The Research On Hydrodynamic Characteristics And Energy Conversion Efficiency Of A Wave Energy Converter In Nonlinear Regular Waves

Wave energy,with the characteristics of high density and wide distribution,is a kind of inexhaustible,renewable and clean energy.The development and utilization of renewable energy,such as wave energy,will play an important role in solving the problems of energy crisis,environmental pollution and climate change.In such a case,the wave energy converters have been developed.It is a kind of device which is specially installed in ocean and used to absorb wave energy.This paper will investigate the hydrodynamic performance and the efficiency of the device in depth.The hydrodynamic performance and efficiency of a wave energy converter will be investigated based on the potential flow theory with the fully nonlinear boundary conditions.The whole boundary,including the body surface,free surface,truncated boundary and bottom,will be divided into a series of linear element through the boundary element method.The free surface is tracked through the nonlinear free surface kinematic boundary condition while the velocity potential on the free surface is updated through the dynamic boundary condition.The auxiliary function method is used to calculate the pressure of body.For a long time simulation,a numerical damping zone of a wavelength is set at the area near the truncated boundary,to eliminate disturbed wave due to the body motion and the reflected wave due to the control boudary.In the time-domain,the fourth order Runge Kutta method is adopted to realize the time integration of free surface.In this paper,the hydrodynamic characteristics of a two-dimensional body in nonlinear regular waves are first studied.The wedge is forced to enter into a 5 order Stokes wave with infinite water depth at a constant speed with two degrees of freedom(vertical and horizontal).The stretched system in the spatial domain is adopted based on the ratio of the Cartesian system in the physic space to the vertical distance the wedge has travelled into the water.A separate treatment at initial stage is used,in which the solution for the disturbed potential is sought to avoid the very large incident potential amplified by dividing the small travelled vertical distance of the wedge.The detailed results through pressure distribution are provided,to discuss the effect of gravity and wave.The deformation of free surface is described,and its effect on the pressure distribution is analyzed.Based on the mathematical model of a two-dimensional body moving in nonlinear regular waves,the hydrodynamic performance of a wave energy converter moving in nonlinear regular waves is further studied,and the effect of fluid gravity,wave force and free surface deformation on pressure distribution is mainly investigated.When the wave energy converter rotates in waves,slamming will occur,the local pressure near the intersection point between the body surface and free surface will therefore become very large and splash jet will therefore be developed.In such a case,the whole computational domain can be divided into two sub-domains and the domain decomposition method will be used.The first sub-domain corresponds to the splash jet while the other one corresponds to the main domain.On the interface of the sub-domains,the continuity of the pressure and velocity is enforced.There is no overlapping in each sub-domain and the conventional BEM can therefore be used.The paper focuses on the efficiency study of wave energy converters installed in nearshore,namely Oyster and Waveroller.The mathematical model of interaction between regular waves and wave energy converters will be developed based on nonlinear time domain method and linear frequency domain method.When time domain method is used,the coupled equation of motion is given under the given condition of wave and power take-off system,in which the equation of motion and the fluid force are both decoupled.The motion of flap,the absorbed energy and efficiency can therefore be obtained.When the frequency domain method is adopted,the equation of steady motion is established based on the condition that the velocity potentials in the fluid field are decomposed,in such cases the explicit equation of linear energy conversion efficiency can be deduced,and the condition of optimal efficiency achieving is analyzed.Through the theoretical analysis in the frequency domain,it is concluded that the efficiency can arrive at the maximum when the natural frequency is equal to the wave frequency and the mechanical damping of the PTO system is the same as the radiated damping.In the numerical analysis,the results based on the nonlinear time domain method are compared with that on the linear frequency domain to verify the accuracy of the numerical procedure.Nonlinear effects are analyzed in severe sea conditions,and the mechanism of wave energy absorption and the method of efficiency improving are both studied in the nonlinear system.