The Investigation Of Hydrodynamic Performance Of WEC-Breakwater Integrated System

The concept of integrating the WEC with breakwater is believed to be an available solution to reduce its cost by sharing part of the essential infrastructure,and some researchers' found that the integrated system could improve the hydrodynamic performance of the wave energy converter.Therefore,this paper analytical investigates the hydrodynamic performance of oscillating buoy wave energy converter-floating breakwater and oscillating water column converter-bottom mounted breakwater integrated system within the framework of linear wave theory.In the model,assuming that the fluid is inviscid,incompressible and the flow is irrotational,the eigenfunction expansion matching method and technique of variables separation are used.The solving process starts from the study of hydrodynamic performance of the integrated system in one degree of freedom(1 DoF),gradually investigates and analyzes coupling motion,including 2 DoF motion of front structure and motion of front structure coupling with air volume change rate in air chamber,and the wave energy capture ability and wave attenuation performance of the integrated system are discussed.Firstly,the hydrodynamic performance of oscillating buoy wave energy converterfloating breakwater integrated system is investigated when the front pontoon moves in single DoF.The effects of structure parameters in hydrodynamic efficiency and transmission coefficient are given.Whether the front pontoon is in surge motion or pitch motion,it is found that the effective working state of integrated system is in relative high frequency range(kd> 2)and the peak value is up to 1.Moreover,when the front pontoon is in surge motion,the increase of the front pontoon width,draft and spacing between pontoon 1 and 2 would shift the peak efficiency to low frequency range.However,the variation trends of hydrodynamic efficiency with structure parameters are relatively complex because the stiffness of the front pontoon in pitch mode gradually increases with the increase of the pontoon width or the decrease of the draft.Secondly,the hydrodynamic efficiency,reflection coefficient,transmission coefficient and free surface elevation inside the gap are compared within three kinds of 2 DoF motion(surge and heave,heave and pitch,surge and pitch).It can be seen that the effective bandwidth(when the transmission coefficient KT < 0.5 and the hydrodynamic efficiency Cw > 0.3)are greatly enhanced in the 2 DoF motion system,and the free surface elevation inside the gap is decrease,which is beneficial to the stable and secure operation of the integrated system.In addition,the optimal power takeoff(PTO)damping is solved,and it is found that the synergism(that is,the ability of wave energy conversion is better in double DoF motion)between the two motions exists but is not obvious.Finally,the effects of geometrical parameters in hydrodynamic performance of the oscillating water column converter-bottom mounted breakwater integrated system are investigated.Comparing with fixed OWC wave energy device,the analytical results show that the peak efficiency(referring to the frequency range when Cw>0.5)of present system is in relative high frequency zone(kd> 2)and the peak efficiency range is broadened significantly.The peak of hydrodynamic efficiency shifts to low frequency with the increase of front wall width and chamber width,and the peak efficiency range gradually increase with increasing the chamber width.