A Numerical Study on a Floating Hemisphere Wave Energy Converter with Hydraulic PTO system

The aim of this study is to conduct a hydrodynamic analysis of a point absorber-type floating hemisphere with Power take-off(PTO) system for an optimum design of wave energy converter(WEC). In general, to predict the behavior of WEC, the equation of floating body motion can be solved in the frequency-domain analysis. A numerical model of PTO system is usually expressed as a linear damper-spring type force. In this study, however, a hydraulic-pump-type PTO system was modeled to reflect the behavior of WEC accurately. To describe the hydraulic PTO system, which is strongly nonlinear, the coulomb damping force was applied. In the time-domain analysis, Cummins equation was employed to consider the coulomb damping force in the equation of motion. Using the developed numerical model for a floating hemisphere WEC, regular wave analysis of hydrodynamic performance was conducted firstly. Heave RAOs were compared for various hydraulic pressures of the PTO system. The generated wave powers, expressed as the combination of buoy velocity and coulomb damping force, were also compared for various WEC conditions. Including PTO system with all possible damping, the modified equation of motion in irregular waves was finally solved to obtain the response of the hemispheric buoy. Significant displacement of the buoy and generated mean power of WEC were then obtained. Based on the parametric study on various wave conditions of selected areas, an optimum design and condition can be proposed to operate the present WEC.