Abstract
This chapter discusses the frequency-domain modelling approach to model wave energy converters (WECs), and presents its advantages and limitations. It is shown that frequency-domain modelling requires the linearization of the forces acting on the WEC. This simplification is acceptable for waves and small-amplitude device oscillatory motions and whenever the mooring system can be modelled by a linear spring and the power take-off (PTO) using either a linear damper or a linear spring-damper system. Under these circumstances, the first step to model WECs is typically based on a frequency-domain approach, where the excitation is assumed to be of a simple harmonic form. Accordingly, all the physical quantities vary sinusoidally in time with the same frequency of the incident wave. Therefore, in the frequency domain the equations of motion become a system of algebraic linear equations that may be solved straightforwardly. However, this requires the calculation of the hydrodynamic coefficients, which can be produced relatively easily using a linear potential flow solver and boundary element methods (BEMs). Hence, frequency-domain models are relatively fast and so widely used to get a first insight on response and power capture of WECs. Nevertheless, despite the advantages of frequency-domain models, it is important to take into account that these models may be relatively inaccurate for large waves, at frequencies close to resonance or where viscous forces are significant.
This is a chapter from Numerical Modelling of Wave Energy Converters: State-of-the-Art Techniques for Single Devices and Arrays.