The dynamics and power absorption of cone-cylinder wave energy converters with three degree of freedom in irregular waves

Highlights: • Motions and wave energy capture of a cone-cylinder WEC with three DOF are presented. • Effect of PTO damping, design parameters on power capture and efficiency are studied. • An optimized WEC is chosen. The absorbed power and efficiency are 38.95 kW and 60.51%. • Power capture is predicted for the optimized WEC at six stations in East China Sea. • At six stations, the absorbed power is 19.18–38.59 kW and efficiency is 37.98–47.95%. Considering the coupling between pitch and surge, the hydrodynamics and power absorption of cone-cylinder wave energy converters (WECs) in surge, heave and pitch, are investigated in irregular waves. The fitting of the infinity frequency added masses and the parametric model identification for convolution items in motion equations are performed. The influences of the power take-off (PTO) damping on the power absorptions in surge, heave and pitch, are analyzed. The optimal PTO dampings are predicted for different cases. The influences of the buoy diameter (D), the draft (T) and the centre-of-gravity position (z_g) on the motion responses and power absorptions in three directions are investigated. The average powers absorbed in heave and pitch increase with increasing D. The mean power absorbed in surge increases with increasing T. The mean power absorption in pitch increases with decreasing z_g. An optimized cone-cylinder WEC is selected with D = 8 m, T = 4 m and z_g = −1 m. The wave power absorption and the absorption efficiency of the optimized WEC at six stations off the coast of Southern East China is predicted. The optimized cone-cylinder WEC can absorb between 37.98% and 47.95% of the incident wave power at six stations.