Time-Domain Models and Wave Energy Converters Performance Assessment

Abstract

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.

Analysis of power extraction from irregular waves by all-electric power take off

“…Thus the evaluation of the peak power (and, consequently, of power ratio) must be performed on a statistical base [10], considering the results coming, in our case, from the simulation of 80 different force profiles. The first set of simulations was aimed at the quantification of the power performance when passive loading is used.…”

“…In order to analyze the WEC behavior in irregular waves and to cope with any non-linearity, a time domain model is required, as the following one based on the Cummins equation [10]:…”

Model Tests and Simulations on a Wave Energy Converter Based on Inverse Offshore Engineering

“…Important insight in the different PTO concepts in reality can be found in [4,[8][9][10]. During this conversion forces are exerted by the PTO on the structure.…”

“…Two main PTO types can be distinquished [8][9][10] In the present test the focus will be on application of Irregular Motion PTOs. The structure motions will decrease as a result of the damping forces provided by the PTO.…”

Mathematical Framework for Hydromechanical Time-Domain Simulation of Wave Energy Converters

“…This is a very powerful approach for most situations, yielding accurate results in many cases with small computational time. Works such as the two-body heave converter from Andres et al, the SEAREV from Babarit et al, and heaving WEC are all examples of recent successful use of linear time-domain approach for WEC design [15][16][17][18][19]. However, the description of the problem through Fourier Transform makes it impossible to incorporate nonlinear (potential flow wave-dependent) effects into these models.…”