maintenance strategies and the capital expenditures of the overall system and the component of focus. For that, the LCOE or Levelised Cost of Electricity seems to be the parameter more relevant to capture all these variations. A sensitivity analysis will consider how varying failure rates, component’s cost and maintenance costs affect CAPEX, OPEX and accumulated energy production along project’s lifetime.

The assessment will focus on selected critical components representative of three wave energy technologies: CorPower, IDOM and Wavepiston. The three user cases of the VALID project will guide the work, though it is the aim that results will be applicable beyond the VALID project.

Initially, a relative quantification of the impact on the LCOE of increasing the performance and reliability of the critical component will be done. As output data comes from the experimental results, this quantification will be refined.

VALID is a Horizon 2020 research project where fourteen partners around Europe are collaborating into developing a new hybrid testing platform and methodology for critical components. VALID aims at integrating both reliability and testing methods together with relevant data on component failures early in the design and testing process to ensure that the proposed testing procedure is built upon past experience. The focus of the present paper is to statistically relate the realibility of the critical component to the costs of whole system.

Heterogeneous WEC array optimization using the Hidden Genes Genetic Algorithm

2023-06-29T15:25:29+01:00

Wave Energy Converters (WEC) are deployed in arrays to improve the overall quality of the delivered power to the grid and reduce the cost of power production by minimizing the cost of design, deployments, mooring, maintenance, and other associated costs. WEC arrays often contain devices of identical dimensions and modes of operation. The devices are deployed in close proximity, usually having destructive inter-device hydrodynamic interactions. However, in this work, we explore optimizing the number of devices in the array and concurrently, the dimensions of the individual devices (heterogeneous) to achieve better performance compared to an array of identical devices (homogeneous) with comparable overall submerged volume. A techno-economic objective function is formulated to measure the performance of the array while accounting for the volume of material used by the arrays. The power from the array is computed using a time-domain array dynamic model and an optimal constrained control. The hydrodynamic coefficients are computed using a semi-analytical method to enable computationally efficient optimization. The Hidden Gene Genetic Algorithm (HGGA) formulation is used in this optimization problem. During the optimization, tags are assigned to genes to determine whether they are active or hidden. An active gene simulates an active WEC device in the heterogeneous array, while the hidden gene results in a reduction in the total number of devices in the array compared with the homogeneous array. The volume of the heterogeneous array is constrained to be close to that of the homogeneous array. These hidden tags do not exclude the associated devices from the optimization process; these devices keep evolving with the active devices as they might become active in subsequent generations. Heterogeneous arrays were found to perform better than homogeneous arrays.

SIMULATIONS OF EXTREME WAVE LOAD ON AN OSCILLATING WATER COLUMN WAVE ENERGY CONVERTER

2023-06-26T19:56:13+01:00

Extreme load analysis is an essential step in the design of structural and mooring systems for wave energy converters (WEC). The current study aims to evaluate the structural loads on the MARMOK Oscillating Water Column (OWC) WEC under both regular and irregular wave conditions, as well as to evaluate the effectiveness of its station keeping (mooring) systems. The project employs the open source computational fluid dynamics simulation software OpenFOAM to model the fully moored floating system. The WEC device hydrodynamics are validated against laboratory data for free decay in heave and pitch motions. In addition, the station keeping model is validated against mooring tension static offset tests and benchmarked against experimental data in irregular waves. An elegant method of numerically recreating irregular wave inflow conditions from empirical measurements is shown which allows for consistent comparison between the model and experimental results. The calibrated numerical model is then employed to study the full-scale system’s responses. Preliminary results of structural loadings on the fixed and floating spar buoy (with one mooring configuration as sample) are presented in the paper.

On the survivability of WECs through submergence and passive controllers

2023-06-27T12:30:53+01:00

The survivability of WECs during extreme seas and heavy storms has proven to be challenging during the deployment of devices as they often fail during extreme storms. The survival mechanism is often inherited with the device design and mode of operation. It is not economically and logistically viable for devices to be taken back onshore in case of storms. Some devices lock their PTOs during heavy storms and others lock all the moving parts all together, in the case of CETO, its design has an advantage where it can be submergepad. CETO is a buoyant actuator WEC composed of a buoy submerged close to the surface. Three tethers connect CETO to the sea-bottom through rotary PTOs, allowing the device to be wound down, submerged, and therefore, less exposed to the extreme loads at the sea surface during large storms. This paper will study the survivability of the device during extreme sea-states and will examine the required depth to bring its response back to operational conditions. This work will also look at the alteration of some passive controllers, such as a conventional spring to minimize the response of the device instead of maximising the power capture. With the PTO objective altered in extreme sea-states to minimize the response instead of capturing power, the possibility of harvesting power during extreme sea-states with the device submerged will be checked. Finally, the survivability strategy of CETO through submergence will be showcased with wave tank experiments conducted at IHC as part of the Europewave program.

A probabilistic framework for fatigue damage of lift based wave energy converters

2023-06-26T18:12:15+01:00

Wave cycloidal rotors are lift based wave energy devices that have shown great potential to harness the energy of the waves through the use of submerged hydrofoils. Although the lift force of the foils can be controlled passively or actively to maximise power extraction, the foils of the rotor are subject to variable loading which can expose these components to premature mechanical failure. Therefore, it is important to develop novel fatigue analysis tools that help predicting the lifetime of this type of lift based devices. In this paper, we test the hypothesis that when cyclorotor operates at constant rotational velocity in irregular waves, the fatigue damage of the hydrofoil stress hot spot can be computed through a probabilistic approach. We compare our results to the fatigue damage computed with well established deterministic methods. We find that for both narrow and broad band stress energy spectra, the probabilistic method works well at high probability sea states, when the range of stresses does not exceed the inflection point of a double slope SN curve. This is a promising outlook since it confirms that the fatigue damage of these versatile lift based wave devices can be treated in a similar fashion to the fatigue damage of conventional offshore structures.

Preliminary design of an OWC wave energy converter battery charger

2023-06-20T14:46:55+01:00

This paper introduces a low-power off-grid oscillating water column wave energy converter with an internal battery bank. The research aims at the preliminary design and devising of the control strategy of a power electronics interface between the turbo generator and the battery bank. The converter comprises a spar buoy, a biradial turbine, a permanent magnet generator, a full-wave bridge rectifier, a braking chopper, a DC-to-DC step-down converter, and a lead-acid battery bank. The power-take-off system was modelled in Simulink/MATLAB, and its performance was assessed with steady-state simulations, considering a wave climate characteristic of Leixões, Portugal. The chamber pressure, the turbine, generator and rectifier performance were taken from experimental data sets. A simple battery model was derived from the manufacturer's datasheet. An ideal step-down DC-to-DC converter operating in discontinuous conduction mode regulates the battery charging current. This converter, in parallel with the braking chopper, adjusts the generator counter torque by regulating the current through the rectifier. Twelve system variables were recorded for selected pairs of input pressure and step-down converter design coefficient. The power at the rectifier's output terminals was mapped for the rotational speed and input pressure. The results show a system rating of 1.4 kW with 400 W of electrical power at 200 rad/s for the most frequent sea states. The range of the duty cycle, the inductance and the braking resistance were derived. Two closed-loop controllers were proposed for managing the step-down converter and the braking chopper. Their set points and saturation limits were derived from the simulation results.

A Methodology to measure the energy flux captured by a submerged U-OWC by using temperature sensors

2023-06-27T15:14:44+01:00

The estimation of the power captured by a wave energy converters (WEC) device, needs to calculate the plant efficiency. In general, it is necessary to measure both the pressure and the discharge fluctuations of the fluid motion inside the plant. Unfortunately, gauges for the direct measurement of the velocity are bulky and provide punctual measures and, especially on converters having a U-duct, the presence of the velocity sensor produces a relevant disturbance on the motion field. To overcome this issue, an alternative method to evaluate the captured energy flux, using the pressure fluctuation and the air temperature inside the plenum, was proposed by [1] and [2]. However, no information about the accuracy of the temperature sensors and consequently about the errors in estimation of the energy flux were provided.

In this work, following the procedure described by [1] and [2], we have analysed the influence of the time response of the temperature sensor in evaluating the variation of the air volume inside the chamber and, consequently the energy captured by the plant. To this aim, the submerged U-OWC, tested directly at sea in [1], has been simulated numerically. The aim of the numerical experiment is having the actual estimation of the air temperature inside the plenum and trough it, the captured energy flux. The computational domain is constituted by a wave-flume, with a piston-type wavemaker, placed at the left extremity and a submerged breakwater embedded a U-OWC plant, in the middle. The numerical 2D unsteady simulation is based on the Eulerian approach, using the commercial code Ansys Fluent v17.0, Academic Version.

Starting from the knowledge of the pressure fluctuation at the upper opening of the vertical duct and, of both pressure and temperature variations of the air in the plenum, we have evaluated the energy flux absorbed by the plant and we have calibrated the mathematical model used in [1] and [2], using as input the time series of the pressure fluctuations at the upper opening of the vertical duct, and the variation of both temperature and pressure of the air inside the chamber. Then, using the time series of the actual air temperature, we have simulated the input of several first order temperature sensors characterized by different time constant t, and we have analysed the percentage differences in term of energy flux as a function of t.

We have observed that the measurements of the temperature inside the plenum are strongly affected by time constant of the sensor, which produce large errors in the evaluation of the captured energy flux.

Finally, we have proposed a method for conditioning the measure of the air temperature, obtaining an excellent estimation of the energy flux.

Boccotti, P. (2003) "On a new wave energy absorber"Ocean Engineering 30(9), pp. 1191-1200.
Arena, F., Filianoti, P. (2007),"Small-scale field experiment on a submerged breakwater for absorbing wave energy", Journal of Waterway, Port, Coastal and Ocean Engineering, 2007, 133(2), pp. 161–167.

Biofilm prevention in the generator of a direct drive wave energy converter

2023-06-25T08:47:37+01:00

Power take off in a wave energy converter has a number of unique requirements. It must convert low speed oscillating motion into electricity in a reliable, low maintenance manner. A direct drive system, where the electrical machine is optimised to operate at low speed, has the potential to offer a mechanically robust and simple solution. Similar to a hydraulic power take off, the only regular maintenance would be to inspect and replace the seal between moving parts. One strategy for removing regular maintenance is to have an unsealed system, i.e. one where sea water is allowed throughout the electrical machine. A fully flooded electrical machine has benefits in terms of cooling, but poses challenges relating to reliability, corrosion, biofouling and lubrication.

Biofilm refers to a thin layer of fouling organisms which can interfere with the operation of components. Recent work has found that submerged surfaces can be kept free of biofouling using projected ultraviolet (UV) light from LEDs. This paper discusses the testing procedure and impact of the UVC irradiation on biofilm prevention within the active part of an electric generator in a systematic manner, with a view to accelerate its translation to full-scale applications.

A prototype generator is being developed which will be installed in the North Sea, consisting of a submerged linear tubular electrical machine. A magnetic tubular translator will oscillate within a cylinder that houses stator coils. Lubrication will be by way of solid polymer bearings. In order that the active part of the electrical machine can oscillate smoothly, it is imperative that biofilm is prevented from colonising on the bearing surface, which also makes up the magnetic gap of the electrical machine.

The system will have a slow reciprocating oscillation, with a peak speed of perhaps 2m/s. For most wave energy converters there will be brief static periods twice in every wave, and in calm seas these could be prolonged to several hours or even days. In low energy sea states oscillation amplitude could be less than the fully rated amplitude, meaning different parts of the bearing surface could be exposed for different amounts of time.

Early-stage work is underway to investigate the use of UV irradiation in the active part of the electrical machine and bearing surface as biofilm prevention. Flat panels (600mm x 220mm) are used to simulate the original surfaces between moving parts. To achieve biofilm growth, an artificial slime farm was deployed which allows test panels to be subjected to a continuous dynamic flow. The light source of UV irradiation was provided by Light Emitting Diodes (LEDs) with 278nm wavelength. The effectiveness of the biofilm prevention by UVC were evaluated by Image Analysis

The results indicate that UVC can significantly control biofilm presence on the panels. It also has demonstrated that intermittent UV can achieve successful biofilm prevention on submerged surfaces. However, observations indicate the actual UVC light intensity may perform below the manufacturer’s specifications, and this could lead to a detrimental effect on its biofilm control performance.

Hydro-elastic interaction of polymer materials with regular waves

2023-06-26T19:04:43+01:00

The use of flexible materials has the potential to offer a step-change reduction in the cost of wave energy devices by enabling them to absorb more extreme wave loads through their structural responses. Flexible wave energy converters are often manufactured from polymer, fabric, or reinforced polymer components. The elastic modulus, fatigue performance, seawater ageing, and manufacturing process determine the effectiveness of flexible components at replacing their rigid counterparts. During design, it is necessary to assess the hydrodynamic response of the WEC structure to different wave conditions. This work investigates the hydro-elastic response of a submerged polymer membrane, held in a horizontal frame, exposed to regular wave loading. Fast-Fourier Transform analysis enabled assessment of the non-linear response of the membrane exposed to the different wave conditions. The ratio of harmonic to measured wave amplitude ratio gives insight into the excitation mode of the membrane as a function of frequency. It is found that the peak response of the membrane tends to coincide with the fundamental frequency of regular waves. By varying the ratio of membrane length to wavelength an understanding is provided of the hydro-elastic response of the polymer membrane which should be useful in validating software used in the design of flexible WECs.

Degrees of Freedom Effects on a Laboratory Scale WEC Point Absorber

2023-06-25T09:15:31+01:00

The number of rigid bodies and operational degrees of freedom of a wave energy converter (WEC) have significant impacts on power production magnitude and efficiency across wave periods, but this concept is not well studied. Investigating the effect of the number of bodies and degrees of freedom will help WEC developers design devices that most efficiently meet power and cost needs. This paper presents experimental and numerical results for a point absorber WEC across three modes of operation in regular waves. The three modes of operation are ‘one body heave only’, ‘two body heave only’, and ‘two body six degrees of freedom’.

The Laboratory Upgrade Point Absorber (LUPA) was used as the test article. This work builds on the 2021 EWTEC paper [1] by completing fabrication and undergoing wave testing. The LUPA experiment took place in the Large Wave Flume at the O.H. Hinsdale Wave Research Laboratory at Oregon State University in Corvallis, Oregon, USA; the installation and a wave test are shown in Figure 1. LUPA is constructed as a 1:20 scale two body point absorber with a buoyancy driven float, and a spar with a heave plate to act as a reactionary body. The spar is 3.7 meters tall and the float has a 1-meter diameter.

Figure 1. (Left) Mooring line installation. (Right) Wave tests in the Large Wave Flume.

The ‘one body heave only’ mode locks the LUPA spar so only the float is moving, and only in heave (vertical). The ‘two body heave only’ mode allows both the float and spar to move but both bodies are restricted to heaving. The ‘six degrees of freedom’ mode has taut mooring lines connected to the spar and LUPA can translate and rotate in all directions.

A damping optimization of the power take off was performed experimentally and numerically by sweeping damping values across a wide range of regular wave periods. The optimal damping value is used to compare the power generation of the three modes of operation.

The LUPA project is an open-source, lab-scale wave energy converter that acts as a testing platform for students and researchers. The data from this testing along with numerical models and engineering design files will be publicly available here: https://github.com/PMEC-OSU/LUPA. This paper presents how increasing complexity of WECs through the number of bodies and degrees of freedom affects power production. It also provides a baseline case for LUPA which other researchers can compare against when applying their research to the LUPA project.

References

[1] B. Bosma, C. Beringer, M. Leary, and B. Robertson, “Design and modeling of a laboratory scale WEC point absorber,” 14th Eur. Wave Tidal Energy Conf., pp. 1–9, 2021.

Effects of projected wave climate changes on the sizing and performance of OWCs: a focus on the Mediterranean and Atlantic European coastal waters

2023-06-07T21:39:19+01:00

Reliable estimations of the Annual Energy Production (AEP) achievable with a certain Wave Energy Converter (WEC) are among the fundamental elements for a sound evaluation of the related Levelized Cost of Energy (LCOE) that plays a crucial role in the investment decision-making process. The lack of reliability in estimates of the device productivity can be a result of the uncertainty in the assessment of the available wave energy resource since the geometry of the device which maximizes the AEP is dependent on the specific wave climate at the foreseen installation site.

The Climate Data Store of the Copernicus Climate Change Service delivers projections of the wave climate along the 20 m bathymetric contours of the whole European coastlines, covering the period 2040-2100, under two Representative Concentration Pathway (RCP) scenarios (RCP4.5 and RCP8.5) [1].

This work addresses the effect of such long-term wave climate changes on the optimal sizing and performances of Oscillating Water Column (OWC) WECs to be installed along the Atlantic European coastline, expanding a previously published work in which the Mediterranean coastline was investigated [2]. The capture width of the OWC WEC under different wave conditions is computed using an empirical model [3] capable of predicting the device performance with acceptable accuracy and limited computational time.

The results show that the optimal geometry of the OWC WEC varies significantly in the different geographical locations, and that the long-term changes in the wave energy resource could cause a slight modification of the optimal geometry in each potential installation site. The AEP of the OWC WEC in the projected wave climate scenario can be significantly different compared to the present one, potentially contributing to reducing the LCOE of this wave energy conversion technology in some geographical locations.

REFERENCES:

[1] Caires, S., Yan, K., 2022. Ocean surface wave time series for the European coast from 1976 to 2100 derived from climate projections. (Accessed on Feb. 2022) DOI: 10.24381/cds.572bf382. Copernicus Climate Change Service (C3S) Climate Data Store (CDS)

[2] Simonetti, I., Cappietti, L., 2023, Mediterranean coastal wave-climate long-term trend in climate change scenarios and effects on the optimal sizing of OWC wave energy converters, Coastal Engineering, 173, 104247.

[3] Simonetti, I., Cappietti, L., Oumeraci, H., 2018. An empirical model as a supporting tool to optimize the main design parameters of a stationary oscillating water column wave energy converter. Appl. Energy 231, 1205–1215.

A Multi-PTO Wave Energy Converter for Low Energetic Seas: Ensenada Bay Case.

2023-06-15T10:21:34+01:00

The paper presents a wave energy converter concept to harvest energy at Ensenada Bay, Mexico. The study area can be classified as a low energy sea due to the mean wave power being around 10 kW/m; the wave conditions are significant wave height of 0.5 to 2.5 and peak period of 5 to 20 s. The wave energy converter is formed by a torus buoy and anchored to the sea bottom by four connection structures distributed every 90°. The connected structures play a piston role, and their response is leveraged to run a power take-off system, for example, a linear or hydraulic system. This work does not focus on designing the power take-off system; therefore, it is simplified as linear damping, and an iterative process calculates its value. The hydrodynamic buoy study is made on the frequency domain with Nemoh BEM solver and the power absorption on the time domain with the WEC-Sim code. The capture width ratio is used to evaluate the wave energy converter performance and is a key factor in choosing the optimal size. The wave energy converter captures an average of 20% of the available energy and is well-fixed between periods 8 and 16 s. The present study achieves its target: providing a wave energy converter system to operate under site wave conditions.

Graphene oxide reinforced room-temperature-vulcanising elastomers for flexible wave energy converters

2023-06-26T19:35:03+01:00

Rubber products are widely used for marine applications, such as fenders, bumpers, ship launching airbags, and hovercraft skirts. Many of these products are made through a vulcanising (crosslinking) process where large-scale tooling for high-temperature and high-pressure moulding is required. The cost will be greatly magnified when manufacturing large-scale components, such as flexible parts for next-generation wave energy converters. Two-part polyurethane rubber (PUR) is a room-temperature, low-pressure curing alternative that could minimise manufacturing costs. However, the general inferior mechanical performance of such room-temperature-vulcanising (RTV) rubber (compared to vulcanised rubber) restricts the application of PUR in the marine industry. It has recently been reported that the addition of graphene oxide (GO) could significantly improve the internal bonding of polymer elastomers and thus provide better mechanical performance with a low filler loading. In the work presented in this paper, different types of silane coupling agents (SCA) were used to treat the surface of GO, and the potential application of PUR/GO composites under marine environments and their mechanical performance were investigated. More than 75 % increases in tensile strength were observed after 1 wt % of GO was added. Moreover, a significant reduction in water absorption occurred during the seawater immersion test. It is suggested that hydrophobic sites provided by silane coupling agents play an important role on the GO surface, where polar groups, such as hydroxyl and carboxyl groups, were replaced during the silane treatment.

A Dual Hardware-In-the-Loop (DHIL) platform for testing and validation of WEC subsystems

2023-06-27T14:09:30+01:00

This paper presents the design process and application of a dual hardware hardware-in-the-loop (DHIL) testing platform targeting components and subsystems of wave energy conversion devices. The DHIL testing methodology combines two HIL-equipped test rigs allowing to simultaneously test components mounted on each rig and connected to the same simulation loop. The platform was therefore developed combining a HIL test rig for drivetrains with a HIL test rig for structural components.
The DHIL testing platform has the capability to address multiple types of tests on critical subsystems and components within a WEC: characterization tests, for defining key performances of the equipment under test; accelerated tests, to assess qualitative and quantitative reliability features; and ultimate load testing, for survivability purposes. The overall aim of these tests is to identify weaknesses in an early design phase of device design or as a qualification activity prior the deployment of an ocean prototype. Additionally, HIL and DHIL tests can be used to assess the influence of a design update on the overall WEC model and to track failure interdependencies at a relevant scale. Finally, all of the above-mentioned activities can inform the development of a more accurate global numerical model, potentially at sub-system / critical component level, to be validated based on the test results.
The definition of the test rig mechanical and electrical input specifications is dependent on the understanding of the load envelope each subsystem / component will be subject to during its lifetime. To define such envelope, a research activity modelling three different device types, three deployment sites and multiple design situations (e.g. power production, parked, shut-down etc.) led to the creation of a load database that was combined with information from WEC developers.
After defining the input specifications, the rigs were designed by identifying the optimal layout, the key components and the setup of the overall test area. Analyses on mechanical, electrical and hydraulic parameters were completed to ensure the required performance of each rig could be achieved, while guaranteeing the safety during their operation.
The signal processing of each rig was also analysed, with the aim of defining the minimum rig latency to allow HIL and DHIL tests to be performed. This analysis took into account the possible interface characteristics of the simulator, the sensors and the main features of the models used for real-time tests.
An overview of the testing activities to be conducted within the IMPACT project will be presented in this paper. Such activities aim to de-risking the technologies at early stages and increasing their maturity through a structured process. The proposed approach has the final goal of demonstrating that the DHIL testing platform is capable to reduce capital-intensive activities, often associated with the development of large-scale prototypes, which is a critical factor for the successful development and time-to-market of a WEC.

Hardware-in-the-loop testing framework for active accumulator wave energy converters

2023-06-01T06:46:32+01:00

The hardware-in-the-loop technique is a hybrid co-simulation approach that joins real physical and virtual components. The real components may be the power take-off while the virtual ones the numerical models of sea wave and wave energy converter hydrodynamics. This simulation approach has been published with some regularity, however a review of the state-of-the-art has not been found, in particular in the research field dedicated to wave energy converters. Thus, the objective of this paper is to provide the ongoing results of such review, such as the generic testing framework and taxonomy, and their articulation on the simulation of a new oil-hydraulic power take-off concept. In this new concept, different power take-off nominal pressures may be regulated according to different sea states, by changing the hydraulic accumulator charging characteristics. Therefore, this active accumulator is intended to allow an increase on the range of possible Power Take-Off damping forces, thus, better wave energy harvesting performance. The paper results are also intended to support novice and advanced researchers to design simulation approaches in a clear and appropriate manner.

Multi wave absorber platform design, modelling and testing

2023-06-26T15:49:44+01:00

The subject of this paper is the development of physical and numerical models and a tank test programme to investigate the performance of a multi wave energy absorber platform (MWAP). The platform is inspired by the proposed designs for large scale platforms to be used for floating offshore wind (FOW). The modular design of the physical model enables a variable number of absorbers to be mounted to the platform, with up to 9 absorbers tested simultaneously. The absorbers used are a simplified version of a submerged pressure differential device, with each absorber incorporating a set of mechanical springs to approximate the response of the real internal air spring. Physical model tank tests will be undertaken during 2023, utilizing a range of environmental conditions representative of those at an exposed site on the west coast of Scotland, leased through the ScotWind programme and which has an appropriate water depth and wave resource for large scale wave energy exploitation. Measurements taken during physical model testing will be used to validate numerical models of the MWAP and will allow subsequent investigation of key drivers of annual energy performance, exploring platform configuration options not tested in the wave tank. The motivation for this project, design considerations and balance between tank scale & full-scale design requirements will be given. Discussion will be provided on the implications of the limitations and assumptions made during the physical and numerical modelling work, as well as next steps for utilisation of the tools beyond the scope of this project.

Analysis of data from the full-scale prototype testing of the WASP – A novel wave measuring buoy.

2023-06-14T20:41:25+01:00

To assess the viability of locations for wave energy farms, and design effective coastal protection measures, knowledge of local wave regimes is required. Current regime-measuring devices are expensive, and the aim of the WASP Project is to develop a low-cost, self-powering wave-measuring device. The Wave-Activated Sensor Power Buoy (WASP) comprises a floating body with a moonpool. The relative motion of the water level in the moonpool to the buoy will pressurise and depressurise the air above the water column, which will be used to drive a bidirectional turbine in the manner of an oscillating water column, which will operate in conjunction with a generator to recharge an on-board battery pack. It has previously been demonstrated through tank testing of scale-models that a sea-state may be estimated from measurements of the pressure within the air above the water column under controlled conditions [1]. Important statistical parameters relating to the sea state, such as the significant wave height, zero-cross period et cetera. may then be estimated from the spectral moments of the wave spectrum [2].

This paper seeks to validate the use of measurements taken of the pressure in a volume of air contained in a chamber above an oscillating water column (OWC) (which is in communication with the ocean) to estimate the sea state which induces motion of the water column.

The WASP was deployed from March to July 2019 at the Smart Bay test facility in Galway Bay, Ireland. The ¼ Scale test site was previously characterised by B Cahill UCC 2011 [3] using data captured by the Marine Institute Datawell WaveRider buoy from 2006 to 2011. Pressure signal data recorded by the WASP at a frequency of 8Hz was processed in to 30-minutes blocks to reflect the presented data from the WaveRider buoy, from which the WASP was moored a distance of 400m from during deployment. Power Density Spectra for 6,500 thirty-minute sea states were generated and an inverse transfer function between the WASP and the WaveRider produced. This allowed for the calculation of the associated significant wave height, Hs, and zero-crossing period, Tz, from the WASP pressure data for each thirty-minute block of data. At this time, outliers and bimodal spectra were removed to simplify the analysis, and the data was binned based on Hs and Tz ranges. For each bin, average inverse transfer functions were generated. A Bretschneider Spectrum [4] was produced using the Hs and Tz values from the WaveRider data for the respective binned WASP data. An average transfer function was then generated between the Bretschneider models and the corresponding sea states for the WASP. Using this final transfer function, a scatter plot of Hs and Tz for the test site was generated and compared against the previously produced scatter plot from the initial resource characterisation of the Galway Bay Test Site in 2011 in order to determine if the WASP is capable of measuring sea-states through measurement of time series pressure signals in the volume of air above the water column in the OWC.

Test rig for submerged transmissions in wave energy converters as a development tool for dynamic sealing systems

2023-06-20T11:42:05+01:00

A submerged transmission, fitted with a dynamic sealing system, in a wave energy converter (WEC) serves the purpose of transmitting the force, absorbed by a wave activated body, to an encapsulated power take-off (PTO) system, while preventing seawater from entering the capsule. Dry generator operation is generally a prerequisite for attaining long technical service life. Little attention seems to be devoted in publications to the study of dynamic sealing systems in WECs, and to test rigs for experimental verification and/or evaluation of the ability/performance of existing dynamic sealing systems in a controlled laboratory environment. This paper begins by presenting some of our earlier research within the focus area of dynamic sealing systems, incl. design considerations and typical operating conditions. This part also presents the 1^st laboratory test rig, used for verifying the sealing ability of the piston rod mechanical lead-through design in the 1^st and 2^nd full-scale experimental WEC prototype from Uppsala University. In 2021 project DynSSWE (Dynamic Sealing Systems for Wave Energy) was initiated. Drawing from experience, the project includes development of a new test rig, representing a tool for further development of dynamic sealing systems. This paper introduces steps in the design and development process of that new test rig, enabling accelerated long-term test runs with a setup of multiple piston rod specimens. The test specimens’ will be surface treated differently with the aim of improving the prospects of a long maintenance free service life. Since the new test rig is in the design stage, seal testing results are not yet reported. The presented work is funded by the Swedish energy agency with the aim of improving subsystem performance in wave energy devices.

The Impact of Uncertainty on the Control of a Multi-Axis Wave Energy Converter

2023-06-18T12:53:04+01:00

As global energy demands and climate concerns continue to grow, the need for renewable energy is becoming increasingly clear and wave energy conversion (WEC) systems are receiving growing interest. Over the years, many WEC systems have emerged but most of these systems are designed to extract energy from a single direction of motion. In reality, there are six degrees of freedom that a conversion device can potentially harness and a device that can operate on multiple axes, should be able to more effectively and consistently produce power. However, the design and control of the power take off (PTO) system for a multi-axis device is challenging due to the system complexity and nonlinearity. WEC systems often utilize optimal control techniques for PTO operation and leverage a prediction of the upcoming wave force to ensure power optimization. Prior work has clearly demonstrated that high power production can be achieved when an exact system model is used and the upcoming wave conditions are known, but uncertainty in the underlying model or the wave prediction can degrade performance. PTO control on a multi-axis WEC must leverage predictions of forces in multiple directions and if model predictive strategies are used, must leverage a simplified model of the WEC dynamics to be able to optimize in real time. The uncertainty in these predictions and the model could severely degrade the WEC’s power output. This work examines the control of a multi-axis WEC system, TALOS, and leverages machine learning to predict wave forces over the upcoming time horizon. TALOS is a point-absorber type WEC with multi-axis PTO system. The PTO uses a heavy ball that is attached to the hull with springs and hydraulic cylinders. When the hull is pushed by the external waves, the relative motion between the ball and hull moves the hydraulic cylinders causing them to pump a fluid through a circuit, thereby driving a hydraulic motor to produce electricity. This design has shown promising results in energy output but is more challenging to control since the PTO can move over six degrees of freedom. This paper seeks to quantify wave prediction uncertainty and its seasonal variation and to examine the impact of the uncertainty of the prediction on a model predictive controller’s ability to optimize the power output of TALOS.

Spectral control co-design of wave energy converter array layout

2023-06-27T14:18:32+01:00

Wave energy systems are designed to maximise energy absorption from the oscillatory motion of waves, which can make a crucial contribution in a new carbon free generation matrix [1]. Thus, the global ocean energy market (including wave and tidal) is expected to grow by more than 700\% by 2028 [2]. However, the harsh conditions faced in the ocean, the strong variability of the resource, and the high force/velocity ratios pose significant challenges to the development of the different ocean energy technologies. Therefore, to date, wave energy converter (WEC) technology is still under development and still needs a significant progress to enhance the energy conversion efficiency and, consequently, to become commercial viable. Crucially, to meet these existing challenges, there are a number of key considerations to progress the effectiveness and efficiency of WEC technologies. In particular, energy maximising control systems, to maximise energy absorption, and the optimisation of WEC array schemes, to take advantage of constructive interaction effects, are currently considered as some of the key drivers for the development of efficient WECs [1].

Recent studies have shown that total design of WECs, analysing the impact of each individual component on the rest of the system, is a key methodology to achieve effective designs. For example, the levelised cost of energy (LCoE) is considered in the objective function in [3], analysing the interplay between the control system based on a spectral controller and the specifications of the power take-off (PTO) system defined as the maximum stroke and force. This methodology has been generally labelled as `co-design', or, when the development is carried out in a control-aware manner, `control co-design'. Similarly, in [4], the interaction between control and optimal WEC geometry is studied. In addition, as discussed before, another key driver in achieving effective wave energy system is the operation of WEC arrays. In particular, in [5] and [6] different array layouts are analysed in terms of computational efficiency and control, respectively.

Considering the results in [5], where methods based on harmonic balance techniques are used to analyse the computational demand related to different array layouts, an assessment of the impact of different array layouts on the LCoE, is performed in this study. Figure 1 illustrates different array layouts (a), and the impact of different separation and number of WECs on the resulting LCoE (b).
(Figure in PDF abstract)
For the implementation of this study, a spectral control methodology is considered to virtually achieve optimal control solutions, even in constrained scenarios. In tandem, different layout templates, composed of multiple point absorber WECs, are considered. To achieve a clear global indicator based on the LCoE, each layout is analysed, in terms of the separation distance and the number of WECs, as well as capital and operation expenditure (CapEx and OpEx, respectively). Following the methodological guidelines considered in [3], a general co-design scheme is designed, essentially based on an exhaustive search method, indicating the interplay between different array layouts and LCoE.

A NEW SEAWATER LOW-HEAD TURBINE FOR THE OBREC

2023-06-28T16:25:10+01:00

The OBREC (Overtopping Breakwater for wave Energy Conversion) is based on the overtopping phenomenon [1] where incoming waves run over an overtopping ramp and fall into a reservoir located above the sea level inside a conventional rubble mound breakwater, or into a vertical caisson breakwater. The wave energy is transformed into potential energy, with some contribution of kinetic energy. Then, the flow is driven through a turbine addressing the final transformation into electrical energy. The OBREC prototype is located in the middle of the San Vincenzo breakwater, in Naples harbour (Italy). The key target of the whole pilot project was to demonstrate the high capacity factor of the system (expressed as the ratio of the electrical energy produced to the electrical energy that could have been produced at continuous full power operation), even in a low-energy wave climate. At the study site, in fact, the yearly average wave power was found to be 1.8 kW/m over the last 42 years [2]. However, during last 5 years, a maximum wave height of over 5 m has been registered, confirming the high reliability under severe wave conditions.

Due to the short monitoring period and to the undersize of the previous power take-off (PTO) systems, no definitive power matrix is available for OBREC [3]. The energy conversion efficiency of OBREC has been deeply investigated over the last years through both physical and numerical model tests. Within the ongoing full-scale monitoring activities in real environments, a new set of power take-off was installed. A propeller-type pico-turbine equipment was in 2021, and its final setting was completed in 2022. Performance and reliability of the entire structure were closely monitored under different “stress tests”, i.e. start & stop cycle in marine environment with a high variable combination of flow rate/hydraulic head.

This paper presents the design stage for the new equipment and preliminary results from these tests numerical and field tests.

Contestabile, P., Crispino, G., Di Lauro, E., Ferrante, V., Gisonni, C., & Vicinanza, D. (2020). Overtopping breakwater for wave Energy Conversion: Review of state of art, recent advancements and what lies ahead. Renewable Energy, 147, 705-718.
Contestabile, P., Russo, S., Azzellino, A., Cascetta, F., Vicinanza, D. (2022). "Combination of local sea winds/land breezes and nearshore wave energy resource: Case study at MaRELab (Naples, Italy)", Energy Conversion and Management, ISSN 0196-8904, 257, 115356, https://doi.org/10.1016/j.enconman.2022.115356
Mariani, A., Crispino, G., Contestabile, P., Cascetta, F., Gisonni, C., Vicinanza, D., & Unich, A. (2021). Optimization of Low Head Axial-Flow Turbines for an Overtopping BReakwater for Energy Conversion: A Case Study. Energies, 14(15), 4618.

Experimental investigation on the hydrodynamic performance of a pile-supported OWC-type breakwater

2023-06-30T09:06:56+01:00

The present study considers the design optimisation of an Oscillating Water Column (OWC) incorporated into a pile-supported breakwater structure. This has been achieved by performing a substantial number of scaled physical model tests and a methodical variation of key device parameters within multiple configurations. The key aspects that have been investigated include: (a) the geometric characteristics of the breakwater structure, (b) the pneumatic efficiency of the OWC, (c) the geometry of the OWC chamber and (d) the relative position of the OWC chamber within the breakwater. The present work considers (monochromatic) regular waves of varying steepness and effective water depth as incident wave conditions. The efficiency of the designed structure in terms of shore protection capacity and wave energy extraction was assessed by quantifying relevant transmission coefficients and the power output metrics. This has been achieved by using a combination of collocated and complementary measuring devices, such as arrays of wave gauges, pressure transducers and a high-definition video camera. The study concluded that systematic refinement of the geometrical parameters can substantially enhance the overall hydrodynamic efficiency of a pile-supported OWC breakwater. Additionally, it was found that a configuration featuring a chamber positioned in front of the breakwater, with a relative chamber breadth of 0.67, outperforms wider breadth configurations in terms of both energy extraction efficiency and reduction of the transmission coefficient. Taken together, the present study provides an in-depth analysis of the effects of key design parameters of a breakwater-integrated OWC, its efficiency and shore protection potential.

Weight Reduction Methodologies for Wave Energy Devices

2023-07-25T08:24:38+01:00

The OE Buoy is a wave energy converter based on the Backward Bent Duct Buoy oscillating water column model which generates electricity through the fluctuation in wave height. Wave energy conversion devices are often faced with a particularly high levelized cost of energy (LCOE) when compared to other renewable energy devices, and various investigations into bridging this gap have been carried out in recent history. Previous studies on the OE Buoy have suggested that a significant reduction in required construction material is possible as a result of reduced differential pressures acting across the hull walls in operational conditions. Various structural analysis campaigns have been conducted on sections of the hull to assess this theory.

A Finite Element Analysis (FEA) was performed on a full-scale model of the OE Buoy under maximum design wave loadings in operational conditions at EMEC’s Billia Croo test facility in Orkney, Scotland using Robot Structural Analysis software. A maximum pressure of 145 kPa was calculated for an 18.7 m peak wave height at Billia Croo. The OE Buoy was modelled for both static and dynamic load conditions under various constraint layouts. A modal analysis was conducted on the model which estimates the natural frequency of the OE Buoy to be approximately 6.67 Hz.

Wave Excitation Tests on a Fixed Sphere

2023-07-20T09:11:37+01:00

Wave excitation tests on a fixed sphere with the center at the still water level were carried out with three different physical wave basin setups. The tests were completed as a continued effort of the working group OES Wave Energy Converters Modelling Verification and Validation to increase confidence in numerical models of wave energy converters by generation of accurate benchmarks datasets for numerical model validation. An idealized testcase with wave excitation of a fixed sphere to be used with the benchmarks was formulated. The three investigated physical wave basin setups included: 1) a six degree-of-freedom load cell mounted to the top of the sphere, 2) a bending beam force transducer mounted to the top of the sphere, and 3) a system of six pretensioned wires mounted to the top and bottom of the sphere with force transducers attached to each wire. The aim of the present paper is to identify the best representation of the idealized testcase. To this end, the three experimental setups are inter-compared in terms of dynamic properties, sensitivity, and disturbances of the water phase from the presence of measurement equipment. Low inter-experiment variability was disclosed, i.e., 5-8% depending on wave-nonlinearity, indicating accurate representations of the idealized testcase across all setups. Setup 3 was found to be the more accurate representation and further work with this setup to release a public benchmark dataset was planned.

Performance enhancement of pitching WECs via oscillating water columns technology

2023-06-30T09:32:51+01:00

This paper describes the coupling of an oscillating U-shaped water tank, also called a U-tank, with a pitching floating Wave Energy Converter (WEC) to expand the response bandwidth. The performance of these energy converters strongly depends on their frequency response, and their resonance period is generally fixed once the geometric and inertial parameters of the system have been defined. The integration and appropriate control of dynamic inertial systems based on water ballast tanks enable slow tuning of the system's resonance with the incoming wave, maximizing energy extraction. The dynamic coupling of the hull with the water tank is then analyzed, and a passive control system is developed that acts on the air contained in the reservoirs of the U-tank by partitioning the volume within. Air expansion is then controlled by discretely adjusting the available, properly partitioned volume allowing the variation of the frequency response of the sloshing water tank and device. The resolution of the dynamics involves linear models based on the Boundary-Element-Method as far as hull hydrodynamics are concerned; a solution of the Euler equation describes the oscillating tank. Finally, the expansion and compression of the air contained in the reservoirs of the U-tank are assumed to be governed by a generic polytropic transformation law, and such a condition is linearized around the operating condition. The ISWEC device is adopted as a case study, and its energy harvesting working principle is based on the gyroscope technology. The results aim to confirm the ability to perform slow tuning of the device frequency response via regulation of the air volume of the sloshing water tanks.

Numerical investigation of the power performance of a wave energy converter comprising a multi-body power take-off

2023-06-20T12:06:53+01:00

This article presents a wave energy converter exploiting the pitch of a floating body moored to the seabed. When the floating body tilts under the action of an incoming wave, a movable mass, placed inside the hull, moves relative to the floating body and actuates an electrical generator. Most devices of this type have the drawback that the moving mass sequentially accelerates, slows down, stops and then repeats this sequence in the opposite direction. This generates an irregular instantaneous power output.

The proposed concept consists of (at least) two eccentric bodies having the same mass and revolving at
opposite speeds around a vertical axis. In this « counterrotating » solution, the oscillations of the float result in the continuous circular motion of the direct-drive PTO, though the global centre of gravity of the eccentric bodies moves back-and-forth along the symmetry axis of the device. If the eccentric bodies move at constant speed, their global centre of gravity moves in a sinusoidal manner along its pathway.

The present study aims to investigate, through modelling and numerical simulations, the influence of the main parameters, such as the phase and the PTO mass moment, on the performance of a counter-rotating device exposed to waves of various heights and various wavelengths. Optimal phase and mass moment are determined numerically. The resulting output power is close to the theoretical maximum power that can be harvested by the floating body.

Hybrid wind-wave systems: The case of the VolturnUS-S semi-submersible platform

2023-06-15T10:23:05+01:00

Wind-wave hybrid floating systems are subject to active research and development in academia and industry. Industrialization of hybrid wind-wave energy systems will benefit from modular designs, allowing floating platforms to be used with or without wave energy converters (WECs). This work explores the design process to use oscillating water column (OWC) WECs as modular add-on features to existing semi-submersible floating platform designs. As the base case design, the IEA 15 MW offshore reference wind turbine with the VolturnUS-S floating platform designed by the University of Maine was used. A design approach was proposed, which involved various tools, such as OpenFAST developed by NREL, and the OWC Modelica tool developed at IST/ULisboa. Aero-hydro-servo-elastic simulations were performed to analyze the influence of the wind turbine energy generation design modifications and damage equivalent loads (DELs). The hybrid platform was modeled in the OWC Modelica tool to assess the wave energy conversion and the interaction between OWCs and the floating platform. The investigated hybrid design did not significantly increase DELs of the wind turbine in aero-hydro-servo-elastic simulations. The hybrid system does not decrease the LCOE
compared to a standalone floating wind turbine. The main cost driver was identified to be the material of the OWCs' ducts. Furthermore, the coupling forces between OWCs and the floating platform were found to be in the same order of magnitude as the wave excitation and radiation forces. The possibilities of including these forces in OpenFAST and the challenges for this are discussed.

Analysis of the viability of a radial Double Decker Turbine for application in Oscillating Water Column devices

2023-06-27T16:11:08+01:00

Oscillating Water Column (OWC) devices are the most widespread among the different systems developed to harness the wave energy available on our coasts. Air turbines, normally used as Power Take Off (PTO) in these devices, are their most controversial part. Unidirectional turbines were first used with rectifying valves to take advantage of both characteristic operating stages of the OWC: exhalation and inhalation. The use of valves was quickly discarded and bidirectional turbines, which rotate in the same direction regardless whence the flow comes, were adopted as the most common solution despite reaching lower efficiencies than unidirectional ones. The Twin Turbine Configuration (TTC), based on the use of two unidirectional turbines, emerged then as a promising system, its main drawback being the duplicity of the equipment. The Double Decker Turbine (DDT) concept has been recently introduced to overcome these limitations since it combines in a single design the two typical solutions: self‐rectifying behaviour and the use of unidirectional turbines. In this work, the performance of a radial DDT, composed of an InFlow Radial (IFR) turbine and an OutFlow Radial (OFR) turbine, is assessed. A CFD model is used to design and optimize an IFR turbine to be mounted in combination with an OFR turbine taken from the literature. Finally, a non‐steady analysis is carried out assuming sinusoidal flow conditions. Results demonstrate that this radial version of the DDT could compete with its axial version and with other alternatives.

An Early Design Phase Method for Characterizing and Comparing Wave Energy Converter Archetypes

2023-06-07T17:54:58+01:00

The ocean is both a precious resource for coastal communities and a vast source of largely untapped renewable energy that can help combat climate change. This presents a unique opportunity for wave energy converters (WEC) to provide both utility-scale and small-scale electricity for resilient, energy-efficient communities that can better withstand the effects of climate change. Designing a WEC is not straightforward as the ocean supports a wide range of uses and the highly energetic environment can threaten survival. This requires WEC designers to deconflict their use area and select deployment locations that minimize risk. Subsurface WEC designs provide a potential solution for these challenges as their deployment depth can mitigate area-use conflicts with other ocean users. A fully submerged WEC also has increased survivability due to being in a less energetic environment and at a depth that they are less likely to collide with ocean vessels. However, a submerged WEC may also suffer from the less energetic environment as it decreases the amount of electricity the WEC can generate. These tradeoffs make it unclear when subsurface WECs are beneficial to deploy. There is little research to date that offers comprehensive understanding of this field of WECs in regard to their capabilities or constraints. This paper presents a method for characterizing and understanding the functions of WEC archetypes by leveraging functional decomposition to examine high level overlaps between archetypes. Functional decomposition is a well-established engineering design method for breaking down complex systems into constituent parts to track flows of energy, information, and materials. The method enables engineers understand how each component contributes to the overall functionality of the system and enables us to analyze functional performance overlaps between WEC archetypes. We apply this method to submerged WECs and present an analysis of the high level functions. The findings of this case study shed light on the functional uniqueness of submerged WEC archetypes and can be used to guide future development in subsurface WEC technology. Better identifying functional overlaps between WEC archetypes will help researchers and developers generate effective designs that build resilient coastal communities.

DESIGN, INSTALLATION, CAPACITIES AND EXPENSES OF AN INDOOR MULTIPURPOSE MODULAR 2D WAVE FLUME AND CIRCULATING WATER CHANNEL

2023-06-27T10:27:18+01:00

In this article all the details related to the design, installation process, working capacities and expenses of a modular indoor laboratory flume are presented. The facility is able to work as wave flume or circulating water channel. Flume structure design distributes appropriately the load, to ensure the structural safety of the building. The length of the wave flume depends on the number of these structures, 2.5 m each, which are attached one to each other. Presented cases are 12.5 m and 25 m length. Width is 600 mm and can be filled up to 700 mm water depth. In this work, a multipurpose 50 m³ underground water storage tank is used to feed the flume through three centrifugal pump pressure group equipped with a variable frequency drive. Main structure is made of a welded stainless steel square-tubular frame. Seabed is flat, made from a folded stainless steel 5 mm sheet metal and a sandwich-type assembly system allows the fitting of the lateral glasses to the seabed frame. A fine-tuning regulation system allows a perfect alignment of each of the modules with the adjacent. This manufacturing and assembly strategy avoids undesirable whirls effects. Two types of wave generators have been tested, an in-house developed one for regular waves and an externally manufactured one with added options: irregular waves, and active absorption system of reflected waves. At the end of the flume, a parabolic beach has been installed as wave energy dissipation system. An in-house developed regulation system sets the optimal position of this device. Several dissipation options have been tested: height, slopes and perforated surfaces. A foldable wave generator paddle and a set of pipes and valves, allows working as water channel by recirculating water to the underground storage tank, and therefore, increasing flume research versatility. In the upper part, there have been mounted guides along the whole facility so that different ad-hoc devices can be attached onto it, such as testing models or necessary instrumentation. Among this, we can mention wave gauge module together with resistive wave gauges; digital, ultra-low pressure sensor that is fully conditioned and temperature compensated; CompactRIO controller as data acquisition system (National Instruments, cRIO-9063 model) with an inputs voltage module (National Instruments-9205, C-series). A computer with a Labview program that has been developed specifically for that purpose controls all data recording. In addition, the expenses of the main components are also presented, which demonstrate the cost effective of this indoor laboratories where small scale models can be tested. In this work the design characteristics and operation of the main components of the wave flume, such as the wave-maker, wave absorber, wave probes and data acquisition system are reported in detail together with the full range of wave parameters achievable in any experimental campaign in the Le Méhauté chart. This technical recommendations provides any research group with useful guidelines and details necessary to construct their own domestic wave flume being good value for money and unachievable otherwise.

Analysis of the North Atlantic offshore energy flux from different reanalysis and hindcasts

2023-06-14T08:36:14+01:00

To date there is a wide range of wave reanalysis and hindcasts available to the scientific and engineering community which are commonly used for different applications, including downscaling or the estimation of the wave energy resource (Morim et al., 2022). These long datasets have been created using different combinations of forcing fields, physical parameterizations, and numerical choices (like spatial and spectral resolution). All these elements have a direct effect on the accuracy of the wave models’ output (e.g., Alday et al., 2021) and thus, they are one of the main reasons for the differences between these products. In the present study we analyze the significant wave heights and peak periods characteristics from a selection of global datasets. We additionally include results from a hindcast created using the WAVEWATCH III model, with adjustments specially aimed to reduce uncertainties of the wave energy resource along the Atlantic coasts of Europe. Models’ output is compared with buoys and altimeter data from the latest ESA (European Space Agency) CCI Sea State V3 product. Preliminary validation of the hindcast we have generated for the North Atlantic already show an important bias reduction for wave heights in the 2.5 to 11.5 range compared to ERA5 wave product. Using the relevant wave parameters, we estimate the power density and quantify the differences between databases. Then, based on scatter diagrams obtained from the joint distributions of significant wave height and peak period, the differences in the power captured by wave energy converters (WEC) related to different wave data sources will be quantified (e.g., Babarit et al., 2011; Henriques et al., 2016).

Wave Spectral Analysis for designing Wave Energy Converters

2023-01-11T15:16:42+00:00

When designing a Wave Energy Converter (WEC) it is inappropriate to consider overall bulk parameters, like for instance, the typical scatter diagrams Hs-Tp. The reason is that in most of the world wave conditions are complex and involve several long-term wave systems, each with different characteristics (Fig. 1a). This is true even for apparently simple basins like the Mediterranean or the North Sea, in the open ocean this is certainly the case with swells arriving from remote places. Naturally, each Wave System (WS) has different characteristics, like their seasonal variability, typical wave height, but most importantly, the wave period and the spectral width are fundamental design parameters. As WECs must resonate on the exciting force to effectively convert energy, that resonance depends on the wave period. On the other hand, the spectral width indicates how disperse is energy in frequency and direction, the broader the spectrum (more entropy) the more difficult to tap its energy. Therefore, a smart WEC’s design cannot aim to the full spectrum, but to the particular WS with the more favorable characteristics (high energy, narrow spectral band, low frequency). For the present application we focus on a site in the Galapagos Islands. Apart from the aforementioned spectral conditions, this tropical site offers a couple of additional advantages, namely a small seasonal variability, and the absence of high extremes (to be withstood by the device even if idle), with still an interesting average resource (~20 kW/m). At this site we find three main WSs (see Fig. 1, a and b). WS1 belongs to the southerly swells, from the Antarctic extra-tropical storms. WS2 corresponds to swells from the northern hemisphere. WS3 is due to the southern trade-winds, active locally in the area (wind-sea). Clearly, the target for energy conversion is WS1, but for a non-directional device (e.g., point absorber), WS2 is also interesting due to the similar peak frequency. In addition, WS1 and WS2 are seasonally complementary, as they are mainly active in the austral and boreal winters respectively. The identification and separation of these different WSs allows us to determine their characteristics and to obtain specific design parameters. The modulation strategy for the WEC design is the subject of a parallel paper.

Fig. 1. a) (left) Spectral wave conditions in the Galapagos Islands. WS1: Antarctic swells, WS2: northern swells, WS3: southern trade-winds, WS4: Central America wind jets. The colorbar indicates number of occurrences within 1979-2015. b) (right) Average one dimensional spectra for the WSs in panel a. Data obtained from GLOSWAC: https://modemat.epn.edu.ec/nereo/.

Long term wave load trends against offshore monopile structures: A case study in the Bay of Biscay

2023-01-26T14:15:23+00:00

This study examines the trend in wave parameters over available annual records in the Bay of Biscay, where EWTEC
2023 is being hosted. There have been various renewable offshore energy projects in the 21st century in this region,
such as the OWC (Oscillating Water Column) plant in Mutriku [1], or BiMEP (Biscay Marine Energy Platform)[2],
the infrastructure for testing ocean devices in Armintza, where better characterization of the wave resource could help
to provide design information to developing wave energy technology. Other studies [3] have shown that wave height
has increased in recent decades, as well as the frequency at which extreme waves occur, which affects the fatigue
experienced by deployed devices. The aim is to study the effect of the maximal wave height, its frequency, and loads
on monopile cylindrical structures, which are basic component structures for wave energy converters and offshore
wind turbines. The effect of this evolution, against these generic structures in offshore wind and wave energy devices,
is studied using the 20th century reanalysis ERA20 (1900-2010) of ECMWF (European Centre for Medium-Range
Weather Forecasts) [3]. A significant increase in two forces, drag forces and inertial forces, of up to 15%, is computed
over 110 years at a gridpoint near the Western Basque coast, which constitutes a strong positive slope that can be also
associated with climate change, and has strong implications for the design of new marine renewable energy technology.

Numerical modelling of wave and tidal current interactions and their impact on wave parameters

2023-01-26T14:48:19+00:00

In regions where both waves and tidal currents coexist, tidal flow can significantly alter wave parameters and hence affects the estimation of wave energy resources. This study uses a coupled wave-current numerical model to evaluate the influence of wave-current interactions on wave parameters. To achieve this, the simulation was performed in 3 stages. At first, a large-scale North Atlantic wave model was constructed using the spectral wave model TOMAWAC to generate wave conditions and boundary inputs. This wave model was calibrated and validated at four sites around the UK using field measurements. Secondly, a small-scale numerical model covering Pentland Firth and Orkney Waters, Scotland, UK, was chosen, and tidal flow and current speeds were simulated by the three-dimensional flow model TELEMAC 3D. As with the wave model, the flow model was also calibrated and validated with site measurements from an ADCP; thus, both models were validated. In the third stage, a coupled TOMAWAC-TELEMAC 3D model was employed for the small-scale region, and the wave parameters generated by the large-scale model were input as boundary conditions. The TOMAWAC-TELEMAC 3D coupled model was validated with field measurements at two locations in Orkney Waters, where waves and currents coexist. Various wave and tidal currents parameters produced from the coupled model are presented in the paper. To evaluate the wave-current impact on wave parameters, a qualitative and quantitative analysis of these parameters is carried out, and the results are presented and discussed in the paper. The large-scale and small-scale numerical models developed in this study are useful tools for generating wave boundary conditions and wave energy resource assessment, helping researchers and engineers better understand the characteristics of wave-current interactions.

On the errors in annual energy yield estimation due to monodirectional wave spectra assumption

2023-07-06T21:07:00+01:00

The wave energy sector has made significant progress, and its potential is tangible. However, in order to make wave energy production a key tool in the energy transition, it is crucial to minimize the uncertainty associated with the energy production assessment. Trustworthy evaluations are reachable only through detailed analyses based on reliable, accurate and representative wave data.
The use of synthetic parameters exclusively derived from a frequency spectrum leads to the neglect of directional information and the combination of wind waves and swells is treated incorrectly. This study investigates errors associated with relying solely on the frequency spectrum and points out the relevance of directionality.
The island of Pantelleria serves as a case study, and the results show that PeWEC (Pendulum Wave Energy Converter) performance is overestimated when the monodirectionality of the waves is assumed. Neglecting the directional information contained in the frequency-direction spectra results in an overestimation of energy production of 40%.

Validation of ERA5 Wave Energy Flux through Sailor diagram in Spain (2005-2014)

2023-06-26T10:09:48+01:00

The aim of this study is to validate the ERA5 Wave Energy Flux (WEF) estimations against observational data. To that end, 0.5° x 0.5° resolution ERA5 reanalisis WEF values and corresponding data from 15 directional REDEXT buoys of Puertos del Estado surrounding the Iberian and Canary Islands’ coast covering a period of ten years (2005-2014) have been used.

In this study, the Sailor diagram and its methodology [1] have been used to compare the skill of the model for a two-dimensional variable (WEF). The methodology on which the Sailor diagram is built proposes a diagram along with different statistical indices. In this particular case, WEF is a vectorial magnitude, and its zonal and meridional components (WEFx and WEFy) have been assessed at the same time.

To compare the observed data with the ERA5 data, well-known indices extended to two dimensions such as RMSE, correlation and bias have been used, as well as variances of each component. Furthermore, to analyse if the WEFx and WEFy components of the ERA5 data are rotated in relation to the observations, their relative rotation angle, eccentricity, and congruence coefficient of the first EOF (Empirical Orthogonal Function) have also been examinated. This way, the analysis is extended to fully cover the dimensionality of the data.

[1] Sáenz J, Carreno-Madinabeitia S, Esnaola G, González-Rojí SJ, Ibarra-Berastegi G, Ulazia A. The Sailor diagram – A new diagram for the verification of two-dimensional vector data from multiple models. Geosci Model Dev. 2020;13(7):3221–40. https://doi.org/10.5194/gmd-13-3221-2020

Impact of Resource Uncertainties on the Design of Wave Energy Converters

2023-07-11T14:34:20+01:00

In order to become economically viable and compete with other more traditional energy sources in the energy market, offshore renewable energies, such as wave energy converters (WECs), tidal energy converters and even floating offshore wind turbines, still need significant development. Key aspects for the development of these technologies include: (i) optimising the designs by reducing material use, (ii) increasing the energy generation capacity, (iii) enhancing the durability of key components, and (iv) improving accessibility and availability. All these aspects rely on accurate metocean data and the use of incomplete or inaccurate metocean data can incorporate a higher uncertainty to a design process; an area where the uncertainty level is already significant.

The uncertainty in metocean data affects the development of any stage in the chain, from the response of the system to the final energy generation capabilities. The most common sources of metocean data in the ORE sector are observation buoys and re-analysis datasets. However, long-term resource variations suggested in various recent studies suggest that long datasets are required for a better understanding of the resource. Similarly, different international organisations recommend considering relatively long periods of data. Therefore, re-analysis datasets, with decades of data since 1950, seem to be crucial, but inaccuracies of such datasets are well-known.

As a consequence, the present paper suggests different statistical bias-correction techniques in order to improve the quality of re-analysis datasets: (i) the delta method, (ii) linearly-spaced quantile-mapping (LQM) and (iii) Gumble-distribution-based quantile-mapping (GQM). In this sense, first the bias of the re-analysis ERA5 dataset is evaluated in three different locations: Bay of Biscay, off the West coast in Portugal and off the West coast in the US. That way, the capacity of the different statistical bias correction techniques will be studied, identifying the advantages and disadvantages of the different techniques, and quantifying the uncertainty in the resource re-analysis data.

Finally, the impact of this uncertainty on the design parameters is studied. Considering that one of the most relevant design parameters is power generation, differences in power generation estimations based on different resource datasets are assessed in the three locations. Figure \1 illustrates part of these results, showing the difference in the annual mean power production (AMPP) on the left and the variation in mean-to-peak ratio on the right. The underestimation of the raw ERA5 re-analysis is clear (about 40%), which demonstrates, on the one hand, the perils of using raw re-analysis data, and, on the other, the need for adequate bias-correction techniques. However, this conclusions are very dependent on the specific location and resource characteristics and, thus, the same analysis is conduction in geographical locations with significantly different resource conditions.

Discussions on Wave energy period in higher wave energy potential marine waters of Taiwan

2023-06-14T08:17:49+01:00

For evaluating wave energy, wave energy period Te is commonly adopted. However, the in-charge government agencies in Taiwan traditionally provided with peak frequency Tp without giving information on the transferring relationship between Te and Tp. This results in difficulties on correctly evaluating wave energy potentials at certain marine waters. Thus, existing spectrum data need to be derived to calculate values of Tp and Te so as to derive representative transferring coefficient Cep for long-term evaluations on wave energy. This study aims at deriving values of Cep at relatively higher potential marine waters of Taiwan. As shown in Figure 1, the data were collected from two stations in Keelung marine waters and one station within marine waters for offshore wind of Changhua. By choosing different criteria for Cep, selected data were regressed for obtaining representative Cep in different seasons. For conservative point of views, the values of Cep ranging from 0.85~0.87 from data in northeaster monsoon months, as shown in Figure 2, being generally slightly smaller than those of the whole year, were adopted at the three stations. It is also found that the representative values of Cep were derived mainly from data comprising of exploitable wave power densities of between 2~80 kW/m². Data collection instruments, selection criteria and associated outcomes shall be detailed in the full paper.

Internal waves

2023-01-03T18:42:10+00:00

Internal waves are ubiquitous oceanographic features that occur in various forms across the world’s oceans. They manifest themselves as interface waves across ocean density layers that represent the interplay between buoyancy and gravitational forces, and are typically classified as linear and nonlinear internal waves. Nonlinear internal waves are characterized by isopycnal displacements that can exceed 30 m, and current velocities that approach 1 m/s. Internal wave energy converters, if developed, could have the advantage of no surface expression and provide for the availability of renewable ocean energy in regions of scant surface wave energy resources.

Here, internal wave energies were computed at two locations: the New Jersey continental shelf and the coast of Central California. The available energy resource calculated for internal waves was then compared against surface gravity wave resources at each of these locations. Results suggest that the internal wave energy flux is comparable to that of surface waves on the New Jersey continental shelf during the summer of 2006 but is two orders of magnitude lower than that of surface waves in Central California during the summer of 2017. When expressed in terms of forces on a cylindrical structure, internal wave forces are an order of magnitude lower than that of surface waves on identically sized cylinders. However, the forces of the two resources are comparable when the diameter of the cylinder is doubled for the internal wave calculations. This suggests that while a larger energy converter would be required to harness internal wave energy, the larger size could be a reasonable tradeoff for advantages such as the lack of surface expression and the availability of energy in regions of limited surface wave energy resources.

These results could potentially pave the way for longer-time scale characterization of internal waves and the development of devices to harness this energy resource (thereby powering the Blue Economy), while augmenting the portfolio of marine renewable energy resources. Further, the dominance of internal wave energy resources during summer seasons when the water column is stratified can augment the surface wave energy resource that can be diminished during the summer season.

Feasibility of wave energy harvesting in the Ligurian Sea

2023-06-27T08:28:37+01:00

A series of short and mid-term guidelines have been established due to the pursuit to offer clean energy and reduce the environmental impact in the Mediterranean and European environment. Currently, the scientific community and the industrial sector promote to find new technologies and means to achieve these regulations. Efforts to provide sustainable ways to supply electricity in Italy have led to the exploration of marine renewable energies (MRE) in the Mediterranean Sea. In particular, in the Ligurian Sea, where the wave climate can provide one of the higher energy sources, represents an optimal opportunity for supplying this energy resource to coastal cities. However, the wave conditions are not as significant as those in other marine regions around the world. There are several devices currently developed which can be applicable to the region. Hence, an evaluation from a technical and economic perspective is advised. Additionally we also investigate the scaling and survival considerations for Wave Energy Converters (WECs) when facing extreme storm events. The proposed study offers the evaluation of a sustainable alternative for powering the electricity mix in the Liguria region, through the exploitation of the wave energy resource. Attractive findings emerge after the assessment of eight floating-body wave energy converters.

Identification of optimal sites for the deployment of wave energy converters: the importance of a technology-centred approach

2023-07-03T14:33:40+01:00

Driven by climate issues and geopolitical uncertainties, Europe faces the need to transform its energy supply dramatically and quickly. Various renewable technologies are proposed as a medium- to long-term solution for an environmentally and economically sustainable energy mix: among the available solutions, wave energy converters (WECs) are attracting growing interest due to the large untapped wave energy potential in European seas. In this context, the choice of optimal locations for the use of wave energy is fundamental to limit the technological gap with other fully developed conversion technologies, and to ensure competitive energy costs. In this paper, we compare different possible strategies to identify suitable sites for the installation of WECs, namely the one based on pure analysis of the wave energy resource, and that considering the productivity of the device in different sea states, i.e., its power matrix. Using the performance matrices of notional WECs, particularly an Oscillating Surge Wave Energy Converter (OSWEC) and a Heaving Point Absorber (HPA), we estimate optimal locations on the Italian coasts and highlight the advantages and disadvantages of the two approaches. The analysis shows the importance of a technology-based approach for the spatial planning of future wave power plants and highlights significant differences compared to the approach that can be used for the preliminary identification of sites for wind farms and, especially, for photovoltaic plants. We use the obtained results to introduce the MORE-EST platform, a novel web-based tool for straightforward estimation of wave resources and WECs productivity in European seas. The proposed platform is able to integrate information on wave resource assessment, bathymetry, marine space use and technological features, and represents a tool aimed at researchers, WECs developers, and policy makers.

Upsampling wave temporal resolution: Investigating wave parameters and the influence on WEC power performance

2023-06-17T21:57:01+01:00

Power production of wave energy converters (WEC) predicted in the time domain utilize wave resource parameters and time-domain hydrodynamic model simulations of the WEC. While the hydrodynamic model provides high temporal resolution of power production (10’s of Hz), the wave resource parameters are often based on frequency-domain calculations with temporal resolution of 30 minutes to an hour. However, real ocean wave conditions vary on much shorter time scales. Relying on frequency-domain calculations will not be sufficient to capture the short-term variability and accurately predict WEC power production for a standardized methodology that follows power system requirements. These requirements need forecasted data with high sampling frequency for accurate energy predictions. Looking specifically at resource characterization, high temporal resolution datasets are not publicly available or do not exist for many coastal locations. Due to data availability, low temporal resolution datasets are being used in a majority of studies to generate representative wave conditions as inputs to numerical simulations. Representative wave conditions are used to generate wave spectrums. The issue with this practice is spectrums are then used to predict the efficiency of systems that will not accurately capture the variability of waves in short timeframes. Creating a standardized methodology to increase the temporal resolution of metaocean conditions to inform model development can provide better forecasting of power production. Random amplitude, Fourier coefficient methods have been suggested for WEC simulations of finite durations to improve the observed variability in wave heights and power production. Variability using this method does increase for finite durations compared to the commonly used deterministic amplitude method. In this paper we will investigate the influence of wave parameters (significant wave height, maximum wave height, and energy period) on the prediction of WEC power production. A better understanding of the influence of these parameters will provide a path towards future standardization methodology for resource inputs for time-domain modeling.

On spatial interpolation of ocean energy source variables: A comparative analysis

2023-06-30T12:36:00+01:00

In the context of wave energy systems development, the estimation of wave parameters such as significant wave height (Hs) and energy period (Te) over the entire ocean surface is of paramount importance. These information are crucial for estimating the energy harvesting potential of deployment sites, designing wave energy converters (WECs), planning optimal maintenance intervention frequency, and assessing the impact of waves on coastal communities. However, measuring Hs and the Te at every point in the ocean is impossible due to the vastness of the ocean and due to the cost and difficulties of installing and maintaining wave instrumentation buoys, since these have to survive in marine environment, which is particularly hostile. As a consequence, the amount of data is too limited and sparse in space for the practical and precise performance of these analyses. To address such data scarcity and sparsity, we analyse in this paper various spatial interpolation techniques employed to fill the spatial gaps in the wave parameter datasets. These techniques are compared in their performances in estimating the wave parameters in space given a set of known measurements in limited sampling locations. Three types of interpolators are considered: linear interpolator, spline interpolator, and radial basis functions (RBFs) interpolator. These algorithms are trained and tested on a public dataset of wave parameters from Copernicus Marine Service in an area between the coastlines of South England and North France. To simulate the available data scarcity and sparsity, only limited percentages of the ocean area are considered covered and available in the training stage (from 0.01% to 1%). The performance of each interpolator is evaluated on the remainder of the considered area in terms of Normalized Root Mean Square Error (NRMSE) and on the Normalized Mean Absolute Error (NMAE) achieved by the algorithm in reconstructing the parameters at these unsampled locations. The results of this study demonstrate the feasibility of spatial gap-filling of wave parameters, and demonstrates that the RBF algorithm outperforms the other two algorithms both in performances and, even more, in terms of robustness to different training points sampling.

The application of temporal gating in the measurement of response amplitude operators

2023-06-07T21:29:26+01:00

Scale model testing of wave energy converters (WEC) in wave flumes and basins is essential for full scale development. One of the parameters commonly measured is the response amplitude operator (RAO), which represents the response of the WEC to wave excitation. These are typically measured at discrete frequencies using regular waves. This approach can be slow, depending on the fidelity and frequency resolution. An alternative is to use a broadband wave source, however reflections from walls can significantly contaminate the frequency response measurement. In this paper, a rapid method for measuring the RAO is presented using a chirp signal to generate waves in the frequency range of interest to measure the transfer function, and a temporal gating technique (commonly used in experimental acoustics and RF engineering) to remove reflections. The technique will be demonstrated on numerical data, as well as two scale experiments.

Analysis of the impact of floater interactions on the power extraction of a dense WEC array with adaptable nonlinear PTO

2023-07-17T06:59:53+01:00

This research focuses on studying the interactions between a closely spaced wave energy converter (WEC) array with an adaptable hydraulic power take-off (PTO) system. The boundary element method is used to extract the hydrodynamic and hydrostatic coefficients, while the power extraction and hydrodynamic behaviour of the array are simulated in irregular waves with a mixed frequency-time-domain (MFT) model to include the nonlinear PTO forces of each array element. The interactions between floaters are assessed and the influence of adaptability on the behaviour of the total array and its elements in comparison to single floater performance (i.e., the q factor) is analysed. The differences in performance of the WEC array are assessed with three levels of adaptability: no adaptability, adaptability per sea state and adaptability per incoming wave.

NEW DESIGN OPTIONS FOR THE IMPROVEMENT OF THE MUTRIKU POWER PLANT

2023-06-14T21:04:52+01:00

The research based on the development of OWC devices continues growing and nowadays there are endless options that have already been studied. From the type of turbine and its control, to the most efficient energy conversion system or design, the OWC technology needs to be further developed in order to be economically attractive.

This research studies several possibilities to improve the efficiency of the Mutriku breakwater wave plant. On the one hand, the L-shape configuration of this plant allows studying a new configuration based on the coupling of the well-known U-shape [1], to create a new L+U-shape breakwater. In addition, the installation of harbor-walls has been proven as a promising option to enhance the interior damping of the camera [2], and this option seems to be very promising because of the design of the Mutriku plant allows its performance. Finally, a design based on the sharp-edges round off its being studied in order to reduce the energy losses and therefore enhance the energy harnessing of the plant.

The experimental work has been carried out in the 12.5 m long wave flume located at the laboratory of Fluid Mechanics of the Energy Engineering Department (UPV/EHU). The physically constructed OWC device corresponds to one of the cameras installed at the Mutriku breakwater wave plant, an isolated camera, according to the construction plan and applying a 1:36 scale. All the experiments were carried out at two tides that correspond to the medium and maximum equinoctial live tides of Mutriku location. According to the information of the Basque Coast sea states [3], a constant incident regular waves of 30 mm were generated at several periods: 0.7<T[s]<1.7 for the medium tide and 0.7<T[s]<2.1 for the maximum tide.

The L+U-shape was created with two lateral walls joined by frontal walls of different heights (W), as shown in Figure 1-left, that were positioned at several distances from the physical model. However, another L+U-shape was considered using a transversal wall of different heights and without lateral walls (TW, Figure 1-right), and placing at several distances form the plant.

The obtained results reveals that the promising configuration corresponds to the one containing lateral walls, so that the influence of harbor walls will be further studied.

References:

[1] C. Xu, Z. Liu, G. Tang, Experimental study of the hydrodynamic performance of a U-oscillating water column wave energy converter, Ocean Eng. 265 (2022) 112598. doi:10.1016/j.oceaneng.2022.112598.

[2] D.H. Yacob, S. Sarip, H.M. Kaidi, J.A. Ardila-Rey, F. Muhammad-Sukki, Oscillating Water Column Geometrical Factors and System Performance: A Review, IEEE Access. 10 (2022) 32104–32122. doi:10.1109/ACCESS.2022.3160713.

[3] Y. Torre-Enciso, I. Ortubia, L.I. López de Aguileta, J. Marqués, Mutriku Wave Power Plant: from the thinking out to the reality, 8th Eur. Wave Tidal Energy Conf. (EWTEC 2009). (2009) 319–328. http://tethys.pnnl.gov/sites/default/files/publications/Torre-Enciso_et_al_2009.pdf.

Intracycle Active Blade Pitch Control for Cross-Flow Tidal Turbines Using Embedded Electric Drive Systems

2023-05-31T09:41:01+01:00

Cross-flow tidal turbines (CFTTs) have proven advantages over horizontal axis turbines in terms of high power density per unit area, simplicity of design, and operation independence from inflow direction. However, they suffer from an unsteady flow regime which can comprise dynamic blade stall and thus problems of material fatigue or even failure. Active pitch control mechanisms on blade level have been shown to provide a potential solution, when continuously adjusting the pitching angle of each individual blade during the whole rotational cycle of the turbine.

As part of the research of the OPTIDE project, in this study, electric drive systems embedded in the blades of a CFTT flume model are proposed aiming to realize an active pitch control with high efficiency and fast response. The blade embedded actuation allows for reasonable flow conditions. For full scaled on-site applications this is required to reduce hydrodynamic losses and to protect the actuators and electronics from the harsh environmental and operation conditions. Based on the expected hydrodynamic loads from numerical flow simulations (CFD), several types of actuators are considered. The first type has brushless DC motors installed at both sides of each blade. Along with a gear box with proper reduction ratio, the actuators are able to provide required torque within expected cycle period. The second type of actuator drives the blade directly, which always results in faster pitching action, higher drive efficiency, more accurate positioning of blades, as well as simpler structure. Specifically, the shaft of each blade is designed as the primary of a limited angle torque motor, while the blades are used as the secondary with magnets inside. To mitigate the potential saturation effect on the iron of the primary, the blade can also be used as the primary, where there always exists much larger space for windings. In this case, the magnets are now located at the shaft. By doing this, it is expected to output larger torque in a wider range of pitching angle as compared with the original one, while almost the same power is required. An experimental test bench for a single blade with both types of actuators is built to verify their ability of a fast and accurate pitching control. This also lays the foundation of identifying the optimal pitching angle for the control and inhibition of dynamic blade stall at various flow conditions and blade positions within the rotational cycle of the turbines. After successful optimization and testing of the model scaled mechatronical design, the actuators will be up scaled for realistic applications.

Numerical optimisation of the active lift turbines using OpenFoam's overset method

2023-01-23T14:23:55+00:00

This work presents a preliminary study of the numerical optimization of "active lift turbine" using computational fluid dynamics (CFD). This active lift turbine is designed to improve efficiency of vertical axis energy recovery turbines (wind and tidal turbines). It uses a crank rod system to convert normal forces into momentum in order to improve the yield.

After a validation work of CFD model in two dimensions, the numerical optimization of the tidal turbine system is carried out. In fact, due to the specific characteristics of the active lift turbine, the blades do not follow a circular trajectory (Fig. \ref{fig:combined}). The OpenFoam overset module is therefore chosen because it allows to control the motion of the blades easily \cite{Chalmers}. The Overset method allows a good performance evaluation and limits the risk of numerical divergence that could be caused by mesh deformation methods; Moreover it limits the computation time compared to remeshing methods. However, it is more expensive than a more traditional Arbitrary Mesh Interface method \cite{Openfoam}, which is not suitable for the active lift turbine simulations.

The impact of several parameters on the performances is studied. The simulations are performed for a range of operating conditions including different inflow velocities, angles of attack and active flow turbine settings (like the amplitude of the radius variation, see Fig \ref{fig:combined}). The results are evaluated in terms of the turbine power output and efficiency.

The results show the importance of numerical simulation for the development of new types of energy recovery systems. It allows a better understanding of the interactions between the turbine and the fluid, and of the operation of such systems.

Further studies are required for the active lift turbine: conventional attempts at optimisation are not perfectly suited to this system. Improving the lift/drag ratio makes less sense when the normal forces should also be taken into account to improves the performances.
Furthermore, other aspects of operation should be analysed: variation of the turbulence rate, implementation of boundary layer re-attachment systems...

Non-dimensional scaling of passive adaptive blades for a marine current turbine

2023-06-26T11:29:13+01:00

For tidal energy to support access to off-shore electricity, further development is needed to decrease costs and increase reliability of current turbines at relevant scales. Blade pitch control strategies can significantly reduce structural loads in above-rated flow conditions by shedding power through decreased angles of attack. This can be accomplished through an active strategy using motorized blades or a passive adaptive strategy using flexible, self-twisting blades. We focus this study on the passive adaptive approach in which the composite fibers of the blade are oriented off-axis to produce a coupling between bend and twist deformations.

Extending laboratory results to larger, open-water designs requires an understanding of hydrodynamic and hydroelastic scaling. While dimensionless scaling relations have been extensively studied for current turbines with rigid blades, relatively few studies discuss appropriate hydroelastic scaling for passive adaptive blades. In this study, we experimentally apply non-dimensional scaling laws to laboratory-scale passive adaptive turbine blades and demonstrate similarity in blade deformation and non-dimensional loads across scales.

When Cauchy similarity is achieved between model and full-scale, the same steady-state blade loading and blade deformation are expected. We define Cauchy number as Ca = ρU_o²/E, where ρ is the water density, U_o is the freestream velocity upstream of the turbine, and E is the transverse flexural modulus of the blade (i.e., elasticity corresponding to bending in the flapwise direction). We tested the effectiveness of Cauchy-scaling by designing an experiment in which blade bending stiffness and flow speed varied, but Cauchy number remained constant. The first blade used a 7-ply carbon fiber spar while the second blade used a 5-ply carbon fiber spar, both fabricated with unidirectional fibers oriented 10° off-axis and cast in a semi-rigid polyurethane using the same mold. All other non-dimensional parameters relevant to hydrodynamic scaling were held constant, where possible.

As hypothesized, we observed agreement in thrust coefficient, deflection, and twist when Cauchy similarity was achieved, particularly when flow remained attached over the entire blade span. Small differences of 0-7% were observed in normalized thrust, deflection, and twist compared to 50-65% when Cauchy number was allowed to vary by 50%. We did not observe this similarity for normalized mechanical power between the 5-ply and 7-ply blades, but hypothesize that the source of the disagreement was a small surface defect in the urethane on the 5-ply blade. The experiment will be repeated to confirm this hypothesis and included in future presentations of this work.

Our experimental result partially demonstrates the effectiveness of using Cauchy number to scale passive adaptive marine current turbine blades and model their steady-state hydrodynamic and hydroelastic behaviors in a consistent, non-dimensional manner. Accurate experimental models are critical to support the development of passive adaptive blades, which may obviate the need for an active pitch mechanism, thereby increasing reliability and decreasing maintenance costs. Finally, we present initial results from a field-scale turbine equipped with rigid and passive adaptive blades, demonstrating a path towards validating our conclusions from lab-scale testing.

Optimal Design of a Submerged Tidal Device for Low Current Environment

2023-04-17T10:57:14+01:00

The tidal current power is one of the reliable renewable ocean energy resources that can be deployed at the region having ocean current. The feasible current speed for the application of tidal device is known to be more than 2m/s. As there are many islands in Asia where the current speed is below than the feasible speed, the submerged device that can produce the power at lower current speed with easy deployment and retrieve is introduce. To simplify the installation and O&M operations, the 4-point taut mooring system is applied that can also secure the designated position. In this study, a HAT (horizontal axis turbine) of 5-kW submerged tidal device is designed and analyzed for a low current region that consist of two buoyancy tanks, a yaw control strut, and a diffuser-type brimmed duct. To analyze the most stable condition, 9 different cases of width and height of the device and 8 different mooring arrangements (total 72 different cases) are investigated both by frequency and time domains. The results show that the dynamic behavior that mainly affects the turbine performance decreases with lower strut height. However, it was observed that the device width has little effect on the dynamic response of the device. The device dynamic motion decreases with decreasing azimuth angle and increasing hang-off angle of the mooring configuration. The optimal design configuration that can produce the maximum power is found that of 4.42 m width, 2 m height, 30° azimuth angle and 60° hang-off angle. According to API RP 2K code, the mooring system is also designed in both intact and damaged conditions. The proposed tidal device could be applied for the low current speed areas with easy implementation and maintenance.

Designing Vortex Generators for Tidal Turbine Blades

2023-06-06T11:09:32+01:00

Increasing tidal turbine performance through innovation is crucial if the cost of tidal energy is to become competitive compared to other sources of energy. The present investigation deals with the application of Vortex Generators (VGs) on tidal turbines in view of increasing their performance. The more mature wind energy industry uses passive VGs either as a retrofit or in the blade design process to reduce separation at the inboard part of wind turbine blades. Tidal turbine blades also experience flow separation and here we examine whether passive vane VGs can be used to reduce or suppress that separated flow.

Vortex generators (VGs) in various forms have been used and studied for flow separation control on wings since the 1940s [1]. Their working principle is relatively simple: they generate streamwise vortices that energise the boundary layer on the surface they are attached to, by bringing high momentum fluid closer to the surface [2]. This mechanism has been described by various researchers [3–6], while a number of studies have provided optimization guidelines under a variety of flow conditions [7–13].

In the present investigation, a VG configuration is selected following a thorough wind tunnel campaign. It is found that sizing parameters for the tidal turbine profile are very similar to the wind turbine relevant literature [13,14]. The best performing vane VG configuration had a height of 0.007c, which corresponded to half the local boundary layer height (0.5δ) for operational Reynolds numbers. The results are also used to validate a Reynolds Averaged Navier Stokes (RANS) VG modelling approach using the BAY model [15]. The validated method is used to simulate the flow past a tidal turbine in both model size (1:8) and full scale, see Figure 1. The results show that VGs do suppress flow separation in both cases. However, and importantly, it is revealed that the significance of rotational effects is such that when deciding VG placement locations, only the full size blade should be considered. In the interest of brevity, the performance increase caused by a standard VG configuration is show in Figure 2, where a power coefficient improvement of 1.05% is predicted at λ=3. Figure 3 shows the effect on the normal and tangential forces on the blade. In the final paper and presentation, the results for different VG locations will be included and analysed in detail.

A Methodology for developing a prediction model for the remaining fatigue life and residual strength of tidal turbine blades

2023-01-26T18:33:30+00:00

As tidal energy nears commercial viability, the reliability and safety of a tidal energy device becomes more prevalent. A key aspect for determining their reliability and safety, along with reducing risk during operational deployment, is the structural integrity of tidal turbine blades. Therefore, a validated model for predicting the structural integrity of tidal turbine blades will aid in de-risking tidal energy technologies. In this study, a three-phase approach was used to formulate a strategy to predict the remaining fatigue life and residual strength of tidal turbine blades, over their operational lifespan. In Phase 1, the parameters influencing the structural properties of tidal turbine blades were identified based on the literature review, and the expertise in the field. Then, parameters were extensively studied and classified into four main impact groups, which include load conditions, design and manufacturing, degradation, and unexpected situations. Loading conditions on the blade are directly linked to hydrodynamic forces, maintenance, operating conditions, and corrosion effects. At the same time, these scenarios can vary with fluid-structure interactions, climate conditions, local site conditions, and maintenance and inspection schedules of the blades. The design and manufacturing category mainly represents the impact of the properties of composite materials, the geometry of the blade, and manufacturing process parameters. Similar to the other structures, tidal turbine blades are subject to deterioration and unexpected accidents during their service life, which significantly compromises the structural integrity of the blade. In Phase 2, a data management strategy was formulated related to identified four impact categories and investigated the possible methods of analysing the data. In this context, finite element analysis of composite tidal turbine blades was identified as the most appropriate tool to comprehensively examine collected data, prior to comparing the results to the field and laboratory-based test data. Mesh properties of the numerical models, test standards, instrumentation, and equipment used for field and laboratory-based structural testing of tidal turbine blades, as well as the accuracy of data acquisition systems, influence the comparison of these results. Finally, with the information gathered, as well as knowledge and experience in the field, a method for estimating the residual strength and remaining fatigue life of tidal turbines at each stage of their operation was formulated. The model will undergo a series of extensive validation processes using experimental testing datasets and will be used in the future to develop vulnerability curves related to the remaining structural life of the tidal turbine blades.

Multi-Actuator Full-Scale Fatigue Test of a Tidal Blade

2023-02-28T13:49:10+00:00

Fatigue testing for tidal turbine blades involves the application of cyclic loads without matching the blade's natural frequency, which is challenging due to their high stiffness and associated thermal issues of composite materials at those frequencies (typically around 18Hz cycles). An auxiliary system is required to load the blades to address this challenge. However, traditional hydraulic systems tend to be highly energy-demanding and inefficient.

To solve this problem, researchers utilized real on-site data to define a series of equivalent target loads and implemented them in FastBlade, which proved an efficient way to perform fatigue testing. They used a regenerative digital displacement hydraulic pump system and achieved a remarkable 75% energy savings compared to a standard hydraulic system. During the testing, they utilized a system of 3 actuators instead of the traditional single actuator system, which produced more realistic and complex loads. We also address such changes in temperature along large composite structures during the test and mechanisms to address these issues.

Throughout the test, a series of measurements were taken on the blade response and FastBlade itself, which revealed exciting results on the mechanical behaviour of the blade and best testing practices for FastBlade. Impressively, the blade withstood 40 years' worth of accelerated fatigue loading without catastrophic failure.

Experimental techniques for evaluating the performance of high-blockage cross-flow turbine arrays

2023-06-09T11:49:00+01:00

Cross-flow turbines show great promise for extracting power from water currents since their rectangular projected area allows them to achieve high blockage. As a turbine’s blockage ratio—the ratio of the rotor projected area to the channel cross-sectional area—increases, its efficiency and structural loading increase since both kinetic and potential energy in the freestream are converted to mechanical power. For an array of turbines deployed in a river or tidal channel, the array blockage ratio will vary due to daily or seasonal fluctuations in the water level, as well as when individual turbines are deactivated for maintenance. Consequently, understanding how the performance characteristics of the array change as confinement is varied is of practical interest.

Here, we characterize the performance of a laboratory-scale two-turbine array at various levels of confinement in a recirculating water channel. The array blockage ratio was varied from 30% to 60%—the upper end of what might be realizable in a natural channel. Two experimental approaches for varying the blockage were considered: 1) altering the channel cross-sectional area via a change in water depth, and 2) altering the array projected area by changing the blade span. Across all tested blockage ratios, the nominal chord-based Reynolds number (4.0 x 10⁴) and nominal depth-based Froude number (0.22) were held constant, and the submergence-based Froude number was minimized to avoid ventilation of the turbine rotors at the upper end of the tested blockages. At each blockage ratio, the turbine performance was evaluated across a range of tip-speed ratios under a counter-rotating, phase-locked control scheme, wherein the turbines rotate at the same, constant speed but in opposite directions, with a constant angular phase offset, Δθ, between them. We focus this work on Δθ = 0°, an operating case in which the lateral forces and reaction torques for a pair of turbines are equal and opposite, which is advantageous for support structure design.

As the array blockage ratio is increased, we observe significant increases in the array performance and thrust coefficients, as well as an increase in the range of tip-speed ratios over which the array produces power. For the highest confinements, peak power coefficients exceed unity and thrust coefficients are substantially higher than in conventional array designs. We observe disparities between the power and thrust coefficients for arrays with the same blockage ratio, but different blade spans. We attribute the higher performance for longer blade spans to differences in the relative magnitude of parasitic support structure losses between the two rotors, as well as free surface effects. Further, we explore the effectiveness of techniques for accounting for these performance differences through the estimation of blade-level performance.

Overall, our results provide a solid foundation for understanding how the performance of cross-flow turbine arrays change as a function of the array blockage ratio, and highlight considerations for the design of cross-flow turbine experiments.

Observations from structural testing of full-scale tidal turbine blades

2023-06-19T09:53:57+01:00

As the world shifts its reliance from fossil fuels to renewable energy, a reliable, predictable and dependable source of energy is vital; tidal energy provides such a solution. In 2021, the cumulative installed capacity of tidal stream energy in Europe, since 2010, reached 30.2 MW, which is roughly three times as much as the rest of the world. The total electricity produced in Europe from tidal energy increased by 8 GWh to a total of 68 GWh to date. The tidal turbine blades convert the energy in the tidal current to useful mechanical energy that can be converted to electricity. Therefore, the reliability of the blades is paramount to the success for the turbine. Structural testing of composite tidal turbine blades is performed to ensure the design and manufacturing processes [4] produce a reliable component that performs for its design life span. In recent years, a number of structural testing programmes have been performed on full-scale tidal turbine blades, as the sector strives for commercial viability.

In this paper, observations during the structural (static, dynamic and fatigue) testing of full-scale tidal turbine blades are presented and discussed. These observations have been made from 5 testing campaigns on full-scale tidal turbine blades at the Large Structures Testing Laboratory in the SFI MaREI centre in the University of Galway. The length of these blades range from 2-8 metres, for devices of 70kW to 2MW. Therefore, primary aim of this paper is to report significant observations from the full-scale structural testing of tidal turbine blades. The experience gained from these structural testing programmes highlighted a number of best practices that could be introduced to the next revision of both the IEC 62600-3:2020 test specification and the DNV-ST-0164 standard, which includes using a combined flapwise-edgewise loading, the use of a multi-actuator load introduction system to ensure a correct load distribution and the duration of the holding time for static testing.

The inclusion of these potential best practices could benefit all of the tidal energy sector by being included in the next version of the relevant testing standards. These activities, along with the successful testing programmes that has allowed the developers to deploy their devices for operational trials, helps de-risk tidal energy technology aiding in lowering the levelised cost of tidal energy.

Experimental flow conditions effects on a bottom-mounted ducted twin vertical axis tidal turbine compared to real sea conditions

2023-06-06T20:03:26+01:00

From 2019 to 2021, HydroQuest tested its 1 MW-rated ducted twin vertical axis tidal turbine (2-VATT) at Paimpol-Bréhat test site, France. At this site, the turbulent intensity is evaluated about 15 % and extreme wave conditions up to 6 m significant wave height with 12 s peak period were observed during the two years of demonstration. In addition, the current vertical velocity profile is sheared with about 20 % velocity difference between the top and the bottom of the turbine, and the average directions of the ebb and flood tides are about 22° asymmetrical. The bottom mounted 2-VATT was instrumented for performance and loads assessment which allowed the certification of the power curve, both in flood and ebb tides, as well as the analysis of the sea state influence on the turbine behaviour. Based on that experience, the company wants to gain confidence in its design process by comparing those results to numerical and lab-scale experiments ones.

Along the past three years, an important amount of work has been achieved to widely characterise the behaviour of the 1/20 scale 2-VATT similar to the 1MW-rated demonstrator. The turbine was tested in many different operating conditions in the Ifremer’s wave and current flume tank to try to reproduce full-scale conditions and turbine response. Those conditions include a wide range of operating points and incident velocities, facing aligned and misaligned currents, with and without vertical velocity shear, with and without bathymetry generated turbulence, with and without surface waves following and against the current. In this presentation, we propose to synthesise the effects of all these conditions on the 2-VATT response to highlight the most critical ones in the design process of such a device, in comparison to the results obtained at sea.

To this date, we showed that the lab-scale model behaviour differs between ebb and flood tide current conditions due to the difference of relative counter-rotation of the two columns of rotors and to the base asymmetry. The wake is also significantly different as it recovers 30 % faster in the flood tide configuration compared to the ebb. We also found that the presence of a sheared and misaligned incident current barely affects the ducted 2-VATT average performance. However, it modifies the torque repartition between the rotors leading to an increase of the power fluctuations. Furthermore, when adding bathymetry obstacle upstream, the high flow shear and turbulence also generate strong power fluctuations on the 2-VATT. The load fluctuations are significantly increased, impacting the structure fatigue compared to a flat floor configuration, but the risks of device drifting or toppling are not affected. The waves impact the power and loads fluctuations too while barely affecting the average performance; and the average performance increases with the Reynolds number in the tank. Those two latter points still need investigations to better characterise their effect on the 2-VATT.

Experimental comparison of the flow-induced loading between a ducted bottom-mounted twin vertical axis tidal turbine at still and an unducted prototype

2023-02-10T16:10:26+00:00

The FloWatt project aims to demonstrate the efficiency of the HydroQuest’s 2.5 MW-rated turbine for commercial farm deployment. The project is dedicated to develop an innovative design of a turbine for high energetic sites and to deploy seven vertical axis tidal turbines on the Raz Blanchard site, France. This technology has already been successfully developed in a previous project at a smaller scale on the Paimpol-Bréhat test site.

Along the past three years, an important amount of work has been achieved to widely characterise the behaviour of the 1/20 scale 2-VATT similar to the 1MW-rated demonstrator. The turbine was tested in many different operating conditions in the Ifremer’s wave and current flume tank to reproduce full-scale conditions and turbine response (wide range of operating points and incident velocities, facing aligned and misaligned currents, with and without vertical velocity shear, with and without bathymetry generated turbulence, with and without surface waves following and against the current). Based on that experience, we gain confidence in the design process by comparing those experimental results to numerical and in-situ ones.

After a quick presentation of the industrial development for the deployment of a pilot farm with the manufacture, installation and operation of 7 turbines on the Raz-Blanchard site, we will focus on the scientific development aiming to prepare large-scale deployments of the next generation of turbine. One important step is to compare the flow-induced loading between the 2 kinds of turbine (ducted and unducted) and to analyse environmental loads during survival conditions. To investigate those points, the behaviour of a 1/20 scale 2-VATT similar to the 2.5 MW-rated demonstrator is experimentally studied in a wide range of operating conditions: with and without vertical velocity shear, and with and without surface waves following and against the current.

In the paper, we will compare at 1/20 scale the behaviour of the two kinds of turbine: 1 MW-rated and 2.5 MW-rated turbines. The experiments have been done in the same basin, under the same experimental conditions. The databases are being processed before launching a fine physical analysis.

A Two-scale blockage correction for an array of tidal turbines

2023-06-22T11:47:26+01:00

It has been shown theoretically that tidal fences consisting of multiple turbines placed side-by-side can make
use of constructive interference (local blockage) effects to raise the energy extraction efficiency of the fence
above that of the Betz limit applicable to unblocked flow problems. For the two-scale problem of a long
array of turbines partially spanning the width of a much wider channel (vanishing global blockage) the
efficiency of energy extraction, normalised on the undisturbed kinetic energy flux, rises from the Betz limit
of 0.593 to the partial fence limit of 0.798 [1]. Experiments on pairs of side-by-side turbines at large
laboratory scale [2] have confirmed the important aspects of the underlying partial fence theory and that
some of the performance benefits offered by constructive interference effects can be achieved in practice.

Experimental validation in wind tunnels, towing tanks and other laboratory facilities are however prone to
global blockage effects not seen in full-scale open flows due to the close proximity of flow boundaries to the
body. These global blockage effects modify the thrust and power performance of the turbines, such that
corrections to experimental curves are necessary to either translate laboratory-scale experimental results to
full-scale conditions, or to calculate the expected loads and power on tidal turbines deployed in blocked-flow
conditions [3][4]. The difficulty applying blockage corrections to turbine arrays is the non-linear interaction
between local and global blockage. These two effects cannot be simply decoupled as for various turbine tip-
to-tip spacings (affecting local blockage), changes in the global blockage have a different impact on turbine
performance.

A number of blockage corrections have been developed for single turbines operating in blocked flow
conditions. These corrections typically seek to describe an equivalent free-stream velocity which, in the
absence of global blockage, would result in the same thrust and velocity through the turbine as in the blocked
case. Thrust and power curves are then scaled non-linearly with the ratio of the experimental tank velocity
and the equivalent free-stream velocity [5]. These single turbine blockage corrections can however only
account for global blockage, and simplifications must currently be made based on the assumption that global
and local blockage effects can be linearly decoupled [2].

This work therefore presents an analytical blockage correction for co-planar arrays of tidal turbines based on
two-scale momentum theory [1]. This correction is then compared to other models, particularly for turbine
array experimental test data. Finally, RANS computations for a turbine array at various global blockage
ratios is compared to the analytical model, demonstrating its validity. A particularly useful aspect of the
theoretical model is to allow for experimental quantification of the local-blockage effect for finite length
fences. For instance, doubling the fence length doubles the global blockage, but increases in fence thrust and
power cannot be attributed only to the change in global blockage due to non-linear coupling. This correction
allows for a decoupling of these two effects, such that the local blockage effect can be isolated and
quantified.

Performance Assessment of a Multi-Rotor Floating Tidal Energy System

2023-06-16T14:55:13+01:00

Performance assessments of full-scale tidal turbines have been carried out for multiple machines deployed globally and reported in literature. The rotor number and diameter of the demonstrated tidal energy converters differs from single rotor devices in the MW class to scalable arrays of comparably small turbines. For multi-rotor systems (MRS), potential interaction, positive or negative, remains a key research question. Previous studies at Sustainable Marine prototype system (PLAT-I 4.63) have implied that variations in the incoming flow field are highly likely to cause deviations in rotor performance.

Sustainable Marine has developed and built the pre-commercial PLAT-I 6.40 platform. PLAT-I 6.40 has been connected to the Canadian grid in May 2022. Between May and end of September 2022 the system has been undergone comprehensive commissioning and performance trials. PLAT-I 6.40, carries six 4m diameter SCHOTTEL Instream Turbines (SIT), each rated at 70 kW. This work presents the results of full-scale field tests that focussed on the assessment of differences in the performance between the individual turbines during summer 2022.

To evaluate the variances in the flow field across all rotors, a specific test campaign has been conducted, where a flow speed sensor was sequentially positioned upstream of each turbine while a second flow measurement device was stationary as reference. For all test configurations the power curves have been determined following the guidance of the IEC TS 62600-200 based on the flow speeds measured with both devices.

This paper presents the experimentally obtained power curves for each test configuration and compares it against the reference values. The results show that there are steady differences in the flow field resulting in varying power outputs across the different rotors (Figure 1). However, a comparison of the individual rotor power curves with the design predictions shows good agreement.

The Influence of the Downstream Blade Sweep on Cross-flow Turbine Performance

2023-01-27T21:47:01+00:00

Because cross-flow turbines rotate perpendicular to their inflow, the blades encounter a continually fluctuating angle of attack and relative velocity that can lead to the unsteady, non-linear phenomenon of dynamic stall. Additionally, because of appreciable deceleration of the flow through the turbine rotor (induction), the relative velocity a blade experiences during the upstream portion of its cycle differs appreciably to that of the downstream portion. Both dynamic stall severity and turbine induction depend on the ratio of the blade tangential velocity to the inflow velocity - the dimensionless ”tip-speed ratio”. Consequently, the power generated by a blade varies substantially with tip-speed ratio and angular position. As the tip-speed ratio increases, the angle of attack range experienced by the blade decreases, dynamic stall weakens, and the relative velocity incident on the blade increases. In aggregate, this increases the amplitude of the performance peak in the upstream sweep and shifts the peak later in the cycle. In contrast, as the tip-speed ratio increases, power losses during the downstream sweep becomes increasingly detrimental to time-averaged turbine performance. Recent works, both experimental and computational, have investigated the near-blade hydrodynamics of cross-flow turbines in concert with performance measurements. However, despite the significance of the downstream sweep on performance, most attention has focused on the power generating phases.

Here, we specifically investigate the impact of the downstream blade sweep on cross-flow turbine performance using a 1-bladed turbine (NACA 0018 foil). Because turbine torque is measured at the center shaft in our experiments, a one-bladed turbine allows us to isolate the performance contributions for the upstream and downstream sweeps (i.e., with a multi-bladed turbine, the torque contribution from each blade is ambiguous). Additionally, flow fields are investigated to understand the hydrodynamic mechanisms for the observed degradation in downstream performance at high tip-speed ratios. For this purpose, two-component, phase-locked, planar particle image velocimetry data is obtained inside the turbine swept area for two tip-speed ratios. We find that the average performance in the upstream sweep continues to increase beyond the optimal tip-speed ratio, with respect to time-averaged performance, of 2.5. In contrast, the average performance in the downstream sweep is net neutral until the optimal tip-speed ratio where it begins to decrease at a faster rate than the upstream performance increases. This indicates that the optimal tip-speed ratio is strongly influenced by the point at which the downstream sweep begins to consume appreciable power. These results highlight the importance of understanding hydrodynamics in the downstream sweep, where induction and upstream disturbances violate simple models to predict angles of attack and relative velocity. An improved understanding may suggest strategies to improve performance in the downstream sweep and increase optimal tip-speed ratios and performance of cross-flow turbines.

Additive Manufacturing for Powering the Blue Economy Applications: A Tidal Turbine Blade Case Study

2023-02-03T20:03:16+00:00

As the marine renewable energy industry continues to expand, innovation in the manufacturing space must grow accordingly to reduce costs and ensure the economic feasibility of new technologies. Additive manufacturing, more commonly known as 3D printing, provides an alternative for rapid prototyping of marine hydrokinetic technologies, particularly supporting Powering the Blue Economy^TM initiatives of the U.S. Department of Energy Water Power Technologies Office. This study explores the application of additive manufacturing in the development of marine hydrokinetic structures, focusing on material and printing method selection, design, and analysis of a 3D-printed spar for an axial-flow tidal turbine blade. Corrosion-resistant metals were deemed ideal due to the loads and harsh marine environment the blade would experience. Laser metal deposition methods were determined to be the most effective and scalable for the considered scale. The designed spar adapts its geometry to the blade—a feature uniquely suited to additive manufacturing—and is intended to serve as the blade's primary structural component. A finite element model was used to study stresses and deformations under loading conditions. The spar was manufactured using 316L stainless steel through direct energy deposition, and defects were assessed and recorded. Future efforts will include mechanical testing of the spar. This research establishes a benchmark process for using additive manufacturing in developing marine hydrokinetic structures, paving the way for future optimization and techno-economic analysis.

Design and Demonstration of a Passive Pitch System for Tidal Turbines

2023-06-29T14:47:23+01:00

Tidal currents are renewable and predictable energy sources that could prove fundamental to the transition to a sustainable use of renewable energy resources. Over a tidal period, changes in the flow speed in a tidal channel require that the blade pitch is adjusted to maximise power extraction. This is currently achieved with active pitch actuation, which however increases the capital and maintenance cost of the turbine. Furthermore, because of turbulence in the tidal stream, turbine yaw, wave-induced currents, etc., tidal turbine blades experience high-frequency velocity fluctuations that result in power and thrust unsteadiness, both of which are transmitted to the generator, the tower, and the active pitching mechanism, shortening the operating life due to fatigue loading.

A passive morphing blade concept capable of reducing the load fluctuations without affecting the mean loads has recently been formulated and demonstrated with low-order simulations (https://doi.org/10.1016/j.renene.2021.10.085) and measurements (https://doi.org/10.1016/j.renene.2023.01.051). The system allows both passive pitch adjustment to changes in the mean flow speed over the tidal period, and the mitigation of high-frequency fluctuations.

In this paper, we present the recent progress on the development of morphing blade technology, including with numerical simulations and experimental tests on a 1.2-m diameter turbine. Two different design concepts have been tested in the FloWave facility at the University of Edinburgh and in the recirculating channel at the Institute for Marine Engineering of the Italian National Research Council, respectively.

Experimental results show that the amplitude of power variations over a wide range of flow speeds is substantially decreased, while thrust variations with changes in freestream speed are essentially suppressed. The detrimental effect of yawed inflow is, in addition, almost entirely cancelled. The fluctuations in the root-bending moment, thrust and torque are consistently reduced over a broad range of tip-speed ratios. We also show that such a system, if improperly designed, could result in a negative starting torque, and we show the steps necessary to avoid this issue.

Furthermore, we present a theoretical and numerical framework that allows the design of passive pitch blades that can cancel either thrust or power fluctuations in specific flow conditions, as well as mitigating both types of fluctuations over a wide range of conditions. Specifically, we show that for any quasi-steady change in the relative flow speed and direction, there is a pitching axis that allows a chosen force component to be kept constant. High-frequency force fluctuations can also be substantially mitigated, and the extent of the mitigation depends on the inertia and friction in the system.

Overall this paper demonstrates experimentally the effectiveness of morphing blades for tidal turbines and presents a theoretical and numerical framework for the future development of this technology.

CFD analysis of hydrodynamic force on a horizontal axis tidal turbine

2023-07-20T16:59:02+01:00

Horizontal axis tidal turbines are similar to wind turbines in both geometry and principle of operation, yet they need to withstand much heavier loadings and extreme conditions in a harsh operating environment. Consequently, the loadings on tidal turbine blades need to be accurately evaluated within the design stage to ensure they can withstand loadings with little need for repair. With the advancement of computational capabilities, computational fluid dynamics offers a relatively inexpensive method of simulating working conditions and estimating loadings, compared to traditional physical testing, while maintaining accuracy and applicability under a range of operating conditions. In this research, a computational fluid dynamics model of a tidal turbine rotor has been developed using commercial code ANSYS CFX, where the three-dimensional blade geometry is developed from the prototype tidal turbine used in the Round Robin Tests in the framework of the H2020 MaRINET2 Infrastructures Network project. The outputs from the numerical model are validated against the experimental results, where it is revealed that with an increasing rotating speed of the tidal turbine, the thrust force that turbine experienced increases, while the torque force experiences a rise firstly, reaching a maximum value and then decreases gradually. In addition, the experimental results show that during the operation of ocean tidal turbines, the turbine blades experience frequent and large-scale fluctuation of hydrodynamic loads, including thrust and torque, which may lead to tidal turbine blade damage due to fatigue. Therefore, the next stage of development with this numerical model is fatigue loading evaluation, in order to improve the present fatigue design and testing of tidal turbine blades, while gaining a greater understanding of the damage mechanisms.

Dynamic Responses of a 1:5-Scale Ocean Current Energy Converter

2023-06-13T11:01:49+01:00

The objective of this study is to numerically evaluate the dynamic responses of the Floating Kuroshio Turbine (FKT)—an ocean current energy converter—installed for field testing. In autumn 2022, the FKT was installed and tested at a 1:5 scale in the open sea area, located 1 kilometre off the LiuQiu Island coast, Taiwan. Because of unfavourable site conditions, the mooring and power cable systems for the FKT had to be customarily designed for the testing conditions. An investigation of the dynamic motions and structural responses of the mooring and cable systems was performed prior to physical testing to ensure the integrity of the testing campaign. The numerical analyses were performed using the commercial software DNV SIMA, where the prospect of the surfaced FKT and the mooring entanglement were evaluated and compared. This paper details the modelling process and the evaluation of the dynamics of the entire FKT system. Finally, the challenges and potential improvements for numerical assessments of a testing-oriented offshore marine energy system are identified and discussed.

The Development of a passive blade-pitch mechanism to reduce the loads on a tidal turbine in high-flow conditions

2023-07-24T10:11:12+01:00

Maintenance of tidal turbines is expensive, and relies upon suitable weather conditions and vessel availability, which can lead to costly delays. Turbine reliability can be improved by eliminating complexity in the design. The use of fixed-pitch blades, for example, can greatly reduce maintenance costs by avoiding complex active blade-pitch mechanisms which are the main source of failure in wind turbines.

Pitching rotor blades to feather in high-flow conditions is, however, the most effective means of reducing loads on the rotor. Turbines with fixed-pitch blades therefore necessarily have smaller rotor diameters compared to those with active-pitch systems, in order to keep the thrust force, flap-wise bending moment, and power output within allowable limits. In low-flow conditions, power output is proportional to the square of the rotor diameter, so turbines with small diameter rotors capture less energy over their lifetime.

This study seeks to develop a novel passive-pitch mechanism for tidal turbine blades. This mechanism must be reliable, and should act to reduce the loads on the rotor in high-flow conditions so that a larger diameter rotor can be installed, increasing power capture.

An initial design for a passive-pitch mechanism, which is actuated by the hydrodynamic forces developed by the rotor such that the blades pitch-to-feather in high-flow conditions, has been developed. The design is compatible with proven passive reversible rotor blade technology, eliminating the requirement for a maintenance sensitive yaw mechanism.

Since initial concept design, work has been undertaken to build a tool, based on NREL’s OpenFAST blade element momentum code, which models the forces acting on the rotor blades, coupled with the mechanical response of the passive blade-pitch mechanism. This tool has been used to predict the performance of the turbine for a range of operating conditions, allowing the influence of parameters such as blade geometry, rotor diameter and passive-pitch response to be analysed in terms of rotor loading and turbine performance.

Initial analysis suggests that installing blades with a passive-pitch mechanism could reduce the loads on the rotor in high-flow conditions down to just 25% of the loads that would act on an equivalent rotor with fixed-pitch blades. This would allow larger diameter rotors to be installed which could improve annual energy yield by over 40% at a typical site.

Despite these significant potential performance improvements, the challenges of developing a passive blade-pitch mechanism for tidal turbines are not well understood, as tidal developers have all so far implemented fixed or active pitch mechanisms. These challenges will be addressed throughout the remainder of this study, which will involve detailed design followed by physical tank testing.

Effects of non-isotropic blockage on a tidal turbine modeled with the Actuator-Line method

2023-06-21T12:20:43+01:00

Blockage effects are a consequence of the interaction between a body and the surrounding boundaries in a constrained flow. For the case of tidal rotors, global blockage (β) is usually defined by
the ratio between the swept area of the rotor and the cross-sectional area of a channel. Increasing
blockage tends to increase the limits of power extraction (Garrett and Cummins, 2007), as well as
thrust on a rotor through an attendant increase of through-rotor mass flow. While these observations have been studied and demonstrated for isotropic blockage effects (e.g., Zilic de Arcos et al.
2020, Bahaj 2007 , Mikkelsen 2002), questions remain regarding the validity of such assumptions
for non-isotropic blockage in channels with, e.g., rectangular cross-sections with varying aspect
ratios.

In this work, we will use CFD simulations to analyze the effect of non-isotropic blockage on a
tidal rotor. The study aims to explore these effects using an Actuator-Line representation of an
axial-flow rotor, simulated under different blockage ratios (1 %, 5 %, 10%, and 19.7 %), aspect
ratios (0.25, 0.5, 0.75, and 1), and tip speed ratios (4, 5, 6, and 7). A total of 64 cases will be
considered. For each simulated case, the power, thrust, and spanwise force distributions will be
extracted as functions of time, and used to understand the effect of blockage on the performance
of tidal rotors.

Our preliminary results, in agreement with existing literature, indicate that blockage affects
wake development, as seen in Figure , along with power and thrust. These results, for a constant
aspect ratio, show power increases up to 26 % for a blockage of 20 %. The bulk of the simulation
matrix, including the different aspect ratios, is currently under production and is expected to be
ready before the paper submission deadline.

References
Garrett, C., Cummins, P. (2007). The efficiency of a turbine in a tidal channel. Journal of fluid
mechanics, 588, 243-251.
Zilic de Arcos, F., Tampier, G., Vogel, C. R. (2020). Numerical analysis of blockage correction
methods for tidal turbines. Journal of Ocean Engineering and Marine Energy, 6, 183-197
Bahaj, A. S., Molland, A. F., Chaplin, J. R., Batten, W. M. J. (2007). Power and thrust measurements of marine current turbines under various hydrodynamic flow conditions in a cavitation
tunnel and a towing tank. Renewable energy, 32(3), 407-426.
Mikkelsen, R., Sørensen, J. N. (2002). Modelling of wind turbine blockage. In 15th IEA
symposium on the aerodynamics of wind turbines, FOI Swedish Defence Research Agency.

Intracycle Control Sensitivity of Cross-Flow Turbines

2023-07-17T10:56:39+01:00

Intracycle control for cross-flow turbines employs sinusoidal perturbations in rotational speed to affect turbine power output and loads. This technique is explored for one- and two-bladed turbines with experiments that combine performance measurements with in-rotor flow-field measurements using non-intrusive particle image velocimetry. Performance enhancement is demonstrated for a wide range of control parameters but is found to be more sensitive to the sinusoidal phase offset than amplitude. Optimal performance is observed under conditions which maintain constant nominal blade angle of attack and consistently high blade-relative velocities once the static stall angle is exceeded during the power stroke. This maintains high lift and delays vortex separation. Perhaps most significantly, kinematics are also identified which decrease peak turbine loading by 12% while still producing marginally higher power output, highlighting the utility of intracycle control for load mitigation. The range of control parameters that produce these beneficial loading properties is relatively narrow and flow-fields are similar to operation at a constant rotation rate. We hypothesize that reductions in maximum loading are achieved by smoothing the changes in angle of attack and blade relative velocity profiles throughout the cycle.

Development of an Unmanned Mobile Current Turbine Platform

2023-06-13T10:52:26+01:00

A prototype low-flow (~0.5 m/s) marine current turbine for deployment from a small unmanned mobile floating platform has been developed at Florida Atlantic University for autonomously seeking and harnessing tidal/coastal currents. The support platform is an unmanned surface vehicle (USV) in the form of a catamaran with two electric outboard motors and with capabilities for autonomous navigation. An undershot water wheel (USWW), aided by a custom flow concentrator, has been selected as the basic design for the marine current turbine, which is mounted on the stern of the USV. The concept of operation is that the unmanned surface vehicle would navigate to a designated marine current resource, autonomously anchor at the location, align itself in the current, and deploy the USWW turbine using a custom cable-lift deployment mechanism. As the USWW harnesses the local current, an onboard power-take-off (PTO) device converts the harnessed mechanical energy to electricity which is stored in onboard batteries. The selected PTO utilizes a spur drivetrain/gearbox coupled with a NuVinci Ball-continuously variable transmission (CVT). It is estimated that the small prototype turbine system will produce over 12W power for currents over 0.5 m/s. The automated anchoring system consists of an electric winch, a Rocna anchor, anchor chain/rode and a line locking mechanism designed to aid in taking tension off the winch. Preparations have been made to test and demonstrate the application of the platform in harnessing tidal currents in the Intracoastal Waterway in South Florida and coastal currents at locations off Fort Lauderdale, Florida. The preparations include obtaining the necessary environmental permits for conducting in-water testing; developing required mitigation measures in protecting local wildlife and their habitats; and identifying potential in-water test sites and surveying them for their suitability in terms of current resource, bottom type, water depth and local boat traffic. Application of the marine current turbine platform to serve as an unmanned mobile floating recharge station for small aerial drones will be demonstrated. For this purpose, the USV includes a flight deck for landing and takeoff of small aerial drones whose batteries would be recharged via a wireless direct-contact recharging pad powered by the onboard batteries. Modeling in support of turbine design and parametric studies in support of optimization of the performance of the system will be discussed. Scaling of the prototype system in terms of size and capacity will be discussed.

Design, Manufacture and Testing of an Open-Source Benchmark Composite Hydrokinetic Turbine Blade

2023-06-13T10:45:46+01:00

In a trend towards clean energy alternatives, recent years have seen great strides in the marine energy space. This has resulted in a pressing need for the design, development, and validation of novel energy harvesting technologies such as hydrokinetic devices, which capture kinetic energy from waves, tides, and currents. However, these devices span numerous concepts and designs that often lack solid benchmark research that can be freely referenced. This work focuses on the design process of an open-source composite hydrokinetic turbine blade for a three-bladed marine turbine rotor assembly with a diameter of 2.5 m. The proposed blade consists of two structural composite skins that are bonded with an adhesive and filled with a foam core. This study will explore and contrast the efficiency and resolution of low-fidelity rapid design methodologies and comprehensive high-fidelity approaches for the blade design, modeling, and analysis efforts, a key objective in this research. Blade hydrodynamic loads were modeled and applied to finite-element blade models to study deformations and potential failure. Ongoing efforts will result in blade manufacture and structural testing at the National Renewable Energy Laboratory. In future work, multiple blades will be deployed at the Living Bridge site at the University of New Hampshire and will be compared to rigid aluminum blades of the same geometry, developed by Sandia National Laboratories. Ultimately, this research will lay foundational groundwork for researchers and manufacturers, establishing a baseline composite blade design that will serve as a benchmark in the development of future hydrokinetic turbine blades.

Wake characterization of tidal turbines in the Pentland Firth using vessel-mounted ADCP measurements

2023-05-31T09:35:51+01:00

Knowledge of the extension and velocity deficit induced by tidal turbine wakes is crucial for the optimization of tidal farm layouts, as the wake induced by an upstream turbine may substantially affect the power and loadings of a device located downstream. The MeyGen project is the largest planned tidal stream energy project in the world: it aims to develop up to 398 MW of installed power in the Pentland Firth, Scotland, a site with current velocities reaching up to 5 m/s. During Phase 1 of the project, four 1.5 MW turbines were installed, providing a valuable opportunity to investigate turbine wake dynamics in full scale. As part of Phase 2, site characterization campaigns were carried out to plan the deployment of additional turbines, representing 28 MW of tidal power capacity. Hence, the influence of the existing devices on the downstream flow was investigated.

This work introduces the method used for studying wakes downstream tidal turbines using vessel-mounted Acoustic Doppler Current Profiles (ADCP) measurements. During two spring tides, the data was collected using an ADCP Teledyne Workhorse 600 kHz configured for along-beam velocities recording with 1 m resolution, as well as a GNSS Hemisphere Vector V102. The aim of this study was to map the flow downstream the turbines already installed on site, in order to identify and characterize the wake. Cross-flow transect measurements were conducted at various along-flow distances downstream from the devices, and repeated several times for improved accuracy. The cycles of repeated transects were performed both at flood and ebb tides, with the turbine running or switched off to spot potential differences introduced by device operation. Mean flow velocity estimates in the cross-section were obtained from raw data using the location-based velocity solver developed by Vermeulen et al. (2014). The method provides improved flow velocity estimates from vessel-mounted ADCP measurements because it strongly reduces the spatial extent over which flow homogeneity must be assumed, thus decreasing the chances that this assumption will fail. The reduction of the volume across which homogeneity must be achieved may represent a significant advantage for the investigation of tidal turbine wakes. To the authors’ knowledge, this is the first time that this method has been applied to wake characterization in a tidal stream energy site.

Outputs of the analysis include mean velocity magnitude and direction, as well as velocity deficits associated with the wakes. The mean velocity estimates obtained in each cross-section are compared for cases when the turbine was running or switched off. Findings reveal that vessel-mounted ADCP transects, coupled with the location-based methodology for velocity estimation, provide a powerful tool for tidal turbine wake characterization. The final presentation and paper will provide the results of this study.

References:
Vermeulen, B., Sassi, M. G., & Hoitink, A. J. F. (2014). Improved flow velocity estimates from moving-boat ADCP measurements. Water Resources Research, 50(5), 4186–4196. https://doi.org/10.1002/2013WR015152

Tidal Turbine Benchmarking Project: Stage I - Steady Flow Experiments

2023-06-09T11:58:56+01:00

The tidal turbine benchmarking project, funded by the UK's EPSRC and the Supergen ORE Hub, has conducted a large laboratory scale experiment on a highly instrumented 1.6m diameter tidal rotor. The turbine is instrumented for the measurement of spanwise distributions of flapwise and edgewise bending moments using strain gauges and a fibre Bragg optical system, as well as overall rotor torque and thrust. The turbine was tested in well-defined flow conditions, including grid-generated freestream turbulence, and was towed through the 12.2m wide, 5.4m deep long towing tank at Qinetiq’s Haslar facility. The turbine scale was such that blade Reynolds numbers were Re=3x10^5 and therefore post-critical, whilst turbine blockage was kept low at 3.1.

In order to achieve higher levels of freestream turbulence a 2.4m by 2.4m turbulence grid was towed 5m upstream of the turbine. Measurements to characterise the grid generated turbulence were made at the rotor plane using an Acoustic Doppler Velocimeter and a five-hole pressure probe. An elevated turbulence of 3.1% with homogeneous flow speed across the rotor plane was achieved using the upstream turbulence grid.

The experimental tests are well defined and repeatable, and provide relevant data for validating models intended for use in the design and analysis of full-scale turbines. This paper reports on the first experimental stage of the tidal benchmarking programme, including the design of the rotor and comparisons of the experimental results to blade resolved numerical simulations.

Tidal Turbine Benchmarking Project: Stage I - Steady Flow Blind Predictions

2023-02-25T17:33:17+00:00

This paper presents the first blind prediction stage of the Tidal Turbine Benchmarking Project being conducted and funded by the UK's EPSRC and Supergen ORE Hub. In this first stage, only steady flow conditions, at low and elevated turbulence (3.1%) levels, were considered. Prior to the blind prediction stage, a large laboratory scale experiment was conducted in which a highly instrumented 1.6m diameter tidal rotor was towed through a large towing tank in well-defined flow conditions with and without an upstream turbulence grid.

Details of the test campaign and rotor design were released as part of this community blind prediction exercise. Participants were invited to use a range of engineering modelling approaches to simulate the performance and loads of the turbine. 26 submissions were received from 12 groups from across academia and industry using solution techniques ranging from blade resolved computational fluid dynamics through actuator line, boundary integral element methods, vortex methods to engineering Blade Element Momentum methods.

The comparisons between experiments and blind predictions were extremely positive helping to provide validation and uncertainty estimates for the models, but also validating the experimental tests themselves. The exercise demonstrated that the experimental turbine data provides a robust data set against which researchers and design engineers can test their models and implementations to ensure robustness in their processes, helping to reduce uncertainty and provide increased confidence in engineering processes. Furthermore, the data set provides the basis by which modellers can evaluate and refine approaches.

On the design of a small scale tidal converter for long time deployment at sea

2023-06-13T10:37:00+01:00

A scale model of a kite-like converter of tidal energy, the GEMSTAR, is designed to be installed at sea for a long term deployment. The aim of the experiment is to develope a digital twin with fault detection and isolation capabilities. To this aim, a fully-functional 1:10 scale of GEMSTAR is designed, starting from measurements of tidal currents in the site of installation. Several spots were investigated searching for the most suitable current profile. The paper describes the principal issues concerning operational functions and the choice of the physical parameters to monitor during the deployment through several sensors placed on board. A scale model of a kite-like converter of tidal energy, the GEMSTAR, is designed to be installed at sea for a long term deployment. The aim of the experiment is to develope a digital twin with fault detection and isolation capabilities. To this aim, a fully-functional 1:10 scale of GEMSTAR is designed, starting from measurements of tidal currents in the site of installation. Several spots were investigated searching for the most suitable current profile. The paper describes the principal issues concerning operational functions and the choice of the physical parameters to monitor during the deployment through several sensors placed on board.

Preliminary performance assessment from towing tank testing of a horizontal-axis turbine

2023-02-23T07:57:28+00:00

Following the Norwegian government’s goal to be a ‘low emission society’ by 2050, there is a significant push to develop offshore renewable energy; principally offshore wind, but also wave and tidal stream. The MarinLab towing tank at the Western Norway University of Applied Sciences, Norway is 50 m long with a 3.0×2.2 m section and was inaugurated in 2016 as an educational and research facility, supporting local industry to achieve a low-carbon transition.

To advance knowledge of aero- or hydrodynamic interaction of future turbines for both wind and tidal stream energy, a model scale test-bed horizontal axis turbine has been developed. This turbine will enable testing of rotor diameters typically in the range of 500-600 mm. The turbine is instrumented with a torque-thrust sensor of 5 Nm/100 N capacity, custom-manufactured by Marin in the Netherlands. The turbine is speed regulated via a 4096 counts angular encoder connected to a 200 W Maxon EC-i brushless motor.

Before testing new rotor geometries, a benchmarking study is being undertaken with an existing known geometry. The benchmark rotor has diameter, D=700 mm and employs the same NACA 63418 airfoil as Mycek et al. (2014), allowing comparison. Unlike Mycek et al. (2014), the nacelle has a nominal diameter of 90 mm and length 760 mm. The size of the rotor risks exceeding the capacity of the torque sensor, such that maximum tow-speed is limited to 0.8 m/s. The benchmark blades are machined from solid aluminium in a four-axis CNC milling machine, following a method similar to Payne et al. (2017). Due to machining limitations, the minimum trailing edge thickness is specified as 0.2 mm. The final surface finish is achieved by manual polishing, and the final dimensions, quantified using a Hexagon ROMER Absolute laser scanner, are found to be within ±0.2 mm accuracy. Future blade sets will typically have a smaller diameter, allowing for 3D printing and consistency of surface finish.

A blade-element momentum model has been run with Prandtl tip and hub losses and 0° pitch (Fig. I). Lift and drag coefficients were calculated for a range of chord Reynolds numbers using XFOIL, for angles of attack between -5 to 16° and extrapolated to ±180° with Viterna’s method. Due to difficulties in estimating laminar-turbulent transition around critical Reynolds numbers, there is uncertainty in the XFOIL coefficients. At peak TSR, the chord-based Reynolds number is approximately 280k at 3/4 span. Curves for both Rec =250k (solid) and 500k (dashed) with Ncrit = 9 are presented, showing a significant drop in performance for lower Reynolds numbers, due to a collapse in Cl/Cd. Initial tests were conducted in near- zero ambient turbulence, though passive turbulence grids are available for future testing. Reasonable agreement on C_T is observed when corrected for blockage according to Bahaj (2007), and further assessment will provide results on C_P, tower loads and wake recovery.

Experimental Optimization Environment for Developing an Intracycle Pitch Control in Cross Flow Turbines

2023-06-09T11:37:13+01:00

Cross-flow tidal turbines also known as vertical-axis turbines have not reached the efficiency as it is usual found for horizontal-axis turbines. This can be mainly accounted to the intensive development effort dedicated to the latter in the past 30 years in the area of wind energy. Less attention has been given to cross-flow turbines as they present mechanical cyclic loads that detriment their durability. Nevertheless, there is great potential for their employment in tidal energy exploitation as they are intrinsically independent from the flow direction and have a higher power output per area in farm installation. For this reason the research for cross-flow turbines have regain impulse.

Among various approaches to improve their hydrodynamic characteristics found in the literature, the intracycle pitch control is one of the most promising one. It consists in actively pitching each blade as function of the azimuth angle.

The present paper will introduce the overall experimental environment and methods for developing an optimal intracycle control for cross-flow turbines in tidal power generation. This is the main component of project OPTIDE at Otto-von-Guericke-University Magdeburg, Germany in cooperation with the LEGI at University Grenoble-Alpes, France and the University of Applied Sciences Magdeburg-Stendal.

The experimental system consists of a turbine flume model equipped with a speed controlled generator. The three bladed turbine model has independent controlled pitch actuators embedded in each blade and a force sensing system in one of the blade holding arms. Local high-dynamic systems allow to control the speed of the generator and the pitch position as a function of the rotational angle of the runner i.e. pitch trajectory.

A superimposed governing system which automates the experiment has been implemented. This allows to perform an optimization process with the experiment in the loop. Evolutionary algorithms are employed to solve the two objectives of the optimization: (1) Maximizing the efficiency by (2) minimizing the alternating structural loads on the runner. These kind of problems are usually solved with numerical simulations using finite element method (FEM) and computational fluid dynamics (CFD). However, this commonly comes with very high computational efforts and uncertainty. For this reason in our approach simulations are replaced by an experimental optimization technique.

The optimization cycle implemented in the superimposed governing system chooses a set of individuals (pitch trajectories) for each generation on base of the genetic algorithm. Each pitch trajectory is performed sequentially while power output and loads are measured. This allows to evaluate each individual in about one minute. The best individuals are then the base for the offspring of the new generation. Cross-over from best performing parents and subsequent mutation ensure both, convergence and a thorough exploration of the entire design space.

It is expected that this method will fast and reliably find the optimal pitch trajectories under various conditions. Selected cases will be analysed in detail by means of particle-image velocimetry (PIV) with synchronized force and torque measurements to research the underlying hydrodynamic mechanisms and the effects of the flow control performed by the pitching motion.

Acoustic Characterization around the CalWave Wave Energy Converter

2023-06-25T09:10:50+01:00

Sound generated by marine energy (ME) installations in the ocean environment remains a particular concern for environmental permitting despite the limited evidence showing low levels of ME sounds relative to other anthropogenic sounds. In an effort to increase understanding of potential environmental effects of marine energy projects and help reduce barriers to marine energy deployments, a new acoustic monitoring technology, the NoiseSpotter®, was developed and recently demonstrated around CalWave’s operational scaled xWave™ wave energy converter (WEC). The NoiseSpotter® improves upon traditional acoustic sensing techniques by use of a cost-effective, compact array of acoustic particle velocity sensors that characterizes, classifies, and provides accurate location information for anthropogenic and natural sounds in real time.

Results are presented from co-deployments of NoiseSpotter® with the operational CalWave WEC that were conducted over a 9-day period in fall 2021 offshore of Scripps Research Pier in San Diego, California, USA. The multi-day deployment of the NoiseSpotter® at 100 m from the CalWave WEC revealed a rich library of sounds that include:

Low-level (~95 dB re 1 µPa relative to an ambient noise floor between 80-90 dB re 1 µPa) sounds from the WEC associated with the deliberate actuation of mechanical components,
Sounds from a hovering helicopter,
Marine mammal vocalizations, and,
Small boat engines.

Sound levels from the WEC were placed within the context of ambient sounds, and reveal little deviation from the ambient soundscape. The azimuthal anisotropy of WEC sound was investigated via deployments along four cardinal directions around the WEC. While a noticeable increase was observed along the north-south orientation, the sound levels along all directions still showed little deviation relative to the ambient noise floor. Analysis of low-level WEC sounds in conjunction with exploratory machine learning techniques demonstrate the utility of directional acoustic sensing in distinguishing marine energy sounds from the myriad other sounds in the surrounding ocean environment.

A Conditional probabilistic encounter-impact model for fish-turbine interactions

2023-06-01T08:31:48+01:00

Knowledge gaps exist in the collective effort to quantify risks and impacts of fish-turbine interactions. Empirical data and modeling studies have characterized stages of fish approach and pass through hydrokinetic turbines, but there has not been a comprehensive model that quantifies conditional occurrence probabilities of fish approaching and then interacting with a turbine in sequential steps. When calculating conditional probabilities, the probability of each step occurring is, in part, dependent on the occurrence of the previous step in the sequence. We combined acoustic data measurements and when data limited, published probabilities to estimate conditional probabilities of tidal turbine impacts on marine animals. The encounter-impact probability model includes approach, entrainment, and impact phases. The approach phase includes probabilities of entry into a spatial domain and zone of influence, followed by entrainment to the device. Each model component of an encounter includes probabilities of active or passive avoidance. Impacts are defined as fish collisions with a device, blade strikes by a rotating blade, and/or a collision followed by a blade strike. Probabilities are estimated using acoustic data from surveys in Admiralty Inlet, WA USA for Pacific herring (Clupea pallasii) encountering an axial or cross-flow turbine during day or night. Conditional probabilities are additive or multiplicative depending if events are cumulative or sequential. Impact probabilities for collisions and blade strikes are estimated individually and then combined to estimate the probability of overall impacts. Probabilities of occurrence for each model component range from 0.0000242 to 0.40. Impact probabilities for collision followed by a blade strike was lowest with estimates ranging from 0.0000242 to 0.0678. Impact probabilities for blade strike had the highest probabilities ranging from 0.000261 to 0.40. Overall impact probabilities had wide variability from 0.00110 to 0.689. Maximum probabilities occurred in each model component and overall impact probabilities for a cross-flow turbine at night with no active or passive avoidance. Estimates were lowest when probabilities were conditional on sequential events, and when active and passive avoidance was applied for an axial-flow turbine during the day. As expected, conditional probabilities were typically lower than equivalent model component and overall probabilities published in the literature. The encounter-impact conditional probability model can be applied to any marine animal in any environment for any hydrokinetic device. Estimating impact probabilities for Pacific herring in Admiralty Inlet for two device types illustrates utilization of existing data and simultaneously identifies data gaps needed to fully calculate empirical-based probabilities for any specific site-species scenario. As additional monitoring data are collected, accuracy of impact risks due to encounters of marine animals with hydrokinetic devices will increase and be confidently used in establishing regulatory policy.

SafeWAVE The contribution of the SafeWAVE EU project to the future development of ocean energy

2023-06-16T11:36:30+01:00

The European Atlantic Ocean offers a high potential for marine renewable energy (MRE), which is targeted to be at least 32% of the EU’s gross final consumption by 2030. The European Commission is supporting the development of the ocean energy sector through an array of activities and policies: the Green Deal, the Energy Union, the Strategic Energy Technology Plan (SET-Plan) and the Sustainable Blue Economy Strategy. The nascent status of the MRE sector and Wave Energy (WE) in particular, yields many unknowns about its potential environmental pressures and impacts. Wave Energy Converters’ (WECs) operation in the marine environment is still perceived by regulators and stakeholders as a risky activity. The complexity of MRE licensing processes is also indicated as one of the main barriers to the development of the sector. The lack of clarity of procedures, the varied number of authorities to be consulted and the early stage of Marine Spatial Planning (MSP) implementation are examples of the issues identified that may delay the permitting of the projects. Finally, there is also a need to provide more information on the sector to the general public. Only with an informed society would be possible to carry out fruitful public debates on MRE implementation at the local level. These non-technological barriers that could hinder the future development of WE in EU were addressed by the WESE project funded by EMFF in 2018. The present project builds on the results of the WESE project and aims to move forward through the following specific objectives:

Development of an Environmental Research Demonstration Strategy based on the collection, processing, modelling, analysis and sharing of environmental data collected in WE sites from different European countries where WECs are currently operating (Mutriku power plant and BIMEP in Spain, Aguçadoura in Portugal and SEMREV in France).
Development of a Consenting and Planning Strategy through providing guidance to ocean energy developers and to public authorities tasked with consenting and licensing of WE projects in France and Ireland; this strategy will build on country-specific licensing guidance and on the application of the MSP decision support tools (i.e. WEC-ERA[1] and VAPEM[2] tools) developed for Spain and Portugal in the framework of the WESE project; the results will complete guidance to ocean energy developers and public authorities for most of the EU countries in the Atlantic Arch.
Development of a Public Education and Engagement Strategy to work collaboratively with coastal communities in France, Ireland, Portugal and Spain, to co-develop and demonstrate a framework for education and public engagement (EPE) of MRE enhancing ocean literacy and improving the quality of public debates.

The project will finish in September 2023. In this contribution we present the main outcomes, results and lessons learnt during the project.

[1] https://aztidata.es/wec-era/;

[2] https://aztidata.es/vapem/

Automated detection of wildlife in proximity to marine renewable energy infrastructure using machine learning of underwater imagery

2023-06-09T09:49:21+01:00

Environmental interactions of marine renewable energy (MRE) projects are challenging to monitor, and key questions remain about their potential impacts. Processing large volumes of environmental data acquired from submarine monitoring and the use of machine learning to identify presence and interactions of marine wildlife with MRE infrastructure are powerful tools for assessing the environmental response to MRE infrastructure. The use of automated image analysis for species identification and enumeration using algorithms like convolutional neural networks can vastly reduce the time required to extract usable data from submarine imagery compared to manual expert processing. We present a novel industry-ready image processing workflow for automated wildlife detection developed using 1000+ hours of underwater video footage obtained by Nova Innovation Ltd. from their operational tidal stream turbine array at Bluemull Sound in Shetland, Scotland. The objective of this work was to develop a workflow and associated algorithms to automatically filter many hours of underwater video, remove unwanted footage, and extract only video containing marine mammals, diving birds or fish. The workflow includes object detection through advanced image analysis, image classification using machine learning, statistical analyses such as quantification of data storage reduction and number of detections, and automated production of a summary report. Blind tests were undertaken on a subset of videos to quantify and iteratively improve the accuracy of the results. The final iteration of the workflow delivered an accuracy of 80% for the identification of marine mammals, diving birds and fish when a three-category (wildlife, algae, and background) classification system was used. The accuracy rose to 95% when a two-category system was used, and objects were classified simply as ‘target’ or ‘non-target’. The entire workflow can be run from video inception to production of an automated results report in approximately 30 minutes, dependent on size of input data, when environmental conditions such as water clarity and key species of interest are familiar to the algorithm. The accuracy and runtime speed of the workflow can be improved through expanding the training dataset of images used in the development of this initial tool by including additional species and water conditions. Application of this workflow significantly reduces manual processing and interpretation time, which can be a significant burden on project developers. Automated processing provides a subset for more focused manual scrutiny and analysis, while reducing the overall size of dataset requiring storage. Auto-reporting can be used to provide outputs for marine regulators to meet monitoring reporting conditions of project licences. Integration of this workflow with automated passive acoustic monitoring systems can provide a holistic environmental monitoring approach using both underwater imagery and acoustics.

Choose Your Own Marine Energy Adventure Game: Collision Risk

2023-06-06T06:46:00+01:00

The risk of collision between marine animals (marine mammals, pelagic fish, and diving seabirds) and underwater turbines continues to be the first question raised by regulators for new tidal or riverine energy projects around the world, and the most significant issue that slows down consenting/permitting. The challenges to understanding collision risk stem from the difficulty in observing close encounters and interactions of marine animals with a tidal or riverine turbine, which rotates much slower than a hydroelectric turbine or a boat propeller. To better understand this risk, the marine energy community and the general public need to be educated on the processes and various stages involved in collision risk of marine animals with turbines. This project aimed to develop an interactive outreach tool for the marine energy community and the general public, to educate the audience on the low risk of collision by highlighting the different stages of a collision event at various spatial scales and featuring a fish species of concern, the Atlantic salmon. This tool is a three-dimension educational interactive experience, accessible from any web-connected platform, and is hosted on a publicly accessible website. Throughout the experience, the player is presented with various decisions to take as the salmon approaches and interacts with a tidal turbine. At each step along the way, the player is provided with bite-size scientific information and weblinks to deepen their understanding of the topic. This short but efficient tool with expert messages is a "hands-on" experience for the players, where they are the animals and make the decisions leading up, or not, to collisions. The outcomes of this project will support broad outreach and education goals in order to reduce barriers to consenting/permitting for future deployment of tidal and riverine turbines.

Measurements of the wake from a floating tidal energy platform

2023-06-25T08:56:28+01:00

Wake measurements are critical for quantifying the hydrodynamic impacts of turbine presence and tidal energy extraction on the tidal flow. Turbine wakes are typically assessed using numerical models and controlled laboratory experiments, with only a few field studies available for the wakes of full-scale operating tidal turbines.

In this investigation we present field observations of the combined wake generated by the four-turbine array mounted onboard Sustainable Marine Energy Canada PLAT-I 4.63. Measurements were conducted downstream of the platform in Grand Passage, a tidal channel in the southwest of the Bay of Fundy in eastern Canada in October 2020. Velocity data were obtained by a suite of mobile Acoustic Doppler Current Profilers (ADCP), both vessel-mounted and free-drifting. Data were collected during ebb and flood tides (and therefore with time-varying inflow velocity), and under different turbine operating conditions. The collected data were organized according to the turbine inflow velocity for ebb and flood tide. For each tide, the wake and undisturbed flow regions to the sides of the wake were identified. Vertical profiles of velocity in the wake were compared to inflow velocity measured by a current-meter onboard PLAT-I and with measurements in the undisturbed flow to the sides of PLAT-I wake.

In all measurements the PLAT-I wake manifests as a reduction in flow speed at the depths spanned by the turbine rotors. The reduction is maximum near the platform for both ebb and flood. For flood, velocity profiles vertically mix less than 5 effective diameters downstream of the array, but velocities remain slower compared to the flow outside of the wake. Flow speed increases downstream, recovering approximately about 20 effective diameters from the platform. For ebb, the velocity reduction persists farther downstream compared to flood, there is less vertical mixing, and the wake shape is still present beyond 10 effective diameters downstream of the platform. Increased turbulence is also observed downstream of the platform, which recovers to levels similar to those of the surrounding undisturbed flow about 10 effective diameters downstream of the turbine for both ebb and flood. Comparisons of results between the two measurements approaches, and between the wake of clean and bio-fouled turbines are also explored.

Siting tidal energy projects through resource characterization and environmental considerations

2023-06-20T16:53:04+01:00

The development of tidal energy technologies has progressed to where devices can be deployed, operated, maintained, and recovered with some level of assurance that they will and produce adequate levels of power. Equally important to further the tidal energy industry is the ability to site and gain regulatory permission to deploy and operate these devices. This paper sets out a framework for reaching preliminary siting of tidal devices, drawing from case studies from three locations in the US where research studies have provided information in support of tidal deployments.

Through the TEAMER funding opportunity in the US, tidal energy device and project developers were able to engage US Department of Energy national laboratory scientists and engineers to provide technical assistance for investigating potential tidal deployment sites within US waters. The bodies of water of interest had already been determined by the proponents at the start of the project and constraints and opportunities within those bodies of water were examined to optimize siting capabilities for the developers. Using numerical models and field observations, we characterized tidal resources at a scale that will allow for optimization of energy extraction. We examined the natural and human infrastructure constraints for deploying and operating tidal devices and arrays including channel widths, bathymetry, vessel traffic, ferry lanes, and grid interconnects, in order to narrow siting options. We also examined the biological resources in the water bodies of interest, with a focus on populations of endangered marine mammals and fish, and the critical habitats that support them. The biological resources were then related to the applicable regulatory requirements in place in US for federal and state statutes in areas where the tidal applicants wish to deploy. Based on these analyses, preferred deployment locations were delineated and processes for meeting regulatory requirements laid out, including post-installation monitoring plans that will be needed. This initial assessment of logistical, regulatory, and environmental conditions for the deployment of a tidal technology is a first step toward the achievement of regulatory compliance for tidal energy projects.

Three locations were considered for tidal energy development in the US. The first one included the area around an archipelago of islands in the northern portion of Washington State, near the US-Canada border, with the intent of installing one or more floating tidal devices to add energy resilience and independence for the single utility that services the isolated islands. The second location was in the coastal waters of Maine where tidal power would be added to the local electrical grid. The third location was in Cook Inlet, Alaska, where the applicant seeks to deploy multiple floating tidal devices to provide renewable energy in place of conventionally generated power for the city of Anchorage.

ITSASDRONE, an autonomous marine surface drone for fish monitoring around wave energy devices

2023-06-26T09:35:24+01:00

The ocean energy development is one of the main pillars of the EU Blue Growth strategy. However, while the technological development of devices is growing fast, their potential environmental effects are not well-known. The nascent status of the Marine Renewable Energy (MRE) sector and Wave Energy (WE) in particular, yields many unknowns about its potential environmental pressures and impacts, some of them still far from being completely understood. Wave Energy Converters’ (WECs) operation in the marine environment is still perceived by regulators and stakeholders as a risky activity, particularly for some groups of species and habitats. The SafeWAVE project aims to improve the knowledge on these impacts trough an Environmental Research Demonstration Strategy based on the collection, processing, modelling, analysis and sharing of environmental data collected in WE sites from different European countries where WECs are currently operating (Mutriku power plant and BIMEP in Spain, Aguçadoura in Portugal and SEMREV in France) representing different types of technology, different types of locations and different types of project scales, therefore, different types and/or magnitudes of environmental impacts. As part of SafeWAVE project, environmental monitoring plans relative to electromagnetic fields, acoustics (noise), seafloor integrity and fish communities need to be put in place. In the case of fish communities, generally speaking, any artefact located in the sea may cause an attraction effect on fish communities, especially if it is floating. In the case of Marine Renewable Energy Devices, during the operation phase, in general, the placement of any artefact in the sea can result in an attracting effect on fish communities, especially if it is floating. In order to study this possible effect, fish communities monitoring around the Penguin WEC-2 of WELLO-Oy localized in BiMEP test site with the ITSASDRONE surface drone developed by AZTI was planned. The study was designed to carry out the tunning and conditioning of the ITSASDRONE and to discover the associations between WECs and fish aggregations. Penguin WEC-2 of WELLO was removed from BiMEP test site due to an alarm of leakage. Consequently, we decided to carry out the monitoring work around the HarshLab, a floating laboratory device constructed by Tecnalia and localized also in BiMEP test site. The study allows us to validate the ITSASDRONE as a viable autonomous vehicle for fish school monitoring. It still needs some technological improvement related to navigation system, but in general, the ITSADRONE meets the objectives for which it was conceived and could be an excellent monitoring technique due to its capacity to work remotely and in near shore areas. Schools of unidentified small pelagic fish were observed distributed throughout the water column, predominantly near the bottom in the device area. But, future studies are needed to further explore the association between WECs and fish aggregations. The aim of this communication is to show the work done by an autonomous marine surface drone, ITSASDRONE, in relation to fish monitoring in BiMEP area.

Empowering Communities to Participate in Marine Energy Planning and Development

2023-04-05T15:06:23+01:00

To meet community priorities and equitable outcomes, marine renewable energy (MRE) projects, such as those for marine energy and offshore wind, require informed and collaborative marine spatial planning (MSP) to overcome significant cultural, societal, and economic barriers that may limit responsible and accelerated deployment. Effective MSP brings together coastal communities and ocean users to coordinate decision-making for sustainable use of marine resources. The goal of this project is to develop digital tools to enable easy access to information, interactive visualization tools, and feedback mechanisms to empower communities to engage in meaningful communications with other stakeholders in the MSP process.

In collaboration with community organization partners, the Santa Barbara Maritime Museum and Redwood Community Action Agency, we held community-based workshops to solicit needs for communities to gain knowledge and communicate concerns about MRE development. The key results from initial outreach activities are:

The MRE planning process must incorporate stakeholder feedback starting in its early stages (as early as siting). Community benefits must be clearly articulated and quantified.
Communities generally distrust offshore energy developers. Communication with regulators and developers together would be useful.
Community members are eager to learn about MRE. Their biggest knowledge gaps are MRE device and array characteristics (size, extent, location) and effects of MRE development on the viewshed, recreational activities, and on wildlife.
There is too much information available over disparate sources. Community members do not know where or how to search for answers to questions about MRE development in their communities (or elsewhere).
Community members want to hit the ‘Easy Button’. Data need to be digestible and tools need to be easy to use with little technical knowledge or experience necessary.

Outreach activities guided the development of the following digital tools:

Interactive infographics to communicate topics of concern (e.g., viewshed impacts and potential interactions with whales) in a visually engaging and interative platform.
News feed relevant to MRE to quickly digest headlines and read articles in greater detail, sort articles by topic, and react to articles or other comments.
Artificial intelligence (AI) driven chat bot, which uses the latest in AI, namely GPT-3 (Generative Pre-trained Transformer 3), to produce real-time answers to user questions. Answers are logged, and can be reviewed and revised by marine energy experts to provide human vetting of the results.
Commenting system to provide feedback on information throughout the site, as well as the tools themselves.

These tools will empower communities with information and feedback mechanisms to promote meaningful stakeholder engagement and communications, and therefore, informed decision-making. These tools represent a major leap forward in ocean multi-use management with MRE.

Assessing the effect of onshore and offshore Wave Energy Converters on seafloor integrity combining image-based and acoustic methods

2023-06-25T08:58:54+01:00

The European Atlantic Ocean offers great opportunities for the development of projects for renewable energy extraction, and the Marine Renewable Energy sector is developing different technologies for energy converters, including Wave Energy Converters. Besides, the European Commission is adopting measures and politics to increase the installed capacity of ocean energy. However, there are still uncertainties on the potential environmental effects of wave energy converters, which led regulators and stakeholders to perceive their operation as a risky activity.

To overcome the non-technological barriers that could hinder the development of marine renewable energies, and improve the knowledge on the impacts on the seafloor integrity (among others), SafeWAVE project set an Environmental Research Demonstration Strategy based on the collection, processing, modelling, analysis and sharing of environmental data collected in Aguçadoura (Portugal), Armintza (Spain) and Le Croisic (France) representing different types of technology, locations and project scales.

Video recordings were carried out, using Remotely Operated Vehicles, to identify the environmental impacts of the moorings on the seafloor morphology. That information was completed by side scan sonar campaigns using an Autonomous Underwater Vehicle.

The video recordings were useful also to assess the attraction effect of the moorings to epibenthic invertebrates and fishes, whereas the side scan sonar provided quantitative information on the area affected by physical alteration (<1% of the total area occupied by the devices at Armintza and Le Croisic).

Effects of the spacing between two hydrokinetic turbines on the bedforms by numerical simulations

2023-06-25T03:24:36+01:00

Accelerating hydrokinetic renewable energy development towards endurance requires investigating interactions between the hydrokinetic turbine and its surrounding physical environment. Interactions between hydrokinetic turbines (HT) and mobile sediment bed are considered as a critical area of assessment, however limited research studies have been published to address this issue. Hill et al. (2015) have shown experimentally that the presence of either single or multiple turbines and the rotation of the blades affect the bed morphology. Musa et al. (2019) have investigated experimentally the local effect of streamwise aligned turbines on the bedload, they found as a result that the geomorphic effects are stronger with increasing shear stress due to the presence of the rotors, inducing an alternating scour-deposition phenomenon. Chen et al. (2017) have investigated the influence of rotor blade tip clearance (distance between blades and seabed) on the scour rate of pile-supported horizontal axis current turbine. The results suggest that the decrease in tip clearance increases the scour depth, hence more sediment transport. Recently, Khaled et al. (2021) have studied the impact of hydrokinetic turbine on erodible sand banks, they showed numerically a significant interaction between the confinement of the turbine and its impact on the near bottom.

In the present issue, we study the impact of two interacted turbines on the near bedform morphology. A modelling framework is derived to predict the significant transport induced by the turbines, such as the Euler-Euler (EE) multiphase model for sediment transport and the Blade Element Method (BEM) to model the forces generated by the turbines, using the open source platform OpenFOAM and the library SedFoam (Chauchat et al. 2017). A phase of validation is presented for the combined model (EE and BEM) using experimental results of Hill et al. (2015). The present study consists in considering one sediment class, sand of diameter of 0.25 mm, and two horizontal axis turbines with an axial flow direction corresponding to the riverine case. The approach is configured with four different axial inter-turbines distances. The wake distribution behind the second turbine is altered by the wake of the upstream turbine in all configurations (fig. 2). This interaction promotes the erosion under the turbines and the deposition along the axis of the turbines in the wake due to relation between the dynamics of ripples generation and the wake effects of turbines (fig. 3).

Underwater noise impact assessment of a wave energy converter in the northern Atlantic (Spain)

2023-07-24T19:34:08+01:00

As most nations around the world commit to decarbonize their economies, the electrification of many sectors is an ongoing trend that will only strengthen in the next years. The energy demands for this transition are considerable, and renewable energies are being pushed to substitute fossil-fuel technologies with great urge.
While marine renewable energy (MRE) is still not as developed as wind, hydraulic or photovoltaic energies, it constitutes a promising and mostly untapped resource. MRE may complement quite well the production curves of both wind and solar power, and can provide non-fossil energy to coastal populations, or those located in islands where land is precious, so its development is important to many countries, given that most of the population is located near the coast.
Even though MRE does not produce CO2 emissions, their possible impacts on the marine environment are already well theorized, such as encounters with moorings/cables, collisions, underwater pollution (among others) there are still uncertainties in the actual impacts of real WECs on their surrounding ecosystem.
This is one of the reasons behind the European research project SafeWAVE - Streamlining the assessment of environmental effects of wave energy (2020-2023), - as well as its predecessor WESE – Wave Energy in Southern Europe (2018-2021)-, which is dedicated to research on the non-technological barriers to the development of the wave energy sector. A fundamental work package of SafeWAVE is the monitoring and modelling of possible relevant impacts of wave energy converters (WEC), including electromagnetic fields, underwater noise, or seabed integrity, using as test cases four different WECs deployed in corresponding test sites, located in Basque Country (Spain), Nantes (France) and Aguçadoura (Portugal).
The aim of the present work is to report the results obtained from monitoring, processing, and modelling of underwater noise around one of the wave energy converters (WEC) studied in the context of this project. The monitoring activities consisted of two campaigns, one before and during installation of the device, and another one during its operation and eventual decommissioning. Each one covered a duration of about 45 days, using one and three moored hydrophones surrounding the WEC, respectively. Acquired data was processed and analysed to obtain Sound Pressure Levels (SPL) in 1/3 octave bands ranging from 20 Hz to 20 kHz for different sea states, operational status, and activity (deployment, decommission, etc.). Lastly, underwater transmission losses were modelled using a full range dependent computational model allowing to assess the extent of acoustic disturbance that could be caused by the WEC, and furthermore, by a hypothetical array of several WECs.
Initial results evidence differences between the SPL distributions before and after the deployment of moorings, as well as during operation (with respect to background levels), at the location of the hydrophones.

Life Cycle Assessment of a wave energy device – LiftWEC

2023-05-31T16:11:35+01:00

The need to move towards a low-carbon economy has brought about the emergence of various renewable energy sectors, including Marine Renewable Energy (MRE). However, after many years of research and development, the MRE industry still faces challenges in achieving commercial viability, especially regarding wave energy. Whilst it remains possible that successful wave energy technologies exist in the traditional research trend, it is also appropriate to explore alternatives that produce energy by different approaches. Wave-induced lift force devices may be the possibility to move beyond traditional wave energy technologies using diffraction and/or buoyancy forces. In this context arises the LiftWEC, a promising configuration of a lift-based wave energy converter. The LiftWEC device couples with the waves through lift forces generated by two hydrofoils that rotate in a single direction aligned orthogonally to the direction of wave propagation.

To fully evaluate the overall advantages of this new technology, it is necessary to go beyond the techno-economic performance and reliability. While capable of producing electricity from clean sources, MRE devices are not entirely environmentally friendly, since energy is consumed and pollutants are emitted during their various life cycle stages. Accordingly, as the MRE sector expands, it is important to ensure that the technologies prove to be sustainable alternatives in terms of their environmental impact.

Life Cycle Assessment (LCA) is a widely recognized methodology to evaluate environmental impacts by considering the technology’s performance over its life cycle. This methodology complies with international standards ISO 14040, which specify the general framework, principles, and requirements for conducting and reporting this type of assessment.

A “cradle to grave” LCA assessment was applied to the LiftWEC device to evaluate the potential cumulative environmental impacts of the system, complete from the extraction of raw materials until decommissioning. Each stage was analysed within the defined system boundaries, and data on the energy, materials, emissions, and waste products associated were gathered. To allow comparison with other MRE technologies and traditional means of electricity generation, carbon dioxide equivalent emissions per produced electricity (gCO2eq/kWh) were calculated for the study.

Since ocean energy is broadly considered to contribute to a low-carbon energy system, special attention was given to the LCA results on the global warming potential (GWP). Besides a set of 18 impact categories, Cumulative Energy Demand (CED) and Carbon and Energy payback time (CPT and EPT, respectively) were also analysed. The CPT and EPT are important indicators that measure the time required to offset the carbon emission and demanded energy, respectively, accounted across all development phases of the device.

This work included the comparison of LCA findings for other MRE devices reported in the literature to validate the viability of the LiftWEC in terms of carbon and energy footprint. In addition, the assessment analysed alternative materials, locations, and recyclability allowing the identification of potential improvement opportunities regarding the reduction of environmental impacts.

Validation of the energy resource assessment with experimental data for the site selection of a tidal turbine in the Tagus River estuary.

2023-06-11T10:36:47+01:00

Lisbon, the capital of Portugal, is located on the mouth of the Tagus River, where the current speed
and direction are mainly governed by the local tides. The narrowest part of the river is located between Lisbon downtown and the western side of the city. This narrowing accelerates the water flow and makes it a potential site for a tidal energy system. A preliminary study based on numerical simulations using the software MOHID was conducted to assess potential energy yields throughout the estuary using freely available hindcast data. This allowed the selection of three potential sites for a tidal turbine in the Lisbon area based on yearly tidal and current energy density: off the coasts of Cacilhas, Bel´em, and Pa¸co de Arcos. However, even if the current model has been previously validated with experimental data, it was only done at two locations in the estuary that are far from the potential
sites. Due to the complexity of the phenomena driving the current speed at these locations, additional validation is necessary before committing to a specific site. This paper presents the numerical analysis, the experimental campaign and the validation of the results at those three locations. Drifters with sails of 3.4m and 4.5m depth were released at least 8 times at each location and retrieved after 15 minutes of free drift. Each drifter was tracked with a GPS and the current speed and direction were derived from the drifters’ trajectory. The analysis of the experimental data shows good agreement with the model, even though an error of 0.3m/s is consistent throughout the tests. This paper concludes with a discussion on the model temporal and spatial discretisation and the forces included in the analysis that could be the source of the differences between the numerical and experimental data.

On tidal array layout sensitivity to regional and device model representation

2023-06-26T10:09:22+01:00

In-stream tidal devices are ready to be deployed, yet the largest operational commercial array is limited to 6MW. Upcoming government support should see the size of such arrays increase by orders of magnitude, and thus the optimal placement of turbines within tidal arrays is an emerging challenge for successful commercial integration. Hydrodynamic models are required to predict the power produced by an array and the impact on the surrounding environment. The influence of common model inputs to layout optimisation are investigated herein. This is achieved using a shallow water equation based tidal array modelling framework, Thetis, coupled with a low cost analytical wake model (FLORIS) that allows for rapid assessment of the impact of small changes in hydrodynamic results on array micro-siting. The sensitivity of array optimisation at an intermediate development point (43 turbines) is interrogated through both artificial flow field manipulation and the variation of model input pertinent to the optimisation. A small margin exists in which an optimised array layout will perform efficiently for a deviation in flow prediction accuracy. However, incorrect flow predictions by a range sensitive to model inputs had a reduction of almost 8% on array efficiency relative to a control case. The sensitivity of flow field variance by model input changes, on extractable energy and array layout are substantial. Comparing arrays sited on flow fields using different bathymetry resolution leads to a discrepancy on average of over 2% to average array power. Arrays sited for different mesh resolution and friction representation also see average changes up to and exceeding 0.75%. For array developers and the future of this nascent industry, re-calibration of the model at every stage of data collection coupled with early acquisition of bathymetry data is critical not just for accurate power quantification, but also array efficiency. At a regional scale, quantification of the potential tidal resource must consider the consequences of uncertainty in model input data and site constraints that cannot yet be accounted for.

An Analysis of the German tidal energy resource

2023-08-25T12:23:55+01:00

A transformation of the energy sector towards a low-emission power generation is needed to mitigate global warming and fight the current climate crisis. In recent years, tidal energy technology has matured and shows potential to balance Europe’s future power grid. While reviews of the tidal energy resource exist for a number of European countries, the potential of tidal energy along the German North Sea coast is overlooked so far. This review closes this gap and provides a first analysis of the German tidal energy resource. Germany’s North Sea coast is characterised by comparatively low current velocities and shallow waters. Using available data from the EasyGSH-DB North Sea Model, Germany’s practical tidal energy resource is estimated at 66.6 GWHy−1, under strong restrictions, excluding the most energetic sites in the estuaries of Elbe, Weser, and Ems. Based on the results, future work for a more detailed analysis is suggested.

Resource assessment using a combination of seabed mounted and semi-stationary vessel-mounted ADCP measurements

2023-06-11T21:07:17+01:00

Accurate resource assessment for tidal stream sites is crucial for calculation of the Levelized Cost of Energy (LCOE) of each project or turbine. The main method as recommended by the IEC’s technical specification on tidal stream resource assessment involves using a numerical model as well as several measurements using Acoustic Doppler Current Profilers (ADCP) mounted on the seabed. Ideally, once the locations of the turbines are known precisely, seabed mounted ADCP measurements will be undertaken at all turbine locations. For large tidal arrays this is however challenging, especially with project development constrained by time and budget. The MeyGen project is the largest planned tidal stream energy project in the world, aiming to develop up to 398 MW of installed power in the Pentland Firth, Scotland, with current velocities reaching up to 5 m/s. During Phase 1 of the project, four 1.5 MW turbines were installed. During Phase 2, site characterization campaigns were carried out to plan the deployment of additional turbines, representing 28 MW of tidal power capacity. Based on learning from Phase 1 around the need to accurately calibrate numerical models at a very detailed spatial resolution, and the costs and time to obtain seabed ADCP measurements at many turbine locations, a new measurement methodology was proposed. This method used semi-stationary vessel-mounted ADCP measurements to characterize the flow at the planned turbine locations, rather than relying on a few seabed-mounted ADCP measurements and a numerical model, or relying on seabed-mounted ADCPs at all the locations of the planned turbines.

This works introduces the method used for calculating the tidal stream velocities at the planned turbine locations. A 35-day seabed mounted ADCP survey was conducted as a baseline within the area of planned turbines. It used a Teledyne SentinelV50 ADCP configured with 2Hz sampling rate, 5-beam velocities and 1 m depth cells. Using harmonic analysis and prediction, the current velocities were obtained for a 18.6 years period.

Additionally, during seven days of spring tide, water velocities were measured using a vessel mounted ADCP. It used a Teledyne Workhorse 600kHz configured with circa 1Hz sampling rate and 1 m depth cells. The vessel mounted measurements were undertaken at each turbine location during 5 minutes every hour, for a 12-hour tidal cycle. During each 5 minutes the vessel kept its position within 15-meter radius around the nominal position. Similar vessel-mounted measurements were also undertaken above the seabed mounted ADCP, as well as continuously for 12 hours.

The semi-stationary vessel mounted ADCP measurements were validated against the seabed-mounted ADCP measurements with the co-located 12-hour data. The current velocity magnitude obtained from the vessel mounted measurements at the turbine locations were then correlated with the simultaneous data from the seabed mounted ADCP. This correlation was finally combined with the 18.6 years timeseries predicted at the seabed-mounted ADCP location to obtain long-term times series and statistics for turbine locations. The final presentation and paper will provide the results of this study.

Measurements of tidal flow variability in Ramsey Sound, Pembrokeshire

2023-06-12T14:07:13+01:00

The nature of the flow at in-stream tidal energy sites is particularly important for predicting array and device performance, and also for operations and maintenance planning. Previous developers have reported issues such as the choice of vessel, cost of operations, and the limits of operation of deployment vessels. The dynamics of the flow around slack water has been of particular interest at Ramsey Sound in Pembrokeshire (UK) for planning the recovery of an existing turbine, the Tidal Energy Limited ‘Deltastream’.

This research presents flow characteristics of Ramsey Sound, based on analysis of Acoustic Doppler Current Profiler (ADCP) measurements and tide gauge data from the nearby Standard Port of Milford Haven. The ADCP was located approximately 300 m across the channel, at the northern end of the channel, where the channel width was 1200 m and the mean depth was approximately 33 m. The flow dynamics were examined specifically to look at times potentially suitable for offshore operations Two weeks of data were used in the analysis, spanning a complete spring-neap cycle.

Results demonstrate that flow velocities exhibited clear asymmetry, with stronger flows on the northerly directed flood tide than on the ebb. There was considerable variation in the measured current speed around the time of the maximum, suggesting large scale bed feature generated turbulence. The flood (northerly) current maximum was approximately in phase with high water at Milford Haven. Cross correlation indicated that the flow generally led the elevation by 20 minutes. In contrast to the expected theory, the current strength at mid-depth was stronger than at the surface on the maximum flood tide. The maximum flow speed in the tide was reasonably predictable from the tide range at Milford. A threshold-based analysis of the ADCP measurements allowed the duration of slow-moving water to be identified for operation planning.

Operations and planning in light of sound understanding of hydrodynamics at tidal energy sites is critical for future economic success of the tidal energy sector. The results shown here from an ADCP deployment in Ramsey Sound have shown the capability to give useful tools for planning recovery operations.

Investigation of Low Order Parameters Affecting Tidal Stream Energy Resource Assessments

2023-06-16T14:21:51+01:00

Tidal stream energy may have the potential to contribute to the baseline energy needs of the UK. There is a significant difference between the existing assessments of the UK’s resource, highlighting the need for a standardised model. The paper investigates some challenges of quantifying tidal stream energy. This study examines the sensitivity of resource assessments to low-order parameters, in order to inform higher order modelling and contribute to defining a standardised model. A combination of channel characteristics, turbine parameters and simulation settings formed the basis of 96+ cases to determine key parameters affecting resource assessments. Two 0-dimensional, idealised channels were modelled using 2-8 constituents from four UK sites spanning the semi-diurnal form factor range. Blockage-corrected blade element momentum theory was used to represent variable-speed, variable-pitch turbines. Three blockage designed rotors and one rotor designed for an unspecified blockage were simulated. Power capping and combined power and thrust capping were implemented.

The study indicates dynamic balance is a key parameter affecting tidal dynamics when turbines are present in a channel. Variability of annual energy production due to the nodal cycle was most significant in the drag dominated channel with increasing blockage. Constituents with large amplitude and phase differences led to greater variability of the resource across the nodal cycle. Results indicate that the maximum deviation from the average annual energy production over the 18.6-year nodal cycle is +/- 8.8%. At a low form factor site, the difference between using 2 or 4 constituents was insignificant. However, for a higher form factor site, that is still semi-diurnal, the peak velocity increases by 0.32 m/s with 8 constituents compared to 2. Random Forest modelling indicates that the most important characteristic point on a variable-speed, variable-pitch turbine performance curve is the rated speed. Comparison of capping strategies highlights that combined power and thrust capping leads to an increase in energy per swept area. Combined capping decreased the variability of the resource over the nodal cycle, which is helpful to maintain a steady supply to the grid and sizing energy storage systems.

Mapping the Unresolved Tidal Resource in Estuaries

2023-06-26T12:31:45+01:00

Estuaries are ideal locations for extracting tidal energy and yet the global resource appears poorly mapped. Estuaries typically have high tidal ranges and strong tidal currents, due to amplification processes; and this resource is juxtaposed to industrial/residential areas with electricity demand [1]. For example, 22 of the 32 largest cities in the world are adjacent to estuaries [2], and UK estuaries collectively worth over £5.5b to UK economy with >1/6^th of the population and ~1b tonnes of cargo traded at their ports [3].

Mapping the tidal energy resource is challenging due to the sensitivity of ocean-model simulated currents to the model resolution [4]. Both tidal amplitude and currents are heavily modified in estuaries [5]. Whilst many estuary-specific resource modelling studies have been achieved (e.g. [6]), no large-scale future tidal energy resource mapping project has been undertaken – likely due to computational cost (e.g. [4]). It is therefore unrealistic to hydrodynamically model all global estuarine systems, to resolve current and future changes to the tidal energy resource; instead we aim to apply a simplified analytical technique that could be calibrated and validated by the new NASA SWOT satellite mission (https://swot.jpl.nasa.gov/), as well as citizen science. Estuarine tidal dynamics have been observed to change rapidly due to changes in river-flow climatology and bathymetry (e,g. dredging); see [7]. Indeed, climate change driven impacts to tidal dynamics (sea-level rise and altered riverine climatology), alongside anthropogenic driven morphodynamical changes and interactions with future tidal dynamics, could increase the future estuary tidal resource [1].

Three physical processes drive mean tidal amplification excluding over-tides): (1) Funnelling - concentration of the tidal energy flux with reducing width; (2) Resonance - when estuary length-scales are close to the natural period of the basin; and (3) Shoaling (reduction in the propagation speed of the tidal wave resulting in an increase in amplitude). Each of these three processes are analytical solved, and a 1D analytical model applied to demonstrate the resource. Sea-Level Rise (SLR) scenarios are included to show SLR can modify estuarine tidal dynamics through both estuarine geometry (e.g. increasing resonance) and boundary conditions (global mean sea-level rise increasing boundary conditions). A normalised result of the analytical approach is shown in Figure 1, which demonstrates a computationally efficient analytical solution that includes future changes to tidal dynamics.

Our research will apply this analytical solution to the Bristol Channel, a region soon-to-be heavily instrumented as part of the “Cal-Val phase of the NASA SWOT project (https://projects.noc.ac.uk/swot-uk/swot-uk-bristol-channel). Finally, we will develop a technique to bring together digital observations from citizens and sensor networks (see https://digitalenvironment.org/), to validate our simplified approach and resolve the tidal energy resource.

Influence of the spatial variation of upstream velocity on a vertical-axis tidal turbine performance

2023-06-11T21:49:28+01:00

Assessment of the upstream flow available to the tidal energy converters (TEC) is key to evaluate its performance. Simultaneously, TEC technology has been innovating on its concepts and designs to expand the potential sites to harvest energy generated by tidal currents and rivers. The Gkinetic CEFA 12 is an easy-to-deploy device suitable to operate on estuary environments. The design consists of two 1.2 m vertical axis turbines attached to the sides of a buoyant platform, which uses a bluff body to accelerate the incoming flow to the rotors. The device was deployed on a single point mooring enabling passive flow alignment.
As part of the Vertical axis tidal turbines in Strangford lough project (VATTS), the upstream flow has been measured using acoustic doppler profilers (ADP) mounted on the TEC during its operation in Strangford lough. Recommendations of IEC-200 were followed when mounting the ADPs relative to Gkinetic. However, the continuous repositioning of the TEC according to the prevailing tidal regime affects the ADPs heading. The implications of the spatial variation of the rotor’s upstream velocity on the resource assessment and device performance are not clear since this situation is not typical.
To investigate the spatial variation of the upstream velocity two ADCP deployment locations were made. A seabed (upward facing) ADCP as per the current standards and a device mounted (downward facing) ADCP upstream of the rotor plane. Evaluation of the influence of the following three factors were made: i) the ADPs direction repositioning according to prevailing tidal regime, ii) the proximity of the sensors to the sea-surface, and iii) the proximity of the sensors to the TEC.
These three factors will be evaluated using ADP datasets collected at Strangford narrows during the VATTS project. The datasets were obtained at approximately the same location of the TEC operation. They enable the study of the following scenarios 1) incoming flow to the rotors during operation, 2) incoming flow on undisturbed conditions (no turbine operation), and 3) a harmonic analysis prediction of the undisturbed incoming flow, which solely captures the tidal-driven flow.
The investigation of these three scenarios will provide a better understanding on the rotor’s upstream flow spatial variation, and the influence of the device’s proximity and near sea surface conditions on the mean flow. These findings would benefit developers of alternative TEC designs that operate near the sea-surface.

Tracking a large vortex at a tidal power site

2023-07-03T12:35:03+01:00

Tidal turbines are sensitive to the high turbulence of tidal flows. Large and intense vortices are generated at the seabed and cause extreme loads, fatigue damage and degraded power production. Thus, these vortices must be characterised prior to the turbine installation. The flow characteristics can be assessed through ADCP measurements, but these measurements are sparse. The vortex characteristics are strongly affected by the seabed macro-roughness (rocks, faults) that induces spatial variations of the flow characteristics at a local scale. These local variations are difficult to catch through measurements [1]. Numerical simulations, that cover large areas, can fill in the gaps of measurements. Reynolds Averaged Navier Stokes simulations cover wide domains, but do not simulate the turbulent motions. Large Eddy Simulations (LES) do simulate the turbulent motions, but are computationally expensive, which reduces their spatial and temporal coverages.

LES has been validated for the simulation of turbulence at a tidal power site, with a spatial coverage of about 0.5 km² and a temporal coverage of about 30 minutes [2,3]. However, the high turbulence intensity complicates the analysis of turbulent motions. The tracking and characterisation of vortices is complex and time-consuming. New methods to automate this work would be very welcome.

In this work, we use Large Eddy Simulations to simulate the vortices generated at the rocky seabed of the Paimpol-Bréhat tidal turbine test site (France). A tracking method is used to follow the movement of turbulent motions (see Figure 1). This tracking highlights the long durability of turbulent motions. The impact of the most intense motions on fictive turbines is assessed and extreme flow variations are observed. This confirms the interest of the method for an easy detection of the most troublesome vortices and the locations where they are generated. It paves the way for the identification of the locations where turbine installation should be avoided due to potential damaging turbulent motions.

[1] Togneri, M. & Masters, I. Micrositing variability and mean flow scaling for marine turbulence in Ramsey Sound. Journal of Ocean Engineering and Marine Energy, 2016, 2, 35-46

[2] Mercier, P.; Grondeau, M.; Guillou, S.S.; Thiébot, J. & Poizot, E. Numerical study of the turbulent eddies generated by the seabed roughness. Case study at a tidal power site. Applied Ocean Research, 2020, 97

[3] Mercier, P. & Guillou, S.S. The impact of the seabed morphology on turbulence generation in a strong tidal stream. Physics of Fluids, 2021, 33

Overview of Resource and Turbine Modelling in the Tidal Stream Industry Energiser project: TIGER

2023-06-17T15:52:02+01:00

Tidal energy projects require numerical modelling for the assessment of tidal site conditions and turbine/array performance. The Interreg TIGER project has offered a unique opportunity to implement a wide range of numerical models. This paper provides an overview and comparison of the different numerical models developed by academic partners in the TIGER project. The models cover a variety of spatial and temporal scales. The largest scale models provide long-term climatic studies covering the entire English Channel region, at relatively low resolution, whilst the highest-resolution models provide detailed information about short-term and small-scale turbulent flow and its interaction with tidal turbines. The models are used for various purposes. At one end of the scale, the models have been used to inform the large-scale techno-economic assessment of tidal energy and its impact on the energy mix in the UK and France. At the other end of the scale, the numerical models provide information that feeds into detailed engineering design of tidal turbines at particular sites, and assessment of the energy yield. The models showcase the range of computational tools available to aid the development of the tidal energy industry. This paper will be useful for investors, technology developers and project stakeholders to help identify suitable numerical models to support and develop ongoing and future tidal stream projects.

Evaluating the performance of turbulence closure models for tidal stream resource characterization

2023-07-20T00:26:51+01:00

Harvesting tidal stream energy from the ocean for electricity generation has been considered as an energy resource alternative to fossil fuels for mitigating the negative impact of climate change and enhancing energy security and coastal resilience. Numerical models have been used extensively to characterize and assess tidal resources at potential tidal energy development sites. Turbulence plays an important role in site selection and tidal turbine farm deployment. However, most of the numerical modeling studies for tidal energy resource characterization do not include turbulence characteristics because of the limitation of Reynolds averaged Navier–Stokes coastal ocean model in resolving the inertial sub-range turbulence scales. However, studies also demonstrated that turbulence properties, such as turbulence intensity and turbulence kinetic energy, simulated by the coastal ocean models based on turbulence closures can be useful in assisting tidal resource characterization at tidal energy sites. In this study, we evaluated four General Ocean Turbulence Model (GOTM) turbulence closure models implemented in a tidal hydrodynamic model –Finite Volume Community Ocean Model (FVCOM) to characterize the tidal energy resource in the Western Passage, Maine, USA – a top ranked tidal energy site. The four turbulence closure models used in this analysis are k–kl (Mellor–Yamada Level 2.5), k-ε, k–ω, and a generic model. Model sensitivities showed that the MY2.5 model performed the best in comparison with the field measurements. In particular, the simulated time series of turbulence intensity and kinetic energy matched the observed data very well in the Western Passage. Detailed analysis was conducted to characterize turbulence properties on the horizontal plane and at selective cross-sections. This study demonstrated that turbulence properties simulated by a coastal ocean model, along with tidal hydrodynamic properties, can be very informative for tidal energy resource characterization and project site selection, as recommended by the International Electrotechnical Commission (IEC) technical specifications (IEC TS 62600-201).

Tidal turbine wake characterization by vessel-mounted ADCP data analysis

2023-07-19T23:38:56+01:00

Wakes generated downstream turbines are one of the key points to be considered for tidal stream energy farm layout design. Distance between consecutive turbines must be optimized considering available resource spatial distribution and the current velocity reduction caused by upstream turbines. Tidal turbine wake characteristics have been analyzed experimentally and numerically, however, there is still a lack of studies presenting data measured in the wake of large-size turbines in tidal sites.

In this paper, the wake generated downstream of a SIMEC Atlantis Energy 0.5 MW turbine in Naru Strait (Nagasaki Prefecture, Japan) is analyzed using data measured with a vessel-mounted ADCP. This analysis is based on the comparison between data measured before turbine installation (September 2020) and during turbine operation (March 2021, May 2021, and September 2021).

Due to the difficulties that measuring at the same position and the same tide moment with a vessel-mounted ADCP would mean, a direct comparison between these four datasets is unviable. To solve this issue, data were pretreated to calculate a ratio (R_D) resulting from the division of current velocity values at every vertical layer by the current velocity measured at the shallowest layer. After that, this ratio is interpolated to a regular 3D mesh. Assuming that the current velocity profile shape for one point does not change with time, resulting R_D values interpolated from data measured in different fieldworks can be compared.

The impact of tidal turbine operation on the current velocities is quantified with a variation ratio R_V calculated by dividing R_D during turbine operation (R_D^*) by R_D before turbine installation (R_D). These results show a clear velocity deficit even at 18D from the turbine, which clearly contrasts with results from other similar studies available in the literature, which may be related to the low turbulence intensity measured at the turbine installation point.

Estimation and characterisation of the wave-induced turbulent kinetic energy and turbulent dissipation from ADCP data

2023-06-16T14:15:58+01:00

Turbulence in the water flow causes small-scale variations in the mechanical stress acting on submerged tidal turbines. As such it increases their fatigue loading and impacts greatly their lifetime. It is therefore essential for engineers to have an accurate knowledge and characterisation of turbulence at a given site as they design the structures to be deployed. The strength of the tidal currents is the main parameter influencing the intensity of turbulence through their friction with the sea bed. However, most potential tidal energy sites are located in a coastal environment with shallow enough water depths so that the direct impact of waves on turbulence can not be overlooked. Steepness-induced wave breaking is indeed observed to increase the turbulent mixing for such applications. In this context, we propose to estimate the contribution of surface processes to the total turbulence in Alderney Race, France, the most energetic tidal site in western Europe. The turbulent kinetic energy (TKE) specifically induced by waves and wind is characterised using measurements from a 5-beams ADCP deployed between 27/02/2018 and 06/07/2018.
Analytical profiles are fitted to the data, the only fitting parameter of the model is an evaluation of the turbulence penetration depth, it determines how deep surface processes impact the water column. Its dependence towards mean wave and current parameters is studied. The results do not allow to conclude on the nature of turbulence observed in the mid water column.

Turbine fatigue load prediction from field measurements of waves and turbulence

2023-06-20T12:57:26+01:00

The interest in this work is understanding the conditions which contribute to the cyclic loading experienced by a tidal turbine. Tidal turbines are placed in locations with high flow speeds in order to extract maximum kinetic energy. These flow speeds combined with other environmental factors can lead to a complex operating conditions. When assessing turbine loading across a site, most models use ambient flow conditions, with very few codes including the impact of waves with currents. Previous studies have analysed the influence of wave conditions from measurements on the loading, through a consideration of the turbine not operating. Although tidal conditions are bi-directional, wave conditions do vary with direction and this combination will be investigated in this study.

Full-scale site data was gathered at Le Raz Blanchard, a potential tidal site as part of the Interreg funded TIGER project. The focus in this study is on data gathered which provides both wave and current conditions over a period of 45 days. Loading on the turbine components is assessed through an efficient blade element momentum theory method, which is combined with a synthetic turbulence model to provide a time-varying onset flow. This work determines the impact of the measured waves, and the way in which they are analysed on the turbulence characteristics in the onset flow, specifically the interaction of waves with current and influence on the loading experienced on the turbine.

Development of a Tool to Optimise Tidal Stream Energy Sites

2023-07-03T10:59:40+01:00

Several prospective tidal energy sites have been identified around the UK, however, little is known about the specific, and often complicated, hydrodynamics in many of these area [1]. Many sites comprise complicated seabed and coastline configurations, which can not only give rise to fast-flowing currents, but also results in highly turbulent and spatially-varying currents. These latter features are undesirable characteristics for devices.

Tidal stream energy developers seldom design devices to suit a particular site, rather devices are designed and suitable sites located subsequently. This reduces the number of viable sites as each device has its own design constraints, e.g. minimum water depth, minimum velocity, etc. In order to maximise the exploitability of these sites, where space is generally limited, it would be more beneficial for developers to design devices once site conditions/constraints are better understood.

An open-source tool is being developed that determines the physical constraints of a site based primarily on bathymetry and current velocities (measured and/or modelled). This tool aims to optimise a site to help developers understand what conditions need to be met in order to maximise energy generation – it will also identify which areas are unsuitable depending on the device design. The flexibility of the tool ensures two key aspects:

For existing devices, the device design parameters can be selected to show which areas are viable that meet these parameters (e.g. maximum bed slope, maximum current velocity, etc.);
For devices yet to be designed, different design parameters can be selected to optimise a site to help maximise the energy that can be extracted from a particular site.

The tool is being written in Python with a QGIS front end to allow the user to make changes readily and choose which parameters are important. Not only will the tool identify suitable sites based on user-defined parameters, it will also calculate how much power is available (in kW/m2) based on velocity data.

Given the data availability and interest from tidal energy developers in Ramsey Sound (Figure 1) and to the west of Ramsey Island in West Wales, UK, this site has been chosen as a case study. This site also experiences significant spatial and temporal variation in flow, caused in part by a narrowing of the Sound, which accelerates flow [2]. Once this tool has been proven it will be expanded to assess the viability of other tidal energy sites, both in the UK and overseas. Sensitivity testing is currently underway to determine how sensitive the tool is to data resolution. For example, do higher resolution bathymetry and velocity datasets provide a marked difference in the areas that are viable.

This tool will demonstrate that developers (and other relevant parties in the supply chain) cannot rely on first-order appraisals, which may make energetic tidal straits attractive sites for development since their resource potential depends on flow conditions that are fundamentally local.

Assessing wave-turbulence separation from ADCP measurements with artifical flow data

2023-06-13T13:26:04+01:00

Accurate assessment of marine currents is critical for meaningful planning of tidal stream energy deployments. Methods for assessing mean flow speed are well-established, but the issue of assessing phenomena that vary over much shorter time scales than the semidiurnal tide (principally turbulence and waves) is not as settled. This is in part due to the limitations of the standard instruments used for surveying currents at potential tidal stream sites i.e., acoustic Doppler current profilers (ADCPs). Because ADCPs use multiple acoustic beams to sample single components of velocity at widely-separated spatial locations, a reliable analysis of the mean flow can be obtained by a suitable average over measurements from all beams: this approach works well for determining mean flow properties, but is less well-suited for the smaller and faster variations associated with waves and turbulence. In particular, conventional analysis approaches such as the variance method do not meaningfully distinguish between wave-driven and turbulence-driven velocity variations.

An important consequence of this is that ADCP estimates of turbulent kinetic energy (TKE) are biased high because the TKE calculation method includes wave-driven velocity fluctuations as well as those caused by turbulence. Previous work by the authors has successfully applied a mixed spectral-statistical filter to ADCP estimates of TKE from a tidal stream test site; this filter showed good performance in distinguishing between true TKE and pseudo-TKE due to wave action. However, for field data it is only possible to assess the filter’s performance with respect to wave properties: this is because independent measurements of wave properties can be obtained from a surface buoy, but there is no alternative instrumentation that can yield independent measurements of turbulence.

The study presented here addresses this shortcoming by generating an artificial flowfield with known turbulence and wave properties, sampling with a virtual ADCP, and applying the spectral-statistical filter to the results. In this way it is shown that the combined filter improves the separation of waves and turbulence over the individual filters by an even greater degree when assessed against turbulence data rather than against independent wave measurements. The study discusses the effects of different artificial turbulence generation schemes, specifically contrasting the spectral or Sandia method with the synthetic eddy method; it also discusses the tuning of spectral filter parameters for optimum separation of wave and turbulence effects.

Multi-criteria analysis to evaluate tidal energy potential in France

2023-07-17T11:29:38+01:00

France benefits from an important tidal energy resource, with the second largest potential in Europe, and one of the biggest in the world. In addition, France has an important industrial and academic fabric, with several tidal developers, shipyards, and research centers, creating, theoretically, an attractive environment. However, though some tidal turbines already installed, such as Sabella demonstrator at Ouessant Island, or Guinard energy turbine in the Etel river, the sector still has a large development potential. One limitation of such a development is the lack of available data to assess the total potential. Previous studies mainly focused on the most energetic sites, such as the Alderney Race or the Fromveur Passage, while other sites are neglected. Yet, those sites, such has rivers or estuaries, offer softer conditions, lowering the risks. Moreover, although identifying a suitable area for tidal array implementation requires technical criteria (maximum current velocity, water depth, installable potential, etc.), it should also consider other aspects such as ecological indicators (migratory fishes, specific habitats, etc.), regulation specificities (Natura 2000, ZNIEFF, etc.) or human activities conflicts (fishery, UXO, etc.). On top of that, socioeconomic indicators, such as potential local content or partnerships with relevant academic stakeholders, are encouraged by authorities, and must be accounted for.

Hence, this study proposes an extensive multi-criteria analysis methodology, to assess tidal energy potential of ten sites in France, including five nearshore locations (Alderney race, Fromveur passage, Raz-de-Sein, Raz-Barfleur, Paimpol-Bréhat), and five estuaries, river or similar (Gironde estuary, Gulf of Morbihan, Adour river, Etel river, Arcachon Bay). The analysis is limited to ten sites, selected based on a pre-screening analysis, which will be detailed in the study.

The different criteria used will be presented, including their weight to the final score which reflect their importance during project development, and the 3-level grading system used to discriminate sites. The methodology developed to assess full-time equivalent employments created by a tidal array, using world input-output tables and other economic parameters will also be detailed. A specific focus will be proposed on methodology used to assess the installable capacity, relying on data from MARS 2D model for nearshore sites and public data for the others.

First analysis shows that Raz-de-Sein and Paimpol-Bréhat are the two most suitable sites for tidal arrays implementation, while the Alderney Race is the most energetic one. Those results are driven by high scores obtained by these sites in categories such as ecological or human activities. Direct, indirect, and induced employments created during construction and operational phase by each project are also calculated, as well as potential growth value added. Hypothesis and sensitivity to them will be discussed.

This research was funded by the H2020 projet ELEMENT, grant number 815180.

Improved Modelling of Vertical Velocity Profiles at a Tidal Energy Site

2023-02-25T18:22:53+00:00

The Minas Passage, one of the Bay of Fundy’s tidal channels, located in Nova Scotia, Canada, presents significant potential for tidal energy development because of its highly energetic flows. As development in the region gains traction, the implementation of floating tidal energy platforms is a topic of growing interest. Tidal energy deployments in Minas Passage have historically been bottom-mounted and stationary, and the transition to arrays of floating turbines requires additional considerations. Particularly, this new application demands characterization of flow over the entire water column, including near the free surface. Complete vertical velocity profiles are essential for the successful deployment of floating tidal turbines, allowing for the estimation of key metrics such as tidal power and shear across turbine blades. Here we explore adaptations to the well-established logarithmic law of the wall with the goal of extending the vertical range over which a fitting regime based in classical turbulence theory can capture the observational records of flow in Minas Passage.

Observational site characterization efforts in Minas Passage have primarily consisted of stationary, bottom mounted Acoustic Doppler Current Profiler (ADCP) deployments. Using historical ADCP records collected in Minas Passage between 2008 and 2021, we fit the vertical profiles of velocity using three methods: logarithmic law of the wall, power law, and an adapted logarithmic law of the wall which includes a “wake function” to improve fits in the upper water column. We find that although the law of the wall results in well-fitted estimations of the vertical velocity profiles near the seabed, observational profiles consistently deviate from the fitted curves in the middle and upper water column, recording significantly faster flow speeds than predicted by the law of the wall. The adapted model, which is rooted in turbulence theory and includes a wake term, is successful in capturing flow in the outer layer of the water column and allows for reverse shear to be captured in the profile. The resulting fits show a sizeable reduction in error throughout the entirety of the water column compared to the law of the wall profiles, and consistently reduce the error in the vertical profile fits when compared to power law fits. In addition to resulting in low error for both individual and averaged vertical profiles of flow, the physical quantities estimated from the adapted model, including drag coefficient, agree well with those computed from the law of the wall, demonstrating the physical usefulness of the adapted model.

Assessment of tidal energy resources in the Strait of Magellan in southern Chile

2023-06-13T15:01:55+01:00

The Strait of Magellan in the Chilean Patagonia (Lat: 53.5°S) connects the Atlantic and Pacific oceans through a narrow passage. Historic and current pressures from navigation, oil and coal industries combined with changes due to anthropogenic climate change and local population growth make the region both complex and relevant. A large tidal range on the Atlantic side of the strait produces very strong currents in the first and second narrows that have a significant potential for hydrokinetic energy. Maximum currents have been measured up to 5 m/s.

With the goal of performing a detailed assessment of the marine energy potential of the strait, field measurements combined with an FVCOM numerical simulations are being used to assess the tidal potential and understand the complex coastal physics of the system. Field observations were carried out in short field campaigns in 2018 and 2019 to characterize the flow in one of the channel narrows. We deployed a moored ADCP for currents and turbulence characterization, measured salinity and temperature, sea-surface elevations with pressure sensors, and made velocity transects with a vessel-mounted ADCP. These characterizations will also help to inform the role of the flow through the Strait of Magellan on the continental shelf.

The field data we collected is critical to understand the factors that control the dynamics of the flow in this section of the channel, and understand its marine energy potential. Measured velocities are being used to validate an FVCOM numerical model which will expand our spatially and temporally limited measurements, to estimate the tidal energy resources in the Magellan strait and evaluate the impacts of varying climatic conditions. The numerical model will also allow to predict the complementarity of tidal energy with other renewable resources, in the context of green hydrogen in the Strait of Magellan.

This is the first study aimed at predicting the tidal energy resources in the Strait of Magellan, combining field observations and numerical models. Future work will focus on the interactions of physical and environmental processes that can help to promote a sustainable development of marine energies in the southern part of the continent.

Principles of ADCP deployment methodologies

2023-07-03T11:25:11+01:00

ADCPs are regularly used for resource characterization in wave and tidal energy sites; however, methodologies and best practices are entirely dependent on the site characteristics and the assets available. EMEC have used their widely varied experience at multiple sites and locations to develop a review of methodologies and the pros and cons of different deployment styles for data acquisition.
When deploying ADCPs there are a number of site criteria that must be assessed, such as seabed type, bed flow speeds, surface flow speeds, recovery windows (slack and weather), and water depth and range. These criteria can significantly affect the success or failure of marine operations, not necessarily for deployment but certainly for recovery. When recovery operations fail then projects lose assets (ADCP units and bed frames), data, and money (in hire/unit costs, and vessel hire for example). When the data is not recovered then this can jeopardise project success, wasting time, effort and funding.
Different deployment configurations and recovery techniques have been used for ADCP deployments. Equipment such as ground lines, acoustic releases, USBLs, and surface buoys are used to mark, locate, and/or recover devices. These are varied depending on a number of factors. Ground type, for example, can affect systems used since a ground line cannot be effectively used in delicate ecosystems since the dredging required to recover may disturb local flora and fauna. Also, for example, flow speed and vessel traffic will determine whether a surface buoy can be used.
Based on multiple ADCP deployments to characterize resource for wave and tidal energy projects in Scotland, England, France and other sites, EMEC have reviewed different deployment methods and recovery techniques. There is a proposed set-up for deployments suitable for a number of active marine energy sites and suggestions for edits based on site characteristics and target data requirements.

Informing Early Design Decisions Through Functional Analysis of Maintenance Drivers

2023-06-06T16:32:02+01:00

Operational expenditures dominate the cost of long field-life systems, with maintenance comprising a significant proportion for many systems. However, engineers lack the tools to assess maintenance during conceptual design. Familiar systems mitigate this problem by providing historical maintenance data from which empirical models can be derived. Emerging technologies, like marine renewables, lack operational maintenance data. As a result, engineers must make decisions with no historical data and minimal, if any, operational experience. The high operations cost incurred from basic maintenance tasks, and the loss of energy production during maintenance, further highlight maintenance as a critical cost driver. This paper develops a data-driven model to estimate maintenance intervals of long field-life systems during conceptual design. The model links the elementary functions of a component to maintenance requirements. Relative maintenance considerations were determined by mining function and maintenance data from manuals of long field-life systems. Machine learning was applied to generate a function-maintenance model from the maintenance data. The model consisted of functions grouped into buckets of increasing maintenance demand. The machine learning model was applied to an exemplary long field-life system, a wave energy converter, to explore possible redesigns to reduce maintenance costs. This paper shows that maintenance costs, actions and intervals can be confidently accounted. The function-maintenance model offers two beneficial impacts: it reduces life cycle cost uncertainty, and allows engineers to make informed decisions during conceptual design when redesign costs are at their lowest.

LUBRICATION OF OFFSHORE MECHANICAL COMPONENTS: TOWARDS SUSTAINABLE & RELIABLE POWER PRODUCTION

2023-07-18T20:51:16+01:00

As different wave and tidal energy generators advance towards the commercial deployment phase, addressing potential issues related to the lubrication of the machine components integrating the Power Take Off system (PTO) due to the harsh operating conditions encountered in the marine environment becomes even more essential. Environmentally Acceptable Lubricants (EALs) based on water-soluble polymers are proposed as a way to reduce friction and wear of the mechanical components in wave and tidal energy generators. Although these fluids have the advantage of being biodegradable and non-toxic, they have not shown to be as effective as other synthetic fluids, or mineral oils in preventing corrosion, severe friction, and wear, thus increasing the risk of moving parts experiencing premature failure.

This work explores the potential of different water-soluble polymers to be used in the formulation of EALs that can meet the strict environmental regulations while providing effective protection against wear, and corrosion in offshore operating conditions. To evaluate the potential of these lubricants as an alternative to replace conventional mineral oils, different polymers were analyzed from the point of view of their ability to form an effective full lubricant film that can keep separation between the contacting surfaces, mitigate wear, and prevent corrosion. The rheological properties of these polymers were also studied at different concentrations in order to optimize the performance in the application. The hydrodynamic film build-up properties of EALs formulated with water-soluble polymers with different molecular weight, concentration, and viscosity are reported. The corrosion resistance exhibited by steel components when exposed to the different formulations compared to seawater was an object of examination. The study also aimed to establish correlations between the lubricant film-build up properties, viscosity, and electrical impedance.

The results showed that high molecular weight polymers can form a separating film at relative high pressure in the low-speed region even at low polymer concentrations. While with the increasing speed, the fluid viscosity becomes more important to sustain a full film between contacting surfaces. With the increasing concentration of polymer in the aqueous solution the open circuit potential (OCP) becomes more negative indicating the deterioration of the steel corrosion resistance.

The results provide new insights into the design of EALs that can effectively protect the mechanical components of wave and tidal energy generators while minimizing environmental impact. The findings suggest that water-soluble polymers are a promising solution for offshore applications, as they can provide efficient full film lubrication, mitigate wear, and prevent corrosion. These polymers can help to improve the performance and lifespan of offshore power generators while minimizing the environmental impact.

SEASNAKE: Impact - Marine operations modelling for evidence-based results detailing the impact of using a new fully dynamic cable design for ocean energy devices.

2023-07-18T19:07:18+01:00

The SEASNAKE project - an OceanERA-NET project – is aiming for developing fully dynamic cables for ocean energy. Through new design and application of novel coatings the project is designing a dynamic cable that better suits the conditions and user requirements. In order to fully understand the benefits, the Wave Venture TEMPEST™ techno-economic analysis software will be used to simulate the performance of a proposed wave energy farm with a special focus on the contribution of the dynamic cable subsystem. The results obtained from the simulations will not only provide a deep understanding of the reliability and cost-risk areas regarding the use of dynamic cables, but will also allow a comparison with a baseline scenario consisting of a more traditional cabling system. The latter will be key to identify the advantages of the SEASNAKE project and its commercial viability. KPIs (Key Performance Indicators) will include farm availability and power production downtime as a direct result of cable related issues. The reliability, maintainability and survivability of the cabling subsystem will be tested and the KPIs will give clear indication on the performance benefits and ultimately the impact on the LCOE will be of greatest consideration.

A method for the growth inhibition of biofouling in Sihwa Tidal Power Plant

2023-06-06T16:33:22+01:00

Many facilities in contact with seawater are constantly affected by marine biofouling.
In particular, marine biofouling cause many problems, such as reducing efficiency of water turbine, interfering with water barrier by stop log, and generating harmful gas (ammonia) due to the death of fouling organisms during Overhaul.
For this reason, Sihwa Tidal Power Plant has been researching various fouling organism reduction technologies for the past 8 years, through this, we have increased tidal power generation and operated and maintained tidal power generation facilities smoothly.
This paper presents an analysis of marine biofouling reduction technology of Sihwa Tidal Power Plant. It shows the status and characteristics of fouling organisms, problems and impacts, improvement plans and effects of the Sihwa Tidal Power Plant.

A Filtering device for improving the quality of cooling water in turbine generator of Sihwa Tidal Power Plant

2023-06-08T08:42:58+01:00

The turbine generator cooling water system of Sihwa Tidal Power Plant is an economical system in which the cooling water cools the turbine generator through a circulation pump and the rising temperature recirculates heat exchange with seawater through a surface cooler

However, as the cooling water facility continues to operate and a long period of time passes, the cooling water quality in the piping deteriorates.

In particular, increasing the concentration of Fe ions in the coolant may promote corrosion of circulating pipes, which may reduce the stability of the water turbine generator operation.

Even if the coolant whose water quality has deteriorated is replaced, additional cleaning agents must be treated, but chemical treatment was difficult due to concerns over pipe damage.

To overcome this, a filtering device for improving the water quality of cooling water was applied.

Through the application of filtering devices for about a month, the water quality was improved by reducing the concentration of cooling water Fe ions by about 70%, and the facility maintenance cost was reduced by about $110,000 per year.

We can greatly contribute to maintaining the economical cooling water system and securing the operational stability of water turbine generators by applying the cooling water filter.

A Numerical study on the effect of solidity on the performance of Transverse Axis Crossflow Tidal Turbines

2023-06-14T15:49:38+01:00

This paper presents a three-dimensional numerical study employing an actuator line model to investigate the effect of turbine solidity on the performance of a Transverse Axis Crossflow Turbine (TACTs). Transverse Axis Crossflow Turbines have a low rectangular form and are ideally suited to relatively shallow tidal and riverine sites due to their ability to handle bi-directional flow intake. Lift based TACTs are not well understood due to their complex hydrodynamics when the downstream sweep passes through the wake produced by the turbine shaft and the upstream sweep. The blade loading, hydrodynamics and ultimately power produced during the downstream sweep depends on the number of blades and turbine solidity. This numerical study is carried out using OpenFOAM and replicates planned fieldwork where the power and in-service blade loading will be monitored as a function of turbine solidity. Turbine solidity is a dimensionless parameter that measures the proportion of blade area to the projected turbine frontal area. Turbines achieve peak performance at an optimum solidity and tip speed ratio. Solidity can be varied by changing the number of blades or changing the turbine radius. An increase in solidity causes the peak performance to reduce and shift to a lower tip speed ratio albeit with a greater torque.

This paper also explores the effect that maintaining turbine solidity has on turbine performance over a range of typical inflow velocities. Turbine solidity is held constant by varying both the number of blades and turbine diameter. Iso-solidity has been investigated for horizontal axis wind turbines but little is known about the effect of maintaining turbine solidity on TACT performance.

Vortex induced vibrations of marine risers: validating turbulence models

2023-01-27T16:12:41+00:00

Marine risers such as cables of cylindrical cross-section continuously encounter ocean currents during their service life, causing vortex-induced vibrations (VIV). Such oscillations can last a long period at high frequencies and produce significant damage that accumulates in structural components. The accumulated damage eventually leads to the failure of the structure.

The Reynolds number (Re) for marine risers in the ocean can be much higher than the Re of flows that can be solved with the direct numerical simulation (DNS) method. Therefore, turbulence models are necessary for predicting the VIVs [1] at high Re, which are close to the situations in the ocean [2].

In this study, the response of flow around a fixed circular cylinder at Reynolds number 3900 is investigated with DNS and large eddy simulation (LES). This value of the Reynolds number (Re) has often been investigated in previous studies concerning flows past fixed cylinders as a typical case of the early turbulent regime. Despite the DNS method being very accurate, the computational cost increases with the increasing Re while 2D simulations may become inadequate. For 3D simulations, however, a tenfold increase in Re corresponds to 1000 times more computational cost. This makes it virtually impossible to solve engineering application problems with DNS. In order to efficiently simulate VIV in turbulent flows, turbulence modeling constitutes one of the most important aspects of CFD modeling.

By investigating the Re 3900 case with both DNS and LES, we can ensure the validity of our turbulence model when comparing our results with literature. With a suitably selected turbulence model, our numerical solution is able to capture most of the real physics of the phenomena, including the Kármán vortex street effects on the lift and drag coefficients. A series of experimental measurements used to validate the simulations are also reported. At Re 3900, the drag coefficient conducted by experiments is 1.01, which is in the range of other researches done before, of around 0.94 - 1.04 [3]. The predicted drag coefficients and Strouhal numbers agree with the experimental data and the values reported in the literature [4].

[1] Liu, Guijie, et al. "A mini review of recent progress on vortex-induced vibrations of marine risers." Ocean Engineering 195 (2020): 106704.

[2] Qiu, Wei, et al. "Numerical benchmark studies on drag and lift coefficients of a marine riser at high Reynolds numbers." Applied Ocean Research 69 (2017): 245-251.

[3] Wornom, Stephen, et al. "Variational multiscale large-eddy simulations of the flow past a circular cylinder: Reynolds number effects." Computers & Fluids 47.1 (2011): 44-50.

[4] Violette, R., Emmanuel De Langre, and J. Szydlowski. "Computation of vortex-induced vibrations of long structures using a wake oscillator model: comparison with DNS and experiments." Computers & structures 85.11-14 (2007): 1134-1141.

Structural testing and numerical modelling of a glass fibre-reinforced composite demonstrator for turbine blades

2023-01-28T15:41:01+00:00

Tidal energy, a clean, predictable and reliable renewable energy source, can play an important role in creating a carbon-free energy system in Europe. The cumulative tidal stream technology deployed in Europe was 27.9 MW in 2020, which contributes to 77% of the global total tidal energy device installations. Under a high growth scenario, about 2388 MW of tidal energy capacities will be deployed in Europe by 2030. The structural performance of a tidal turbine blade is vital as it guarantees the safe operation of a turbine within its lifespan in marine environments. Experimental testing is a reliable way of investigating the structural performance of a tidal turbine blade. In this research, the structural performance of a composite demonstrator is carried out. The 5-m long demonstrator represents a tidal current turbine’s spar cap, the strongest region of a typical rotor blade. The demonstrator consists of two spar cap and two webs, which are manufactured with glass-fibre reinforced composite materials. Steel inserts are drilled into the root of the spar cap to connect to the support frame. A hydraulic actuator is used to apply loading to the tip region of the demonstrator to simulate the operation loading. Instrumentations, including strain gauges, accelerometers and displacement transducers, are installed to monitor the demonstrator responses. Besides testing the demonstrator, the mechanical properties of the glass-fibre reinforced composite material are also obtained through coupon tests. The test results are utilised to develop a finite element model, which will be used in the blade design and optimisation case study in the future.

ANTIFOULING AND ANTICORROSIVE PREVENTION WITH CERAMIC COATINGS ON OFFSHORE STRUCTURES FOR RENEWABLE ENERGY

2023-02-09T17:52:04+00:00

In the past, ships, port facilities and offshore platforms dedicated to the exploitation of fossil resources were the only man-made structures that were exposed to seawater, currently the exposed structures are extended to all those used in the field of renewable ocean energy sources, such as waves, tidal flows or oceans streaming and offshore wind energy. Therefore, this study highlights the need for offshore structures to consider the choice of ceramic coatings in the field of surface treatment and marine corrosion control without neglecting another of the main problems that affects structures in contact with seawater, which is the phenomenon known as biofouling. Corrosion is a major problem in offshore environments due to extreme operating conditions and the presence of aggressive corrosive elements. The corrosion resistance can represent the difference between trouble-free long-term operation and costly downtime. On the other hand, biofouling, which is defined as the undesirable phenomenon of adherence and accumulation of biotic deposits on an artificial surface that is submerged or in contact with sea water, can cause variations in the weight distribution of a floating structure, affecting its stability. In addition, biofouling leads to corrosion in the same way that corrosion leads to biofouling, so both factors are studied in parallel.

This study evaluated differences in the total of seawater biofouling attached on coated paints and ceramic coatings in carbon steel for offshore structures. All three different ceramic coatings were made of incorporating active ceramic particles against biofouling as titanium, cobalt and manganese. In this study, the ASTM-D3623 test method, for the protection of submerged marine structures, was used. This method covered the procedure for testing antifouling coatings exposed for a period of two year at an immersion site with a high biological activity in shallow marine environments.

The results of the investigation showed that the cobalt-based coating had the best antifouling properties at the end of the experimentation, although there was no significant difference in the biofouling attached during the two years of exposure, but great differences were shown with respect to the antifouling paints. Biofouling adhesion resistance was greatest when a coating thickness of 217 μm was used and when the substrate surface roughness (Ra) was 0.245 µm. The results indicated up to more 30% total area covered by biofouling in paint coatings than ceramic coatings. On the other hand, the results showed a progressive degradation of the antifouling paint coatings, which meant an exponential increase of biofouling adhered to the samples, but not in ceramic coatings during the two years experiments.

Understanding the force motion trade off of rigid and hinged floating platforms for marine renewables.

2023-06-28T08:51:50+01:00

In this work, we compare the motion and structural response of a rigid and hinged floating structure subject to regular waves. We do this to understand better whether what is the best option for floating marine renewable installations. The hinged structure has two hinges and three pontoons, whilst the rigid structure is made by replacing the hinges with rigid steel bars. We instrument the pontoons with motion detection spheres and with strain gauges to measure vertical point loads. We find that the motion response of the platforms is similar between hinged and rigid at low and high frequencies. However, at intermediate frequency waves, single and triple sagging occur for rigid and hinged structures, respectively. We find significant load alleviation for the hinged structure in the range of frequencies where sagging behaviour occurs. These insights reveal that hinged design can contribute to long term survivability by reducing loads in the structure, whilst identification of motion patterns and natural frequencies are necessary to select operating modes for marine renewable generators mounted on the platforms.

Reducing the uncertainty of ULS load estimates in offshore structural design

2023-07-09T13:36:01+01:00

The design process of any offshore structure, including Wave Energy Converters (WECs), aims to assess a series of critical conditions, often encapsulated in the concept of ‘limit states’, that define the ‘design boarders’ across which a structure is unlikely to respond satisfactorily, potentially leading to functional failure(s). The Load and Resistance Factor Design (LRFD) method aims to quantify such ‘design boarders’ with adequate safety margins on both the loading and the resistance sides of the design inequality. An inherent risk of such approach is that the definition of ‘adequate’ may be clouded by multiple sources of uncertainty, making it challenging to assess if a system is likely to be under- or over-designed from inception.

Although overall guidance for the design process of a WEC can be sought from related industries – see e.g. [1], and while noting that generic guidance for WEC design is available in e.g. [2], there is at present limited (practical) guidance on how to derive estimates of long-term return period loads. In recent work, comparative assessments of different load estimation methods have been made, illustrating the dependence of the outputs on the underlying method – see e.g. [3]. Additionally, and in connection with the assessment of extreme loads acting on WECs, the load post-processing methodology was identified in [4] as the major contributor to the uncertainty in Ultimate Limit State (ULS) load estimates.

This paper presents a novel post-processing methodology to assess the uncertainty when estimating ULS loads acting on an offshore structure. To assess which statistical distributions are best suited to estimate long-term extreme loads, goodness-of-fit tests were performed using a series of input load time-series. The methodology was applied to evaluate the 50-year return load acting on the foundation of a generic Submerged Pressure Differential (SPD) WEC, based on the generic configuration originally defined in [5]. Results suggest that current conventional practices based on visual inspection(s) may lead to the selection of non-representative fitting functions which, in turn, likely lead to inaccurate extreme load estimations.

Ultimately, the methodology described in this paper aims to contribute to a probabilistic approach to the definition of suitable safety factors, which in turn is expected to reduce the uncertainty in key design metrics and the risk of either under- or over-design.

This study was conducted as part of the H2020 VALID and H2020 IMPACT projects.

References

[1] DNVGL-ST-0437. (2016). Loads and site conditions for wind turbines.

[2] IEC/TS 62600-2. (2019). Marine energy — Wave, tidal and other water current converters Part 2: Design requirements for marine energy systems.

[3] Michelen, C., Coe, R. Comparison of Methods for Estimating Short-Term Extreme Response of Wave Energy Converters, Sandia National Laboratories, SAND2015-6890C.

[4] Atcheson, M., Cruz, J., Martins, T., Johannesson, P., Svensson, T. (2019). Quantification of load uncertainties in the design process of a WEC, Proc. 13^th European Wave and Tidal Energy Conference (EWTEC 2019), Naples, Italy.

[5] Babarit, A., Hals, J., Muliawan, M.J. et al. (2012). Numerical benchmarking study of a selection of wave energy converters. Renewable Energy 41, 44-63.

Critical Feature and Seawater Testing of Cross-Flow Rotor Components Fabricated with Additive Manufacturing

2023-06-28T19:19:55+01:00

Cross-flow tidal turbines are an attractive option for powering remote or off-grid applications because of their simplicity as compared to axial-flow turbines. For instance, when oriented vertically, they harvest power from any current direction with a single degree of freedom and no yaw mechanism. Additive manufacturing (AM) offers an opportunity to print parts out of a wide variety of materials that can result in components that are lighter, stronger and/or less expensive to produce than with traditional manufacturing techniques. When coupled with cross-flow turbine rotors, which require critical features (blade-strut, strut-shaft connections) to be both structurally stiff and hydrodynamically shaped, which can be challenging for typical fabrication processes, AM offers the ability to do both well. This paper describes work on the feasibility of using advanced AM techniques to fabricate small cross-flow turbine rotors for applications at sea and near remote coastal communities.

AM materials were categorized into 3 classes – plastics, metals, and ceramics – and reviewed for suitability based on a set of engineering requirements and criteria related to turbine characteristics, material properties, and AM process capabilities. Two plastics and two metals were selected to undergo further testing: Essentium CF25, CarbonX Ult 9085, Titanium Ti-6Al-4V, and Inconel 718. Testing is conducted in three phases: the first is a long-term, 5-month submersion test in the seawater tanks at PNNL-Sequim to study corrosion, water uptake, and biofouling potential; in the second, materials are tensile tested on a load frame to find their failure parameters to compare to material standards; the third test is a fatigue test consisting of cyclically loading test parts with a known force on the order of that exerted on rotor blades in a 1.5 m/s current flow. These tests are designed to discern the suitability of AM materials since their properties from 3D printing processes are known to vary from published parameters. The test samples undergoing submersion testing will be tension tested and compared to control samples not subjected to extended seawater immersion. For fatigue life testing, a small rotor is expected to complete 100 million cycles over the course of a year-long lifespan, but for the case herein is restricted to 1 million for a preliminary performance evaluation. The first 10k cycles are run on an MTS 312.21 load frame at a rate of 0.2 Hz, with the remaining on a custom-built cyclic-deflection test rig at 0.8 Hz.

Material Characterization of Elastomeric Bearing Elements in Wave Energy Converters

2023-07-09T13:41:55+01:00

According to the European Council, 75% of the greenhouse emissions are caused by energy production and use. In order to decarbonize the energy sector, more research is focused towards the renewable energy sources. Among the many available resources, wave energy is one of the key resources to consider. As part of the research, KTH is developing a winch based point absorber wave energy converter. The winch consists of a chain link with elastomer bearings wound around a drum. The winding and unwinding motion of chain converts wave motion into electricity. A typical winch based wave energy converter undergoes around 80 million cycles in its lifetime. This calls for a durable system design requiring minimal service and maintenance. With an elastomer bearing, the winding and unwinding of the chain over the drum is realized as deformation of the elastomer thereby eliminating sliding. While the use of elastomer enables an efficient design, it also makes it more challenging due to its highly non-linear and viscoelastic behavior. A constitutive model is necessary to determine material characteristics of an elastomer for different loading conditions. In this work, a systematic design process is outlined and an attempt is made to determine a suitable hyperelastic material model for the elastomer. The study is focused on two materials – silicone and polyurethane. The test samples are compressed and followed by shear in deformation. For material model determination, the test data is curve fit and later verified using finite element method. The material is assumed incompressible.

Fatigue-life prediction methods of a dynamic power cable for a floating testing platform – a numerical approach

2023-07-29T17:36:37+01:00

Floating testing platforms and marine energy devices use Dynamic Power Cables (DPC) to connect to the testing centres or the national grids. These power cables are, therefore, an inherent component in the reliable operation of the floating structure, and their integrity throughout the design life of the device should be ensured from the early stages of the project. Fatigue failure is one of the most common factors affecting the cable's performance due to the cyclic loads caused by waves and currents. However, accurately estimating the fatigue life of DPC has proved challenging due to their material and geometric nonlinearities. In addition to fatigue failure, the cable design life is also characterised by its ability to withstand extreme conditions on-site.

This paper addresses the design, analysis and specification of a dynamic power cable that will be deployed at the Biscay Marine Energy Platform (BiMEP) for the connection of the HarshLab 2.0, a floating testing platform for materials, subsystems and components developed and operated by Tecnalia. BiMEP is exposed to severe waves that induce large motions on the HarshLab buoy, and different numerical models for the analysis of the hydrodynamic response are reviewed and calibrated against model tests.

To address a comprehensive study of the power cable at the design stage, this paper focuses on reviewing different fatigue-life prediction methods employed for a DPC, considering the damage in the steel armour and the conductor cores. It also covers the cable's response when the system operates under extreme sea conditions and the potential implications of sensitive variables such as loss of buoyancy and marine growth. To perform the fatigue-life study, long-term cyclic loading in each cable component was compared with its resistance to fatigue damage. In this case, the damage for each variable was derived from stress- and strain-based curves for the high-strength steel (armour) and the copper (core conductor) depending on the selected approach.

The design iterative process using the studies performed in this paper led to the definition of a Lazy Wave design with a high-level optimisation of some components, e.g. the ballast and buoyancy modules and the bend stiffener. In general, the behaviour across the cable length is similar between the different fatigue-life approaches, with the most significant expected fatigue damage found in localised hot spots near the bend stiffener and the belly (sag-bend region) of the cable, before the buoyancy modules. It is found that, despite a significant difference across the magnitude of fatigue life estimated in sensitive areas, the final design complies with all design criteria and reaches a satisfactory fatigue life.

Beta-version Testing and Demonstration of the Design Load Case Generator

2023-06-13T17:11:36+01:00

International standards for the design, type-classification and certification of marine energy systems, including wave and current energy converters, are essential for the commercialization of these technologies, but their compliance requires significant effort and resources by project developers; e.g., finding the appropriate met-ocean datasets, processing and analysing this data to estimate the design load conditions, design type-class and load response. Herein we present efforts to address these challenges by developing, beta-testing and demonstrating a web-based tool, the “Design Load Case (DLC) Generator.” This tool integrates a host of data search, processing and statistical tools to streamline the analysis of design load conditions and to determine the design load requirements as in the International Electrotechnical Commission (IEC) 62600-2 design standard. It is demonstrated for a test DLC analysis case for the Reference Model 3 (RM3) point absorber at the PacWave South test site. This test case highlights some of the challenges determining design load requirements and the benefits of facilitating a complex workflow within a single web-based platform that leverages a diverse set of data processing and statistical tools. The DLC Generator facilitates and streamlines DLC analyses for significant time and cost savings on a variety of tasks in a complex workflow, including site data search and retrieval, data quality control, extreme value statistical analyses, and archiving of dynamic load response model inputs and outputs.

Fatigue Life Assessment for Wave Energy Converter Mooring Lines under Realistic Wave Climates

2023-06-25T09:09:23+01:00

Despite the untapped resource stored in ocean waves, none of the suggested wave energy converters (WECs) has yet demonstrated economical viability. This viability lies in two counter-productive aspects: enhancing energy absorption and generation, and reduction of the loads in the critical elements such as mooring lines and dynamic cables. Both objectives are counter-productive in the sense that the former implies enhancing the motion, while the latter requires reducing this motion. Hence, a trade-off between the two objectives must be found.

To that end, the accurate lifetime estimation of the most critical components is crucial, which depends on the fatigue and extreme loads. The latter depends on extreme events, which are highly nonlinear and need to be characterised by fully-viscous fully nonlinear numerical models. In contrast, fatigue effects cause cracks in the material, which appear due to cyclic loading and, thus, computationally more efficient numerical models are required in order to study all the relevant loading conditions within the operating region.

The present study will evaluate different methodologies to assess the impact of fatigue loads in WEC mooring lines, estimating the lifetime of the mooring lines under realistic wave climates. The method follows the following steps:

Mooring line tension is evaluated via hydrodynamic simulations under different realistic resource conditions,
Statistical characteristics of the load cycles are extracted from the time history,
The damage (D) corresponding to each load cycle condition is computed,
The lifetime (L) is estimated by combining realistic resource conditions and the damage corresponding to each condition

In the present study, this approach will be applied to different mooring line configurations (including wire ropes and chains), materials (stud and studless) and dimensions (length and diameter of mooring lines), as shown in Figure 1 for stud and studless chains as a function of the diameter.

The lifetime estimation will be performed in different locations across the European coast, analysing the impact of resource conditions on fatigue lifetime. In addition, the most relevant design parameters will be identified.

Numerical Study on Overtopping Performance of Multi-stage Overtopping Wave Energy Converters

2023-05-16T17:21:20+01:00

The characteristics of wave conditions with large tidal ranges and small wave heights in China's coastal waters limit the operation time of traditional single-stage overtopping devices, while the multi-stage overtopping wave energy converters can increase the overall operation time of the device by distributing the upper and lower reservoirs. To study the overtopping performance of the multi-stage overtopping wave energy converter under real sea conditions, a two-dimensional numerical model of the device is developed and verified by physical model tests. The effects of the lower reservoir opening width, slope angle combination, and slope inundation length on the overtopping performance are investigated under regular wave conditions. It is found that a smaller opening width and two 30° slope angles improve the overtopping performance of the device, while the slope inundation depth has less effect. Further, based on the full-scale prototype device, the database of overtopping in a complete tidal cycle (12h) were constructed under real sea conditions in different seasons to provide the basic data for the Wave-to-wire model of power generation simulation.

Leveraging Explainable Artificial Intelligence for Real-time Detection of Tidal Blade Damage

2023-05-03T12:42:57+01:00

Our current reliance on fossil fuels is the primary contributor to global warming and threatens our survival. Renewable energy is currently considered the leading solution to reduce greenhouse gas emissions. Energy extraction from the ocean tides (tidal turbines) can help fulfill the global renewable energy demand and combat world climate crises. Installations at this scale will have the associated benefit of reducing the levelised cost of tidal energy (LCOE) towards a target of EUR100/MWh, which will make ocean energy a viable option along with other renewable energy sources such as offshore wind. To achieve the target, increased performance and reliability of tidal energy devices are required. Tidal blades are a primary component of tidal turbines, possess a heterogeneous nature and can suffer from complex non-linear damage modes. For example, harsh marine environments can cause impact damage, delamination, matrix crack, fiber breakage or rupture, and others in tidal blades, which could lead to catastrophic failure of the system. Achieving reliable operational health and performance for the tidal blade is thus crucial for tidal energy companies. Fault diagnoses and maintenance operations are challenging in the sea; performance degradation, failure, or breakdown of the entire tidal energy system are more likely if unattended. Therefore, there is the need for real-time and reliable structure health monitoring (SHM) of tidal blades. The existing damage detection techniques have a limitation when dealing with the real-time environment, and do not take into account the along with uncertainty feature, we also addressed the issue of trusthworthiness in system decision employed with explanable artificial intelligence (XAI). This paper presents a real-time damage detection framework, information communication technologies (ICT) based infrastructure for real-time monitoring and proposes a novel model to classify/ detect the damages over blade structure. In addition a XAI based approach is proposed which based on supervised machine learning (ML) and uses an optimized convolutional neural network to classify from the heterogeneous data streams. Testing and evaluation of proposed approach in laboratory and operational settings is the future concern of this study.

Spectral-Domain Modelling of Wave Energy Converters as an Efficient Tool for Adjustment of PTO Model Parameters

2023-06-26T08:58:24+01:00

The power take-off (PTO) system is a core component in WECs as it plays a critical role in power production. In numerical models the PTO systems are commonly represented and simplified through a combination of linear stiffness and damping terms in the equations of motion. These parameters are influential to the dynamic response and thus affect the power performance of WECs. In the preliminary design and optimization of WECs, proper tuning of the PTO damping and stiffness could reflect better the potential of the concept. In practice, the PTO damping and stiffness are tuned to maximize the absorbed power by achieving the desired velocity amplitude or phase of the velocity with respect to the excitation force. However, recent literature has indicated that the selection of PTO parameters for maximum mechanical power absorption is not necessarily optimal for the maximum production of electrical power when the conversion efficiency of the electrical machine is included. To obtain these parameters which maximize the delivered electrical power, wave-to-wire models are widely used. Nevertheless, wave-to-wire models are predominately established by using time-domain models which can be associated with large computational efforts from the perspective of early-stage design and concept evaluation. To tackle this challenge, a spectral-domain-based wave-to-wire model is proposed to cover both hydrodynamic and electrical responses. In this paper, a spherical heaving point absorber integrated with a linear permanent-magnet generator is used as reference. The relevant nonlinear effects are incorporated by statistical linearization using spectral-domain modelling. In particular, the nonlinear effects considered in this work include the viscous drag force, the electrical current saturation and the partial overlap between the translator and stator components of the linear generator. The model results are then verified against a nonlinear time-domain-based wave-to-wire model. Subsequently, the proposed model is applied to identify the PTO parameters for maximizing the electrical power in various wave states. The computational efficiency and accuracy of the proposed spectral-domain model are compared with the time-domain model, with regard to the identification of the proper PTO damping and stiffness. Based on the results, the advantage of using the spectral-domain-based wave-to-wire modeling in PTO tuning is demonstrated.

Numerical modelling of a box-type and bottom-detached oscillating water column wave energy conversion device: a comparison with experimental data and between BEM and CFD numerical modelling

2023-05-30T12:40:17+01:00

Utilization of Boundary Element Method (BEM) based on linear potential flow for modelling Oscillating Water Column (OWC) devices has gained popularity in the last two decades. The commercial BEM solver WAMIT has been used widely for modelling OWCs and validated using experimental modelling (Delauré et. al. 2003, Bingham et. al. 2015, Faÿ 2020). The open-source BEM solver NEMOH has however been mostly ineffective in modelling OWCs since the main approach adopted previously modelled the imaginary piston as a thin disk. In this research, the multi-body interaction problem has been adopted in modelling a box-type and bottom-detached OWC device in NEMOH, where the imaginary piston has been modelled to the length of the internal water column (Penalba et.al. 2017) and compared with experimental data. A further comparison is drawn with the numerical method of Computational Fluid Dynamics (CFD) , which has shown to be accurate for modelling OWC devices (Simonetti et. al. 2015), yet requires significantly higher computational resources than BEM. A two-dimensional CFD numerical wave tank, developed generating and absorbing waves with the waves2Foam toolbox (Jacobsen et al., 2012) of the open-source package OpenFOAM, is used for comparative purposes.

REFERENCES:

Delauré, Y. & Lewis, A. (2003). “3D hydrodynamic modelling of fixed oscillating water column wave power plant by a boundary element methods.” Ocean Engineering. 30. 309-330. 10.1016/S0029-8018(02)00032-X.

Bingham, H.B.; Ducasse, D.; Nielsen, K. et al (2015). “Hydrodynamic analysis of oscillating water column wave energy devices.” J. Ocean Eng. Mar. Energy 1, 405–419 (2015). https://doi.org/10.1007/s40722-015-0032-4

Faÿ F.-X. (2020). “Modelling and Control for the Oscillating Water Column in Wave Energy Conversion.” PhD Thesis, Universidad del País Vasco.

Jacobsen, N.G., Fuhrman, D.R. & Fredsøe, J., (2012). “A wave generation toolbox for the open-source CFD library : OpenFoam”. International Journal for numerical methods in fluids, 70, pp.1073–1088.

Simonetti, Irene & Cappietti, Lorenzo & Elsafti, Hisham & Oumeraci, Hocine. (2015). “Numerical Modelling of Fixed Oscillating Water Column Wave Energy Conversion Devices: Toward Geometry Hydraulic Optimization.” 10.1115/OMAE2015-42056.

Penalba, M., Kelly, T., Ringwood, J. 2017, NEMOH for Modelling Wave Energy Converters: A Comparative Study with WAMIT. 12th European Wave and Tidal Energy Conference , Cork, Ireland. https://www.researchgate.net/publication/319160625.

Numerical and experimental studies of the effects of WEC motion on a combined wind-wave energy platform

2023-05-31T12:23:01+01:00

The development of eco-friendly carbon-free marine energy is actively underway to alleviate the impact of global climate change due to excessive emission of carbon dioxide. To reduce the variability of energy production according to marine environment conditions and maximize energy production, the need for research on a suitable ocean energy platform is increasing. Therefore, various studies on motion control of ocean energy platforms are being conducted.

In this study, the motion response of a combined wind-wave energy platform equipped with multiple wave energy converters (WEC) capable of controlling the motion of a floating offshore wind turbine (FOWT) was numerically analysed and verified through a wave tank experiment. The combined energy platform with multiple WECs attached to the FOWT not only produces stable energy by controlling the motion response of the FOWT with the movement of the WEC, but also produces wave energy by the relative motion of the WEC.

The FOWT is a spar platform with taut moorings, and WEC is a moving cylinder hinged to the FOWT. The motion reduction effect of the FOWT by the movement of the WEC was investigated. Numerical analysis used the ANSYS AQWA program, which is a potential flow-based hydrodynamic program. To consider the restraining force of the FOWT due to the motion of the WEC, the multi-body dynamics theory was applied. The accuracy of the numerical model was improved by applying mooring dynamics, weakly nonlinear Froude-Krylov force, and nonlinear hydrostatic force. The experimental model was fabricated on a scale of 1/100 of the numerical model and was performed in a two-dimensional mini wave tank. To prevent the sidewall effect of the tank, the experiment was conducted only for the incident waves with various periods under the head sea conditions.

The numerical analysis results were in good agreement with the experimental results. In particular, the pitch of FOWT decreased significantly near the pitch resonance period because of the motion of WEC. The change in motion characteristics of the combined energy platform according to the movement of the WEC under various wave conditions was analysed.

Fast time-domain model for an array of interactive point-absorbers

2023-05-27T13:15:38+01:00

A fast time-domain model is developed for an array of six interacting point-absorber wave energy converters, based on the design originated at Uppsala University. The point-absorbers are placed in a symmetric grid, were each row contains one pair. The devices interact by scattered and radiated waves, while they are restricted to move in one degree of freedom, heave. Under the assumption of linear potential flow theory, the hydrodynamic coefficients for the excitation and radiation forces are obtained using an analytical multiple-scattering method. The equations of motion are solved directly in the time domain following the Cummins’ formulation. Modelling an array of wave energy converters in the time domain comes down to solving a system of integro-differential equations, were convolution terms appear in the computation of the excitation and radiation forces. In the majority of wave farm models, frequency-domain approaches are used to solve the equations of motion, since time-domain models are more computationally demanding and significantly more challenging to develop. This is not only because of the numerical integration involved, but especially due to the computation of the convolution term accounting for the radiated water waves on the free-surface, implying that waves radiated by the body in the past continue to affect the dynamics in the future. Regardless the computational effort associated with time-domain approaches, their use is required for realistic control applications and complex device dynamics, like non-linearities due to the power take-off configuration. In particular the non-linear effects that arise during the wave energy conversion chain are treated as time-varying coefficients within the system of differential equations describing the motion. Input to the numerical scheme are irregular, long-crested waves, obtained from the Bretschneider spectrum, corresponding to four, different sea states. It is of high interest to study whether the linear numerical model simulates accurately the performance of each interactive surface buoy in response to the irregular waves. Therefore, the numerical results for the full array configuration are compared with experimental data. At this point we emphasize that there are not a lot of experimental works considering arrays of point- absorbers, due to the complexity and costs associated with the task. Therefore, finding a set of data to validate the array model is not trivial. The experimental results we use were carried out in the COAST Lab at Plymouth University, UK, corresponding to a 1:10 scaled prototype of an array of point-absorbers. The set-up consists of six ellipsoid floats free to move in six degrees of freedom and connected via ropes and pulleys to individual power take-off systems. Despite the highly non-linear effects in the physical experiment, the free motion of the buoys in all directions, and the power take-off configuration, the numerical scheme is able to accurately capture the heaving motion of the buoys and their power absorption.

A CFD-FEM analysis for Anaconda WEC with mooring lines

2023-05-31T12:29:56+01:00

Anaconda wave energy converter (WEC) [1] is a typical flexible tube WEC design. In the past decade, the hydro-elastic performance of Anaconda model has been studied through experiments [2-3] and reduced-order numerical simulations [4-5]. However, to date, most of the undertaken research assumes that both ends of the tube are fixed, neglecting mooring lines that exist at designed conditions. The tube is allowed to move freely head to waves when mooring lines are considered. This is evident by an experiment conducted by Checkmate Flexible Engineering Ltd., where they observed a significant heave motion of Anaconda WEC, which may impact the dynamic response of WEC and further the energy conversion [6]. In our previous work [7], supported by the EPSRC project BASM-WEC (No. EP/V040553/1), a coupled numerical analysis tool using computational fluid dynamics (CFD) and finite element method (FEM) has been proposed to perform a numerical study for an Anaconda WEC model without considering mooring lines.
In this paper, the coupled fluid-structure interaction (FSI) analysis tool [7] based on CFD-FEM method will be used to investigate Anaconda WEC with mooring lines. To account for the effects of free motion of WEC, we develop a sub-module to be embedded into the existing tool. The fluid and structure are solved by a two-phase CFD solver developed based on OpenFOAM and a three-dimensional FEM code CalculiX, respectively. The strong coupling between OpenFOAM and CalculiX is achieved by a multi-physics coupling tool preCICE.
For the flexible tube, a commercial Natural Rubber material is studied. The hyper-elastic model YEOH is utilized to describe the nonlinear behaviour of material, and the model constants are calculated from the bi-axial tests. With the tool developed, numerical simulations are performed for different given regular waves. The flow details, velocity and pressure variation, structure deformation and stress distribution will be fully examined to better understand energy conversion performance of Anaconda WEC with mooring lines.

References
[1] Farley F J M D, Rainey R C T. Distensible tube wave energy converter: U.S. Patent 7,980,071[P]. 2011-7-19.
[2] Chaplin J R, Heller V, Farley F J M, et al. Laboratory testing the Anaconda[J]. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2012, 370(1959): 403-424.
[3] Heller V, Chaplin J R, Farley F J M, et al. Physical model tests of the anaconda wave energy converter[C]//Proc. 1st IAHR European Congress. 2000.
[4] Farley F J M, Rainey R C T, Chaplin J R. Rubber tubes in the sea[J]. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2012, 370(1959): 381-402.
[5] Babarit A, Singh J, Mélis C, et al. A linear numerical model for analysing the hydroelastic response of a flexible electroactive wave energy converter[J]. Journal of Fluids and Structures, 2017, 74: 356-384.
[6] Checkmate Flexible Engineering, https://www.checkmateukseaenergy.com/, accessed 15th December 2022.
[7] Huang Y, Xiao Q, Idarraga G, et al. Numerical analysis of flexible tube wave energy converter using CFD-FEA method[C]//International Conference on Offshore Mechanics and Arctic Engineering. American Society of Mechanical Engineers, 2023 (submitted).

CMIP6 wave climate simulation in the European North East Atlantic Basin using WaveWatch III

2023-05-27T12:56:56+01:00

Climate change is expected to have an impact on wind patterns, and therefore the generation of waves. Phase 6 of the Coupled Model Intercomparison Project (CMIP6), provides various realization of outputs integrated global coupled models for different centuries. Wind quality is a cornerstone for wave energy as it is the primary generation driver in any wave model. Therefore, proper quantification of wind wave interactions are key in the evaluation of future wave energy potential. In this study, a wave hindcast for the North-East Atlantic, using the WaveWatchIII model forced by CMIP6 winds is presented. The model uses a grid of 0.25° of spatial resolution, covering a longitude range of -21.0° to 10° (west to east) and a latitude range of 18° to 80° (south to north).
The main objective of this work is to assess the quality of historical winds from all the CMIP6 wind data that are available under the first realization criteria (r1i1p1f1) at the time of this study. This leads to understanding limitations and proposing a selection method to choose the optimal wind dataset to force the wave model within the analyzed area.
Thus, the optimal CMIP6 historical winds for the North-East Atlantic are used to create a 10 years hindcast(from 2003 to 2012). To further assess the suitability of the selected winds dataset for wave generation, results are compared with the ERA5 wave product. The available CMIP6 models show region-specific variations depending on the Regional Climate models used for their developments. The results show the impact of zonal and, meridional wind intensities, on wave characteristics in different regions over the domain.

Operating and Extreme weather conditions for testing Offshore Devices at Marine Renewable Energy Lab (MaRELab)

2023-06-14T19:55:57+01:00

Marine Renewable Energy Lab (MaRELab) is an onshore/offshore infrastructure for testing full and model scale prototypes aimed at harvesting energy from marine renewable sources.It is a real environment testing site located in the port of Naples, in proximity of the final part of San Vincenzo artificial breakwater. The laboratory covers an area of about 4 km², including 40 meters along the breakwater, and moving 200 meters in the seaside from this. Just few meters from the breakwater, it is possible to reach about 30 meters deep, allowing the correct scaling of the behavior of platforms in deep and intermediate waters. Due to its facilities, MaRELab enables to test different kind of devices. On the breakwater area for example is currently installed the OBREC device (Overtopping BReakwater for Energy Conversion), that exploits the overtopping phenomenon in order to produce energy. In the sea area, instead, floating wind turbines with scaling up till 1:7 approximatively can be installed. Furthermore, other floating devices, such as solar islands, innovative breakwater, can be tested, both individually or combined.The opportunity of investigating these technologies in a real environment allows to evaluate the effective dynamic, structural and energy performances, as well as the effective resistance of materials. Moreover, some physical phenomena are clarified due to higher scaling with respect to indoor laboratory tests. The optimal testing of the different technologies requires an extensive knowledge of the meteorological and marine conditions at the pilot site. For this purpose, in this work, wind and wave energy resources are assessed. In particular, wind and wave hourly data from re-analysis ERA5 dataset (ECMWF) are considered. Data cover the period 1979-2020 and are available for fixed geographical points. In order to characterize more in detail nearshore conditions, wave data have been propagated through the software MIKE21 SW.The energy resource assessment represents a practical guide in defining the optimal testing conditions. It provides information on the distribution of wind and wave energy resources at MaRELab during the year. Moreover, it is possible to investigate the correlation of the two resources [1]. The characterization of the site and the knowledge of the technology to be tested, suggest when the optimal meteorological and marine condition occurs. An Extreme Value Analysis has been carried out to define extreme wave conditions with several return period [2]. Operational and extreme condition, depending on the scaling of the devices, can thereby be realized.

REFERENCES

1. Contestabile, P., Russo, S., Azzellino, A., Cascetta, F., Vicinanza, D. (2022). "Combination of local sea winds/land breezes and nearshore wave energy resource: Case study at MaRELab (Naples, Italy)", Energy Conversion and Management, ISSN 0196-8904, 257, 115356, https://doi.org/10.1016/j.enconman.2022.115356

2. Dentale, F., Furcolo, P., Pugliese Carratelli, E., Reale, F., Contestabile, P., & Tomasicchio, G. R. (2018). Extreme Wave Analysis by Integrating Model and Wave Buoy Data. Water, 10(4), 373. ISSN: 2073-4441. https://doi.org/10.3390/w10040373

An Experimental Study for Wave Energy Converter of Wavestar Type using Real-Time Hybrid Model Testing Technique

2023-06-09T09:39:17+01:00

An experimental investigation of the performance of a wave energy converter of wavestar type was conducted. A PTO system of downscale is difficult to directly apply in the model test because the experiment in the wave basin is based on Froude scaling. In case of the hydraulic PTO system, modelling of oil pressure and accumulator is more difficult in the wave basin test. To overcome the limitation of modelling of hydraulic PTO system, new model test technique was applied in this study. The model tests were performed in regular and white noise waves, and existed model test technique and new model test technique were compared in each other. From the results, characteristics of new model test technique were introduced.

Demonstrating real-time hydrodynamic motion response in force control for regular waves in a robotized dry test rig with a point-absorber WEC

2023-06-06T19:39:02+01:00

A 6-Degrees-Of-Freedom robotized dry test rig has been developed at Uppsala University to test point absorbing WECs (Wave Energy Converters). Using a six joint industrial robot as a buoy movement emulator, the robot's outermost point (joint 6) is connected to the wire from the generator concept WEC PTO (Power Take-Off). The robot's movement in joint 6 thus corresponds to the buoy movement on the sea surface. The test rig can be used for various point absorbing WEC PTO units. In this project, the test rig has been used with a WEC-PTO prototype. The point absorbing WEC-LRTC concept is being developed at Uppsala University. The generator concept is made up of two identical rotating generators. A wire is used as a connection between the generator concept at the seabed and a buoy on the sea surface.

The goal of this article is to demonstrate and evaluate how the test rig interacts with the LRTC-WEC PTO in regular waves. In the presented experiments, a hydrodynamic model with force control method has been used.

The results show a clear difference in the use of the hydrodynamic model with different sizes of the buoy. The test rig with the force control model can be used easily to test different theoretical buoys and different load settings for WEC PTOs. Effective experiments can be performed with real PTO forces instead of simplified simulations.

Future work is to experiment with the position control method and also experiments with irregular waves.

Data-base Hydrodynamic Coefficients Interpolator for Control Co-Design of Wave Energy Converters

2023-06-16T19:08:12+01:00

Ocean waves store an enormous amount of energy that is still untapped. Several wave energy converter (WEC) prototypes have already been suggested by different developers, but none of these prototypes has demonstrated economical viability, meaning that none of them is ready to compete in the energy market against other energy sources.

In order to improve the economical viability of WECs, two main actions have been identified: (i) the energy absorption capability of the devices must be enhanced, for which the design of advanced energy maximising control strategies is crucial; and (ii) cost reduction is a key action, meaning that more reliable prototypes must be designed with an economic perspective. In the traditional design process, the most critical aspects of WECs, the floater, the power take-off system, and mooring lines, are optimised based on the energy absorption capabilities and the loading on critical elements under a simplified control strategy: commonly, an unconstrained passive resistive control. Once the design is determined, an advanced control strategy is developed for that design to maximise energy absorption and generation capabilities.

However, the implementation of more advanced control actions significantly alters the behaviour of the device, substantially enhancing its motion. As a consequence, the pre-defined system may not suit the behaviour of the WEC under such control strategies. As a solution to this design imbalance, the control strategy is adapted so that it can extract the maximum energy allowed by the design, which is far from optimal. However, it is likely that the characteristics of the design include unnecessary overdesigns.

To avoid such problems, an alternative design approach has been suggested, which articulates the information about advanced control actions from the early stages of the design: Control co-design (CCD). The design of the WEC can be decided considering the behaviour of the device under the final control actions. However, analysing and redesigning geometry variations within a CCD loop requires a recomputation of hydrodynamic coefficients, which implies running a boundary element method (BEM) software at each iteration, which can render the computational demand of the CCD optimisation loop prohibitive. In that case, the only solution may be reducing the resolution of the BEM simulation in order to make the CCD optimisation numerically feasible. Another potential solution is computing an extensive pre-defined hydrodynamic coefficient database covering all the potential geometry variations in advance or having a reduced database combined with an interpolator.

The present paper suggests an efficient solution for the computation of hydrodynamic coefficient in CCD loops, which avoids the need for using BEM methods in CCD schemes. Based on an advanced data-based interpolation model for identifying the hydrodynamic coefficients for any variation of a base case geometry. To that end, the interpolation model is provided with hydrodynamic data for an extended base case, including the coefficients for the base case geometry and a limited range of expected variations. Based on this extended space, the data-based interpolator provides accurate information on any variations beyond the original base case, significantly reducing the computational cost of the CCD approach.

Review of TEAMER Awards for WEC-Sim Support

2023-06-22T15:10:27+01:00

Testing Expertise and Access for Marine Energy Research (TEAMER) is a U.S. Department of Energy Water Power Technologies Office sponsored program, overseen by the Pacific Ocean Energy Trust, which aims to advance the state of marine energy technologies. The program connects technology developers with experts at U.S. facilities, including numerical modeling and analysis facilities, like WEC-Sim. The WEC-Sim facility is supported by the WEC-Sim development team at Sandia National Laboratories and the National Renewable Energy Laboratory. WEC-Sim (Wave Energy Converter SIMulator) is an open-source software for simulating wave energy converters. WEC-Sim can model the multi-body dynamics of devices comprised of bodies, joints, power take-off systems, and mooring systems. Since TEAMER’s first round of support in 2020, there have been eighteen TEAMER awards focused on numerical model development in WEC-Sim. TEAMER awards for WEC-Sim support have modeled a wide range of wave energy converter archetypes, including point absorbers, attenuators, oscillating water columns, and many other novel architectures. A wide variety of studies have been conducted, leading to important insights for TEAMER partners and software improvements for WECSim. This article highlights several successful WEC-Sim TEAMER awards. The awards described herein include TEAMER recipients Ocean Motion Technologies, AquaHarmonics, iProTech, East Carolina University, Virginia Tech, Maiden Wave Energy, and the University of Massachusetts Dartmouth. The awards of these seven partners contain a wide range of investigations and cover the creation of baseline hydrodynamic models, PTO modeling, geometry optimization in both boundary element methods and WECSim, and model tuning and validation.

Performance Enhancement of Fluidic Diode for a Wave Energy System through Genetic Algorithm

2023-06-06T18:36:24+01:00

The oscillating water column (OWC) is an extensively studied wave energy converter that produces pneumatic power from the motion of the sea waves, which can be harvested using a pair of turbines without additional devices. However, its efficiency is hampered by poor flow blockage. Researchers have proposed a fluidic diode (FD) to improve flow blockage. Its performance is given by diodicity, which is the ratio of pressure drop in reverse to forward flow. A higher resistance in the reverse path signifies enhanced flow blockage, while a lower resistance in the forward flow minimises power loss at the turbine entry. In the present study, the numerical investigation was performed by solving three-dimensional unsteady Reynolds-Averaged Navier Stokes equations using ANSYS-Fluent 16.1 to simulate the flow behaviour inside the FD. Five geometrical parameters for FD were varied to obtain its optimal shape leading to a lower pressure drop in the forward direction and higher in reverse. The optimal shape was obtained through the genetic algorithm, showing a 12% improvement in performance compared to the base model. Detailed fluid flow and performance analysis of both base and optimum models are presented in this article.

Parametric resonance: a risk to be avoided or an opportunity to be exploited? A case for a 2:1 wave energy converter

2023-05-27T09:20:21+01:00

Accurate models for wave energy converters are paramount for appropriate and effective assessment of the system behaviour under operational conditions; if such models are also computationally efficient, they can be used for control and design applications. Crucially, the underlying design and working principle can be substantially modified only at early development stages and typically based on fast mathematical models with several iterations; since such models are often linear or weakly-nonlinear, complex nonlinear phenomena are rarely embedded into holistic design tools or optimization schemes. A secondary consequence is that nonlinearities are dealt with at a later verification and assessment stage and are typically a burden or an issue to limit. The aim of this paper is to highlight how nonlinearities can actually be a beneficial resource to be exploited and leveraged, rather than a bother. To do so, a computationally efficient nonlinear model is used, able to articulate relevant phenomena at a convenient computational time. The system under analysis is a prismatic floating wave energy converter and the nonlinear model implements an analytical formulation of Froude-Krylov forces computed on the instantaneous wetted surface; this modelling approach is able to articulate a 2:1 parametric resonance which, when activated under certain conditions, is a type of instability able to amplify the amplitude of motion. Note that the 2:1 proportionality refers to the ratio between wave and natural frequencies of the system. Therefore, this paper embeds parametric resonance into the notional design of a wave energy converter purposely designed to experience and take advantage of such an instability. Results are promising, since a substantial amplification is achieved in the 2:1 region, whereas similar or higher oscillation amplitude is obtained in the 1:1 region.

Verification and validation of MoodyMarine - A free simulation tool for modelling moored MRE devices

2023-01-28T19:20:15+00:00

This work presents the verification and validation of the freely available simulation tool MoodyMarine, developed to help meet some of the demands for early stage development of MRE devices. MoodyMarine extends the previously released mooring module MoodyCore (Discontinuous Galerkin Finite Elements) with linear radiation-diffraction bodies, integrated pre-processing workflows and a graphical user interface. It is a C++ implementation of finite element mooring dynamics and Cummins equations for floating bodies with weak nonlinear corrections. A newly developed nonlinear Froude-Krylov implementation is verified in the paper, and MoodyMarine is compared to CFD simulations for two complex structures: a slack-moored floating offshore wind turbine and a self-reacting point-absorber with hybrid mooring.

A Hybrid linear potential flow - machine learning model for enhanced prediction of WEC performance

2023-06-26T10:08:39+01:00

Numerical models based on the linear potential flow equations are of paramount importance in the design of wave energy converters (WECs). Over the years methods such as wave stretching, nonlinear Froude-Krylov and Morrison drag have been developed to overcome the short-comings of the underlying assumptions of small amplitude wave, small motion and inviscous flow. In this work we present a different approach to enhance the performance of the linear method: a hybrid linear potential flow – machine learning (LPF-ML) model. A hierarchy of high-fidelity models – Reynolds-Averaged Navier-Stokes, Euler and fully nonlinear potential flow – is used to create training data for correction factors targeting nonlinear hydrodynamics, pressure drag and skin friction, respectively. Long short-term memory (LSTM) networks are then trained and added to the LPF model. LSTM networks are heavy to train but fast to evaluate so the computational efficiency of the LPF model is kept high. Simple decay tests of generic bodies (sphere, box, etc) are used to validate the LPF-ML model. Finally, the LPF-ML is applied to a model-scale point-absorber WEC to assess the power production.

Design Wave analysis of the M4 wave energy converter device

2023-06-07T11:05:07+01:00

We present physical results for a Design Wave analysis of the M4 wave energy converter (WEC), which
is currently being developed for a kW scale deployment in King George Sound, off the coast of Albany, Western Australia. The M4 wave energy converter is a hinged multifloat device utilising relative pitch as the power-producing mode of motion. We have conducted wave basin experiments at the Australian Maritime College in Launceston, TAS Australia, at a scale of 1:15 compared to the ocean trial. We present an experimental analysis of the hinge rotation in a severe sea state identified for the King George Sound location. We identify the most extreme response of the hinge rotation and the wave that causes it – the so-called Design Wave. By averaging the largest structural responses measured in long irregular wave realisations of the extreme sea states, we identify the most likely extreme response.
The Design Wave is found to be the average of the surface elevation signals occurring simultaneously with instances of the largest response. The Design Wave thus identified is then produced in the basin and the M4 response measured.

On the state-of-the-art of CFD simulations for wave energy converters within the open-source numerical framework of DualSPHysics

2023-06-24T20:20:52+01:00

There are currently several types of devices capable of harnessing wave energy, exploiting a broad variety of physical transformation processes. These devices – known as Wave Energy Converters (WECs) – are developed to maximize their power output. However, there are still uncertainties about their response and survivability to loads induced by adverse environmental conditions, with a consequent increase of the Levelized Cost of Energy (LCOE), which prevents in fact their commercial diffusion. As evidenced by a large body of research, marine renewable energy devices need to have more robust design practices. To address this issue, we propose the CFD-based DualSPHysics toolbox as a support in the design stages. DualSPHysics is high-fidelity software inherently suited to numerically address most challenges posed by multiphysics simulations, which are required to reliably predict WEC response in situations well beyond operational conditions. It should be noted that WECs, generally, may be connected to the seabed and comprise mechanical systems named Power Take-Offs (PTO) used to convert the energy from waves into electricity or other usable energies. To reproduce these features, DualSPHysics benefits from coupling with the multiphysics library Project Chrono and the dynamic mooring model Moordyn+. In this work, the augmented DualSPHysics framework is utilised to simulate a range of very different types of WECs with a variety of elements, such as catenary connections, taut mooring lines, or linear and nonlinear PTO actuators. Version 5.2 of the open-source licensed code was recently released, making the numerical framework publicly available as one unit. This work aims to provide a numerical review of past applications, and to demonstrate how the same open-source code is able to simulate very different technologies.

Specifically, this paper proposes routine modeling and validation procedures using the SPH-based solver DualSPHysics applied to five different WEC types: i) a moored point absorber (PA); ii) an oscillating wave surge converter (OWSC); iii) a floating OWSC (so called FOSWEC); iv) a wave energy hyperbaric converter (WEHC); and v) a multi-body attenuator (so called Multi-float M4). For each device listed above, we provide validation proof against physical model data for various components of the floater(s) and PTO related quantities, performed under specific sea conditions that aim to challenge their survivability. Within the scope of this research, we present the WEC response with respect to the degrees of freedom that really matter for each of the floatings due to hydrodynamic interactions (i.e., heave, surge, and pitch), along with quantities more intimately connected to the anchoring systems (e.g., line tension) or the mechanical apparatus (e.g., end-stopper force). The quality of the results, the discussion built upon them and the demonstrated solver exploitability to a wide range of WECs show that one software model can run all cases using the exact same methodology, which is of great value for the marine energy R&D community. Finally, we discuss future research objectives, which include the implementation of automation to apply open control systems and possible applications to subsets of WEC farm arrays and other floating energy harnessing devices.

A Study on Wave Energy Conversion Problem of Turbine-Integrated OWC Chamber

2023-06-14T18:07:35+01:00

An oscillating water column (OWC) device is a promising wave energy converter (WEC) to utilize ocean wave energy resources. The OWC chamber absorbs the kinetic energy of ocean waves and converts the water column motion to reciprocating airflow. A power take-off system (PTO-system) of the OWC-WEC consists of an air turbine, generator, and power control system. An airflow drives an air turbine, which rotates a generator to produce electricity. The air turbine induces a pressure fluctuation due to its aerodynamic characteristics, which directly affects the fluid motion inside the chamber. In this study, the wave energy conversion problem of the OWC-WEC was discussed based on the experimental and numerical simulation results, considering the turbine-chamber interaction. The wave energy conversion problem from wave to power was solved using the finite element method (FEM)-based numerical wave tank in the time domain. The air turbine was numerically modeled based on aerodynamic coefficients and inertia properties. The validity of the present numerical method was examined by comparing it with the experimental results conducted in a two-dimensional wave flume at the Korea Research Institute of Ships and Ocean Engineering (KRISO). The effect of the turbine-chamber interaction on the energy conversion performance was investigated regarding the various rotational speeds of the air turbine based on experimental and numerical analysis. It was observed that the rotational speed conditions of the turbine, where the hydrodynamic performance of the OWC chamber and the aerodynamic performance of the turbine could be maximized, were different from each other. Therefore, it can be seen that a control technology considering the combined performance of the chamber and air turbine is required to maximize the energy conversion performance of the OWC-WEC.

Dynamic Simulation of Wave Point Absorbers Connected to a Central Floating Platform

2023-06-24T20:38:46+01:00

AmAmong the challenges currently being faced by the wave energy industry, there are the ones related to the mathematical and numerical modelling of Wave Energy Converters. Because various levels of physical complexity are reflected in the dynamics of wave converters, the mathematical modelling of such systems usually comes up with nonlinear dynamic equations to be solved. The nonlinearities, however, may appear in many ways. In this paper, the nonlinear geometric constraints that arise naturally in hinged structures are investigated for floating multi-body systems including wave point absorbers. To achieve that, a method of constraint linearization is proposed and applied to a realistic case study. The method is based on generalized coordinates and generates a robust first-order dynamic matrix to characterize the multi-degrees of freedom hydrodynamic system. The simulation outputs the motion response for all floating bodies, as well as the constraining forces responses, among other parameters. The method requires knowledge of the geometries of the system but rather few assumptions, namely, to perform the linearization of constraints. The method is illustrated with a case study, where three wave point absorbers are concentrically attached to a Floating Offshore Wind Turbine platform with an onboard hydraulic Power-Take Off system.

Hydrodynamic and Static Stability Analysis of a Hybrid Offshore Wind-Wave Energy Generation

2023-07-04T12:15:05+01:00

Marine structures like Floating Wind Turbine (FWT) is exposed to the oncoming waves and wind that can cause oscillatory motions within the system. These undesired oscillations can have negative impacts on the efficiency of the system, reduce its lifespan, hinder energy extraction, increase stress levels, and raise maintenance costs. To mitigate these negative impacts, the integration of Wave Energy Converters (WECs) into the FWT system has been proposed. This hybrid system may be capable of extracting coupled wind-wave energy and transferring electrical power to the shared grid. This paper presents an investigation of the use of Oscillating Water Columns (OWCs), a type of WECs, within a FWT system. The purpose of using an OWC to increase the hydrodynamic damping and reduce the resonant motions of the floating wind turbines under environmental loads, including both wind and wave loads. This is because the wave energy from OWC would be very small as compared to the wind energy. However, OWCs can provide a damping source for reducing the resonant motions of the floater, especially the pitch resonant motions. This would be very beneficial for the power performance of the floating wind turbine and the structural design of the floater. The purpose of this paper is to redesign the original FWT platform to accommodate the additional OWCs by considering the hydrostatic stability and hydrodynamics since the new elements, the OWCs, can significantly change the response of the platform. The redesign of the original FWT involves the integration of OWCs within two out of three columns of an existing semisubmersible platform for a 12 MW FWT. To do this, two moonpools, which are consistent with OWC air chambers, have been created within two columns of the FWT. The water ballast was designed for the columns with and without OWCs. After that the redesign is done hydrostatic stability and hydrodynamics analyses are evaluated. The hydrodynamics properties are discussed in terms of the hybrid platform response as compared to the original platform. The hybrid platform was modeled using GeniE and the hydrostatic stability and hydrodynamics of the system was evaluated by HydroD, tools developed and marketed by DNV. The results of this study demonstrate the potential benefits of integrating OWCs within a FWT system in terms of reducing the platform oscillatory motion.

Study with Large Eddy Simulations of energy dissipation due to backwash flows in wave overtopping.

2023-06-28T19:56:35+01:00

Renewable energy market growth is imperative to reach a sustainable society. This necessity demands improving existing methodologies to exploit green energy sources and explore new technologies. Wave energy is one of the sources with the larger offer in power available in the oceans to be harvested; however, the remoteness and extreme conditions of the best locations to harvest wave energy have made the development of wave energy converters expensive and less attractive than other green energy sources that operate on land. Overtopping wave energy converters (OWEC) operate on the shore, facilitating their construction and maintenance. They can be integrated into coastal defences, sharing the cost of construction and their environmental impact and creating new benefits from the coastal defence. The effectiveness of an OWEC relies strongly on their ability to capture wave-overtopping volumes. The geometry of the run-up ramp (slope and shape) plays an important role in the efficiency of an OWEC. An optimum geometry for each location and wave condition needs to be found to maximise the operation of any OWEC. The efficiency of these devices, quantified by the hydraulic efficiency, must be improved to make them an attractive solution for energy conversion. Three distinguishable processes occurred on the ramp of an OWEC; the wave collapse, run-up and run-down (backwash). The first two have attracted more attention from researchers, specially oriented to the design of coastal defences, but fewer studies have analysed the impact of the backwash on the energy losses during the collapse of the waves. When an incoming wave approaches the run-up ramp, it crashes against the backwash, composed of the water volume that didn't overtop the structure in the previous wave. These interactions increase turbulences and, therefore, energy dissipation, affecting the performance of an OWEC. This study analyses the hydrodynamics during the backwash flows' and incoming waves' interaction on sloped structures. A numerical code Hydro3D, based on Large Eddy Simulations, is used to conduct the present study. As a reference condition, a structure with a freeboard high enough to not allow wave overtopping is tested; in this case, all the water that run-up the slope runs down after the water tongue reaches its highest elevation. Structures with lower freeboards are tested to allow different wave overtopping levels and backwash flows. The impact of different backwash flow conditions on energy dissipation is then evaluated. The founding of this study allows us to verify if a reduction in the backwash flows can improve the efficiency of a OWEC.

Nonlinear WEC modeling using Sparse Identification of Nonlinear Dynamics (SINDy)

2023-06-08T12:32:22+01:00

Modeling oscillating surge wave energy converter (OSWEC) systems to accurately predict their behavior has been a notoriously difficult challenge for the wave energy field. This is particularly challenging in realistic sea states where nonlinear WEC dynamics are common due to complex fluid-structure interaction, breaking waves, and other phenomena. Common modeling techniques for OSWECs include using potential flow theory to linearize the governing equations and ease computations, or using CFD to solve the full Navier-Stokes equations coupled with rigid body motion. However, both of these options have significant limitations. Potential flow theory breaks down in realistic sea conditions where nonlinear WEC dynamics are present, and CFD is often too computationally expensive for many applications such as real-time state prediction and optimal control, two areas of active research in the wave energy field. In particular, OSWEC dynamics are dominated by diffractive and viscous forces, often making common assumptions and linearization approximations (including small-body approximations) unreasonable, and CFD computationally intractable.

To bridge this gap in modeling methods, we propose using Sparse Identification of Nonlinear Dynamics (SINDy) to build nonlinear reduced-order models (ROMs) that describe OSWEC behavior in response to large-amplitude regular waves. SINDy is an equation-free, data-driven algorithm that identifies dominant nonlinear functions present in system state dynamics using a library of nonlinear functions created from time series measurement data. The result is an ordinary differential equation (ODE) in time that can be solved from an initial condition to model and predict time behavior of the states. SINDy is parsimonious, meaning it uses a sparsity-promoting hyperparameter with the goal of only including the minimum number of terms to capture dominant dynamics, resulting in interpretable and generalizable results that are not overfit to the data. Using the discovered ROMs and integrating in time, not only can SINDy provide time series models and future state predictions of OSWEC dynamics, it can also give insights into which variables are critical in describing the underlying dynamics of the state.

In this study, we use SINDy to describe the nonlinear dynamics of a lab-scale OSWEC in a wave tank subjected to large-amplitude regular waves. We use nonlinear simulation data to generate kinematic, force, and torque data and use it as input to SINDy to identify ODEs that describe the measurement variables in time. We then integrate the ODEs to recreate the time series as well as predict future system behavior. We directly compare the resulting time series to the original data input to assess the accuracy of the SINDy model. We also interpret the dominant terms in the ODEs to gain insight on underlying mechanisms of the observed nonlinearity.

Early results show SINDy is a promising tool for modeling nonlinear OSWEC dynamics. We are able to build ROMs for variables such as angular kinematics and the moment about the hinge that generate an accurate recreation of data measurements. We found strong dominance in cubic and quintic terms of the ROMs, suggesting higher-order nonlinearities in the system dynamics. These findings inspire future work in identifying underlying mechanisms driving nonlinearity.

Numerical and Experimental Characterization of Rotational Floating Body Drag

2023-06-24T20:34:17+01:00

Hydrodynamic drag plays a significant role in the motions and response of floating bodies – whether it be a wave energy converter, floating wind structure, or offshore oil & gas platform. Existing literature provides significant overview of the methodologies (both experimental and numerical) to characterize translational drag, however, there is limited research on the contributions (and methods of application) for rotational drag.

This paper will detail both numerical modelling and a physical experimental campaign to assess how rotational drag impacts floating body dynamics, and best practices for numerical model inclusion. Specific focus will be on 1) the variety of methods used to input rotational drag into numerical models; 2) processes and lessons learnt from the experimental derivation of rotational drag coefficients; and 3) how does weakly non-linear wave stretching methods influence rotational drag.

The experimental campaign is currently underway to classify the significance of rotational drag coefficients in characterizing floating body behavior. Translational and rotational drag coefficients of a simplified, inertial property matched, 1:50 floating body is being determined through a series of calibration tests. Both traditional free decay tests and forced oscillation tests will be implemented to evaluate these coefficients across multiple degrees-of-freedom. The final paper will present an overview of the experimental campaign, the results and lesson learnt.

On the numerical side, the floating body will be modelled in the open-source wave energy converter modelling tool, WEC-Sim, and validated against the experimental results. Numerical results will be presented to review general body responses, with and without rotational drag, and generic wave conditions plus those expected at the PacWave wave energy test site in Oregon, USA.

The inclusion of rotational drag coefficients and weakly nonlinear hydrodynamics are expected to improve computational model results, especially in the nonlinear wave excitation range, providing a better understanding of floating body behavior.

A Development and validation of the in-house hydrodynamics code and the DNV software for TALOS wave energy converter

2023-06-14T17:56:11+01:00

The Lancaster in-house code is a time-domain analysis code, developed especially for the TALOS wave energy converter, a novel wave energy conversion technology. The TALOS wave energy converter is a point absorber-like wave energy converter, but with a unique power take-off (PTO) system. The whole PTO system is fully enclosed within the structure, hence no moving parts are exposed to the harsh marine environments.

The PTO of the TALOS WEC consists of a mass ball within the structure, with a number of springs and dampers (for instance, direct drives or hydraulic systems) connecting between the mass ball and the structure. Such an arrangement of the PTOs would make the PTO essentially non-linear, regardless whether the actual PTO dampers are linear or nonlinear as well as the linear springs. Therefore, a time domain analysis must be established for the TALOS WEC. Towards the goal, an in-house time-domain model has been developed at Lancaster University, and now it is to be validated via the delicated numerical models built using well-established commercial software (and in the future via experimental data). Within this framework, the present paper presents an effort for validation using the DNV SESAM commercial hydrodynamic and structural analysis software.

To build a numerical model for TALOS WEC, the Lancaster in-house time domain code is based on the hybrid frequency-time domain approach. That is, the basic hydrodynamic parameters are analysed using the panel methods (WAMIT, HAMS, NEMOH), and then the relevant parameters are transformed for the Cummins’ time-domain equation, such as the added mass at infinite frequency, and the memory effect (the convolution terms). This work is an extension of the comparison between the in-house code and the DNV SESAM software, and the comparisons would include the relevant hydrodynamic analyses, the time-domain analyses with and without the TALOS PTO system, with the aims towards developing the validated tools of both the in-house code and the commercial software.

Wave Amplification inside an Open Circular Caisson for Wave Energy Conversion in Waters with Medium Energy Density

2023-07-18T10:19:42+01:00

Wave energy is one of important marine renewable energy resources. Many studies have been devoted to harnessing the energy for human use. Though they are almost everywhere in the sea, waves can be much more significant in some sea areas than others. For example, the wave energy density in Asian waters is usually much less than that in European west coasts.

To make the wave energy harvesting more viable in Asian waters with medium wave energy density, we propose to employ an open caisson to amplify the wave locally and to combine it with a wave energy converter to tap the amplified wave energy. In this study, we focus on the effect of incident wave height on the amplification factor which is defined as the ratio of the wave height inside the caisson to that of the incident wave. Shown in Figure 1, the caisson is mounted vertically on the horizontal seabed in the open sea. At the edge of the opening, it has two guides on the two sides of the opening. They are identical in geometry and part of a solid cylinder. The purpose of the two guides is to enhance the wave amplification inside the caisson.

The study was conducted primarily by CFD computations and partially verified by experiments. In computations, the finite volume method was employed to discretize the Navier-Stokes equations. A multi-block grid was generated for computational purposes. The volume-of-fluid (VOF) method was used to capture the free surface. The nonlinear iterations were conducted with the PISO method. And the implicit time marching scheme was adopted in the time direction. It is interesting to find that the amplified wave height in the caisson is not linearly related to the incident wave height. Furthermore, the amplification factor is also a function of the incident wave period. The wave period at which the peak value of the amplification factor appears is insensitive to the wave height. The amplification factor is usually greater than unity for a wide range of incident wave period.

System Identification for Modelling M4 Wave Energy Converter

2023-06-22T17:19:48+01:00

As one of the renewable energy resources, wave energy has the potential to be one of the major electricity generation resources in the long term. However, the industrial-scale implementation of wave energy is still at an early stage and requires deeper research and development. Currently, only several different wave energy conversion techniques such as oscillating bodies, oscillating water columns, and over-topping modules, are available. Among these energy converters, the 6-float, multi-mode, moored wave energy converter (WEC) M4 has been developed by the University of Manchester. M4 consists of a bow float, three mid floats and two stern floats with beams hinged above the outer mid floats. The mooring to the bow float includes an elastic cable from the bed to a spherical buoy and an inelastic cable linking the buoy to the bow float. The M4 converter has been modelled numerically. However, these numerical methods often require large computational resources and therefore they can only be used offline and they cannot be implemented in real-time applications. It is highly desirable to develop a computationally efficient model for the M4 system.

In this paper, system identification methods are used to model the M4 WEC system with the experimental data. First, two linear models including autoregressive with exogenous inputs (ARX) and the Box-Jenkins (BJ) model are used to build three different types of models for 6 float M4 WEC system using three different input and output variables: wave surface wave surface elevation, the mooring forces (top and bed forces) and the bow float motions (pitch, surge and heave). Further, when constructing the model of the M4 system, the impacts of both sampling rate and model orders are considered. The modelling results are compared and analysed using three selected criteria including fitting percentage, mean squared error and final prediction error. Finally, the performance of identified models are validated by wave basin experiments which covers seven major wave conditions including different wave heights and mean wave periods. Results show that both ARX and BJ models are feasible to model the M4 WEC system, with low orders and sampling rates for high short-term prediction accuracy. These models could be potentially used for real-time applications.

Semi-analytical and CFD formulations of a spherical floater

2023-07-18T12:22:33+01:00

Today, humanity is facing the great pressure of fossil fuels exhaustion and environmental pollution. This obliges governments and industries to make accelerated efforts on producing green energy. The focus is spotted on marine environment which is a vast source of renewable energy. Among several classes of designs proposed for wave energy conversion, spherical Wave Energy
Converters (WECs) have received considerable attention. The problems of water wave diffraction and radiation by a sphere has been examined by a substantial amount of literature, i.e., [1]–[4], whereas in [5]–[8] linear hydrodynamic effects on a spherical WEC have been examined. All these research works are based on potential flow methodologies. Nevertheless, over
the last decade there has been a significant interest on Computational Fluid Dynamics CFD modelling due to its detailed results, focusing also to spherical WECs [9]–[10].
In the present work a semi-analytical model is applied to solve the wave radiation problem around a spherical WEC (Figure 1), in the context of linear potential theory. The outcomes of the theoretical analysis are supplemented and compared with high fidelity CFD simulations (Figure 2 for a semi-submerged sphere). Furthermore, the two methodologies are compared with a theoretical approach for the hydrodynamic analysis of floating bodies with vertical axis as being presented in [11]. The method is based on the discretization of the flow field around the body using coaxial ring elements, which are generated from the approximation of the sphere’s meridian line by a stepped curve.
Numerical results are given from the comparison of the three formulations, and some interesting phenomena are discussed concerning the viscous effects on the floater.

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A231, 1-7.
[2] Hulme, A. 1982. The wave forces acting on a floating hemisphere undergoing force periodic oscillation. J. Fluid
Mech., 121, 443-463.
[3] Wang, S. 1986. Motions of a spherical submarine in waves. Ocean Engng., 13, 249-271.
[4] Wu, G.X. 1995. The interaction of water waves with a group of submerged spheres. Appl. Ocean. Res., 17, 165-
184.
[5] Srokosz, M.A. 1979. The submerged sphere as an absorber of wave power. J. Fluid Mech., 95, 717-741.
[6] Thomas, G.P., Evans, D.V. 1981. Arrays of three-dimensional wave energy absorbers. J. Fluid Mech., 108, 67-
88.
[7] Linton, C.M. 1991. Radiation and diffraction of water waves by a submerged sphere in finite depth. Ocean Engng.,
18, 61-74.
[8] Meng, F., et al. Modal analysis of a submerged spherical point absorber with asymmetric mass distribution.
Renew. Energy 130, 223-237.
[9] Shami, E.A., et al. 2021. Non-linear dynamic simulations of two-body wave energy converters via identification
of viscous drag coefficients of different shapes of the submerged body based on numerical wave tank CFD simulation.
Renew. Energy, 179, 983-997.
[10] Katsidoniotaki, E., et al. 2023. Validation of a CFD model for wave energy system dynamics in extreme waves.
Ocean Engng., 268, 113320.
[11] Kokkinowrachos, K., et al. 1986. Behaviour of vertical bodies of revolution in waves. Ocean Engng., 13, 505-
538

A Multiquery analysis of a PeWEC farm

2023-07-18T08:48:24+01:00

The maximization of the power output of an array of wave energy converters (WECs) and its impact on the surrounding area are two fundamental, strictly interconnected, aspects that must be considered in the design of a wave farm. The effect of those two elements combined, usually considered separately, is evaluated, in this study, for a small farm of floating devices. The numerical simulations are performed using a coupled model between the BEM solver Capytaine (near-field solution) and the wave propagation model MILDwave (far-field solution). The farm is located off the coast of Pantelleria Island and is composed of three PeWECs, WECs designed for the Mediterranean Sea. The power output and the perturbed wave field around the PeWEC farm are compared to the single device, for different distances among the WECs. Moreover, a Power Take-Off (PTO) is implemented for assessing the difference in the far-field effects between a controlled and an uncontrolled case. Results show that the wake produced by the PeWEC farm is more prominent and more complex with respect to the single device. The inclusion of the PTO contributes to further attenuate the wave field in the lee of the PeWEC farm.

Effects of control strategies on the performance of floating WEC point absorbers operating attached to a breakwater by time-domain simulator

2023-05-30T14:15:29+01:00

The estimation of the performance of Wave Energy Converter (WEC) arrays of the type of simple floaters operating in nearshore and coastal areas, characterized by variable bottom topography, is important for the estimation of the wave power absorption and determination of the operational characteristics of the system and could significantly contribute to the efficient design and layout of WEC farms. For this purpose, full 3D models based on Boundary Element Method have been developed, supporting the systematic use for optimization studies. Apart from WEC arrays in nearshore and coastal regions a promising possibility is the installation at the exposed side of port breakwaters with significant energy potential. In this work, the analysis of the above system is extended to the prediction of WEC performance in multichromatic incident waves under latching control strategy using a time-domain simulator, showing improvement of performance in particular conditions.

Numerical investigation of a new hybrid floating wind turbine concept

2023-06-24T20:31:11+01:00

In today’s energy scenario dominated by the need to develop new methods of generating electricity, energy-intensive infrastructures are a promising solution towards energy self-sufficiency and environmental sustainability. Floating offshore wind turbine represents one of the major solution to exploit renewable energies. Currently, the increase of offshore wind market opens up the possibility of integrating different technologies to take advantage of marine energy potential, in particular wave energy.

This work presents a new wave-wind hybrid floating platform with three Oscillating Water Columns (OWC) integrated in a floating offshore wind spar buoy. The design methodology is described showing the size of geometric characteristics of OWCs and the size of the energy conversion system. The hybrid system is investigated by a time-domain model that integrates the wind turbine model, the hydrodynamics of the floater, the thermodynamics of the OWC air chambers, and the damping effect induced by the OWC air turbine. The thermo-aero-hydro coupled numerical framework is described to highlight the implementation of the thermodynamic model in WEC-Sim/MATLAB environment. Specifically, the water column dynamics are solved as pistonrigid body representation, enabling the time-domain analysis of the OWC dynamic behaviour. Impulse turbines are considered as OWC Power Take-Offs (PTO). The resource scenario of Mediterranean Sea is considered as case study, in terms of both wind and wave conditions. The analysis focuses on the power extraction capabilities of the OWCs and on the impact of OWCs on the productivity of the wind turbine. Further developments on multi-objective design tools and on optimal PTO control design aiming power maximisation could be conducted for hybrid energy platforms based on this established numerical framework.

Quantification of uncertainty in linear wave energy hydrodynamic models from experimental data

2023-01-27T19:30:19+00:00

Considerable testing and modeling are required in order to fully realise, efficiently develop, and successfully industrialise the wave energy converters (WECs). Numerical modeling, full-scale measurements, and scaled prototype testing are the various methodologies that can be applied to model WECs and predict the dynamic response. Mathematical WEC models form the basis of model-based energy maximising control and directly affect the ability of model-based controllers to maximise energy capture. Linear WEC models are attractive in leading to simpler control designs, but may not cover the complete operational space. One solution is to identify a range of linear models at different operating points, which give a measure of the underlying nonlinear behaviour [1], [2]. This model set can then be used to extract a nominal model, and an associated uncertainty region, which can be used as a basis for a robust WEC controller synthesis process, such as articulated in [3].

Recently, such an approach has been adopted using data generated from a high-fidelity numerical computational fluid dynamics (CFD) model [4]. However, numerical wave tanks (NWTs) and physical wave tanks differ significantly in terms of the range of tests which can be performed, and the contamination which can affect the measurements used to determine the data-based models e.g. measurement noise, numerical effects, wave reflections, etc [5]. As a result, the determination of nominal models and uncertainty regions in a physical wave tank may provide some advantages (and disadvantages) which need to be examined carefully. In addition, the range of post-processing techniques which could, or should, be applied to the different experimental/numerical domains, to improve the fidelity of the identified models, may differ between domains.

In this paper, experimental testing of a WEC, by recreating a wave field similar to real-life conditions and a small-scale version of the device, is used to understand the hydrodynamic behaviour and to obtain an accurate dynamic model for WECs, which are considered to be essential towards optimal WEC design. Physical wave tank experiments, even though having their own disadvantages, overcome some difficulties of CFD-based NWT experiments, most notably huge computation time, problems in accurate representation of viscous fluids, uncertainty in the specification of an appropriate turbulence model, and propagation of incident waves [6].

In this study, representative linear models of a point-absorber type WEC from a physical wave tank in the wave basin at Aalborg University are determined which give insight into the system dynamics and provide a basis for robust control of WECs. Among different stimulation techniques to excite the system dynamics in physical wave tank tests, the particular types of excitation signals covering the complete range of frequencies and amplitudes of the system dynamics, while considering limitations on the range of excitation signals or the wave tank reflections, are adopted for the determination of representative linear models. Moreover, a full investigation is carried out to ascertain the appropriate post-processing needed to optimise the signals as the basis for model identification. The model identification includes the non-parametric frequency response by means of empirical transfer function estimate (ETFE).

An Overview of an experimental campaign for arrays of wave energy conversion systems

2023-07-17T13:04:47+01:00

It is already well-known that the vast energy available in ocean waves can provide a massive contribution towards effective decarbonisation. Nonetheless, due to the irregular reciprocating motion of ocean waves, convergence towards a single type of technology becomes rather difficult, and tailored research, aiming at ultimately providing reliable WEC systems, is still required to achieve commercialisation.

Once a given concept is established, numerical models become almost automatically required for a (very) large variety of tasks, including e.g. dynamical and performance assessment, control technology, geometry optimisation, and mooring design, among others. Depending on the requirements associated with a specific task, different types of models are used, with very different levels of complexity (both computational and analytical) and fidelity. For instance, a fully nonlinear estimation of the hydrodynamic response of a WEC would often require high-fidelity numerical modelling techniques, based on e.g. computational fluid-dynamics (CFD), while control applications prefer analytical expressions able to capture the main underlying dynamics in a simplified form.

Regardless of the associated complexity, validation is always required for the reliable utilisation of a specific model for a given application, with experimental validation being the most valuable tool to secure this objective. In recent years, different initiatives were put in place, aiming at the identification of numerical model accuracy through a set of widely available case studies. For instance, the OES-Task 10 has been running a number of experiments followed by blind numerical model validations for several single-body wave energy converters. While the proposed methodologies proved to be efficient, the cases under study are all focused on single-body devices.

Since the sector is slowly approaching a pre-commercial stage, it is important to prove the capabilities of numerical models also for farm configurations which are, ultimately, the way in which WEC systems will be effectively deployed. Motivated by this, we present, in this paper, an overview of an experimental campaign performed at Aalborg University during the period of September-October 2021, where 8 different array layouts of up to 5 devices have been tested, using a 1:20 prototype of the Wavestar WEC system as a baseline device (see PDF file for reference). Data has been collected systematically for regular and irregular wave conditions, including e.g. free-surface elevation at different points in the wave tank (with 19 wave probes in place), effective wave excitation forces acting on each device and layout, and associated motion variables. Furthermore, each different layout has been also tested under (reactive) controlled conditions, providing experimental data on PTO forces and associated motion (i.e. under controlled conditions). The results of this experimental campaign will be available as part of an Open-Access dataset, being an extremely valuable tool for reliable modelling within the WEC research/industrial community.

This paper intends to provide a detailed account of the technical aspects concerning the full experimental campaign, including setup, test design, synergies between collected data, and examples of how these results can be used to validate a variety of models, from different input/output points, depending on the requirement of each specific application.

Solution verification of WECs: comparison of methods to estimate numerical uncertainties in the OES wave energy modelling task

2023-07-21T06:53:56+01:00

High-fidelity models become more and more used in the wave energy sector. They offer a fully nonlinear simulation tool that in theory should encompass all linear and nonlinear forces acting on a wave energy converter (WEC). Studies using high-fidelity models are usually focusing on validation of the model. However, a validated model does not necessarily give reliable solutions. Solution verification is the methodology to estimate the numerical uncertainties related to a simulation. In this work we test four different approaches: the classical grid convergence index (GCI); a least-square version (LS-GCI); a simplified version of the least-square method (SLS-GCI); and the ITTC recommended practice. The LS-GCI requires four or more solutions whereas the other three methods only need three solutions. We apply these methods to four different high-fidelity models for the case of a heaving sphere. We evaluate the numerical uncertainties for two parameters in the time-domain and two parameters in the frequency domain. It was found that the GCI and ITTC were hard to use on the frequency domain parameters as they require monotonic convergence which sometimes does not happen due to the differences in the solutions being very small. The SLS-GCI performed almost as well as the SL-GCI method and will be further investigated.

HydroChrono: An Open-Source Hydrodynamics Package for Project Chrono

2023-06-24T20:28:23+01:00

In this paper we present the development and verification of HydroChrono, a hydrodynamics package for the Project Chrono physics engine. This package includes the implementation of hydrodynamics equations, the added mass for multibody systems, the development of I/O functions as well as a Python API, and comparison against standard reference cases and other existing tools. HydroChrono provides a flexible, fully open-source solution for simulating wave energy converters (WECs), floating offshore wind turbines (FOWTs) platforms, and other hydrodynamic systems.

Here we show, via comparisons with existing tools for benchmark verification cases, that HydroChrono accurately models hydrodynamic forces - making it a useful tool for the design and optimization of these systems. Additionally, the integration of HydroChrono with Project Chrono offers access to finite element modeling capabilities and high-fidelity modelling – with Chrono’s existing coupling to CFD and SPH codes. This provides numerical modelers with a multifidelity simulation framework for designing and validating these systems.

The development of HydroChrono provides a new, open-source solution for simulating hydrodynamic systems. Its compatibility with other simulation tools enables a more streamlined and efficient design process, advancing the field and providing new opportunities for innovation in this area.

Nonlinear hydrodynamics of a heaving sphere in diffraction, radiation, and combined tests

2023-07-24T19:17:00+01:00

We report on an experimental campaign designed to shed light on critical nonlinear hydrodynamic effects of heaving wave energy converter (WEC) buoys undergoing large-amplitude motions in operational conditions. The experiments carried out with a spherical model comprised radiation, diffraction and combined tests, where the vertical motion was prescribed and delivered via an actuator. As such, we had independent control of incident waves and motions, enabling isolation of different nonlinear terms by combining recordings from multiple phase- and amplitude-manipulated runs. All tests utilised short-duration wave groups and/or corresponding transient motion signals.

We focus on analysis of nonlinear changes in the hydrodynamic forces, and free surface, in the first-harmonic frequency range - this is of most importance to WECs. In a series of radiation experiments, with progressively increasing imposed motions, the radiated wave field and the force in phase with the body velocity are found to decrease nonlinearly, pointing to the WEC's reduced ability to radiate waves under larger oscillations. In the combined tests, we are able to isolate various high-order cross-terms. We attempt to explain the observed trends through third-order potential flow interactions and consider a simple method to approximately describe these.

A new type of wave tank: prototype and proof of concept

2023-06-24T20:38:39+01:00

The prototype and proof of concept of a new type of wave tank are described. If successful, this innovation would enable many wave tank experiments for only a few percent of current costs. Tank testing remains indispensable for new designs, and its high cost is a significant inhibitor of offshore innovation. To date, all wave tanks generate and propagate scaled but real water waves towards a physical model. This new type of tank instead reproduces the velocity field that such waves would have, but only near the model. In practice, this can be approximately achieved only for cases where the marine structure is much shorter than the wavelength, and its draft shallow compared to the wavelength and depth. In such cases the wavefield is time-varying but approximately spatially uniform near the structure. Therefore, it can be reproduced by moving a container of water like wave orbitals; and ensuring slosh modes are minimised. A prototype was built, and after initial challenges in controlling slosh excitation, a proof of concept was successfully achieved: a cylinder was excited by surge oscillations and its response appears consistent with rough calculations. Range of potential applications of the new type of wave tank, lessons learnt by prototyping, and next developments are discussed.

Development of the Exowave Oscillating Wave Surge Converter

2023-06-28T19:37:15+01:00

With increasing demand for renewable energy resources, the development of alternative concepts is still ongoing. The wave energy sector is still in vast develop- ment on the way to contribute to the energy production world wide. The present study presents the development of the Exowave wave energy converter made so far. A numerical model has been established supported by wave flume tests performed at Aalborg University during the first phase of the development. Furthermore, a successful open sea demonstration has been performed on 7 meters of water at Blue Accelerator, Belgium, from which the concept has been proven. As part of the ongoing research, verification of the numerical model will be made through experimental testing in the wave tank of Aalborg University, and an open sea demonstration at 14 meters of water depth will be executed off the coast of Hanstholm, Denmark.

Comparison of physics-based and machine learning methods for phase-resolved prediction of waves measured in the field

2023-07-11T13:21:59+01:00

Phase-resolved predictions of surface waves can be used to optimize a wide variety of marine applications. In this paper, we compare predictions obtained using two independent methods for field data, with horizons sufficient to control wave energy converters.

The first method is physics-based prediction. In this method, a set of optimal representative angles, obtained using an optimization algorithm given time histories of a wave buoy motion in 3D, are used for forward propagation based on linear wave theory. The second method is a machine learning method using an Artificial Neural Network (ANN) which requires longer records for training.

Field measurements were obtained from the Southern Ocean of Albany, WA. The field data were collected by an upwave ‘detection’ array of 3 Sofar Spotter wave buoys and a downwave ‘prediction’ point coincident with a Datawell Waverider-4. All buoys were soft-moored, and data were collected over 3 months in 2022. Selected intervals during this period are presented in the paper to compare and contrast the predictions made by the two different methods. We find that some wave fields can be predicted well over more than a period in advance, all that is required for active control of a wave power take-off in a renewable energy application. In contrast, highly spread sea states remain a challenge. The methods are also compared in terms of the complexity and time required for making predictions. Further discussions are made on the applicability of the results to other locations.

Hydrodynamic studies of a 15 MW semi-submersible FOWT to assess the suitability of the inclusion of a damper system

2023-06-13T17:06:12+01:00

Floating Offshore Wind Turbines (FOWT) can exploit the high energy density found in the offshore environment, with turbines now reaching up to 15 MW in size. At the same time, however, the energetic environment and the massive size of the device present significant challenges in the motion stabilization and mooring system. To overcome these challenges, a tuned mass damper (TMD) has been considered for integration in the FOWT for peak motion reduction. This paper investigates the baseline responses including motion, dynamic response, and tensile loading of the mooring line for a 15MW FOWT on a semi-submersible platform without TMD to identify the damageable motion and the impacts of the TMD on the motion response under wave-wind environmental loadings. The comprehensive analysis is conducted in a package for the dynamic analysis of offshore marine systems, named as Orcaflex. The dynamic and motion characteristics of the 15MW FOWT are analysed and compared under different environmental parameters. The wave and wind parameters are quantified by the 20-years statistical data of the Celtic Sea including both operational and extreme conditions (with a 50-year return period).

Subsequently, the key parameters of TMD are investigated by configuring different combinations of mass, damping coefficients and stiffnesses. The preliminary results of the study show that the TMD system can successfully mitigate extreme motion characteristics, however this is strongly dependent on damping properties. Unsuitable TMD designs may increase the motion responses of FOWT and the tensile loading on the mooring line. Therefore, the TMD properties have to be adjusted based onsite environmental conditions). With this consideration, an active TMD with changeable damping properties will be conducted in future research.

Comparing Numerical Models of the TALOS Wave Energy Converter

2023-06-15T11:01:45+01:00

In this study, we present the development of a numerical model of the TALOS Wave Energy Converter (WEC) in WEC-Sim and compare it with existing numerical in-house models from Lancaster and results from DNV's Sesam code. The objective of this work is to validate the performance of the TALOS WEC using WEC-Sim and compare the results with those obtained from other numerical models. The TALOS WEC is a promising technology for the generation of clean, renewable energy from ocean waves. The development of a reliable numerical model for the TALOS WEC is crucial for its design and optimization. To achieve this, we have implemented the TALOS WEC in WEC-Sim and compared the results to in-house models from Lancaster and to results from DNV's Sesam code. Our results show good agreement with the results from Lancaster and DNV, demonstrating the accuracy and reliability of the numerical model developed in WEC-Sim. This work advances the field of wave energy conversion by providing a verified numerical model of the TALOS WEC, which can be used for further optimization and design studies.

Control synthesis via Impedance-Matching in panchromatic conditions: a generalised framework for moored systems

2023-07-01T17:26:46+01:00

This study focuses on addressing the challenge of integrating the tangled mathematical model of the mooring system into an effective control synthesis. The presented synthesis framework utilises the impedance-matching technique to achieve the desired controller performance by adapting the control parameters to align with the dynamic characteristics of the moored wave energy device. By leveraging this technique, the simulation framework provides a means to effectively incorporate the intricate mooring dynamics into the control synthesis process. Furthermore, this paper aims to delve into the concept of defining a representative control action by examining the input-exciting force of the feedback-controlled system. Through a straightforward case study, the authors demonstrate the significant impact of the mooring on the system dynamics and underscore the applicability of the proposed simulation framework. Moreover, the paper verifies the importance of considering the controlled system’s exciting input when addressing control synthesis, particularly in panchromatic conditions.

Hydrodynamic Response of Mocean Wave Energy Converter in Extreme Waves

2023-06-19T07:21:47+01:00

The design of moored floating wave energy converters (WECs) must take into account extreme responses and mooring line loads in order to ensure their survival and continued wave power generation in the ocean environment. This study focuses on Mocean Energy's hinged raft WEC and aims to provide a comprehensive understanding of its hydrodynamic characteristics in survival wave conditions. To achieve this, a physical model study was conducted on a Froude scale of 1 in 50 at the FloWave Ocean Energy Research Facility, University of Edinburgh.

The experiments involved the use of NewWaves focusing of crest and trough at the model hinge location, as well as long irregular waves. Motion responses of the fore and aft bodies of the WEC were measured using a Qualisys camera, and single component load cells were used to measure the forces in the 3-point catenary mooring line. The hydrodynamic characteristics of the WEC were evaluated in terms of response amplitude operators and non-dimensional mooring line loads.

Results indicate that the fore and aft bodies of the WEC exhibit similar motion responses, except for the pitch motion. The aft body has a pitch response 2 to 3 times higher than the fore body. Concerning the moorings, the wave load on the mooring line in line with the wave direction was found to be higher than the other two mooring lines which were arranged at an angle to the wave direction.

In this paper, a brief discussion of the model set-up, parameters, test procedure, analysis of results, and discussion will be reported. The results will provide insight into the behaviour of the Mocean device in survival wave conditions and will aid with the determination of appropriate design parameters for optimal performance and survival in the ocean environment.

The Dynamic response of floating offshore renewable energy devices: Sensitivity to mooring rope stiffness

2023-06-28T11:41:51+01:00

The offshore renewable energy sector has seen a rise in floating devices, all of which require mooring and anchoring systems. Synthetic ropes have emerged as a promising technology for cost reduction in this system. However, characterising the behaviour of these materials, which exhibit complex non-linear, visco- elastic and plastic structural properties, presents challenges. Numerical modelling and tank testing are the available tools for developers to overcome these challenges, however, there is a lack of guidelines for test facilities regarding the design of tank-scale mooring systems. The present work focuses on the numerical design of a typical semi-taut mooring system using synthetic materials suitable for future-generation floating offshore wind turbines. A coupled time-domain hydrodynamic model was employed to explore the dynamic sensitivity of the device to changes in mooring rope stiffness. The results demonstrate that changes in line axial stiffness have a greater impact on platform surge and mooring line tension than on heave and pitch responses. These findings establish preliminary margins for target stiffness values, which are valuable for selecting mooring materials for scaled tank test models. Although the case study was floating wind, the results have broader applicability to wider floating marine energy device design.

Experimental measurements of two elastic taut-slack mooring configurations for the multi-float M4 WEC

2023-06-28T11:33:23+01:00

Moorings are a vital and often problematic component of any floating offshore renewable energy system, whether for wind or wave energy conversion, and here we consider the multi-float WEC system M4 in a 122 configuration. Previous experimental work has shown elastic cables to reduce extreme snap loads by a factor of about 6 when considering single cables between the bow float and a mooring buoy, and between the mooring buoy and the bed [1]. Here we compare results for two alternative configurations designed to reduce the mooring footprint as well as extreme snap loads. The first system, shown in Figure 1 (a), consists of a large surface buoy connected to the seabed with three equi-spaced elastic cables in pre-tension, with a single elastic cable to the bow float. The second system, shown in Figure 1 (b) has a smaller submerged buoy, again connected to the seabed with three equi-spaced elastic cables in pre-tension, however, this is connected to a smaller surface buoy which is subsequently connected to the bow float. Both systems are taut-slack. In this paper we present experimental measurements taken at the COAST laboratory at the University of Plymouth. Both configurations are tested in irregular (JONSWAP) wave conditions up to limiting steepness, with run times of 35 mins at 1:40 scale or about 3.5 hours full scale. Peak, mean and rms bed loads for both configurations are presented along with rms and peak relative angular pitch response with drive train disengaged (free hinge between mid and stern floats).

References
[1] P. Stansby et al., “Experimental study of mooring forces on the multi-float WEC M4 in large waves with buoy and elastic cables,” Ocean Eng., vol. 266, no. P4, p. 113049, 2022.

Choosing wave energy devices for community-led marine energy development

2023-06-20T05:11:38+01:00

As we pursue more justice and community driven renewable energy projects, more decision making is put in the hands of community leaders. Appropriate tools and resources are necessary for communities to determine what wave energy developer might fit their community needs and wave resource. While there are methods and tools for communities, researchers and wave energy developers to determine the wave resource or potential locations for wave energy development, we are lacking a tool to help communities take the next steps in pursuing wave energy.

A community driven design process was undertaken in Sitka, AK, in conjunction with evaluating the local area for wave energy resource. The community had interest in wave energy development, a decent wave resource area accessible, and existing maritime expertise that could aid in operations and maintenance. Further, as a remote island, the community viewed alternative energy development positively as a way to achieve more energy independence. Still, in this community driven process, we identified a gap that while the community may be interested in a wave energy project, there was little data on which wave energy devices would be appropriate for the community and few tools to help them choose.

This presentation will highlight questions that the participants from Sitka, AK still have after engaging in a community centered design process. We compare the requirements that the community prioritizes with the tools that researchers and developers have to evaluate those requirements, allowing assessment of whether these tools are useable and adequate for community use. We also show an analysis of a selection of current wave energy developers with some of the community focused factors and proposals for tools that may be useful in future community energy development. This research ultimately highlights what factors must be presented to a community for them to choose to pursue installation of a device.

A comprehensive assessment tool for low-TRL current energy converters

2023-06-02T12:15:51+01:00

Along with a market-competitive levelized cost of energy, a current energy converter technology strongly benefits from an extensive consideration of socioeconomic, environmental, and regulatory factors early in the design process. As part of a technology performance level assessment, a series of assessment questions and guidance are developed and presented to evaluate an early-stage current energy technology on holistic criteria considering the entire device lifecycle. The assessment represents an accumulation of industry and research experience to date and relies on regular updates to ensure alignment with industry best-practices, regulatory requirements, and up-to-date technical understanding. A cradle-to-grave (materials, manufacturing, installation and deployment, operations and maintenance, and end-of-life) assessment of capabilities and functional requirements % MA- currently calling functional requirements engineering specifications within the PBE team, not sure if we should start using "engineering specifications now and be consitent with our language considering the potential desal paper. If I'm interpretting this incorrectly pls ignore me.
for tidal, river, and ocean current technologies has been completed. This work presents the evaluation questions and qualitative %(i.e., high, medium, and low)
performance criteria for current energy converters deployed in tidal, ocean current, and/or river applications. Key considerations related to manufacturing and installation include supply chain robustness, manufacturability and related job creation in the end-user and/or adjacent communities, and the time-to-repayment of the embodied energy debt. During deployment and maintenance operations, the safety of the device and subsystems during disconnect or grid failure, the difficulty and frequency of offshore heavy-lift activities, the avoidance or mitigation of area-use conflicts, the sea-states and weather conditions that permit maintenance access, and the availability of contingency plans (should conditions change unexpectedly) are a portion of the considered assessment criteria. Results include the potential impact of early-stage design decisions on the socioeconomic, environmental, and regulatory performance of a technology that allows developers to increase the product value and probability of success, and minimize costly late-stage design iterations through early and broad consideration of factors affecting overall performance and acceptability.

Wave energy communication and social opposition

2023-06-23T12:34:51+01:00

Despite the benefits of marine renewable energies (MRE) to the decarbonisation, public opposition has often been posed to MRE projects. This opposition is one of the reasons slowing down Europe´s energy transition towards clean energies. Aside from one wave energy production farm operating in Europe, other developments are still at pilot or prototype phases. In the context of European SAFEWAVE project (https://www.safewave-project.eu/), we aim to understand the causes that trigger opposition to wave energy projects and identify how communication could improve the perception and attitudes towards MRE projects. To achieve this aim, a systematic analysis of ongoing wave energy projects, scientific bibliography and social media has been carried out. Outputs of this research indicate that opposition to wave energy is rather limited and primarily posed by national and local communities, as well as NGOs. Opposition emerges after envisaging negative affection on i) economy (e.g., conflict with existing uses), ii) social aspects (e.g., local communities for which the benefits are unclear), and iii) the environment (e.g., uncertainty on environmental impacts). Despite much of the wave energy information available on the media is produced and communicated by scientists and engineers (which should be considered a reliable sources of information), most of the communicated content focus on the drivers, the technological developments and benefits. Limited information on potential impacts of wave energy projects is shared. The target audiences vary between channels; YouTube, Facebook and Google have a wider audience than Twitter, which seeks a more professional audience. A holistic communication approach, in which both expected benefits and impacts are communicated may reduce opposition and help society to become more marine energy-literate, allowing for informed decisions and responsible behaviours/attitudes. Availability of official documents, participatory approaches, and transparency are crucial for improving the perception of future wave energy projects.

Do recent renewable energy policy changes in Ireland satisfy the requirements of a nascent wave energy technology development sector?

2022-12-21T16:22:19+00:00

Regulatory frameworks for European countries are influenced by international and EU policies. These policies have a direct effect on the renewables sector with European countries as signatories to the Paris Agreement[1]and the European Green Deal[2].

As a result of emissions targets being ratified into national statute books, governments have agreed to reduce greenhouse gas emissions by 51% in 2030 and to net zero by 2050, and with recently heightened energy security concerns, renewable energy policies must adapt to keep pace. One country undergoing major changes is Ireland. This study examines the changes to renewables policy in Ireland in general and offshore renewables policy in particular, and whether they are sufficient to enable the country to forge what has the potential to become a vibrant wave energy technology development sector. This study will compare these policy changes with some of the successful wind energy policy measures implemented in Denmark and with examples set by other international institutions.

This study will examine policy changes in Ireland since the ratification of the Climate Action Act 2021[3] and Marine Area Planning Act 2021[4]. The study will attempt to determine whether the new policies meet the requirements of Ireland’s nascent wave energy industry. It will look at areas of interest for wave energy technology developers, specifically: climate-related legislation, test facilities, consenting procedures, feed-in tariffs, environmental impact, public support, intellectual property protection and government financial support. This study will examine examples set by Wave Energy Scotland (WES) and EuropeWave; pre-commercial, government funded procurement models, and ask whether the newly established Maritime Area Regulatory Authority (MARA) which will be responsible for issuing foreshore licences in Ireland, is Ireland’s version of these institutions and how it is likely to differ. Questions will be raised about whether the focus on offshore wind by policy architects helps or hinders wave energy technology development and whether it is possible to balance public concerns with technology development through policy. Finally, it will look at Denmark’s wind energy development and examine which policies contributed most to its enduring success and whether these have been absorbed into new offshore renewables policies in Ireland.

Conclusions will show that many policy requirements that had been formerly lacking have been addressed. It will be shown that the changes have largely come about due to the demand for offshore wind and that wave energy technology developers in Ireland will benefit and lose from this. Further analysis and the passage of time are required to determine whether the new policies are fully enforced by newly formed MARA but that Ireland needs to develop an institution that embraces the new policies.

[1] The Paris Agreement | United Nations

[2] European Green Deal | Think Tank | European Parliament (europa.eu)

[3] Climate Action and Low Carbon Development (Amendment) Act 2021 (irishstatutebook.ie)]

[4] Maritime Area Planning Act 2021 (irishstatutebook.ie)

Integration of wave energy into Energy Systems: an insight to the system dynamics and ways forward

2023-05-24T09:00:33+01:00

Wave energy is a rich and highly accessible renewable energy resource, that has largely been under-developed. Studies from the sector have tried to show the potential of benefits wave energy in “simple cases” or via small hybrid systems, the large scale incorporation of wave energy has not yet been fully investigated. Our approach uses a fully dynamic climate driven energy system model, which has undergone modifications to include wave energy converters and their associated dependencies.

This study explores the system dynamics and important elements that will be used for large scale wave energy integration; in a fully coupled European Energy System. We explore the cost pathways of different wave energy converters, the impact of climate data, and the impact of transmission capacity expansion under cost-optimal configurations of a multi-renewable European power system. From this preliminary approach we aim to provide the boundary conditions, and assumptions that will govern the integration of wave energy into the European Energy System up to 2050.

Can Risk-Based Approaches benefit future Marine Renewable Energy deployment, planning and consenting processes?

2023-06-22T14:50:50+01:00

The need to harness the vast energy resources of the oceans has led to a significant increase in the design, testing and deployment of novel technologies for Marine Renewable Energy (MRE). However, growth in this area has been slowed in part by several non-technological challenges, among them the ability to gain permissions to test and deploy installations. These consenting processes are often characterised by long bureaucratic procedures (with many authorities involved) and excessive environmental impact assessment studies, resulting in delays and additional costs to developers.

One option which may help to release this block is to adopt a Risk-Based Approach (RBA) to energy consenting, whereby an assessment of risk is used in the decision-making process. The EU-funded SafeWAVE project (www.safewave-project.eu) has focussed on this possibility in France and Ireland, building on the work of an earlier EU-funded project, WESE (https://www.researchgate.net/project/Wave-Energy-in-the-Southern-Europe-WESE), in which similar work was undertaken in Spain and Portugal. Here we present some of the findings from these projects, in particular the process to work towards a set of guidance for the use of RBAs in MRE consenting processes.

RBAs have already been used in the context of Marine Spatial Planning and Ecosystem-Based Management and have been found to be useful for interpretation of data from experts, indicators and ecosystem models. Indeed, a number of RBAs have also been developed that are appropriate for Marine Renewable Energy (MRE) consenting processes. A thorough review was undertaken of several recognised RBAs and common components were identified across five of the most useful and relevant of these. From these common components, a ‘stepwise process’ was formulated, specifically designed to be embedded into MRE consenting systems. This stepwise process has been constructed such that it can be presented to regulatory stakeholders in France and Ireland with a view to determining the feasibility of its implementation. Ultimately the outcome of these discussions will form the basis for the development of a guidance document on risk based, adaptive management consenting processes with recommendations on how the process can be taken forward and utilised by regulators, planners and developers. While the development of a prescriptive procedure is not feasible (due to the varying nature of the MRE installations devices themselves as well as differing environmental conditions and impacts where devices are deployed), there is scope for providing guidance to assist regulators in taking a robust and holistic risk-based approach. Such a set of guidelines could facilitate a broader understanding and thus the wider use of RBAs, which in turn has the potential to remove one significant barrier to progress in the field.

Towards increased social acceptability of marine renewable energy

2023-01-27T16:58:29+00:00

The development and deployment of novel technologies, including those associated with marine renewable energy, are vital for the ongoing decarbonisation of our energy system. However, technology on its own is not sufficient to realise the energy transition away from fossil fuels. Non-technical barriers – such as regulatory, economic, environmental or social aspects – can be a substantial impediment to widespread adoption of renewable energy technologies. One such impediment, is public opposition to specific deployments of renewable energy and/or indeed to the use of particular technologies. Within the SafeWAVE project, we are cognisant of the importance of good community relations and the need to develop two-way communication with stakeholders to facilitate the successful scaling up of ocean energy device deployments. Accordingly we have worked to co-develop and demonstrate a framework for education and public engagement (EPE), specifically aimed at ocean literacy for communities in France, Ireland, Portugal and Spain. This EPE framework aims to go beyond social acceptance, which is often equated to acquiescence to a fait accompli, and is designed to contribute to the development of projects which exhibit inherent social acceptability.

This paper presents the work within SafeWAVE project to develop the EPE framework with input from the communities. The first part of the paper reports on critical review of selected EPE programmes associated with marine energy test sites and infrastructure deployments, with methods adopted in each case analysed, key challenges faced identified, and best practices documented. Next a framework for public education and engagement is presented, which builds on this review and draws from literature across multiple disciplines – including sociology, psychology, political science, education. Finally, leveraging this developed knowledge the collaborative process of deriving tailored EPE programmes on ocean literacy around marine renewables for the four focal communities is outlined and described. These programmes are intended to reach out to, and engage with, local communities, interested stakeholders and the wider public on issues surrounding the deployment of ocean energy devices, including but not limited to: effects on local communities, implications for marine life, potential impacts for water users, etc.

The paper concludes by discussing the experience of developing the bespoke programmes, outlining emergent feedback from trialling of key elements of the programmes, discussing experiences and proffering guidance based on lessons learned.

Environmental Effects of MRE: Advancing the Industry through Broad Outreach and Engagement

2023-05-29T09:59:56+01:00

Marine renewable energy (MRE) can benefit from broad outreach and engagement with a wide variety of audiences to raise awareness, address concerns about potential environmental impacts, generate public support, build a future workforce, share progress on research and development, and succeed within the larger blue economy. OES-Environmental is an international initiative of 16 countries that brings together the MRE community to increase understanding of environmental effects of MRE. OES-Environmental has focused on sharing research outcomes and knowledge to broad audiences that are best addressed with a variety of learning formats. Throughout these diverse outreach efforts, several important lessons have been learned. These include the importance of clarifying the main message for each audience and for each resource created; finding the best venue to deliver materials to different audiences; using language, complexity, and context that suits audience requirements and is age-appropriate; utilizing a variety of methods to share messages; and leveraging creative ideas to produce an assortment of engaging, accurate products. Creating materials tailored for specific groups has made outreach efforts more effective and has allowed for the information and findings from OES-Environmental to increase awareness of environmental effects of MRE by reaching new audiences.

Based on OES-Environmental’s 2020 State of the Science Report – a comprehensive technical report providing the most up-to-date review of scientific information on environmental effects of MRE – and additional research to date, products have been designed to engage with different audiences, mainly: the MRE community (which includes regulators and advisors, project and device developers, consultants, and researchers); science, technology, engineering, and math (STEM) students; and the interested public. For the MRE community, a brochure that provides a high-level overview of environmental effects and a series of guidance documents which provide in-depth information to aid application of scientific evidence for consenting have been developed. In addition to these new resources, webinars, workshops, and Tethys – an online knowledge platform – have been used to share information and collaborate with the MRE community to advance the state of understanding. For STEM students, an animated MRE video series and a coloring book were created to offer a visual format to learn about environmental interactions with MRE devices and current research. For the interested public, an open-access magazine article and several podcast episodes were developed to expand general understanding of MRE. OES-Environmental also engages the public through ongoing content creation on various social media platforms.

As OES-Environmental continues to synthesize current scientific knowledge around environmental effects of MRE, outreach and engagement will continue to be a focus to ensure important findings are communicated to diverse audiences. Sharing lessons learned by OES-Environmental around outreach and engagement can help other researchers and the MRE community broadly to communicate more effectively and accelerate industry progress.

Techno-economic Optimization of an Offshore Hybrid Power System: Argentine Basin Case Study

2023-01-28T00:03:33+00:00

Ocean observation and monitoring relies on data gathered from multiple sources including gliders, cabled observatories, underwater vehicles, and moored buoys. Moored buoys, augmented by resident autonomous underwater vehicles, are potentially well-suited to long-term oceanographic data collection with the resolution and scope of data collection increased by utilizing in-situ power generation. While power needs could be provided by a single generation source (e.g., diesel generator, solar photovoltaics, or wave energy converter) with battery backup, a hybrid power system can potentially smooth seasonal variations that would otherwise require large generation and storage capacities, and be tailored to a locations’ resource availability. By reducing system size through hybridization, we have the potential to reduce overall system cost. To this end, we developed an optimization model that defines the generator and storage capacities of the cost-optimal hybrid power system for a given location and load profile. The model considers generation by wave energy converters, wind turbines, solar photovoltaics, diesel generators, and current turbines with a battery for energy storage. The model uses the defined load profile and location specific time series of resource availability for a time-domain simulation of the power generated and battery state of charge. Based on the power system size, mooring, and mission specific maintenance schedule, the model calculates capital and operating costs for each potential combination of generation and storage capacities. The optimization model searches the 6-dimensional design space to find the lowest cost system that can satisfy the load requirements. In this paper, we focus on a case study of a hybrid power system serving an ocean observation buoy with a resident autonomous underwater vehicle located at the Mid-Atlantic Shelf Break. Diagnostic metrics such as the cost-breakdown and generator capacity factors are evaluated to understand cost drivers and draw comparisons to the costs of single generation systems. Additionally, the model sensitivity to key assumptions such as battery life, maintenance vessel cost, survival loads, and required persistence are discussed.

Ensuring Resilience in Ocean Energy Power Plants: A Survey of Cybersecurity Measures

2023-02-08T14:19:53+00:00

Offshore Renewable Energy (ORE) is a promising solution to address the challenges of climate change and the depletion of fossil fuels [1]. Wave power, a form of ORE, is considered one of the purest energy sources with significant growth potential [2]. In addition to investing in these energy sources, nations are also working to enhance the protection of Critical Infrastructure (CI). CI encompasses all services crucial to the functioning of society and the economy, including electric power systems and their various forms of generation, such as renewable energy sources. Hence, in addition to exploring various forms of power generation, the cybersecurity of the networks connecting the devices in these systems is a crucial aspect to consider to prevent attacks and minimize the risk of cyber threats to suppliers and customers [3]. For instance, the European Commission states that reducing CI vulnerability and increasing its resilience is one of the main objectives of the European Union.

However, to date, a comprehensive review that synthesizes the various approaches to cybersecurity in ocean energy is yet to be published. The objective of this study is to present a comprehensive survey of the application of cybersecurity measures to renewable energy sources, with a specific focus on ocean energy. A systematic review of the literature was carried out, following the steps outlined by Kitchenham [4]. The methodology steps are illustrated in the flowchart (see Figure 1). Of the 49 articles selected, three main study topics emerged: i) smart ocean, ii) cybersecurity for renewable energy systems, and iii) marine data security. These three topics are interrelated as a smart ocean can be considered as an integrated sensing, communication, and computing ecosystem that connects marine objects in surface and underwater environments [5]. Once the wave energy converters (WECs) are installed, it is also essential to develop safety systems for these devices, as demonstrated in the first report on cybersecurity guidance for MRE (Marine Renewable Energy) systems [6] prepared by the Pacific Northwest National Laboratory (PNNL). In preparation for this report, researchers reviewed the cyber threats and vulnerabilities of information technology (IT) and operational technology (OT) equipment used in various WEC models. Figure 2 presents an example of the possible threats and attacks on WEC devices.

In conclusion, this article provides a comprehensive survey of the application of cybersecurity measures in ocean energy, highlighting the importance of reducing vulnerability in the cybersecurity of power plants in this sector. Through a systematic review of the literature, three main study topics were identified and analysed, providing a valuable resource for future research in this area. The findings of this study can inform and guide the development of more secure and resilient systems, contributing to the overall improvement of critical infrastructure in the field of ocean energy. As such, this article offers a significant contribution to the ongoing effort to address the challenges posed by the changing energy landscape and the need to protect critical infrastructure from cyber threats.

Please refer to uploaded PDF to see references and figures.

On the complementarity of wave, tidal, wind and solar resources in Ireland

2023-06-27T12:12:46+01:00

The use of renewable energy sources has been recognised as a key strategy for combating anthropogenic climate change. These energy sources are regarded as sustainable because they are naturally replenished and do not produce greenhouse gases. A vital step in achieving a low-carbon economy and addressing the global challenge of climate change is the implementation of renewable energy alternatives. This “green revolution” has been led by solar and wind energy. Incorporating new forms of renewable energy resources, such as wave and tidal energy, into the current mix of resources will aid in the transition to a fully 100% renewable energy future due to the abundance of such resources [1]. The complementarity assessment of renewable energy resources is crucial to design the optimal mix of these resources to meet load requirements in a jurisdiction. Multiple studies in the literature discuss the complementarity of renewable energy modalities for different jurisdictions. A review of such studies is presented in [2]. The review shows that most of these studies focus on wind, solar and hydro-power generation, with most focusing on just two of these three modalities. However, recent efforts have been made to assess the temporal complementarity of more than two resources, including marine renewable energy resources (wave and tidal) for US and UK jurisdictions [3], [4], concluding that marine renewable resources may have significant value to a future power system in terms of reduced balancing requirements and valuable capacity contribution. Similarly, Ireland may enjoy similar benefits from combined resource exploitation due to its island topography and good marine resource. Fig. 1, by way of example, presents renewable resource generation profiles (hourly resolution) for four seasonal weeks in 2017 and illustrates the potential benefits of combining marine resources at Inishtrahull Sound, Ireland. For example, wind and tidal are low in the autumn week (highlighted section), while wave and solar resources are available. However, the summer week sees low wind and wave resources, while tidal and solar provide complementary benefits. Another critical aspect of the complementarity studies is the set of metrics and indices used to assess the complementarity. Such metrics and indices are reviewed in [5]. Correlation coefficients are commonly used in the literature to evaluate complementarity between energy sources. However, there are several issues related to the correlation metrics reported in the literature, including the inability to handle nonlinearities in the data and the inability to handle more than two resources [6], to name a few.

In this paper, we present a complementarity assessment of the four renewable resources, i.e. wave, tidal, wind and solar, around the island of Ireland (including Northern Ireland) using the new complementarity indices based on the mathematical concept of the total variation [6], which allow for complementarity assessment of more than two resources. We also comment on the possible benefits of the temporal characteristics of the marine renewable resources on the Irish generation mix.

Please see attached file for Figures and references.

A Comparison of the European Regulatory Framework for the deployment of Offshore Renewable Energy Project

2023-01-27T15:49:10+00:00

The REPower EU Plan has set a minimum of total renewable energy generation capacity of 1,236 GW by 2030. Achieving this target, and emission reductions by 2050, will require the extensive deployment of offshore energy facilities, especially offshore wind (OW) and wave energy converters (WECs). However, an incomplete and sometimes unfavourable regulatory framework still jeopardises the feasibility of both prototypes and large-scale installations. There are, for example, significant differences between the permitting procedures in different Member States and regions. Moreover, following the transposition of the Directive 2014/89/EU “establishing a framework for maritime spatial planning”, important differences pertain to the way environmental and heritage protection is dealt with. An overview of the offshore permitting schemes for offshore wind in ten European counties (Germany, Denmark, Scotland, Sweden, France, Spain, Portugal, Italy, Belgium, and Ireland) is provided, demonstrating a mismatch between the current members’ complex regulations and the future offshore wind targets. Using customised key performance indicators, we describe and assess the extent to which the regulatory frameworks are conducive to installing industrial devices in achieving the country’s 2030 target. Finally, we propose actions to facilitate the installation of OW while ensuring both environmental protection and industry development in the countries under investigation.

Ocean Energy: Markets – Currency – Impact. Dimension of & Choices in the Technology Development Space

2023-02-10T23:04:58+00:00

Reflecting in the acceleration of the changes in ecological, climatological and generally planetary health, it is critical to employ those energy source as well and those energy used that are of the highest ratio of benefit to effort and deliver the most valuable and impactful, tangible contribution in service of natural commons and common societal good. Such considerations hold for all ocean energy types and especially for ocean wave energy.

Thus, it is not only important to consider the resource that is converted into the standard form of usable energy, that is, in the form of electricity and to deliver to the most prominent marketplace, that is, the continental grid; it is equally important to consider the use, purpose and impact after converted energy.

Reflecting on the entire value chain from marine renewable energy to a) usable energy to b) the actual use and purpose of the energy may lead to highly impactful implementations with more direct delivery of the renewable energy to the valued application. In such more direct paths from resource to impact the extracted energy and the applied energy is a mean to the purpose rather than a means to an end.

Assessments of marine renewable energy markets other than powering the continental grid, such as Powering the Blue Economy, have been investigated and numerous research efforts are underway. However, to fully maximize the achievable impact of ocean wave energy it is critical to extend the consideration from replacing the energy source in existing applications to the enablement of highly impactful applications that are currently not existing or are not operated at the magnitude when powered by ocean waves.

Highest effectiveness is achieved when the uniqueness of the renewable energy form matches the unique needs of the targeted application. For ocean wave energy the uniqueness lies in relative consistency, high degree of forecastability, energy density and the ubiquitous nature across the oceans. These provide a plethora of opportunities to serve markets and purposes that are directly in or based on the oceans.

Technology development progress indicators such as Technology Readiness Levels (TRL) and Technology Performance Levels (TPL) are typically used to span up the technology development space and provide a framework and orientation for the desired technology development trajectories. Based on the description of the motivations above the paper and presentation with introduce and describe additional and alternative technology development indicators that directly point and guide the development towards impactful application and purpose as the desired technology development goal.

In order to provide a clear understanding of impact markets and the associated values, thus, their specific currency these are best served in, three impact markets are presented in detail as concrete examples. These support planetary and ocean health in different ways through carbon capture, acceleration of the deployment of all forms of marine renewable energy types and enabling the implementation and enforcement of environmental legislation. Beyond these, a dozen of others impact markets are listed to provide an overview of impact oriented markets and applications.

Informing development of a socioeconomic data collection toolkit for marine energy: a literature review

2023-01-27T22:56:31+00:00

Marine energy projects have the potential to create significant benefits by stimulating economic growth, improving local infrastructure and services, and providing energy security and resilience. Collecting social and economic data is necessary to anticipate potential benefits or adverse impacts, and to develop and appropriately site marine energy projects that suitably address community needs, incorporate and align with community values, and satisfy consenting requirements. Despite the importance of this information, consistent methodology for social and economic data collection to inform marine energy development is lacking. We review the literature from marine energy, other renewable energy industries, and relevant coastal sectors to identify common metrics, methods, and applicable tools for collecting data on social and economic effects. From this, we synthesize our findings and identify lessons learned that will form the foundations of a methods toolkit and template for data collection. This literature review and the eventual development of the toolkit will enable marine energy projects to identify, avoid, and mitigate potential negative effects at the forefront. By sharing findings from the literature and the lessons learned in the process of creating the toolkit, we hope to continue to advance the marine energy industry in a way that promotes energy equity, ensures environmental justice, and centers community values and needs.

Using human-centered design to develop a national research landscape for marine energy in the United States

2023-06-29T03:05:18+01:00

In 2021, the United States Department of Energy (DOE) awarded the Pacific Ocean Energy Trust a grant to act as the coordinator of a foundational research network, ultimately named the University Marine Energy Research Community (UMERC). The community aims to facilitate connection between U.S. university researchers, industry, and government research laboratories to close common gaps in foundational research that are prohibiting the pathway to commercialization. To achieve this goal, UMERC held a series of workshops to create a Research Landscape (Landscape), which identified current challenges, gaps, research capabilities as well as uncovering additional questions about where the sector is headed. A human-centered design (HCD) approach was used throughout the three-workshop series.

HCD is a problem-solving and design technique that uses human perspective and emotion to develop solutions. The stages of human centered design include inspiration, ideation, implementation and validation, or testing, in an iterative, or cyclical process that results in ongoing refinement. HCD is carried out with the acknowledgement that values vary from context to context and are subject to change as people and technologies interact over time (Zachry and Spyridakis).

It is through this approach that we are able to identify the current gaps and challenges and through the HCD approach, we will continue to refine the Landscape as current challenges and gaps are retired and new challenges and gaps arise. This will help account for the fast pace of innovation in the marine energy sector, where human-technology interactions are changing as the technology develops, and there are new entrants into the market. With the current state of fluidity in technology design and application, what works at one location may not work at another location. Using HCD methods and sensibilities, workshop participants, including individuals from universities, private sector companies and the national laboratories, we able to bring in their individual perspectives to develop the Landscape.

Through the HCD process, the workshops revealed a set of values, tools, research interests and gaps and challenges. These were synthesized into what is now the current Landscape that can be found on the UMERC website. The values are themes that should be considered when designing marine energy projects. These include community, innovation and new technologies or applications, education, sustainability, equity, blue economy, and collaboration. The main challenges were condensed into four categories that include creating markets and a trained workforce, management and logistics, understanding and protecting the environment, and marine energy engineering, research and development. The tools are actions that can be carried out to overcome the main challenges. Finally, a list of common research areas was identified under each main challenge area.

Following our HCD methodology, our cycle of iteration will soon start again. While the current Landscape serves as a benchmark, the next steps include a series of industry-academic brainstorming sessions, with the aim of creating collaborative projects to address challenges, as well as come up with a list of common technology agnostic challenges, in hopes to push future research funding.

Floating wind and wave energy technologies: applications, synergies and role in decarbonization in Portugal

2023-07-18T11:57:56+01:00

Floating offshore wind and wave energy are both promising marine renewable energy technologies that are still at an early stage of development. This paper reviews synergies between the two technologies that can be mutually beneficial and facilitate the path to commercial exploitation. The analysis focuses on three different types of combined wave and wind systems: Co-located arrays, hybrid systems and island systems. The decarbonisation potential of co-located systems is discussed. Existing carbon emissions from marine technologies are compared and how synergies can achieve these reductions are explored. A case study is presented to calculate the impact of multi-purpose platforms on the Portuguese energy system. Portugal was chosen because it has an excellent climate for marine resources, deep waters, positive political support for renewable marine energy and previous success in deploying first technology demonstrators for floating wind and wave energy systems.

Marine Renewable Energies and Maritime Spatial Planning: different national proposals for their legal and spatial context

2023-06-07T16:14:20+01:00

In 2022, the European Commission adopted the REPower EU package which proposes to further increase the 2030 target for renewables from 40 % to 45 % and revise the Renewable Energy Directive to accelerate permitting. Moreover, the European Offshore Renewable Energy Strategy estimates to have an installed capacity of at least 60 GW of offshore wind and at least 1 GW of ocean energy by 2030, reaching 300 GW and 40 GW of installed capacity, respectively, by 2050, which would require less than 3% of the European maritime space.

The marine environment is an ecosystem that supports a set of uses and human activities, contributing to economic and social development. Most of the uses and activities require the use of maritime space, either temporarily or permanently, increasing the potential for conflict with existing and traditional marine uses. The European Directive 2014/89/UE establishes a framework for marine spatial planning (MSP) and requires the competent authorities in each Member State to develop national maritime spatial plans, analysing and organizing human activities in marine areas, applying ecosystem-based approach, involving stakeholders, in order to achieve ecological, economic and social objectives. Moreover, this MSP framework not only intends to mitigate possible present and future spatial conflicts but also contemplates the emerging sectors and the most recent technological development in the sea.

In the context of the EU-funded SAFEWave project (https://www.safewave-project.eu/), the different National Maritime Spatial Plans in Europe and how the Marine Renewable Energies (MRE) have been considered or integrated in different national plans have been analysed (i.e. Spain, France, Ireland, Portugal and United Kingdom). The outcome of this analysis has identified: (i) the approaches to implementation of MSP plans vary by country and sometimes within countries; (ii) differences in the stakeholder or public involvement in MSP (ii) differences in national MRE targets; (ii) different strategies and priorities for raising the issue of MRE within the national MSP; (iii) for which countries MREs are strategics in their national plans; (iv) conflicts with other uses (i.e. Marine Protected Areas), and mechanisms to solve them; (vi) technical, socio-economic and environmental factors considered for MRE polygons identification in national MSP; (vii) the development of national MSP requires a review of the authorization or concession procedures for the allocation or reservation of areas for MREs.

Ultimately, comparing the integration of national plans for marine renewable energy (MRE) into national marine spatial plans (MSP) can inform and guide management strategies, legislation, and policies to support management actions and efficiently plan future MRE deployments.

Techno-economic analysis of marine hybrid clusters in two potential Latin American markets

2023-05-31T09:23:43+01:00

Contemporary communities require innovative solutions to cope with projected increases in demand for natural resources. Diversification and modernization of the energy matrix through the affordable and efficient harnessing of marine renewable energies (MRE) are possible means to mitigate the vulnerability of coastal communities and climate change. While the offshore wind sector has reached sufficient maturity to compete in the energy market, other MRE, such as wave energy, are still in the development phase, which limits their financing and commercial deployment. This study aims to evaluate the techno-economic feasibility of maritime hybrid clusters (MHC) powered by wave energy converters (WEC) and offshore wind turbines (OWT) to electrify households and the marine aquaculture sector, where electricity surpluses can be stored in lithium-ion batteries or for green hydrogen production. Different scenarios for WaveDragon (WD) and Pelamis (PEL) WEC farms were studied in two coastal communities, Coquimbo (Chile) and Ensenada (Mexico). The mean annual wave power availability at Coquimbo is high (26.05 kW/m) and of Ensenada, moderate (13.8 kW/m). The wave energy shows less inter- and intra-annual variability in Coquimbo than in Ensenada. The hybridization between WECs and OWT covers the total electricity consumption, where the PEL-OWT system is the cost-effective option in Ensenada and WD-OWT in Coquimbo. Ensenada demonstrated a higher electricity surplus than Coquimbo, profitable result of storing it to sale in the electricity market or for hydrogen production. For both selected WECs, the seaweed aquaculture integration in a blue economy framework generates higher returns than households, higher in Coquimbo than Ensenada.

A time domain approach for the optimal control of wave energy converter arrays

2023-06-16T10:51:08+01:00

Wave energy converters typically use various control methods to extract energy from ocean waves. The objective of the control system is to optimize the energy extraction process, taking into account the dynamics of the system and the wave conditions. The task of deriving the optimal control laws of wave energy converter arrays for regular and irregular waves using the Pontryagin minimum principle was previously investigated in the literature. The result is a combination between the singular arc and bang-bang control laws. For irregular waves, some complexity arises due to the radiation state-space representation, which requires ignoring the hydrodynamic coupling terms related to the added mass and radiation-damping coefficients; this reduces the computational complexity of the control force but adversely affects the solution's accuracy. Also, the derived control laws are specific to a particular wave condition. Recently, the optimal control of a flexible buoy wave energy converter was derived using the convolution representation for the radiation force. In this work, the optimal control laws of flexible buoy wave energy converters are modified to simulate wave energy converter arrays; then, the results are compared to those obtained by dropping the hydrodynamic radiation coupling terms. Although using a convolution representation adds computational complexity to the optimal control problem, it generates an equation that is generic to any wave condition, can be used with any wave spectrum, and provides an expression for the switching condition.

Experimental validation of rollout-based model predictive control for wave energy converters on a two-body, taut-moored point absorber prototype

2023-06-07T12:55:23+01:00

Model predictive control (MPC) has proven its effectiveness in improving the energy capture efficiency of wave energy converters (WECs) under physical constraints. Further application of MPC requires to speed up its online computation for an industrial controller. To this end, the rollout-based MPC (RMPC) for WECs has been developed. The idea of rollout is to decouple the optimization horizon and the prediction horizon, so as to achieve long-horizon performance with short-horizon optimization. In this paper, the RMPC that has previously only been validated by simulation is further put through wave tank testing. The experimental device is a realistic two-body, taut-moored point absorber prototype. The RMPC is based on a simplified model of the device and implemented in real time with wave force estimation and prediction. Experiment results confirm RMPC’s energy efficiency as well as constraint satisfaction, so its computational advantage against conventional MPC is highlighted.

Control co-design and uncertainty analysis of the LUPA’s PTO using WecOptTool

2023-06-14T15:44:09+01:00

Control co-design has been shown to significantly improve the performance of wave energy converters (WEC). By considering the control and WEC design concurrently, the space searched by the optimization routine is greatly expanded which results in better performing devices. Recently, an open-source WEC co-design code, WecOptTool, was released to perform control co-design research and facilitate its adoption in the community. In this study, we use WecOptTool to perform control co-optimization and uncertainty analysis of the LUPA device. The Laboratory Upgrade Point Absorber (LUPA) is a new open-source laboratory-scale WEC that provides a platform for testing new concepts, innovating control schemes, and validating numerical models. The LUPA can be adjusted to different configurations, including changing the number of bodies, the degrees of freedom (DOF), the float and spar geometry, and the diameter of the drive sprocket pulley in the power take off (PTO) system, as well as providing different control algorithms and input waves. The drive sprocket diameter influences the torque vs speed of the generator, which allows for more flexibility in operating under different wave conditions or with different control schemes. In this study we optimize the drive sprocket diameter, while considering the optimal control algorithm for each potential design, to identify the optimal diameter for electric power production at the PacWave South WEC test site. This case study demonstrates several new capabilities of WecOptTool including a multi-body multi-DOF system and multi-directional irregular waves. The PTO dynamics are modeled using first principle methods for a parametrized model of the mechanical subcomponents in combination with generator model obtained using a power-invariant Park transform. The case-study will be made available to serve as a design tool along the LUPA hardware. Users can readily use this model to perform their own design optimization prior to testing with the physical LUPA device. Finally, we use the automatic differentiation capability of WecOptTool to perform a sensitivity and uncertainty analysis of the LUPA device.

Tidal barrage operation optimization using moment-based control

2023-06-28T21:20:19+01:00

As decarbonization of energy systems becomes an imperative and the deployment of intermittent renewables increases, the operation of electrical grids becomes challenging. In this sense, tidal barrage schemes can supply clean and predictable energy with more flexibility than the more traditional wind and solar plants. However, the high infrastructure costs associated with tidal barrage plants hinders its deployment. An optimal operation is key to maximize energy output and assure the project’s feasibility.

Recently, Ringwood and Faedo [3] introduced a novel approach to tidal barrage control by applying wave energy control techniques, with promising results. The model consists on a two-way tidal barrage scheme, where an analogy is made between latching/declutching algorithms applied in WEC control and holding/sluicing from tidal barrage. The optimization was done using the moment-based framework developed in [1] and [2], which can formulate linear and non-linear problems in a concave quadratic environment, thus allowing the application of efficient numerical solution algorithms. This preliminary study has a number of simplifying assumptions, leaving a pathway to further examine the optimal control problem.

This paper extends the results of the analysis from [3] by adding complexities to the model. Crucially, the basin is modeled such as the area is a polynomial function of the water level, instead of assuming constant surface area. This will affect the operational head of the optimal solution, as the volume of water at the top of the basin will be higher than at the bottom. Another added constraint will be the minimum head that the turbine requires to generate electricity. Furthermore, a cost function is introduced to penalize the energy consumption of the sluice gates, as shown in figures 1.a) and 1.b). This not only increases the accuracy of the model, but in many cases improves the convergence of the algorithm by adding a quadratic term. Figure 2 shows the value of the overall energy yield (as a fraction of the energy output without penalty) and simulation run time (as a fraction of the run time without penalty) with different penalty coefficients.

1.a) Sluice gate area without considering cost function

1.b) Sluice gate area with penalty factor of 3

2. Energy and run time variations with penalty

References:
[1] N. Faedo, G. Scarciotti, A. Astolfi, and J. V. Ringwood. Energy-maximising control of wave energy converters using a moment-domain representation. Control Engineering Practice, 81:85–96, 2018.

[2] N. Faedo, G. Scarciotti, A. Astolfi, and J. V. Ringwood. Nonlinear energy-maximizing optimal control of wave energy systems: A moment-based approach. IEEE Transactions on Control Systems Technology, 29(6):2533–2547, 2021.
[3] J. V. Ringwood and N. Faedo. Tidal barrage operational optimisation using wave energy control techniques. IFAC-PapersOnLine, 55(31):148–153, 2022. 14th IFAC Conference on Control Applications in Marine Systems, Robotics, and Vehicles CAMS 2022.

Laboratory Tests Assessment of a Mechanical Sensor-less MPPT Control Strategy for Tidal Turbines

2023-06-16T11:06:24+01:00

The purpose of this study is to demonstrate through towing tank experiments the effectiveness of a novel sensor-less Maximum Power Point Tracking (MPPT) control for Tidal Stream Turbines (TST) under fluctuations of the onset flow.

Ocean energies will play a crucial role in the renewable energy sector over the next decades. Instream turbines for tidal currents are a rapidly maturing technology to exploit a highly predictable energy source. However, short-term fluctuations on the inflow velocity caused by waves or turbulence determine fatigue loads that affect system reliability. Power control strategies to maximize the energy yield can be also used to mitigate the effects of transient loading on drivetrain components.

Aim of this paper is to present a straightforward and robust MPPT control method based on the linear relationship between the current and voltage squared of the generator's DC outputs. The method requires pre-determined turbine characteristics to establish the control reference that is effective across operating conditions. The proposed MPPT model was derived mathematically through linearization and simplifications of the turbine power conversion system and validated by model tests carried out in the wave-towing tank facility of CNR-INM in Rome, Italy, using the 1.5 m diameter Tidal Turbine Testing (TTT) device developed at the Queen’s University Belfast (QUB).

In the study, a conventional TSR control method was also considered in order to perform a comparative analysis of system response to inflow speed fluctuations with time scales comparable to turbine revolution periods. TSR control was tested using two control references: TSR = 5 (design point) and TSR = 6 (over-speed zone) to verify the operation of the turbine under different loading conditions. The tests were conducted in two scenarios: calm water (steady state) and unsteady inflow with a regular (sinusoidal or monochromatic) waveform, with amplitude chosen to simulate an extreme wave case.

The power output was measured from the turbine during regular wave conditions and compared to results from steady flow to assess the impact of wave-induced velocity on turbine performance (Fig. 1). Test results showed that by using the proposed MPPT control strategy, the algorithms converged to the maximum power coefficient (Fig. 2), which validates the proposed methodology. Results also demonstrated the capability of the proposed MPPT to significantly reduce mechanical loads fluctuations as compared to the TSR control.

In the full-length paper, the proposed MPPT control strategy is outlined, the test methodology, set-up and conditions are described, and main results are presented and discussed.

Design considerations for a hybrid wind-wave platform under energy-maximising control

2023-06-07T12:48:02+01:00

The environmental impact of emissions of fossil fuels and their rising prices, together with countries' commitment to mitigate the effect of the alarming and rapid climate changes, have been a crucial thrust to investigate new solutions to have an energy supply depending on renewable energies.
In this scenario, offshore wind-wave hybrid platforms have been recently promoted: sharing facilities, infrastructure, and grid connections, give these systems the potential to increase energy production at a lower cost. However, an efficient realisation of these two combined technologies requires two potentially conflicting control objectives: On the one hand, for the wind turbine, a reduced movement of the platform is required, which essentially translates to enhanced stability of the structure, so that its behaviour resembles standard onshore wind technologies. On the other hand, to maximise the energy produced, wave energy converters (WECs) require optimal control technology, which often leads to large amplitude motion, potentially conflicting with the stability requirement for the wind turbine.

The aim of this study is to investigate the effects of design changes on the dynamics of the hybrid wind-wave platform under energy-maximising control, which can be analysed in terms of the principle of impedance-matching. A semi-submersible platform with an incorporated flap-type WEC will be analysed both from a closed-loop and open-loop perspective, and the control system will be designed to maximise the energy produced by the WEC. Design changes on the wind-wave conversion platform will be in terms of flap dimensions, starting from a nominal geometry based on the so-called Oyster system. Analyses will be conducted by both increasing and decreasing the flap depth from the nominal case, to investigate the effect of the different geometries on the interactions between the WEC and the platform. A frequency-domain analysis of the overall input/output (velocity) system will be presented, highlighting the situations that can enhance the potential of both devices and exploit their synergies.

Wave Excitation Force Estimation for a Multi-DoF WEC via a Cubature Kalman Filter: Improved Design and Results

2023-06-28T21:30:23+01:00

In this paper, we develop a multiple degree-of-freedom (multi-DoF) wave excitation force estimator based on the square-root cubature Kalman filter (SCKF) [1] combined with an online empirical covariance matching technique for a CETO-like, submerged wave energy converter (WEC) device with three PTOs.

Though wave excitation force estimation is required by many advanced WEC control approaches, the literature is still quite scarce for multi-DoF devices. Indeed, there are several difficulties in designing a multi-DoF excitation force estimator, especially when the full motion of the WEC is considered. The high dimension of the underlying dynamic system leads to difficulties in on-line implementation and in estimator parameter-tuning. Moreover, nonlinearities may significantly deteriorate estimation performance if a linearized model is used by the estimator to reduce computation time. Papers [2-4] present solutions to multi-DoF wave excitation force estimation, ranging from (extended) Kalman filters to feedforward neural networks, but only up to three DoFs, the surge, heave and pitch directions. In [5], results for a CETO-like WEC are presented for all the six DoFs, based on a CKF wave estimator, which uses a linear state-space model. Undesirable large estimation errors on the yaw direction were observed for several sea states, mainly caused by the model errors introduced by linearizing the kinematics linking buoy motion to PTOs motions. Moreover, even though the CKF is known to be relatively easy to calibrate, guessing covariance matrices of the system and measurement model noises remains troublesome.

In this paper, we propose to use the SCKF, together with an empirical online covariance matching to over overcome the difficulty of parameter tuning. In our estimator, a nonlinear dynamic system of dimension 72 is used to predict the system dynamics at the cubature points, and the 6-dimensional excitation force is estimated. In addition, we show that a careful system state scaling can substantially reduce estimation errors on the yaw direction, even when using the linearized dynamic system. Moreover, thanks to the moderate computation complexity of the SCKF, and its parallelizability, online implementation can be achieved by taking slightly larger sampling time step, at the price of a small degradation in performance.

REFERENCES

[1] I. Arasaratnam and S. Haykin. “Cubature kalman filters”. In: IEEE Transactions on automatic control 54.6 (2009).

[2] Ling, B. A., 2015. “Real-time estimation and prediction of wave excitation forces for wave energy control applications”. Master’s thesis, Oregon State University.

[3] Hillis, A., Yardley, J., Plummer, A., and Brask, A., 2020. “Model predictive control of a multi-degree-of-freedom wave energy converter with model mismatch and prediction errors”. Ocean Engineering, 212, p. 107724.

[4] Bonfanti, M., Hillis, A., Sirigu, S. A., Dafnakis, P., Bracco, G., Mattiazzo, G., and Plummer, A., 2020. “Real-time wave excitation forces estimation: An application on the ISWEC device”. Journal of Marine Science and Engineering, 8(10).

[5] Nguyen, H.-N., Tona, P., Mérigaud, A., Cocho, M., Pichard, A. “Wave Excitation Force Estimation for a Multi-DoF WEC via a Cubature Kalman Filter: Design and Preliminary Results”, In Proc. of OMAE 2021, virtual, online.

A comparison of AC and DC collection grids for marine current energy

2023-06-23T17:46:42+01:00

Important questions to enable the use of marine current energy are how the electrical system is designed, how multiple energy converters are interconnected offshore and how the power is transmitted to the shore. The Division of Electricity at Uppsala University have constructed and deployed a marine current energy converter in the river Daläven in Söderfors, Sweden. In the study presented in this article, a model of a near-shore low-voltage AC collection grid and a near-shore low-voltage DC collection grid is presented for the technology at the Söderfors test site. The models are implemented in MATLAB/Simulink. For collection grids of five turbines, it is shown that the proposed control schemes are able to deliver power to the distribution grid. The controllers are able to achieve this even when one turbine is suddenly disconnected from the grid. Furthermore, it is shown that the conduction losses of the DC system are higher than the losses of the AC system for nominal and high water speeds. However, in a qualitative comparison between the systems it is concluded that despite the higher losses, the DC system can be an interesting option. This is because fewer components need to be placed in the turbine, which is beneficial in offshore systems where space is a limiting factor. Furthermore, a DC system can be less expensive since fewer cables are needed.

Power quality assessment of a wave energy converter using energy storage

2023-06-23T10:29:29+01:00

Wave energy has been an immense area of interest in research and industry in our move towards a sustainable energy production society due to its high energy density and surface area. However, the grid connection of wave energy converters is still one of the major challenges due to the complexity of varying wave resources (amplitude and frequency). Wave energy converters grid integration can lead to several potential challenges, such as voltage fluctuations, harmonics and flicker. Using an energy storage system can help to mitigate few challenges by balancing the grid demand with the wave energy converter power supply. Hence, improving the power quality. This study assesses the power quality of wave energy converters equipped with energy storage against the scenario without any energy storage at different power levels. The power quality in this paper is investigated using total harmonic distortion (THD) of the grid current, dc-link voltage ripple and battery current ripple. The study shows that the addition of a hybrid energy storage system lowers the grid current THD at the point of common coupling (PCC), stabilizes the dc-link voltage ripple and reduces the stress of the battery.

Dimensioning and optimization of multi-source offshore renewable energy parks

2023-06-07T12:29:58+01:00

Multi-source offshore renewable energy parks, consisting of wind, solar and wave converters, can utilize the available offshore area much more efficiently than single source wind parks. They can also produce power with a higher capacity factor than single source parks, utilizing the available grid connection more efficiently. Dimensioning and financial optimization of such parks will be driven by geographic constraints like available area, water depths, shipping lanes, exclusion zones etc., but also by the available resources of wind, solar and wave power. Another major driver in the optimization will be the electricity price. With the raising share of weather dependent renewables in the electricity mix, the electricity price will become more volatile. Therefore, an optimized design process of a multi-source park should also incorporate a pricing mechanism that can produce hourly electricity prices based on actual weather conditions. The paper will present the results of a numerical model that can integrate solar and wave power in a wind park area as well as optimize the export cable capacity. Battery storage can be added to this multi-source park to shift part of the production to hours with higher electricity prices. A case study has been performed for the planned offshore wind park TNW, North of the Dutch Waddeneilanden. Since such a multi-source park will have an expected lifetime of about 30 years, it will even reach the year 2050, in which weather dependent renewable energy will be much more dominant than today, with far stronger electricity price volatility. For the electricity price calculation, assumptions are made for the installed base of solar, wind and wave power in the whole of The Netherlands, as well as the geographic spread of this installed base over the land area and the offshore exclusive economic zone in the North Sea. This installed base is simplified by concentrating it in about ten locations divided over this area. Other necessary assumptions are the future electricity demand pattern, the future capacity of the interconnections with the surrounding countries and the capacity of the flexible load that will be available at that time (electric cars, electric heaters etc.). From literature, the cost of conventional power fueled by natural gas and/or hydrogen is derived which serves as back up power for hours with low wind, solar and wave power production. Based on all these assumptions, a pricing curve is constructed reaching from sub zero at abundant renewable supply to a maximum value at zero renewable supply. With the model, scenarios of future developments in installed wind, solar and wave power, but also in e.g. electric cars, electric heaters and other flexible loads, can be examined and the sensitivity of the optimization of the multi-source park design can be determined. The relevance of wave power can be determined from the average price per MWh that wave power can earn compared to wind and solar.

A Novel proposal of PTO direct-drive linear generator, an Azimuthal Multi-translator Switched Reluctance Machine (AMSRM): mechanical, characterization and performance tests.

2023-06-14T15:42:36+01:00

Research and development on power take-off (PTO) in wave energy has been gaining relevance in recent years, appearing as a priority in various forums (JCR reports [1], SRIA in wave energy development [2], specific research funding program in WES, etc.).

The new proposal of a linear electric generator to act as a PTO in wave energy extraction applications has been developed in the European project SEATITAN [3].

The linear generator is based on a switched reluctance machine type topology in which the magnetic flux is azimuthal. This allows eliminating the yoke, reducing the amount of magnetized material needed and increasing the force density, besides having a cylindrical shape that allows adapting perfectly to the spar of point absorber type WECs.

The Azimuthal Multi-translator Switched Reluctance Machine (AMSRM) has been developed, designed and a prototype has been manufactured in the project. The prototype is composed of two modules of 35 kN of force and 3 m/s of peak velocity.

The complete manuscript will detail the testing facility, as well as the tests performed for the static characterization of the force, electrical losses, and air gap achieved during manufacturing, as well as the dynamic characterization of frictional losses and a preliminary setting of force control parameters.

The dry test facility (figure 1) consists of the necessary electronic converters to conveniently operate the generator and impose a reference force controlling current per generator coil. In addition, an end-stopper is provided to ensure the safety of the tests (this equipment is essential for dynamic tests). For static tests, a load cell for force measurement as well as precision electrical power meters are used.

A test protocol has been defined for each test, as well as an analysis procedure to obtain the required results: for example, for the evaluation of friction, dynamic tests based on a back-and-forth motion with drifting sections are proposed to evaluate the braking force with the generator off at various speeds.

The description of the test protocol, the raw data obtained, the data analysis carried out, and de results obtained will be presented and detailed in the manuscript, as well as the description of the test-based parameterization of a simplified AMSRM model to be used in the dynamic modeling of WECs. The simplified model has been previously developed and presented in EWTEC 2021.

[1] D. Magagna, R. Monfardini, and A. Uihlein, “JRC Ocean Energy Status Report 2016 Edition,” 2016

[2] J. L. Villate et al., “Strategic Research and Innovation Agenda for Ocean Energy,” Etipocean, no. 826033, p. 64, 2020.

[3] WEDGE GLOBAL, “SeaTitan,” 2018. [Online]. Available: https://seatitan.eu/. [Accessed: 29-Nov-2021].

[4] Marcos Blanco, Miguel Santos‐Herran, Gustavo Navarro, Jorge Jesus Torres, Jorge Najera, and Luis García-Tabarés, “Simplified model of a novel direct-drive PTO based on an azimuthal linear switched reluctance generator,” 2021.

Observer-Based Fault Estimation Applied to a Point Absorber Wave Energy Converter

2023-06-13T12:08:31+01:00

The economic viability of wave energy conversion systems is one of the open points among the research community. To lower the energy cost, the devices in charge of extracting the wave power, called wave energy converters (WEC), are often controlled by means of optimal control (OC) strategies. Such OC systems, which have proven their effectiveness in wave energy applications, often rely on a mathematical model of the device (and, in some cases, on the wave excitation force) to optimize the control action provided to the system. Nevertheless, the marine environment results hostile for general device safe operations, potentially triggering a variety of faults in the WEC system. Such condition (for example, a sensor failure or additional friction inside a gearing) directly affects the system dynamics. If this deviation is not considered by the control algorithm, the energy production performance can degrade considerably, or the control action itself can cause a more serious fault. A possible solution is that of designing an algorithm capable of compensating for eventual faults in the system, while still respecting the initial design performance or, when not possible, preserving the main device functionalities. Such a control strategies belong to the family of Fault Tolerant Control (FTC) techniques, which can be divided into two macro-categories: Passive (PFTC) and active (AFTC) algorithms. While PFTC systems are designed offline and can account only for a predefined set of system faults, AFTC algorithms are more suitable to tackle significant system deviations from the nominal model. For this purpose, such algorithms may require some routine to detect, isolate and eventually estimate the specific fault. This task is accomplished by Fault Detection and Identification (FDI) routines. According to the AFTC algorithm, the FDI module must accomplish different tasks. Furthermore, the FDI module accuracy plays a crucial role in some AFTC strategies, since the poor estimation of a faulty signal can induce the controller to behave incorrectly.
This paper presents an FDI algorithm applied to a point-absorber wave energy converter (WEC). The proposed structure consists of an observer-based strategy in charge of detecting, isolating, and tracking effectively faulty signals occurring in numerical simulations. The results demonstrate the proposed observer effectiveness for a predefined set of actuator and sensor faults, both in the case of independent, and simultaneous fault occurrence.

Control of multiple PTOs in single OWC air chambers

2023-06-29T07:54:51+01:00

The paper presents the study of multi-PTO interaction in single-air chamber oscillating-water-column (OWC) wave energy converters (WECs). This strategy seeks: i) tunning system's performance to sea states, ii) increasing reliability through modular designs that are easier to install and replace, and iii) increasing conversion efficiency through better matching between available power and electric conversion equipment. The strategy was assessed through a novel non-linear time-domain model for OWC WECs implemented in the multi-physics object-oriented language Modelica. The model considers multiple degrees of freedom associated with the various PTOs. Different case studies are presented to show the proposed approach's versatility, some of the constraints expected in real implementations, and potential pathways to overcome them. The wave-to-wire model considers air compressibility and non-linear power take-off systems, which is fundamental to assessing the damping level variation required for each sea state considered. The cases presented are for a fixed-structure coaxial-duct OWC WEC, but the results are generalisable for floating structures. Results show a significant increase in mean annual power, and control strategies are proposed for power maximisation. Furthermore, some critical points are shown in the operational conditions' envelop for the system under a selected Portuguese wave climate. The identification of these critical values is important for the control of the OWC WECs. This study represents an advance in the control strategies considering multiple PTOs for a single OWC air chamber to foster innovation actions.

Optimisation of Air turbines for OWC Wave Energy Converters: Sensitivity of Realistic Wave Climates

2023-07-12T15:05:44+01:00

Wave Energy, Oscillating Water Column, Air turbines, Optimisation, Genetic Algorithms, Wave climates.

Among all the Wave Energy Converter (WEC) technologies suggested in the last decades, the Oscillating Water Column (OWC) technology seems to be the most robust and reliable technology. Different are currently in operation, such as the Mutriku Wave Power Plant installed in a harbour, or are being developed, such as the MARMOK floating OWC device developed by IDOM and tested for over a year in the Biscay Marine Energy Platform (BIMEP). One of the key elements of the OWC technologies is the power take-off (PTO) system that converts the pneumatic energy trapped in the chamber into electrical energy. Such PTO system consists of an air turbine coupled to an electric generator, and has been the object of several studies, including numerical and experimental works that cover a wide range of different air turbine configurations, and some of the proposed research lines even reaching to combine both approaches. The most common turbine, mainly due to its relative simplicity both on the conceptual and
mechanical aspects, is the Wells monoplane turbine, including variations such as the biplane and the counter-rotating configurations. However, other configurations such as the impulse turbine or the more recent bi-radial turbine have also been analysed.

The preliminary design of these turbines usually relies on analytical models based on the blade element method, using dimensionless parameters for representing the behavioural charts of the different configurations. In fact, in order to better represent the behaviour of air turbines in realistic conditions with polychromatic waves, it is usual to consider the stochastic version of these dimensionless parameters so that they provide an overall indicator of their sea-state-related behaviour. However, the air turbines, regardless of their configuration, include a large number of different geometrical parameters, which complicates the optimisation procedure and leads to a decision-making process that relies on an expertise-based intuition. In this sense, suggests an optimisation method based on a Genetic Algorithm (GA) that enables the articulation of all the relevant parameters. This GA-based optimisation method articulates the information about the hydrodynamic behaviour of the WEC and the pneumatic conversion within the chamber. Hence, the optimisation is sensitive to the characteristics of the wave climate and, thus, the behaviour of the WEC in that specific wave climate.

However, in order to make wave energy economically viable, mass production of the WECs, including their PTO systems, is a crucial point. As a consequence, standard WEC floaters and PTO system elements may need to be used in the different locations under different resource conditions. In order to evaluate the sensitivity of the optimal air turbine designs to the characteristics of specific wave climates, the present study will define optimal air turbines for different locations worldwide, comparing the characteristics of the different designs.

Integrated hydrodynamic-electrical hardware model for wave energy conversion with M4 ocean demonstrator

2023-07-03T19:34:27+01:00

Wave energy is well known to be a renewable energy resource with worldwide capacity similar to wind. However there is to date negligible generation of electricity from wave. Many devices have been proposed without convergence on a particular design as there has been for wind. We are here concerned with a multi-float attenuator type M4 which has been widely tested in wave basins and modelled by linear diffraction/radiation methods. Potential of MW capacity for grid supply has been demonstrated at many sites. To advance development, small scale ocean tests are being planned for Albany, Western Australia where summer wind-wave conditions in King George Sound will excite the device giving principal absorption with mean periods in the range 2 - 3.5 seconds (or peak periods of 2.5 – 4.5 s). The aim is to learn about most aspects of ocean deployment from wave climate and environment planning to realistic electricity generation, albeit at kW scale. In this paper the emphasis is on the specification of electrical drive train (power take off) which requires the input of torque time variation for the wave conditions on the site, as described by a scatter diagram. First a linear time domain wave multi-float model (Fortran) is set up for the particular 121 configuration, shown in Fig. 1. Such models have been used and validated against wave basin tests for similar configurations. This is then converted into state-space form in Matlab. This is highly efficient and suited for real time PTO control in Simulink. Fig. 2 shows the main components of the electrical drive train, including the gearbox, generator, super-capacitors, power electronic converters and resistor bank to dissipate electricity. Bespoke Matlab models will be run for the wave conditions in the scatter diagram to check that components are suitably rated for normal sea-states, and are safely protected through electrical power-limiting control in high sea states. Simulated electrical generator results will be shown for typical sea states, with some power-limiting. Instrumentation will be specified. Only uni-directional waves are considered in this paper. Ultimately the efficacy of the system will be demonstrated in ocean conditions.

On data-based control-oriented modelling applications in wave energy systems

2023-06-22T06:49:44+01:00

The development of effective energy-maximising control strategies has a crucial role in the empowerment of wave energy technology, and in its improvement towards economic viability. Within the state-of-the-art, most of the strategies adopted to maximise the absorbed energy exploit a model of the wave energy converter (WEC) to be controlled, i.e. they are model-based. These models attempt to replicate the WEC dynamics with a sufficient degree of fidelity, trying, at the same time, to minimise their associated computational burden. However, due to the presence of the hydrodynamic effects , which inherently characterise wave energy systems, simultaneously achieving high-fidelity and computational efficiency is not trivial. Oversimplification of the problem through, for example, linearity assumptions, could lead to non-representative models and/or large uncertainty levels. To overcome these issues, in the last decade, several approaches based on data have been proposed in the wave energy field. These approaches, falling under the umbrella of system identification techniques, exploit data coming from experimental tests or high fidelity simulations, and build control -oriented models with a pre-defined level of complexity. In this paper, we analyse the different strategies that have been adopted in the literature to build data-based control-oriented models for WECs, highlighting the characteristics of each approach, together with their opportunities and inherent drawbacks. An analysis of eventual “partial” data-based modelling of WEC subsystems (e.g. moorings, PTO, or hydrodynamics only) is also reported. Moreover, considerations on the choice of inputs and outputs depending on the WEC type are reported, in an attempt to highlight the different issues that characterise the system identification problem depending on the WEC technology. Finally, conclusions are drawn regarding the capabilities that this type of approach has in (at least partially) solving the modelling issues that affect WEC control system design, and the pitfalls that pure adoption of these strategies has when applied on larger scales, or in the operational stage.

The Performance evaluation of 30kW class OWC wave power plant integrated with breakwater

2023-06-12T08:26:53+01:00

In 2016, Korea started to develop a 30kW class wave power plant connected to a breakwater. After designing, manufacturing and performance testing of each energy conversion device, a demonstration plant was installed in Mukri Port, Jeju Island, Korea in 2021. After passing the completion inspection of the power generation facility, a full-scale grid-connected trial operation began in October 2021. The power plant consists of a sloped Oscillating Water Column, impulse air turbine, permanent magnet synchronous generator, AC-DC converter, energy storage system and integrated control system.

This study introduces the performance evaluation results based on real sea operation data. The performance evaluation of the wave power plant under various wave height and period conditions was performed to evaluate the output power and efficiency of each bin. In addition, performance evaluations were conducted according to wave direction and tidal conditions to examine the effects. The correlation coefficient was derived by analyzing the correlation between wave height, period, wave directions, tide level and output power.

Investigation on the extreme peak mooring force distribution of a point absorber wave energy converter with and without a survivability control system

2023-06-13T12:04:17+01:00

To determine the optimal design of the wave energy converter (WEC) that can withstand extreme wave
conditions, the short- and long-term extreme responses of the system need to be determined. This paper focuses on the extreme peak force distribution of the mooring force for a 1:30 scaled point absorber WEC. The basis of this analysis is the mooring force response obtained from a WEC-Sim model calibrated by wave tank experimental data. The extreme sea states have been chosen from a
50-year environmental contour. Here, first, the long-term extreme response using the full sea state approach is obtained for three constant damping cases of the power take-off (PTO) system. Then, using a contour approach, the expected value of the extreme peak line (mooring) force distribution is computed for the sea states along an environmental contour. Further, for the most extreme
sea state, the extreme peak line force distribution is also computed where a survivability control system, based on a deep neural network (DNN), changes the PTO damping to minimize the peak mooring force in each zero up-crossing episode of surface elevation. The results show that in the absence of a control system, the zero PTO damping case is a conservative choice in the analysis of the long-term response and the design load. For the most extreme sea state along the environmental contour, the survivability control system slightly reduces the expected value of the extreme peak force distribution when compared with lower constant PTO damping configurations.

Wave Farms Integration in a 100% renewable isolated small power system -frequency stability and grid compliance analysis.

2023-06-28T21:22:24+01:00

In general terms, the variable penetration of RE in power systems has some inherent drawbacks, such as lack of manageability and resource variability [1]. Medium (in the range of minutes) and short term (in the range of seconds) variability has a negative impact on system reliability, causing a deterioration of system frequency quality in both interconnected and, moreover, isolated systems [1-2]. Specifically, the variability of the wave energy resource is medium- and short-term. Therefore, although wave energy could be very suitable to be integrated in islands due to its location, the variable nature of wave energy could negatively impact the stability of the power grid [3].

The case study of the work focuses on the island of El Hierro (Canary Islands, Spain). It is an isolated electrical system with a very high penetration of renewable energy sources. The generation of the electrical system is composed of a wind farm, a pumped hydroelectric power plant and conventional generation by means of a diesel power plant.

In a previous analysis [4], the integration of energy storage systems based on flywheels was analyzed. Based on this previous analysis, the manuscript studies the influence of the integration of the wave energy park in the electrical system of El Hierro.

On the one hand, a wave farm will be proposed to evaluate the generated power and its associated oscillation [5]. The wave energy resource at different locations along the coast of El Hierro will be taken into account. On the other hand, an aggregated inertial dynamic mode of the electrical power system will be used to evaluate the impact of the generated power on the electrical frequency and the aging/degradation effects of the hydropumping elements. The Spanish Grid Code will be taken into account regarding frequency regulation mechanisms in isolated systems.

The degradation of the hydraulic pumping systems due to additional frequency regulation stresses and electrical frequency deterioration will be calculated and evaluated in relation to the penetration of wave energy into the system, with and without the flywheel energy storage plant. This will allow quantification of certain technical limits to wave energy penetration in isolated systems and to draw conclusions with reference to the size of such a power system.

[1] R. S. Kaneshiro et al. “Hawaii Island (Big Island) Wind Impacts” Proc. of Workshop on Active Power Control from Wind Power, Broomfield, CO, USA, 2013.

[2] H. R. Iswadi et al. “Irish power system primary frequency response metrics during different system non synchronous penetration,” IEEE Eindhoven PowerTech 2015, doi: 10.1109/PTC.2015.7232425.

[3] Isabel Villalba et al. “Wave farms grid code compliance in isolated small power systems,” IET Renewable Power Generation, 2019, doi: 10.1049/iet-rpg.2018.5351.

[4] Hilel Garcia-Pereira et al. “Comparison and Influence of Flywheels Energy Storage System Control Schemes in the Frequency Regulation of Isolated Power Systems,” IEEE Access, 2022, doi: 10.1109/ACCESS.2022.3163708.

[5] M. Blanco et al. “Study of the impact of wave energy generation in the frequency of an island electric grid,” Proc. of the 12th European Wave and Tidal Energy Conference (EWTEC). 2017.

Experimental passive and reactive control of a Laboratory Scale WEC Point Absorber

2023-06-14T15:46:13+01:00

Scaled testing is an important and valuable step in the process of determining best practices of WEC development, validating numerical models of WEC systems, and/or preparing for larger scale testing. Validation and verification of numerical models are very important as there are physical phenomena that are hard to numerically model such as non-linear frictional effects. This paper builds on the 2021 EWTEC paper [1] in performing and evaluating experimental testing of the Laboratory Upgrade Point Absorber (LUPA). Since this last paper, LUPA has been fabricated and deployed and an initial characterization has been performed. Particularly passive (damping), and reactive (damping and stiffness) control methodologies are employed in regular waves to characterize and evaluate the mechanical power extracted from the waves. Reactive control allows us to invest energy
in the system to get a greater average energy out as compared to damping control.

The LUPA project has just finished its first deployment in the Large Wave Flume at the O.H. Hinsdale Wave Research Laboratory at Oregon State University as shown in Figure 1. Three modes of operation were tested, namely one body heave only, two body heave only, and six degrees of freedom. The one body heave only mode restricts the motion to linear and vertical and fixes the spar body so that the float is the only body free to move. The two body heave only maintains the vertical linear motion, but unlocks the float such that the float and spar are free to heave. The six degrees of freedom case is a floating moored mode with no restrictions on motion.

This paper will focus on comparing passive and reactive control for the one body heave only case and the six degrees
of freedom case with preliminary results shown in Figure 2. Regular waves were tested, focusing on a single wave height
and sweeping the wave period. Results will be presented in power (W) units and in capture width (m). Fig. 2. Top shows max power output vs input wave period for one body and two body configurations and just damping and damping and stiffness cases.
Take note of differing input wave height. Bottom shows capture width.

The LUPA project will provide a valuable testing platform for students and researchers. It will also provide a publicly available open source design and dataset for the research community here: https://github.com/PMECOSU/LUPA. This paper will serve as documentation of the initial testing of the system, providing baseline control results to be compared in future testing.

Wave-to-Wire Control of an Oscillating Water Column Wave Energy System Equipped with a Wells Turbine

2023-06-13T11:50:05+01:00

Wave energy is a significant and relatively untapped source of renewable energy [1], which can considerably contribute to decarbonization. The oscillating-water-column (OWC) [2] is one of the most promising wave energy converters (WECs) for harnessing wave power, especially due to its relatively simple operating principle and the fact that all the moving parts are above the water level.

To improve the commercial viability of WECs, the levelised cost of energy should be minimised and, to this end, comprehensive control strategies to maximise electric energy are essential [3]. Due to the important issue of turbine efficiency, the vast majority of OWC control strategies [4] focus on a simplified control objective, namely turbine efficiency maximisation. While maximising turbine efficiency is a primary focus of OWC control strategies, it is important to note that rotational speed control impacts generator performance. Additionally, for Wells turbines [5], rotational speed control also affects the hydrodynamic performance, specifically the wave-to-pneumatic energy conversion process. Therefore, Wells turbine rotational speed should be ideally modulated to improve the overall wave-to-wire (W2W) efficiency of the OWC system [6], rather than just turbine efficiency.

In this paper, a control strategy for maximising W2W efficiency of a fixed OWC WEC equipped with a Wells turbine is proposed. A schematic of the W2W power train of the OWC WEC considered in this paper is depicted in Figure 1. The proposed control strategy comprises of two parts. Firstly, a `complete' setpoint, which considers the entire OWC W2W model (WEC hydrodynamics, Wells turbine, and generator dynamics), is derived. Secondly, a Lyapunov-based nonlinear controller is designed to track the aforementioned setpoint. Preliminary results from the numerical simulation show that, in comparison to the somewhat traditional turbine efficiency maximising control approach, the proposed W2W control strategy leads to a significant improvement in the electric energy production for all the considered sea states.

Maximizing Wave Energy Converter Power Extraction by Utilizing a Variable Negative Stiffness Magnetic Spring

2023-06-14T15:30:13+01:00

Complex conjugate impedance matching is a key concept for wave energy converter design. Matching the impedance of the power take-off (PTO) system to the complex conjugate of the wave energy converter's (WEC) impedance ensures efficient transfer of energy from the WEC body motion to electrical power. In low frequency waves, impedance matching often requires a negative PTO stiffness. In this paper, an adjustable stiffness magnetic torsion spring will be presented and modeled to understand its potential to improve WEC performance. The spring has the ability to provide a negative stiffness, allowing the PTO impedance to more closely match the complex conjugate of the WEC impedance at low frequencies. The spring also supports an adjustable stiffness value, meaning it can be tuned according to the incoming wave conditions. The spring's tunability may put less stress on the rest of the PTO system in wave conditions outside its normal operation zone without sacrificing electrical power output. The adjustable magnetic spring's effects are modeled and explored in this paper by examining the resultant average annual electrical power and capacity factor. The study suggests that the tunable magnetic spring has the potential to significantly improve capacity factor while maintaining a high average electrical power.

Development of control strategies for novel systems of a full scale OWC for the WEDUSEA project

2023-06-14T06:51:35+01:00

The WEDUSEA project is a €19.6 million European wide joint venture between 14 partners spanning industry and academia from Ireland, the UK, France, Germany, and Spain that will culminate with a two-year, grid connected deployment of a 1MW rated oscillating water column (OWC) wave energy converter (WEC) at the EMEC test site in Orkney, Scotland, UK.

The WEDUSEA project will incorporate a number of novel systems and control strategies to improve device performance, annual power production, and grid integration. During the initial planning and design phases of the project, a wave-to-wire numerical model has been created to investigate the impact these new systems will have on device performance and allow for the testing and development of the control strategies necessary to operate the 1MW power take-off (PTO) system as efficiently as possible. This paper will detail the novel systems added to the OWC, the control strategies developed for the new additions, and the modelled performance of the OWC.

The WEDUSEA OWC will rely on a Wells turbine for the pneumatic-to-mechanical conversion. As Wells turbines are susceptible to aerodynamic stall, a flow control system using 4 bypass valves of varying size will be added, and it will allow for up to 16 different variations for flow control. The performance of the valves will be tested via CFD and MATLAB-Simulink modelling.

The nature of WECs result in fluctuations in the power delivered to the grid, and this can lead to undesirable impacts on the grid. To mitigate these power fluctuations, the WEDUSEA OWC electrical system will include a controllable super capacitor energy storage system that will be used to improve the power quality of the electricity delivered. The objective will be to absorb power peaks and use that stored energy to minimise energy troughs. The power output is aimed to be levelized by mitigating fluctuations in a certain frequency range. The ability of the super capacitor to improve power delivery to the grid may also facilitate improved performance and increased annual power production by the mechanical PTO by enhancing the turbine-generator control system flexibility.

To incorporate the bypass valves and the super capacitor into the WEDUSEA OWC, more complex algorithms will need to be created for control of the mechanical and electrical components of the PTO. These algorithms will be tested and sharpened through testing using the wave-to-wire model developed using MATLAB Simulink and Simscape. The MATLAB-Simulink software simulations will allow for in-depth analysis of the impact the bypass valves will have on the behaviour of the pneumatic-to-mechanical section of the PTO. The Simscape Electrical software will allow for thorough modelling and evaluation of the electrical system and the impact the control strategies will have on the output power quality. The full model will allow for estimations of annual power production, and with improved kWh/yr, the LCOE could see significant reductions. The model testing and control development performed during this early stage of the project will help to maximise the deployment phase of the project and estimate annual power production.

Grid value of co-located offshore renewable energy

2023-06-16T11:34:37+01:00

The paper investigates the added grid value of co-locating wave energy converters with offshore
wind turbine generators and floating photovoltaic solar panels. Grid value is evaluated in terms of
power generation variability and electrical infrastructure utilization. Using meteorological reanalysis
data for the region of the North Sea, wave, solar and wind energy converters are modelled on an
hourly basis to form the power profile of the joint operation. Necessary power ratings of the
transmission system are provided both for stand-alone installations and the hybrid parks and is
shown to be of higher utilization for the latter case. Suitable locations of hybrid parks are also
analyzed and provided in terms of water depth, solar irradiation and favorable wave and wind
climate. The joint operation is found to smoothen the power profile on all time scales but influences
long term variations most heavily. Added grid value strongly depends on the park configuration as
well as metocean conditions. It is shown that by co-locating wave, solar and wind energy converters
the utilization of the electrical transmission system is increased compared to stand-alone
installations.

Enhancing energy system resilience using tidal stream energy

2023-06-13T11:45:07+01:00

During autumn 2021, high global gas demand led to a 400% increase in imported wholesale gas prices in the UK. This coincided with (i) an extended period of low wind resource, where wind turbines provided 60% less energy than typical levels for the time of year, (ii) low nuclear power availability and (iii) depleting domestic natural gas reserves. These simultaneous events led to UK wholesale electricity prices more than doubling. The 2022 military and political impact of Russia’s invasion of Ukraine has led to additional increases in imported fuel prices. These types of disruption have the potential to cause damage worth years of sector revenues [1]. In the future, energy resilience challenges must be overcome whilst also achieving net-zero to limit global warming to within 1.5 degrees Celsius above pre-industrial levels, whilst electricity demand at least doubles.

This research investigates the role of tidal stream energy in enhancing energy system resilience during periods when the energy system is highly stressed, such as during autumn 2021 in the UK. The concept of resilience refers to the ability of the system to survive strong and unexpected disruptions and to recover quickly afterward [1]. This research is motivated by the acknowledgement that the commercial viability of marine power relies on the sector being able to explain how it can provide contributions beyond decarbonisation [2].

We build on an energy system modelling case study of the Isle of Wight [3], using the EnerSyM-RC energy system model. Wind data between 2012 – 2020 is analysed to identify and characterise periods of low wind resource. These data form the inputs to the EnerySyM-RC modelling. The model then applies a simple brute force optimisation method to investigate the most suitable mix of solar PV, offshore wind, tidal stream and energy storage that enhances system resilience on the Isle of Wight.

Initial results show that the adoption of tidal stream energy compliments solar and wind generation by reducing reliance on reserve energy (e.g. imported gas) during high-stress periods. Reduced reliance on imported energy is also achieved because of the distinct generation pattern of tidal power (i.e. 4 periods of power generation, each separated by slack tide, every day), which enhances the utilisation of local energy storage.

References

[1] Jasiunas J, et al., 2021, Energy system resilience – A review, Renewable and Sustainable Energy Reviews, 150:111476

[2] Pacific Northwest National Laboratory, 2021, Grid Value Proposition of Marine Energy: A Preliminary Analysis, PNNL -31123, Technical report

[3] Coles DS et al., 2023, Impacts of tidal stream power on energy system security: An Isle of Wight case study, Applied Energy, 334:120686

Analysis of Ocean Energy Integration in Ibero-American Electric Grids

2023-06-28T21:35:27+01:00

The development of the marine renewable energy, as referenced in the objectives achievement of many energy roadmaps [1], is conditioned by the possibilities to integrate the marine generation power plants in the electric systems. Offshore wind, floating solar PV, tidal, waves and even saline gradient are the technologies considered. The success of these plans needs to fulfil, among others, the compliance of the electric grid codes, ensuring the stability of electric grids.

The integration of marine renewable energies is still not a problem at the electric grids, since the penetration level is not so high. However, the objectives established in the roadmaps invite to analyse the expected effects of new power plants from the point of view of performance at the electric systems. Actually, some research works have identified an increase in the number of frequency and voltage events in electric systems due to higher penetration of marine energies [2].

The connection of the marine renewable generation is usually taken place in nodes of the electric grids not very favourable in terms of power capacity and voltage drops. Moreover, many of the locations where the marine renewable energies result economically viable are linked to weak areas of the electric grid, meaning that frequency and voltage are more sensible. That is the case of remote isolated areas, as usually coastal areas, or islands. Additionally, some of the technologies considered, such as wave energy, present very oscillatory power profiles which produce an unbalance with the consumption profiles, leading to stability problems.

More and more countries are putting an eye on new sources of energy, integrating them in their energy policies in order to search alternatives to face energy crisis. This is the case of many countries in Centre and South America, that have included in their roadmaps the renewable marine energies [3]. One of the main issues in these countries is the lack of robust electric grids or the condition of weak grids in coastal areas.

This paper presents the result of the analysis of different electric grids in Ibero-American countries. Three different regions have been considered: Mesoamerica, South America and Iberian Peninsula and Macaronesia, and particular areas or countries have been selected from each region as representative of the critical conditions in terms of grid integration. Costa Rica, Peru, Brazil and Argentina, and the Canary Islands in Spain, are the selected case studies. The results presented will summarize: power limits, time response, energy storage and backup generation requirements, power ramps, power oscillation allowed, etc.

This work has been accomplished in the frame of the activities carried out by the groups involved in the thematic network REMAR (Opportunities of Integration of Ocean Energies in Ibero-American Electric Grids), promoted and funded by the Ibero-American Programme of Science and Technology for the Development (CYTED).

Combining offshore wind and wave energy to supply a big size desalination plant

2023-06-07T12:44:10+01:00

This research analyses the feasibility of supplying a large size desalination energy demand by marine renewables. The case study is Las Palmas III seawater desalination plant, the largest desalination plant in the Canary Islands (Spain), which is located in the northeast of the island of Gran Canaria. Its average daily water production is 62,614 m³/day, consuming a total of 90,669.52 MWh/year. A constant energy production is needed for the optimal plant operation which raises the possibility of using different renewable technologies in order to reduce the energy fluctuations. In this case, the sea and its wind and wave energy resources are key technologies for supplying desalination plants near the coast. For this reason, different configurations have been simulated combining both technologies and analyzing their pairing in hourly terms to achieve a more stable energy production. The proposed methodology contemplates the identification of the hotspot for the technologies location in terms of environmental constraints and resource assessment (both wind and waves). The subsequent selection of the wind turbine and the wave converter and the energy coverage evaluation. Results tried to establish if the combination of offshore wind and wave energy improves the demand coverage in overall terms and in terms of seasonal match.

Tidal turbulence in medium depth water, primarily a model study

2023-06-19T11:03:03+01:00

Tidal energy is considered to be one important future source of renewable energy. There is presently a strong development in new technology, and there is an emerging need to e.g., describe the turbulence intensity and its characteristics in a tidal stream. In this study we consider a high resolution Large Eddy Simulation (LES) in water with 80 m depth and a tidal stream with a maximum volume mean flow amplitude of 2 ms^-1. the simulation is designed to describe turbulence characteristics at a developer site outside Holyhead in the Irish Sea. We find that the turbulence intensity and characteristics has clear time dependence, and that it is 100% stronger on the retarding tidal current as compared to the accelerating tidal current. We also find that it depends on the distance from the bottom. The turbulence is highly anisotropic with much longer length scales in the flow directions than perpendicular to flow direction. The simulation setup and results for mean flow quantities and turbulence measures are discussed in the presentation. Finally, results are compared with results from a ship mounted ADCP for mean flow characteristics and for turbulence quantities.

EXPERIMENTAL CHARACTERISATION OF THE WAKE OF A BOTTOM-MOUNTED TWO TANDEM OF CYLINDERS PLACED IN A HIGH VELOCITY AREA

2023-06-13T18:30:52+01:00

Following the development of renewable marine energies, the characterization of areas with strong marine currents has become necessary. The Normandy coasts (France) are among the sites suitable for the installation of tidal turbine parks because they have significant energy potential. Projects to install machines in these sites raise questions about various factors affecting the performance of the turbines that would be placed there.

We are currently working on the understanding of the mechanisms leading to the generation of ambient flow turbulence on the seabed. More particularly, we are interested in the impact of the complexity of the bathymetry on the organization of the wake generated. To simplify the modeling of the bathymetry of the seabed, we use generic cylindrical obstacles of rectangular section. The obstacles are placed on the bottom of the study area of the Hydrodynamic Tunnel of the LUSAC laboratory and occupy its entire width. Measurements are made using Particle Image Velocimetry (PIV-2D) for a velocity flow of 3 m/s.

In a first time, in a previous contribution, was evaluated the impact of the ratio between the Height and the Width (H/W) of the cross section of the cylinder on the organization of the near wake. Indeed, the average velocity fields obtained for six different obstacles, highlighted the modification of the organization of the flow topology. The variation of this ratio H/W leads to the modification of the length of the average vortex formation zone, implies the presence of two or even three average recirculation zones in the near wake of the cylinder and leads to the possible presence of a recirculation zone average placed upstream of the cylinder.

Nevertheless, in the seabed the structures are not isolated. In the contrary, on the seabed we can observe successions of structures anchored on the funds, leading to the interaction of a structure with the wake generated upstream by another one structure. The experimental study with the characterization of the flow around a tandem of “long” cylindrical obstacles of square section (side H). The distance between these two cylinders is equal to 2H.

As expected, in the present study, it can be observed the modifications of the topology of the mean flow and the distribution of the turbulent kinetic energy. Indeed, the interaction between the mean recirculation, generated downstream each cylinder, is shown.

Large-eddy simulations of interaction between surface waves and a tidal turbine wake in a turbulent channel

2023-06-13T18:43:21+01:00

Tidal stream turbines are now being developed for array deployments, largely at sites with relatively shallow water depths on either bed-supported, or floating support structures. Proximity to the free-surface presents design challenges with increased exposure to wave-induced kinematics leading to potential for increased peak- and fatigue-loads. Free surface proximity can also alter wake recovery rates which can influence the siting, and operation, of further turbines. To-date the impact of waves on turbine loading and on wake recovery has received
limited attention, generally for specific combinations of conditions reproducible in experimen tal facilities [1-2] or numerical models [3]. Improved understanding of how waves affect both turbine loading, and wake dynamics is necessary to inform the development of appropriate load prediction and mitigation methods and to further parameterise wake recovery to inform array siting.

This work presents CFD analysis of the loading and wake of a three-bladed horizontal axis tidal turbine within a wide, shallow channel with a turbulent inflow developed using a synthetic eddy method and free surface waves modelled using a Level-Set Method (LSM). The turbine and wave conditions considered are based on prior experimental studies [1] within a channel depth h = 0.45 m and mean flow U = 0.47 m/s. The simulated channel is width 3.67h and length 88h and long-crested regular waves are imposed, with the focus of this study on waves with non-dimensional wavenumber kh = 2.8, comparable with experimental conditions. Simulations are accomplished using the in-house large-eddy simulation (LES) code DOFAS
(Digital Offshore Farms Simulator) [4], which adopts an Actuator Line Method (ALM) to resolve the turbine blades. LSM is used to model the air-water interface in an accurate manner [5] with waves can be generated in DOFAS using linear or second-order Stokes theories with an absorption layer placed at the outlet of the domain to avoid wave reflections. At the inlet a mean logarithmic velocity profile is imposed over which artificial turbulence is added. Simulations were run for 400 s of physical time on 8,000 cores using ARCHER2.

The waves studied are shown to alter the rate of wake recovery, increasing the rate in the near-wake of the turbine and slightly reducing the rate beyond eight diameters downstream. Analysis of turbulence characteristics indicates a significant variation across the water column due to wave action. Propagation over the turbine wake also introduces directionality to the wave field, which is associated with the change of wave-speed over the wake region. Modal decomposition analysis of the velocity fluctuations with Proper Orthogonal Decomposition
(POD) reveals that the wake dynamics behind the turbine change due to the waves.

Actuator-Line CFD Simulation of Tidal-Stream Turbines in a Compact Array

2023-06-26T15:09:00+01:00

An actuator-line CFD model is used to simulate tidal-stream turbines acting alone or in a compact 3-turbine staggered array. CFD results confirm that the accelerated bypass flow between two proximal turbines in shallow water can enhance the power output for a close downstream turbine, with additional smaller effect of the upstream turbines’ operating point. Comparison with other authors’ experimental data in a narrow flume (IFREMER) and circular wave-current tank (FloWave) show some difference in relative loads between the turbines, possibly associated with ambiguity in overall blade pitch and difficulty in characterising onset flow in the FloWave tank. The accelerated bypass flow is persistent and largely established on the rotor plane of the upstream turbines, indicating how local array effects might be incorporated in simpler blade-element/momentum-theory design tools.

High-fidelity modeling of a vertical axis tidal turbine model under realistic flow conditions

2023-07-07T17:44:18+01:00

In the context of the global energy crisis, developing renewable energies is of primal importance. Among the renewable resources available are tidal currents. Tidal currents can be exploited with tidal turbines. Different concepts of tidal turbine exist, we here investigate a vertical axis tidal turbine or VATT prototype made by HydroQuest and CMN. More precisely, we carry on numerical investigations on a Hydroquest/CMN tidal turbine model.

Experimental studies are limited to turbine models and thus suffer from scale effects. In situ measurement are still expensive and difficult to acquire. Computational Fluid Dynamics (CFD) is an interesting alternative in order to go beyond what experiments can offer. One major drawback of CFD is that it requires validation of the models used. A validation of our CFD tool is included in this paper. Because the hydrodynamic of vertical axis tidal turbines is highly unsteady, Large Eddy Simulation (LES) are well adapted here.

Blade-resolved Computational Fluid Dynamics (CFD) approaches are appropriate to study vertical axis tidal turbines, but their cost is still prohibitive for real size tidal turbine. Simplified approaches are preferred solutions. Literature showed that Actuator Line Model (ALM) LES is well suited to model vertical axis turbine.

Most tidal sites are characterized with strong turbulence intensities. The presence of turbulence in most tidal sites has led companies and academics to study the effects of ambient turbulence over tidal turbines. It has been shown that turbulence can have a non-negligible influence on the turbine performances and wake. To account for the effects of turbulence, synthetic turbulence is introduced at the inlet of the simulation.

The LBM is an unsteady weakly-compressible CFD approach. It solves the Boltzmann equation using an explicit time discretization and a uniform Cartesian grid called lattice. It has been proven to be an efficient approach for modeling tidal turbines using LES and ALM, Grondeau et al. (2019).

In this paper, we study a tidal turbine model of the Hydroquest/CMN VATT using an ALM-LBM-LES approach. This model was tested at the Ifremer Boulogne-sur-Mer testing facilities. We first compare our results with the experimental ones in a scenario without turbulence and for flood and ebb tide configurations. A numerical investigation of the influence of upstream turbulence on the turbine performances and wake is then realized. Prospects of this study include among others the effects of turbines interactions and waves.

Synthetic eddy generation and modelling of turbine operation in a turbulent tidal flow

2023-06-26T09:29:20+01:00

In the operation of hydrokinetic turbines in tidal sites, the effect of onset flow turbulence on device performance and reliability deserves particular attention. Computational Fluid Dynamics (CFD) models capable to describe this complex phenomenology are necessary to predict harmful operating conditions and to implement design strategies to mitigate the impacts. Similarly, the description of the interaction between turbine wakes and eddies in the onset turbulent flow is important in the study of tidal arrays.

An original CFD methodology to simulate the operation of a tidal turbine in an arbitrary turbulent flow is presented. The methodology falls within the class of Synthetic Methods (SM). A randomly fluctuating velocity field is generated by a volume force distribution acting as forcing term in the Navier-Stokes equations. With respect to existing volume-force SM, an original definition of the forcing terms, and a control strategy to enforce a prescribed turbulence metrics are proposed. Volume force terms are imposed in a thin layer in the upstream region of the computational domain by a sinusoidal distribution with random variation of both intensity and phase. A Proportional-Integral-Derivative (PID) control is applied to minimize the difference between the intensity of the generated flow turbulence and the prescribed conditions.

In [1] the model was applied to describe a turbulent stream in an unbounded flow by Detached Eddy Simulation (DES). The capability to generate eddies that evolve into a homogeneous, isotropic turbulent flow was demonstrated. Here, the model is applied to analyze the interaction between the generated onset turbulent flow and a tidal turbine. A computationally efficient approach is used here in which the turbine is described by a volume force method, in a similar fashion as turbulence generation is modelled. Specifically, a hybrid viscous/inviscid methodology is applied in which the DES solver is strongly coupled with a Boundary Integral Equation Method (BIEM) solver. At each time step, the blade load distribution is calculated by a time-accurate BIEM solution and recasts as volume force terms that are plugged into the DES solution. Examples of BIEM validation studies are given in [2, 3, 4] and in the results of a recent blind-test benchmark to be presented at the EWTEC 2023 Conference [5].

The numerical application described here deals with a 3-bladed horizontal-axis tidal turbine in a 16% intensity, isotropic turbulent onset flow. In the full paper, the overall methodology is described, and numerical results are presented and discussed. Particular attention is given to characterize the generated turbulent stream in terms of key metric quantities like turbulence intensity, Power Spectral Density (PSD), time and spatial scales, isotropy, Probability Density Function (PDF). The results are analyzed to discuss the capability of the methodology to render a physically-consistent description of the phenomenology governing turbine operation in a real tidal flow.

Gregori, M. et al., Mar. Sci. Eng. 2022. (https://doi.org/10.3390/jmse10101332)
Sarichloo, Z. et al., Marine Energy Journal, 2022. (https://doi.org/10.36688/imej.5.77-90)
Dubbioso, G.A. et al., Proceedings of EWTEC 2019.
Salvatore, F. et al., Mar. Sci. Eng. 2018, 6, 53. (https://doi.org/10.3390/jmse6020053)
Willden, R. et al., Submitted to EWTEC 2023.

A study on tidal rotors under the combined effects of currents and waves using actuator-line CFD simulations

2023-07-04T13:59:07+01:00

Determining loads and power on a tidal rotor under realistic flow conditions is challenging. Methods used by offshore industries for the design and deployment of marine devices rely on simulations of a significant number of cases that combine different environmental and operational conditions. Design studies of tidal rotors rely on engineering models of low to medium computational cost that often omit important hydrodynamic effects such as blockage, multi-rotor interactions, wave diffraction, etc., and have a questionable accuracy under conditions such as high-thrust regimes or highly unsteady flows. Alternatively, high-fidelity CFD simulations can implicitly capture most of the relevant physics, but their computational cost often limits their application in engineering practice.

This study will present a numerical study of a tidal rotor operating near the free surface and affected by combined currents and surface waves. The simulations will be performed with an intermediate fidelity actuator-line model implemented within OpenFOAM. The AL model is embedded within a simulated tank discretised with the finite volume method, that employs a RANS turbulence model with a Volume of Fluid (VoF) approach for the free surface. Relaxation zones, implemented with the Waves2Foam library, are used for the generation and absorption of surface waves and reflections. A preliminary render of the assembled model simulating a test condition can be seen in fig. 1.

The paper manuscript will present the results of an initial validation of the modelling strategy that will compare simulated cases with towing-tank experiments. The validation will be followed by a quantification of the interactions between the rotor and the free surface, as well as an analysis of the transient loads and power fluctuations for different operating conditions.

A turbines-module adapted to the marine site for tidal farms layout optimization

2023-06-13T18:46:31+01:00

The ocean energy exploitation is arousing growing interest in the renewable energy sector. In the
short term, horizontal axis tidal turbines are the most promising technology due to the accumulated
know-how in the field of wind energy. In order to maximize the performance of the devices in a
cluster, it is essential to optimize the layout. The marine environment offers different conditions than
atmospheric situations, in terms of confinement and turbulence intensity. Moreover, tidal currents
exhibit a highly predictable pattern in speed intensity and direction unlike the wind resource, which
has a more random behaviour. Nonetheless, most of tidal sites are characterized by the inversion
of flow where the two prevailing directions are not perfectly aligned and opposite, hence the angle
between those directions should be a design variable. In this work we will consider as a case study
the site proposed in [1], where this angle is ±20°.
For those sites with a flow inversion of almost 180°, the staggered configuration is preferable to
avoid wakes interference as mentioned in [2]. Furthermore, many studies [3] had analysed positive
interaction between neighbouring devices in a cluster, hence it is important to establish the optimal
relative position accounting for fluid dynamic positive effects, and not only negative aspects such
as wake interactions. For this reason, in this work we present a novel approach to determine the
best configuration of a cluster of few turbines, a ”module”, which will be the optimized ”building
block” for the whole farm. The procedure to be followed consist of two phases in which both
the characteristics of the site and those of the turbine are taken into consideration. To place the
devices in an optimal configuration, we first consider the change of flow direction during the tidal
cycle for the site of interest, allowing only those configurations which avoid wake interference for
both prevailing flow directions; then, we assess the best layout by exploiting positive interactions
between devices in the cluster. The mutual fluid dynamic influence is analysed by means of a 3D
Blade Element Momentum model of the turbine [4] implemented in the Open Source SHYFEM
code. A series of simulations is performed to outline the power production trend of the module, and
consequently find the optimal distancing between the machines. CFD simulations are also used to
extract the module wake characteristics.

High-fidelity modelling of a six-turbine tidal array in the Shetlands

2023-06-27T11:12:36+01:00

Micro-siting tidal stream turbines in a confined seabed area requires a extensive understanding of the flow dynamics over the water column at turbine deployment locations so that operating conditions are assessed, wake effects can be estimated to infer the energy yield [1], or bathymetry effects can be quantified. Tidal currents have the advantage of being highly predictable and mostly bidirectional but the uneven bathymetry found at most of sites introduces a high variability to the flow conditions within relatively short distances. Considering future tidal turbine arrays will comprise dozens of devices, deploying ADCPs at each turbine position would be very expensive or very time consuming, outlining the need for accurate modelling tools to be used as digital twins in micro-siting. Shallow-water models are widely adopted in preliminary design of tidal arrays but fail to capture the three dimensional nature of the flow and predict deflected wakes whose streamwise length is also over-predicted [2]. Thus, eddy-resolving method are required to fully capture the turbulence from the free-stream flow, induced by turbines and from bathymetry. \\

This study provides a real-project application of the state-of-the-art large-eddy simulation (LES) code DOFAS (Digital Offshore Farms Simulator) [3] to the six 100kW-turbine array deployed by NOVA Innovation Ltd. in the Shetlands, UK [4]. The bathymetry data has been obtained from EMODnet database with the velocity profiles set at the inlet condition of both ebb and flood tides imported from Macleod et al. (2019) [4]. The deployment site is characterised by steep slopes with a maximum depth of 45 m at the cross-section where turbines are located. The bathymetry has a downwards slope when the flow goes in the ebb tide direction whilst upstream during flood tide. The tidal rose indicated a slight deviation of about 20$^{\circ}$ between ebb and flood directions. The turbines have a diameter ($D$) of 9 m attached to a 10 m long hub whose diameter is 1 m. Three array configurations have been studied: (i) single row of three turbines, (ii) two rows of turbines spaced 8$D$, and (iii) two rows of turbines spaced 12$D$. The computational domain extends over 600 m by 300 m in the horizontal plane yielding 540 million cells, requiring 32,500 CPU hours to compute 30 min of physical time.\\

Results show that bathymetry effects at this site play a larger role when designing the location of the secondary row of turbines compared to wake effects from upstream turbines. During the ebb tide, the increase in water depth reduces the wake recovery far downstream so that the array with 12$D$ row spacing has a lower performance than the 8$D$ one with approx. 30\% decrease in energy yield. Conversely, the uphill shape of the bathymetry during the flood enables a fast wake recovery so that the downstream row experiences almost no energy yield loss due to wakes.

Instabilities in tidal turbine wakes

2023-06-27T15:49:57+01:00

We report on the observation of vortex instabilities in the wake of a tidal turbine undergoing harmonic unsteady axial inflow (e.g. due to surface waves). The work was carried out using unsteady RANS (URANS) modelling using the open-source CFD software OpenFOAM, using the `pimpleDyMFoam' solver. The PIMPLE algorithm used in the URANS solver is a combination of the PISO (Pressure Implicit with Splitting of Operator) and SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) algorithms. URANS simulations usually have a limitation on the time step set by the Courant number (C = u (delta t)/(delta x), where delta x is the minimum cell length and u the local velocity. In the PISO algorithm, C<1 is usually required. However, the PIMPLE algorithm allows stable transient simulations at C>>1 by applying under-relaxation to each time-step until a convergence criterion is met, before allowing the time-step to complete with no applied relaxation factors. The turbulence model used was komega-SST, which has been used extensively in tidal turbine modelling.

As with a wind turbine, the response of a tidal turbine to unsteady flow is heavily influenced by the behaviour of the helical wake that returns in close proximity to the turbine blades times per revolution ( – number of blades). Unsteady inflow causes the blades to shed vorticity into the wake, which in turn leads to a spatial variation in wake vortex strength, such that the vorticity of adjacent returning wake segments can differ, and thus there is a spatially varying velocity deficit in the wake. This spatial variation in velocity deficit triggers instability in the blade tip vortices, the emergence of which we demonstrate is governed by the non-dimensional group relating the blade passing frequency to the gust frequency. If the ratio between these two frequencies is an integer, the wake is stable. If not, the tip vortices exhibit various forms of instability. If the wake is unstable, adjacent wake segments will eventually coalesce as they move downstream, such that the wake consists of larger regions of vorticity with lower spatial frequency, compared to the wake immediately behind the turbine. There is also some indication that these unstable, coalesced wakes persist further downstream, compared with stable wakes. This has implications for wind and tidal farm design, where interaction of a row of turbines with the wakes of upstream turbines is an important consideration.

On the accuracy of BEMT and CFD on the power and trust prediction of tidal turbines

2023-06-23T19:57:42+01:00

We compute the loads on a model-scale tidal turbine with Blade Element Momentum (BEM) theory and Computational Fluid Dynamics (CFD) simulations, and we compare the results with towing tank tests. CFD simulations are wall-resolved, steady, Reynolds-averaged Navier-Stokes simulations with a k − ω SST turbulence model, where only a 120^◦ wedge domain with a single blade is resolved in a non-inertial frame of reference. We undertake a detailed uncertainty analysis to identify the sources of error. BEM uncertainty is computed with a Monte-Carlo approach based on the differences in the predictions of CFD and Xfoil for the sectional lift and drag coefficients,while CFD uncertainty is based on the errors due to the finite number of iterations and spatial resolution.

The maximum error of CFD (8.0%) with respect to the experimental data is about half of that of BEM (15.5%) for the power (C_P) and the thrust (C_T) coefficients and both errors are within 4.1% for CFD and within 7.2% for BEM around the optimal tip-speed ratio (λ = 6.03). The BEM error is within the uncertainty associated with the imprecise knowledge of the sectional lift and drag coefficients. The sectional forces from CFD and BEM disagree at both the tip and the root, resulting in a substantial BEM underprediction of C_P at high λ values (up to 15.5%), yet C_T is well predicted (within 2.3%) at every λ. The CFD uncertainty is markedly smaller than the error, which is thus mostly due to a modelling error such as the turbulence model, the neglected effect of the support structure, the free surface, and the imprecise knowledge of the input conditions. Overall these results suggest that CFD provides both a maximum error and uncertainty that are substantially smaller than that of BEM, but both methods suffer from modelling errors that require further investigation.

THE PERFORMANCE OF COUNTER-ROTATING TIDAL TURBINE IN DIFFERENT SEA STATES

2023-06-23T15:33:05+01:00

This paper will report on work being undertaken to evaluate the performance of the counter-rotating tidal turbine in a wave-current coupled operation environment based on Blade Element Momentum Theory (BEMT). The counter-rotating tidal turbine has two rotors rotating in opposite directions on the same axis. It has been proposed on the basis of the theory, which states that a configuration of two rotors having a similar swept area on the same axis has a higher maximum power coefficient than a conventional configuration of a wind turbine with a single rotor. BEMT is a reliable and effective theory for rotor design because it is based on solid physical principles and has a remarkably low computing cost.

Comparison of Actuator Line Modelling of Tidal Power Kites with ADCP Measurements

2023-06-20T15:40:17+01:00

The potential for tidal energy to be a part of future renewable energy systems is expanding. Tidal turbines deployed in open waters or channels are effective methods to harness energy from tidal currents. Sharing similar functionality with wind turbines, horizontally mounted tidal turbines require a minimum tidal current velocity to operate effectively. The Deep Green (DG) power plant which is based on a tethered kite model aims, however, to operate efficiently in tidal current velocities as low as 1.2 m/s. The kite wing is steered in a lemniscate trajectory ( ) almost perpendicular to the tidal current. In the trajectory, the relative flow velocity through the turbine attached to the wing reaches several times the tidal current velocity, enabling efficient operation of the turbine in relatively low-velocity tidal currents. This could reduce geographical limitations in installing large-scale tidal power arrays.

In a previous project, numerical modelling of the DG was carried out in a tidal flow using Large Eddy Simulations (LES) and Actuator Line Modelling (ALM) implemented in OpenFOAM solver. ALM using momentum sources has been used in modelling wind turbines and has been validated against experimental data and observations. The ALM has been further developed in order to be able to model wings (here the kite) that move in arbitrary paths compared to horizontally mounted turbines with rotational paths. This numerical model for the DG kite in a tidal flow has, however, up to now not been validated against observations, which is beneficial before further analyses are made, e.g., optimization studies.

In this study, Acoustic Doppler Current Profiler (ADCP) observations taken in the wake of a DG are compared to the results from the numerical model under similar conditions. Comparing the numerical results directly with the observations leads to discrepancies, hence the model data is resampled in a similar way that the ADCP would measure and process data using a virtual ADCP for the model. Flow properties such as the instantaneous and time-averaged stream velocities are in turn compared for both the tidal flow without the DG and with a DG. The effect of the DG on the tidal flow is analysed using the model and the observations. After resampling the model data, the model and observations show good agreement. This suggests 1) that the DG model using ALM can be used for further analysis and 2) that whilst comparing model data with ADCP observations for studying the small-scale effects of tidal turbine wakes, care should be taken to sample the model data consistently with observations.

Development of a modified BEMT model for the analysis of helical bladed vertical axis tidal turbines

2023-07-18T16:26:38+01:00

Development of a modified BEMT model for the analysis of helical bladed vertical axis tidal turbines

Mohammad Fereidoonnezhad¹, Seán Leen¹, Stephen Nash¹, Tomas Flanagan², Patrick McGarry¹

¹ School of Engineering, University of Galway, Galway, Ireland

² ÉireComposites Teo., Údarás Industrial Estate An Choill Rua, Inverin, Galway, Ireland

E-mail: patrick.mcgarry@universityofgalway.ie

Keywords: Blade element momentum theory, Helical vertical axis tidal turbine, FEA Analysis

Classical blade element momentum theory (BEMT) formulations are not capable of accurately simulating complex blade geometries, such as spiral or helical blade geometries. In this paper, we develop a modified BEMT model to calculate hydrodynamic forces acting on the helical vertical axis tidal turbines. Our framework accounts for curved blade geometries where the leading edge is not orthogonal to the freestream velocity, and chord vectors are not in the azimuthal-radial plane. We validate the model using experimental data for a prototype VAWT. We then perform a parametric analysis of helical bladed VATT designs. Both the helix angle of the blade and the relative orientation of the chord strongly influence the power output of a turbine. Our modified BEMT model also predicts that an increase in blade helix angle results in reduced power fluctuations. Additionally, computed fluctuations in tangential and normal forces acting on blades are shown to reduce significantly with increasing blade helix angle, suggesting a reduction in risk of fatigue failure. We also uncover a complex distribution of normal and tangential forces along the length of a helical blade. Fluctuating hydrodynamic forces computed by our modified BEMT model are input into a finite element (FE) framework to compute the stress state in a fibre reinforced composite blade material for a range of blade azimuthal positions. Analyses reveal that blade deflections are three orders of magnitude lower than the turbine radius, suggesting that sufficient structural stiffness is achieved by the blade design. Results also uncover stress concentrations in the region of strut-blade connection, revealing a higher risk of fracture and fatigue failure at this location.

A comparative study of power production using a generic empirical model in a tidal farm

2023-06-26T12:03:10+01:00

This study presents a generic model for estimating the velocity deficit and turbulence intensity in
a tidal turbine farm. The proposed model considers a range of ambient turbulence intensity, the rotor diameter-to-depth ratio, and the rotor thrust coefficient in realistic applications. We evaluate the power generation of a large-scale tidal farm composed of 16 turbines in an in-line and staggered configuration in an ideal channel similar to the Alderney Race in the English Channel. The added turbulence effect is taken into account when assessing the velocity deficit in the farm. As supported by previous studies, the results show that the staggered array produces more power than the rectilinear array. The staggered arrangement benefits from flow acceleration and wide turbine spacing, which improves wake recovery. According to the results, the farm can be resized by decreasing the lateral spacing in the rectilinear array and decreasing the longitudinal spacing in the staggered array without affecting the farm’s efficiency. The reduction in farm size will reduce cable costs and provide an opportunity for future expansion. For the tidal turbines in shallow water regions, the ratio of rotor diameter to depth is shown to affect the power generated by the turbines. The power produced in the farm decreases with an increase in the rotor diameter-to-depth ratio due to the limited wake expansion along the vertical plane. This low-computational model can be useful in studying the wake interaction of tidal turbine parks in different configurations.

Objective Functions for the Blade Shape Optimisation of a Cross-Flow Tidal Turbine under Constraints

2023-06-12T12:24:32+01:00

Hydro-kinetic cross-flow tidal turbines (CFTT) are omni-directional and offer higher area-based power density compared to horizontal-axis tidal turbines, making them very attractive for tidal energy exploitation. However, the rotating motion around the vertical axis results in continuously varying angles of attack, causing alternating loads, which may lead to fatigue failure and structural damage. The OPTIDE Project addresses these challenges by implementing intracycle blade pitching to individually control the angle of attack, increasing the power coefficient C_P and reducing structural loads. For this purpose a Darrieus turbine is designed with embedded actuators in each blade. Firstly a blade shape optimization will be conducted to fit the actuator at the quarter-chord position while ensuring sufficient thickness. The optimization procedure couples Computational Fluid Dynamics (CFD) with a Genetic Algorithm. The employed optimizer OPAL++ sets ten variables for each individual, which describe the hydrofoil shape, length and tip speed ratio (TSR). A smooth hydrofoil shape is generated from the variables, followed by an automatic mesh generation. Subsequently, numerical simulations of each individual at the desired TSR are conducted, while keeping the blade pitch angle constant. Simulation results provide the C_P and stress acting on the turbine blades, which are the two optimization objectives (maximize C_P while minimizing stress). This process is repeated during the optimization, aiming to determine the most suitable blade shape, that fits the actuator, and operating point (TSR) in a trade-off between C_P and structural loads. This will lead to the increase of efficiency and a longer turbine lifetime.

Investigating the impact of multi-rotor structure shadowing on tidal stream turbine performance

2023-07-17T18:23:01+01:00

As the tidal stream energy sector develops, reducing the Levelised Cost of Energy (LCOE) is essential to sustain commercialisation. Modular multi-rotor foundations, with bi-directional turbines, reduce offshore operational complexity through smaller turbine diameters and lift weight, in turn reducing the device Operational Expenditure (OpEx). With the introduction of modular, multi-rotor foundations, the wake-induced impacts that these structures have on turbine performance must be investigated to better estimate energy yield, loading, and fatigue life. This study sets the scene for investigating the relationship between the turbulent wake generated by a modular ballast weighted foundation and 2-bladed Horizontal Axis Tidal Turbine (HATT) motivated by the HydroWing multi-rotor device concept. The presented work aims to determine the broader magnitude and severity of the loads and establish a robust methodology to be followed with further high-fidelity modelling. Initially, a transient RANS Computational Fluid Dynamics (CFD) simulations environment with a sliding mesh is configured and validated against experimental data. A turbine in free-stream isolation is simulated as a benchmark case with the modular foundation sequentially introduced to analyse the impact of the structure. Key findings suggest that operating turbines downstream of the multi-rotor foundation could cause a 20% fluctuation in thrust loading at a 1.82 Hz frequency resulting in a mean Cp reduction of 8% over a revolution.

A methodology to capture the single blade loads on a cross-flow tidal turbine flume model

2023-06-14T15:15:01+01:00

The OPTIDE project aims to improve the efficiency and durability of Hydro-kinetic cross-flow tidal turbines (CFTT). These turbines are attractive for the exploitation of tidal energy, as the area-based power density of such turbine arrays is higher in comparison to those from horizontal-axis turbines. CFTT also generally feature a simpler design and the ability to operate under varying flow conditions. Nevertheless, the efficiency of single CFTT is lower relative to the most commonly used axial turbine type. Furthermore the life time can be affected by alternating and pulsating stresses, caused by continuous variations of the angle of attack and hydraulic loads during the rotation. These stresses may lead to structural damage and fatigue-induced material failures. A promising approach to overcome these drawbacks is intracycle blade pitching. In this case the angle of attack is continuously adjusted individually for each blade during the rotation. The consequence is smoothed peaks of the load alternations and a higher power coefficient CP . The project aims to explore the influence of active blade pitching on CFTTs and to optimize it with numerical and experimental means. Therefore, a lab-scaled three-bladed experimental turbine with embedded pitch actuators is developed. The model will subsequently be tested in the lab flume of the Institute of Fluid Dynamics and Thermodynamics of the Otto-von-Guericke University Magdeburg. Blade forces in tangential and radial components as well as the machine torque and the rotational speed are measured during the experiments. The turbine is equipped with two full-bridges of strain gauges for the detection of the blade loads, from which the structural stress is calculated subsequently. To ensure the turbine model’s mechanical durability, weakly coupled fluid-solid-interaction (FSI) simulations have been performed and will be presented. To this purpose, a 2D flow analysis, employing the open-source CFD toolkit OpenFOAM (v2206), has been coupled with a 3D structural analysis, using the Mechanical module of the commercial software package Ansys Workbench (2020 R2). The FSI simulations show that the current setup only allows for the measurement of the radial blade load component, because the pitching moment at blade level interferes with the measurements as soon as the profile stalls. The measurements are further distorted by secondary force paths from the loads on the other rotor blades. Possible measures for an improved instrumentation strategy on the flume model will be presented and discussed. It is shown that multiple equipment options allow for a decent investigation of the forces on blade level.

Modelling the effects of boundary proximity on a tidal rotor using the actuator line method

2023-06-12T11:54:58+01:00

For a turbine in unconstrained flow, the maximum power coefficient (CP ) is limited to 16/27 of the undisturbed
kinetic energy flux through the rotor area according to work attributed to Betz, Lanchester, and Joukowsky [1].
This maximum, often referred to as the Betz limit, occurs when the rotor presents the optimum resistance to
the incoming flow, imparting enough force to generate power without overly choking the flow through the rotor.
However, in the context of tidal stream energy, the flow is often constrained by the seabed and the free surface
changing the balance of optimal rotor resistance. Thus, the limit for maximum power extraction is modified
to the form first presented by Garrett and Cummins, CP = (16/27)(1 − B)⁻²[2]. The factor B, known as the
blockage ratio, represents the fraction of the channel cross-section occupied by the rotor swept area, and allows
rotors operating in confined conditions to theoretically exceed the Betz limit. Subsequent theoretical work by
Nishino and Willden extended this model to demonstrate that constructive interference effects between closely
spaced turbines in a co-planar fence can allow efficiency increases above the Betz limit, even in an infinitely wide
channel where the global blockage ratio is negligible [3]. This phenomenon, attributed to the concept of local
blockage, defined as the ratio of rotor swept area to the surrounding flow passage area, has been observed by
several studies both experimentally, and numerically using actuator disk and blade element momentum methods
[4, 5, 6]. However, neither the experimental nor the numerical studies provided detail on unsteady loading effects
stemming from azimuthal variations in the flow field caused by anisotropy in the local blockage, such as when
the rotor is not centred in the flow passage. In this study, a single tidal rotor is simulated using the actuator
line model embedded in a Reynolds-Averaged Navier-Stokes solver with varying degrees of anisotropic blockage
imposed by proximity to a non-deformable upper boundary. The investigation is carried out in the context
of the Supergen ORE Unsteady Tidal Turbine Benchmarking Project using a 1.6 m rotor in a computational
domain equivalent to the towing tank dimensions at the Qinetiq Haslar facility in which the turbine was tested
experimentally [7]. The discrete blade representation and unsteady nature of the actuator line method allows
investigation of variations in loads and the importance of boundary proximity by extracting the spanwise load
distributions and local flow parameters at different positions around the azimuth. The effects of two different
tip-loss correction models on the spanwise force distributions and overall loads are investigated. Analysis of the
integrated rotor loads shows potential for an increase in the maximum CP of ∼1% with changing proximity to
the upper boundary without any detriment to the power-to-thrust ratio. However, anisotropy in the local flow
passage can result in azimuthally varying blade forces, introducing an additional source of fatigue loading to
the rotor and drive train.

Characterisation of turbulent flow and the wake of a tidal stream turbine in proximity to a ridge

2023-06-26T11:35:08+01:00

Fast tidal currents are generally found in shallow water depths where tidal turbines can be deployed to operate. In complex environments in which there is an irregular bathymetry, seabed shape changes can induce pressure gradients that accelerate or decelerate the flow depending on the slope and relative depth, affecting turbine wake recovery. In this study, a laboratory scale turbine, represented numerically using an Actuator Line Method (ALM), is computed using Large-Eddy Simulations (LES) over a flat-bed and in presence of three short ridges with varying streamwise lengths. Turbines are positioned at several locations, namely at ridge-centre (0D) and at 2D upstream and downstream from the foot of the ridge to establish the influence of location on wake recovery rate. Turbine operating point is selected to yield a constant tip-speed ratio based on the disc-averaged velocity at each location obtained from a precursor simulation run without turbines. Results show that, relative to the same turbine in a flat channel, there is a noticeable enhancement of the rate of wake recovery when turbine is sited upstream of the ridge and it experiences high fatigue loads when sited downstream of the ridge. Implications for array design regarding performance losses due to turbine- and bathymetry-induced wakes are drawn.

Verification and validation of blade-resolved viscous-flow tidal turbine simulations

2023-06-29T16:01:32+01:00

Tidal turbines are a renewable energy source on the rise. The exceptional predictability of tidal currents contributes to a high reliability of this technology, which represents a key advantage in the endeavor to become a major contributor to the energy mix. To foster the development and to support the design process of tidal turbines, reliable numerical modeling techniques are required. This paper presents verification and validation work performed within the framework of the Supergen ORE Tidal Turbine Benchmarking Study. Viscous-flow CFD code ReFRESCO is used to conduct blade-resolved simulations of the towing tank experiments. In a first approximation, a steady-state frozen-rotor approach is chosen. A transition model, γ-Re_θ, is employed to predict the flow state transition on the turbine blades. In the process, the sensitivity to input turbulence quantities is highlighted. The numerical uncertainty is estimated based on mesh refinements. Finally, a conclusion is drawn to which accuracy the presented numerical models can predict the outcome of the experiments.

Impact of lateral turbine spacing on the performance of a multi-rotor tidal energy device

2023-07-04T14:04:51+01:00

In this work, the impact of local blockage on the power production of a tidal array due to multiple turbines positioned in close proximity is studied. A numerical model of the HydroWing tidal energy device, which features multiple turbines on a retrievable wing, is being developed using geometry-resolved computational fluid dynamics (CFD). For this paper, the influence of two turbines at several spacings is considered.

The CFD model is used to perform quasi-static steady-state simulations of two turbines in a twin rotor configuration, where a multiple reference frame (MRF) approach is used to simulate rotor rotation. The lateral spacing between the rotors is varied and the resulting impact on the axial loads and power performance of the two turbines is studied, with the aim of identifying the optimal turbine spacing for the HydroWing device. The results will be used in future design optimisation work to minimize the levelized cost of energy of a large scale array using HydroWing technology at the proposed site for the Morlais tidal energy project.

Offline version of the Proceedings of 15th EWTEC 2023, Bilbao

2023-10-03T22:33:36+01:00

Please note: this is a snapshot of the Proceedings of the 15th EWTEC made on 2023-10-01 and provided for convenience as a single download for offline use. The definitive version of the EWTEC Proceedings is the online version. To extract the files, use 7-Zip.

Photos of the 15th EWTEC

2023-10-05T16:38:51+01:00

Photosets of the 15th EWTEC. Please credit 'EWTEC' if using the photos and if online, use twitter handles #EWTEC and #EWTEC2023.