Integrated hydrodynamic-electrical hardware model for wave energy conversion with M4 ocean demonstrator

Abstract

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.