Experimental assessment of combined sliding mode & moment-based control (SM²C) for arrays of wave energy conversion systems

Motivated by the lack of comprehensive experimental implementation and assessment of the potential benefit that can be achieved with energy-maximising optimal control solutions for arrays of wave energy converters (WECs), we present, in this paper, the development, design, experimental implementation, and performance appraisal, of optimal moment-based control for arrays of WEC systems. Both centralised and decentralised controllers are evaluated. Four different WEC array layout configurations are considered, with up to three 1:20 scale prototypes of the Wavestar WEC system operating simultaneously within the basin, subject to a variety of sea state conditions. In particular, the proposed controller, termed sliding-mode-moment-based controller SM²C, is composed of a receding-horizon moment-based reference generation process, and a subsequent proportional–integral–derivative-like continuous sliding mode tracking controller. This composite control structure is implemented and assessed experimentally, providing a detailed analysis of key performance metrics. We show that the proposed SM²C strategy is able to maximise energy absorption for all the considered WEC array layouts, with up to 2.8 times energy improvement when compared to the benchmark controller case. The findings of this experimental study show tangible proof of the performance enhancement that can be achieved in real arrays of WEC systems with the use of appropriate control technology, demonstrating not only the feasibility of the proposed SM²C strategy in itself, but the key role that control systems have to play in the pathway towards effective exploitation of the yet largely untapped wave energy resource.