Performance Evaluation of Floating Two-Body Wave Energy Converter with Hydraulic Power Take-Off System
In the present work, the dynamic coupling between a self-reacting floating two-body wave-energy converter (WEC) and hydraulic power take-off (PTO) system is evaluated. The WEC is surface meshed; the hydrodynamic properties are calculated using WAMIT, a boundary element method code. The multi-body dynamics of the WEC was analyzed using the open-source code WEC-Sim. The hydraulic PTO system considered for the present analysis is a constant pressure system with valves which rectify the flow to high-pressure and low-pressure accumulators. The heave response amplitude operator of the WEC for varying wave period shows that the float and torus have different peak heave response which makes them oscillate with a phase difference. In the combined mode, the WEC exhibits a peak heave frequency which is less than the natural frequency of the float. Various parameters that include the shaft power, absorbed power, and usable electric power for varying hydraulic motor rpm and the area of the hydraulic piston is calculated. The most probable sea state condition that exists on the US East Coast (wave height = 5.5 m and wave period = 7 s) is used for analyzing the WEC. The absorbed power of WEC initially increases with an increase in the size of the hydraulic piston. Once the WEC reached maximum absorbed power, increasing the hydraulic piston diameter decreases the absorbed power. The increase in hydraulic piston area increases the pressure energy stored in accumulators. Since the volume flow rate to the hydraulic motor is constant, increase in the area reduces the limiting velocity of the PTO system. Therefore, hydraulic motor and hydraulic piston should be chosen such a way that the PTO system will work at maximum efficiency. The maximum RPM of a hydraulic motor is determined by the size of the hydraulic piston, smaller the size of hydraulic piston lower the limit of hydraulic motor RPM and vice versa.
KeywordsWave energy converter Hydraulic PTO system Constant pressure hydraulic system Hydraulic motor RPM PTO efficiency
The authors would like to acknowledge the U.S. National Science Foundation for financial support for this work through the CYBERSEES program (Award # 1442858 ) and the GOALI program (Award # 1400164).
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