An efficiency-aware continuous adaptive proportional-integral velocity-feedback control for wave energy converters

The main objective in hydrodynamic control of wave energy converters (WECs) is the maximization of the energy captured from the waves. Latching control, model predictive control and “PI” control are examples of implementable strategies surveyed in the literature. “PI” control is the common name of a form of hydrodynamic control where the control force applied to the captor is a proportional-integral feedback of captor velocity. While suboptimal, it has the merit of being simple, requiring only straightforward computations and can be considered a standard solution for WECs with a four-quadrant power takeoff (PTO) system. Adaptive “PI” control has been already discussed in the literature, usually using a gain-scheduling approach, with optimal gains precomputed off-line for a representative set of sea states and applied as a function of estimated sea state conditions. In most literature, only average on-line estimations of sea states have been proposed, with time windows of several minutes. Such intermittent adaptive control laws are clearly suboptimal in terms of energy recovery, since the control gains are not continuously updated whereas the sea state is continuously time-varying. In this paper we present a continuously adaptive “PI” control strategy, whose gains are adapted on-line on a wave-to-wave basis, based on a real-time estimate of the dominant wave frequency of the wave force. The PTO efficiency is taken into account. The proposed control method is validated and compared through experiment for irregular sea states.