Hydrodynamic Performance Study Of An Oscillating-body Wave Energy Converter

Compared with solar and wind energy, wave energy is a kind of renewable resources with larger density of power. Various types of wave energy converters(WECs) have been proposed and studied including oscillating water column WEC, overtopping WEC and oscillating body WEC. This thesis focuses on the oscillating body WEC, with only the heaving motion considered. The hydrodynamic response and power capture performance of the oscillating body WEC are studied.The thesis firstly studies an oscillating WEC with nonlinear snap through power-take-off(PTO) systems. The time domain equation of the motion of the nonlinear WEC is established based on the numerical results calculated in frequency domain. Then state space model is used to replace the convolution term in the time domain equation, after which the dynamic response of the WEC is numerically calculated using 4th order Runge-Kutta method. Based on the calculated results, the effects of wave parameters and the damping coefficient of the nonlinear PTO systems on the power capture performance of the nonlinear WEC are discussed. Also comparison of the captured power is made between the nonlinear WEC and linear WEC.Then this thesis establishes the time domain motion equation of two oscillating body WEC with nonlinear PTO systems. The conventional interaction factor q and modified interaction factor qmod are introduced to study the influences of wave frequencies and directions on the power capture performance of the wave energy array.Control strategies are often implemented to enhance the power capture performance of WECs. The thesis conducts researches on latching control and declutching control of an oscillating body WEC in both regular and irregular waves. The control sequences are determined using the optimal command theory which is based on the Pontryagin’s maximum principle. Results show that latching control can greatly enhance the power capture performance of the WEC especially when the wave frequency is lower than the natural frequency of the WEC. And declutching control can slightly enlarge the captured power in resonant frequency region.Since a reliable prediction of wave excitation forces on the WEC is essential for both the latching and declutching control strategies, this thesis finally investigate the effects of wave excitation force prediction deviations on the latching and declutching control performance of the WEC, which takes into account both the amplitude and phase deviation in regular waves.