Hydrodynamics Of Breakwater-power Generation System With Irregular Bottom Based On Semi-analytical Method

As a sort of green energy with potential,the commercial application of wave energy is limited by the high costs in the manufacture and installation of wave energy devices.A hybrid system(hereinafter referred to as breakwater-power generation system)integrating a power take-off(PTO)system into a floating breakwater,which can capture wave energy while attenuating transmitted wave energy,is a solution for both shoreline protection and commercial application of wave energy.Although it is proved that geometrical shape has great influence on the hydrodynamic performance of the breakwater-power generation system according to the relevant research,the influencing mode and corresponding mechanism remain unclearIn this study,a two-dimensional semi-analytical mathematical model dealing with floats with arbitrary bottom shapes is established based on the potential flow theory.The semianalytical model is validated by a hydrodynamic coefficients comparison to a program based on high order boundary element method,WAFDUT-2D,and the energy conservation test.To quantify the geometric asymmetry reflected by breakwater-power generation system with different contours,the degree of asymmetry and the absolute asymmetry are newly defined mathematically.A set of symmetric and asymmetric breakwater-power generation systems are selected in comparative studies to demonstrate the effect of PTO damping and geometric asymmetry on the transmission coefficient,the reflection coefficient,and the energy conversion efficiency.The result shows that no matter the hybrid system is symmetric or asymmetric,a larger PTO damping is beneficial for wave attenuation in longer waves,particularly at the heaving natural period of the device.On the premise that the PTO damping is optimized,an increase in the degree of asymmetry greatly improves the energy conversion efficiency.An increase in the absolute asymmetry slightly improves the ability of wave energy attenuation.In addition,breakwater-power generation system is also suggested to be a replacement of traditional wave absorbors with a boundary condition of a partially reflective wall introduced into the above semi-analytical model.The model is validated by the comparision of the results of the published paper and energy conservation test.The influence of partially reflective wall and degree of asymmetry on the hydrodynamic performance of breakwater-power generation system is studied based on the semi-analytical model.First,the maximum wave amplitude in front of the partially reflective wall wall is studied.The wave amplitude is found to change greatly and regularly.Then the influence of the distance between breakwater-power generation system and the coastal wall on the hyrodynamic performance of breakwater-power generation system is studied.The hydrodynamic performance of the breakwater-power generation system is optimal when it is close to the wall.A set of symmetric and asymmetric breakwater-power generation systems are selected and placed closely to the wall in the comparative research.The influence of the degree of asymmetry and the reflection coefficient of the wall on the hydrodynamic performance of the breakwater-power generation system are studied.The existence of the wall can significantly improve the hydrodynamic performance of breakwaterpower generation system.And the improvement of the hydrodynamic performance of the breakwater-power generation system with a smaller degree of asymmetry is more significant.Finally,the impact of the degree of asymmetry,changed by varying the bottom slope,and reflection coefficient of the wall on the hydrodynamic performance of breakwater-power generation system is further studied.The influence of the degree of asymmetry decreases as the reflection coefficient of the wall increases.And in the situation of total reflection,the difference between different breakwater-power generation system is mainly reflected by the corresponding periods of the peak of the energy conversion efficiency peak or the trough of the reflection coefficient.It is necessary take the incident wave condition into account to choose a suitable breakwater-power generation system.