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

Abstract

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.