Study On Hydrodynamic Performance For Oscillating Floater Buoy Wave Energy Converter

With the development of our society, the fossil fuels crisis turns out to be an irreversible stimulant for the development of ocean wave energy conversion technology. The technology of oscillation floater has a bright development prospect for the advantage of simple construct, strong wave resistance, higher energy conversion efficiency and wide frequency response range. Some oscillation floater wave energy devices are nearing commercialization abroad, while they are still in the initial stage with lagging development at home. Due to the huge difference in wave conditions at home and abroad, it’s the only way to develop the oscillation floater wave energy conversion device independently and systematically combined with the character of short offshore wave period and relatively low energy density of our country, and the study of the hydrodynamic performance is the basic work. This paper studies the hydrodynamic performance and the characteristics of wave energy trapping of the floating wave energy convertor via theoretical analysis, numerical calculation and model test.This paper gives an overview about the international research situation and the development tendency of the wave energy generation devices, in which the research progress of the floater wave energy generation device home and aboard is especially introduced. Then the content of this paper is pointed out based on the analysis and comments of the main problems of the existing devices.According to the features of the floater wave energy generation device, the first order diffraction theory and Froude-Krylov theory are used to analyze the wave load on the floater and pontoon. Then the solution of the first order diffraction velocity potential is found with the characteristic function method and matching asymptotic coefficient method and the semi-analytical expressions of floater’s heaving and surging wave exciting force and pitching wave excitation torque are obtained. After that, the programming solution is accomplished. Finally the equal-diameter floater’s heaving and surging added mass coefficient, added damping coefficient, pitching additional rotational inertia coefficient and additional rotational damping moment coefficient are deduced by using first-order radiation theory and the programming solution is accomplished. Studies have shown that the calculation results of the two programs are the same to that of the SESAM software at high frequency so that it can be used in the hydrodynamic performance analysis of the wave energy generation device. It is more in line with the actual situation that the vertical wave force on the floater with the full consideration of the wave diffraction effect is20%lower than that without the wave diffraction effect under the same frequency. The vertical wave force on the floater is5%of the force on the floater under the same frequency, which reveals the working mechanism of the oscillation floater wave energy device.On the basis of hydrodynamic analysis, the coupled equations of the motion of floater and pontoon in the frequency domain are established separately when only considering the heaving motion of the floater and supposing the power output system is linear and adopting a slack mooring method. The frequency response function of the floater wave energy convertor system is obtained and the system’s capture power and energy capturing width under the regular wave are deduced with the equations as well. The effects on the energy capturing width of the floater’s mass, the pontoon’s mass, the floater draught line and the external damping coefficient are studied with the frequency domain model. The results show that the device has the largest power output when the mass ratio of the floater to the pontoon is0.14. With the same size, the shallower waterline of the floater has the bigger capture power and energy capturing width. The hydraulic damping system has the biggest effect on the power output system. With the same device, more energy is captured with higher damping at low frequency while the damping has little effect on the energy capturing at high frequency.Based on the theory and analysis above, a physical model with a ratio of10:1is accomplished with the similarity criterion. A model test in a towing tank is performed for the further study on the affecting factors of the device’s hydrodynamic performance such as the damping of power system, the height of the wave, the wave period, the proportional relationship among the mass of floater and pontoon and the diameter of the floater. The results of hydrodynamic model test are basically consistent with the results of the theoretical analysis, which further verifies the correctness of the theoretical analysis.