Increasing the efficiency of the passive loading strategy for wave energy conversion

The control strategy applied to the oscillating system of the wave energy converters largely determines the overall system efficiency and thus the energy production. One of the first proposed control strategies for this type of device is the passive control. This type of control continues currently to be considered because it can be implemented in a simple way: short-term predictions about the performance of the wave are not needed when passive control is applied. Moreover, the power flow is unidirectional, and the extraction system is subjected to moderate peak powers so that costs and dimensions of the wave energy converter components can be reduced. The main disadvantage of passive loading control is the low energy production compared with other control strategies, such as the so-called reactive control which requires bidirectional power flow between the oscillating system and the generator. This way, the device can extract more energy from the waves, but the global system requires large energy exchanges between the oscillating device and the generator. These energy exchanges cause high losses, and thus the overall system efficiency significantly decreases. Furthermore, the reactive control system involves high oscillating excursions that can become unacceptable. In this paper, a point absorber converter that uses a linear direct-drive generator and power electronics converters jointly is considered. This article presents a passive control strategy that optimizes the power transferred from the generator to the power electronic converter considering the copper losses in the electric generator. Applying this approach, the generation system efficiency increases significantly, and similar values of the oscillating system's heave excursions and the peak-to-average power ratio for both the conventional and the proposed passive loading are found. The formulation of the proposed method is presented as well as time domain wave-to-wire simulations in both regular and irregular waves.