In this study we use a hybrid frequency-time domain model to study the dynamics of a two-body wave-energy converter with hydraulic power take-off subject to phase control. Both bodies - a buoy and a semi-submersible platform - are restricted to move in the heave mode only, and the power is extracted from their relative motion.
The geometry of the buoy and the platform is chosen so as to obtain so-called force compensation, i.e. the vertical wave excitation forces on the two bodies are opposite and approximately equally large. The effect of force compensation is studied by varying the geometry of the platform in order to reveal its effect on the system dynamics and power absorption. Furthermore, one-body oscillation is compared to two-body oscillation for power absorption, and the effect of viscous damping of the platform motion is studied.
Simulation results are given both for regular and irregular waves, and they show that the platform geometry and degree of force compensation strongly influences the performance of the system. It is further shown that flow losses in a hydraulic power take-off system can be substantial, and should thus be thoroughly assessed. Also, the large potential increase in power output using phase control is demonstrated.