Power capture and output of a three-degree-of-freedom wave energy converter with a parallel hydraulic cylinder system

Enhancing the capture efficiency of wave energy converters is a challenge for wave energy generation. A three-degree-of-freedom wave energy converter (TDOFWEC) with a parallel hydraulic cylinder system is proposed. A three-dimensional nonlinear coupled dynamic model of a converter interacting with parallel hydraulic cylinders was established using the Lagrangian approach. Pierson-Moskowitz spectrum is adopted for irregular waves. The coupled radiation force between pitch and surge is included. The nonlinear hydrostatic restoring forces caused by displacement volume changes of different shapes and the influence of pitch and roll motion on the heave restoring force are modeled. The hydrodynamic coefficients, wave excitation forces, motion responses, accumulator pressures, power capture, and motor output dynamic performance are investigated. Using a genetic algorithm, power take-off parameters are optimized. The effects of rotation pair angles, hinge center radiuses, accumulator initial pressure and volume, hydraulic cylinder length and area, motor displacement, generator damping coefficient, wave states, geometry parameters, and shapes on the capture power, output power, capture efficiency, and hydraulic conversion efficiency are investigated. As three angles and hinge center radiuses are optimal, a smaller motor displacement matches with a piston area, a higher capture power can achieve. As the wave height is 2 m, the maximum capture and output powers are 70.2 kW and 48.5 kW, the capture efficiency is 85.2 %. The TDOFWEC improves the capture efficiency by 53 % compared with a cone in heave. As the diameter is 8 m, the maximum capture efficiency of the semiellipsoid, cone, and truncated cone is 90.8 %, 88.7 %, and 88.3 %.