| Due to the energy crisis and environmental issues,converting clean and renewable wave energy into electricity has been a research focus in recent years.The oscillating-body wave energy converter(WEC)is a promising approach,especially for offshore areas.However,conventional oscillating-body WECs with linear power take-off(PTO)systems are less efficient under off-resonance conditions and have a narrow power capture bandwidth.In order to enhance the power capture performance,this study proposed novel PTO systems and control method to conventional WEC in order to improve the power capture performance.At first,the thesis studies the hydrodynamic performance the oscillating WEC with linear PTO system.The frequency domain equation and expressions of best parameters of the WEC are established based on potential theory with only heaving motion considered.An optimized oscillating floater is designed.Then this thesis examines oscillating-body WECs with three new types of PTO systems: bistable impulsive PTO,coupled linear PTO,and coupled bistable PTO.Governing equations for the heave motion of a WEC were established based on the linear potential theory by coupling the buoy and PTO systems and were numerically calculated with the fourth order Runge–Kutta method.The influence of PTO parameters such as the spring constant,stable equilibrium position,and mass ratio on the power capture ratio was investigated.The differences among the WECs with different PTOs were analysed to determine the optimum device.Compared with the linear PTO system,the WEC with the bistable impulse PTO system enhanced the power capture ratio for low-frequency regular waves,and WECs with coupled linear or coupled bistable PTO systems increased the power capture ratio for low-frequency regular waves and expanded the power capture spectrum bandwidth when proper PTO parameters were applied.This thesis finally applies a magnetic transformation control method to the WEC with nonlinear PTO systems.The control system responds based on the velocity of the inner magnet but not the unpredictable irregular waves.The power capture performance of the WEC with control is studied and the influence of stable equilibrium position and response time is investigated.Results show magnetic transformation control can improve the power capture performance of the WEC and the advantage will be more prominent with a higher stable equilibrium position and less response time. |
