Time-Domain Models and Wave Energy Converters Performance Assessment

To evaluate the performance of a Wave Energy Converter (WEC) with realistic Power Take-Off (PTO) configurations, moorings, control systems and other contributions, time-domain models are required to deal with the non-linearities arising from the different elements of the energy chain. Future developers, in order to give a correct estimation of the expected power output of their devices, will have to apply these models and will be asked about the accuracy they can provide, particularly on what concerns the performance of the device in a determined location. A general mathematical outline of this approach was firstly proposed by Cummins by using, under linear assumptions, a classical way of representing the equation of motion of a floating body with a system of integro-differential equations with convolution terms that involve frequency-dependent coefficients. Many methods have been proposed, in literature, to solve this system in the most efficient and accurate way. Some of them relied on a direct numerical integration using standard methods for the solution of Ordinary Differential Equations, while, in turn, others are based on the approximation of the radiation convolution term with a determined number of linear sub-systems or properly chosen transfer functions. This paper presents a general scheme for a simple heaving single-body WEC, whose hydraulic Power Take-Off is coupled to a gas accumulator that serves as a storage device. Different time-domain methods will be used and compared. Particular attention will be paid to the accuracy of the performance calculation of this WPA. It is expected that the results of the simulations provide deeper understanding of the importance of the numerical parameters used in the estimation of the device performance and in this way will constitute an additional suggestion for the choice of a time-domain model for the evaluation of a WPA performance.