Hydrodynamic Investigation Of A Land-based OWC Wave Energy Converter

Exploring renewable energy resources to realize the sustainable development of human society has become the consensus of the world.The ocean possesses abundant renewable energy.The wave energy,one of the marine energies,is considered to be a kind of promising renewable energy due to its high energy density and enormous reserves.The oscillating water column(OWC)wave energy converter(WEC)has become the most implemented in the world due to its structural and mechanical simplicity.However,there are still lot of problems that require deeper and systematical investigation,including,but not limited to:the effects of the wave nonlinearity and the viscosity on the hydrodynamic performance and the survivability of such devices;the issues of the low hydrodynamic efficiency and the narrow effective bandwidthIn this study,both numerical and experimental investigations are carried out to improve the hydrodynamic efficiency and the survivability of the OWC device.The main research contents are outlined as follows:(1)Based on the time-domain higher-order boundary element method(HOBEM),a fully nonlinear numerical model has been developed to simulate the hydrodynamic performance of an OWC device.The energy loss resulting from the flow separation and the vortex shedding are considered by introducing an artificial viscosity term in the dynamic free surface boundary condition inside the chamber.Both the hydrodynamic performance of the single-chamber and dual-chamber OWC devices are modeled.The developed numerical models are validated by comparing with experimental measurements.It is found that the numerical model captures well the main hydrodynamic behaviors of the OWC device,and provides accurate predictions of parameters of interest,including the free surface elevation,air pressure inside the chamber and the hydrodynamic efficiency.(2)The wave-induced forces on the OWC have been investigated by a modified Bernoulli equation.An artificial viscous damping term and the air pressure inside the chamber are introduced in the equation according to the OWCs' structural characteristics.Firstly,the modified Bernoulli equation is validated by comparing with the physical experimental measurements.The wave loads on both single-chamber and dual-chamber OWC devices are then calculated using the validated model.The disadvantage of the physical experiment that only pressure measurements at limited locations can be acquired is overcome.The effects of the wave conditions and the OWC chamber geometries on the wave forces are investigatedsystematically.(3)A set of carefully instrumented experiments was carried out to investigate the hydrodynamic performance of the single-chamber OWC device.The effects of the wave conditions,chamber geometries(including the chamber width,front wall draft and orifice dimensions)and bottom slope on the hydrodynamic efficiency are investigated and discussed systematically.Building on this foundation,the hydrodynamic performance of a dual-chamber OWC device is experimentally investigated for the first time.The effects of the incident wave conditions,chamber width ratio and chamber wall draft on the hydrodynamic performance of the dual-chamber are investigated and discussed.The results indicate that the dual-chamber structure can not only improve the optimum efficiency of the OWC device,but also could broaden the effective bandwidth when compared to a single-chamber OWC device that has the same size.These results form the basis for further optimizing the OWC systems.(4)The effects of the nonlinearity and viscosity on the hydrodynamic efficiency of a single-chamber OWC device and wave forces on the front wall of such device are investigated by using numerical simulations complemented by scale model testing.The results indicate that the wave nonlinearity enhances the wave reflection and the wave energy transformation from the fundamental waves to the higher-order harmonic waves outside the chamber.The transmission ability of the higher order harmonic waves is relatively small,which in turn,lead to a decreasing efficiency of the OWC device.The effects of the viscosity on the hydrodynamic efficiency reach its maximum when the device is operated near its resonant frequency.Hence,the hydrodynamic efficiency decreases more quickly with the incoming wave amplitude near its resonant frequency.The effects of the viscosity on the wave force on the seaward surface of the front wall is larger than that on the wall surface that faces the inside chamber.