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Drag coefficient effect on efficiency of Wave Energy Converter of “Power Buoy” type

The purpose of this study is to examine the effect of drag coefficient on efficiency of a wave energy converter (WEC). The considered floating WEC is a Power Buoy device that is a cylinder connected to the seabed by an extensible Power Take Off system. This study focuses on the case of plane movements of the WEC due to the action of waves, which propagate perpendicular to the axis of the cylinder. The Newton laws written for the WEC device results in a nonlinear second-order differential equation which is solved numerically by a fourth order Runge-Kutta method. The recovered energy is calculated for different drag coefficients. The Morison equation is used to add drag and added mass effects. In the first step, the drag (Cd) and added mass (Cm) coefficients are estimated by a comparison of the results obtained by the numerical code NEMOH and a “Morison” based method. By using last square method and evolutionary algorithm, the values for Cd and Cm corresponding to a non viscous fluid case are determined. Subsequently, for a given Cm, the Cdand the damping coefficient (ß) of the power take off device are changed for the sake of analyzing the effect of drag coefficient on recovered energy. The results show the effect of Cd on recovered energy in various situations.