Effect of non-ideal power take-off on the electric output power of a wave energy converter under suboptimal control

Abstract

This work investigates how the physical limitations and losses of an electric power takeoff (PTO) system affect the production of power for a wave energy converter (WEC) under suboptimal control strategies. The damping of the PTO system is either continuously tuned to the time-varying wave excitation force frequency, which is referred to as passive control (PC), or tuned to the mean centroid frequency of the spectrum, which is referred to as passive loading (PL). The time-varying frequency is estimated by the Hilbert-Huang transform (HHT) method. Numerical simulations with the wave-to-wire model of a WEC including a non-ideal PTO show that the limitations and losses of the system modify the output power for both control strategies, when compared to the ideal PTO case. Furthermore, the benefit of applying the time-varying damping obtained from HHT for certain types of wave spectra differs significantly from the cases when only the hydrodynamic model with an ideal PTO system is considered. While for an ideal PTO system, the greatest improvement of PC over PL is obtained for wideband spectra, this behaviour is not observed when the non-ideal PTO is considered.