Influence of hydraulic power take-off unit parameters on power capture ability of a two-raft-type wave energy converter

In this paper, a two-raft-type wave energy converter (WEC) consisting of two rafts and a hydraulic power take-off (PTO) unit is considered. The hydraulic PTO unit composed of two hydraulic cylinders, two gas accumulators and a hydraulic motor coupled with a generator is available to capture wave power by using the relative pitch motion between the two rafts. To investigate the influence of hydraulic PTO unit parameters on the power capture ability of the two-raft-type WEC, a combined model based on the linear wave theory and basic-hydraulic equations is presented. The model is validated by comparison of the present results with the data obtained by a hardware-in-the-loop (HIL) test rig, and there is a good agreement. Based on the validated model, the influence of piston area, rod-to-piston area ratio and mounting position of the hydraulic cylinder, displacement of the hydraulic motor, initial volume and pre-charge pressure of the high-pressure (HP) gas accumulator and effective damping of the generator on the power capture ability of the two-raft-type WEC is given and discussed. The variations of the optimal hydraulic PTO unit parameters and their corresponding peak capture width ratios with the wave states are also illustrated.