Assessment of Time-Domain Models of Wave Energy Conversion Systems

Time-domain models are necessary for the analysis of wave energy conversion systems, due to the presence of nonlinearities which may not be neglected for accurate prediction of their performance and behaviour. Such nonlinearities are contributed in varying degrees by drag, Coulomb friction, fluid compressibility, and also control mechanism. In time-domain, the equations of motion for the system will contain hydrodynamic radiation terms expressed as convolution integrals due to the frequency dependence of the radiation coefficients. The evaluation of the convolution integral is time-consuming and is difficult to carry out by standard adaptive solvers. Hence, various approximations to the convolution integral have been proposed to avoid these problems. The purpose of this study is to systematically assess the quality of some selected time-domain models. Generic models of wave energy conversion systems will be developed, with the possibility of varying the relative importance of the nonlinear terms. The time-domain models are categorised according to the convolution approximation and the numerical integration method used. Selected assessment criteria include computation time as well as the statistics of device motions and converted power. A model is always a trade-off between efficiency and accuracy. It is hoped that this study will provide some guidelines in the choice of time-domain models suitable for simulation of wave energy conversion systems.