Determining required valve performance for discrete control of PTO cylinders for wave energy

Within wave energy a large challenge remains in designing a Power Take-Off (PTO) system capable of converting the slow oscillations induced by waves into electricity. Fluid power is an essential part of most PTO-concepts. To implement an efficient control of the load force produced by a cylinder on a floating body, throttle-less force control by discrete variation of the effective cylinder area has been investigated and found feasible for the Wavestar wave energy concept. However, the feasibility study assumes adequate valve performance, such that only the compression loss remains. This paper investigates the required valve performance to achieve this energy efficient operation, while meeting basic dynamic requirements. The components making up the total energy loss during shifting is identified by analytically expressing the losses from the governing differential equations. From the analysis a framework for evaluating the adequacy of a valve’s response is established, and the analysis shows the results may be normalised for a wider range of systems. Finally, the framework is successfully applied to the Wavestar converter