Hydrodynamic Research Of A Bottom-hinged Flap Wave Energy Convertor

Ocean wave energy is attracting much public attention at present. Among various wave energy technologies, the bottom-hinged flap technology is very prospective because of its broad bandwidth response, high conversation efficiency, strong wave-resistance ability, and good expandability. Some bottom-hinged flap wave energy converters (WECs) are to be commercialized overseas, while the research and development of bottom-hinged flap device is comparatively lagging domestically. Due to the large difference of sea states, to successfully apply the technology of wave energy utilization domestically, it is necessary to develop independently on the basis of China’s practical situation. Hydrodynamic performance research is only a basic work for the whole technology. Take the sea states—comparatively smaller daily wave and frequent extreme weather condition—f China into consideration, hydrodynamic performance research of WEC appears especially important.In allusion to China’s sea states, a combination of theoretical analysis, numerical modeling and model experiment is adopted to study the hydrodynamic performance of wave energy converters (WECs) of the bottom-hinged flap style systematically and analyze the PTO damping characteristic and parameter influence on the device’s hydrodynamic performance.Based on the linear wave theory, the two-dimensional analytical solution for wave’s action on bottom-hinged flap was deduced, the analytical solution was employed to study the basic dynamic characteristics of the bottom-hinged flap device, and the influences of parameters such as water depth, flap width, flap thickness, flap density, etc. on the device’s hydrodynamic performance were analyzed. The research result shows that on the two-dimensional condition, the bottom-hinged flap device’s maximum capture factor is50%; the influence of water depth on the device’s hydrodynamic performance is comparatively larger while the influence of flap thickness and flap density on it is comparatively smaller. When the PTO dumping adopts a low-frequency radiation damping, the capture factor of the device is almost the same as when the matched damping condition is satisfied. When the difference between the PTO dumping adopted and the low-frequency radiation damping is comparatively larger, the capture factor will be significantly reduced.The frequency-domain potential model was used to analyze the influence of four parameters---water depth, flap width, flap thickness and flap density---on the device’s hydrodynamic performance, the condition when the period of wave is around5.0s was discussed, and a comparative analysis was made in allusion to the two typical damping conditions. The analytic method was applied to discuss the influence of flap density on the device’s hydrodynamic performance, propose the concept of’optimum density’, figure out the frequency interval of the optimum density, and thus make a briefly optimization analysis of the device’s hydrodynamic performance in allusion to China’s typical sea states. The research result shows that for the short wave whose usual period is around5.0, the device’s hydrodynamic performance should be improved on the realm of a wave period that is smaller than the natural period as far as possible, and for the short wave whose usual period is less than6.0s, to achieve a good hydrodynamic performance for the device, the PTO damping adopting the optimum linear damping of period5.0s is an ideal choice; on the condition of adopting an optimum damping with a period of5.0s, the deeper the water is, the smaller the peak value of the power capture and the capture factor is, the smaller the influence of the flap width on the device’s hydrodynamic performance is, and the better the device’s hydrodynamic performance is when the flap thickness increases. On the sea states that the water depth is5.0m and the wave period is5.0s, adopting the optimum linear PTO damping, the device’s optimization width may be8.0m, the optimization thickness may be1.6m and the flap density should better be thinner.The time-domain potential model was adopted to analyze the bottom-hinged flap device’s hydrodynamic performance under the condition of constant PTO moment (a typical non-linear damping), and the stress situation of the bottom-hinged flap was briefly analyzed. The research result shows that when adopting linear damping, the dynamic response of the bottom-hinged flap is more regular and stable, while adopting a larger constant moment, the dynamic response of the bottom-hinged flap is obviously irregular; under the two conditions of linear PTO damping and constant PTO moment, the influence of the flap parameter on the device’s hydrodynamic performance is similar, and when the parameter of the wave is same to that of the flap, the device’s maximum capture factor is almost equal. Under the condition that the water depth is5m, the period of the incident wave is4.0s-11.5s, adopting a bottom-hinged flap of8m’s width,1.6m’s thickness and a density of300kg/m3, when TPTO=0.4～0.6Tex, the power capture of the device is25kW-60kW and the capture factor is0.6～1.0.A physical model of the bottom-hinged flap device was developed, and a magnetic powder brake was used for the first time to simulate the wave energy device’s PTO damping. Through large-scale flume experiment, the natural period of the bottom-hinged flap, the wave power’s nonlinear influence, the PTO damping characteristics, etc. were analyzed; the variation of pitch amplitude, power capture and capture factor along with the wave and the excitation current under the condition of irregular wave were investigated; the influences of wave amplitude, variation of water level and flap density on the device’s hydrodynamic performance were discussed; and the device’s hydrodynamic performance and the wave pressure on the flap surface under the condition of irregular wave were briefly analyzed. The research result shows that the nonlinearity of the test wave has little influence on the wave power; the PTO moment’s simulated result has a significant difference with the constant moment, and the wave period has an obvious influence on the device’s PTO damping characteristic and the amplitude of the bottom-hinged flap. According to the experimental result, under the prototype sea states (with the period of5.0s and the amplitude of0.34m), when the excitation current is0.6A, the power capture of the device is about14kw, and the capture factor is about60%. When the wave period is equal to the excitation current, the device’s hydrodynamic performance lessens as the amplitude increases, increases slightly as the water depth decreases. The experimental result of the flap density’s influence on the hydrodynamic performance is identical to that of the frequency domain digital analog.