| Ocean wave energy is considered to be one of the most promising renewable energy. In the numerous of wave energy utilization technologies, the floating multi-body wave energy converters (WECs), with the great advantages of big single-machine capacity, high energy conversation efficiency, strong wave-resistance ability and good sea condition adaptability,have exhibited good prospects for development. Deep study of this type of WEC is of important significance in relieving the energy crisis and environmental pollution. At present,the research on this WEC is relatively lagged in China. And the hydrodynamic performance analysis is the basic work to carry out the research of WEC. This paper systematically studies the hydrodynamic performance of the device by means of theoretical analysis, mechanism study in time-frequency domain, sea test verification and numerical simulation, against the background of practical engineering application. The mechanisms of the buoy motion and wave energy conversion are the key research contents.Based on the theory of multi-rigid body dynamics, the mathematical model of the motion of WEC in waves is established by considering the constraints and forces of PTO system(Power Take-off). The motion constraint equation is introduced to build an interconnected system for these multi-section buoys, and make the model solvable. On the basis of diffraction theory, the analytical expression of the wave excitation force and the wave excitation force moment of the double-section buoys are deduced. Then the analytical solutions are verified by numerical simulation method.Based on the multi-scale theory, the time-domain approximate expressions of the heaving-pitching motion of buoys in waves are deduced and solved in still water and waves.The motion responses of buoys and synthesis mechanism of displacement and velocity are analyzed from the deducing process and calculational results. Through the impact analysis of PTO system, the results show that the relative capture width of wave energy continues to increase with the coefficients of PTO damping and the return spring in a great extent.Therefore, it is not feasible to optimize the PTO system of this type of WEC in theory. The nonlinearity of the PTO damping has an impact on the pitching motion of buoy. And the relative capture width decreases with the increase of the PTO damping velocity index.According to the periodic characteristics of the buoy motion in waves, a linear equation set with four complex variables are established to express the amplitude-frequency response of buoys. The analysis results show that the pitching angles of two buoys are the same with a phase difference. By analyzing the effect of geometric size on the resonance, phase difference and wave energy capture, a geometry optimization strategy is put forwarded. The dimensionless optimal length, radius and draft, relative to wave length, are LN = 0.5 ,RN = 0.065 (T> 3.0s) and DN = 0.08 (T > 3.0s) in using this optimization strategy.Considering the environmental load of ocean current and wind, and restraint of mooring system, the CFD numerical model of the WEC in waves is established. Then the influences of wave parameters, mooring system, load of wind and current, and array, on the hydrodynamic performance of the three-section buoys WEC, are calculated and analyzed. Numerical results show that there is no clear rule between the transfer powers of the two sets of PTO under linear wave condition. In the case of small wave height, the relative capture width is basically the same, but when the wave height is large, the relative capture width decreases with the wave height. For the floating WEC studied in our paper, the water depth usually do not affect its motion and wave energy capture performance. Under irregular wave conditions, the wave energy transfer power of the device is approximately 38% -65% compared to the linear wave condition. And the transfer power of the two sets of PTO is roughly equal. Under the second-order Stokes conditions, the wave energy transfer power of the device is greater than that in the linear wave condition, which is about 110% -145% compared to the linear wave condition. And the relative capture width is about 55% -65% to the linear wave condition.When the cable is connected to the middle of the WEC, the relative capture width can be effectively improved. When the cable is connected to the two ends of the WEC, the transfer power can be improved with low-weight cable. The relative capture width is usually reduced slightly when the wind speed is relatively small. While the larger wind speed is favorable for increase of the relative capture width. In the stream, the motion and energy conversion of the WEC are very complicated. The stream mainly affects the device through two aspects: Firstly,the interaction of wave and stream cause the change of the motion period and amplitude of the buoys. This factor can be analyzed using the wave encounter period theory and the conservation principle of wave-current interaction. Secondly, the stream load acting on the WEC makes the tension of cable changes. In the study of array of WEC, the minimum distances between the WECs arranged in the array are about 1.0? in horizontal spacing and about 2.25, in longitudinal spacing,with which there are no obvious effects on the wave capture capability in the wave periods of 2.0s, 3.0s, 4.0s and 5.0s. |
PDF Full Text Request