Design and Simulation of a Wave Energy Converter with Hydraulic Power Take Off (PTO) system

Abstract

In the battle against climate change, transition to renewable energy is an important and practical strategy. Wave energy has a large energy density and is largely unexploited. Waves are more predictable and consistent than wind energy and has larger potential. However, there are still drawbacks and continuous new developments in the field have tried to tackle these issues. The focus of the study was on a design of a wave energy converter to be mounted on a conventional Korean buoy (LS-24 model) with a hydraulic PTO system. The system is deployed nearshore with waves of significant wave height of 0.58m and period of 4.66s. Hydrodynamic analysis using ANSYS Aqwa, performed on the WEC model indicated motion response of the floating WEC and mechanical output of the wave absorbers. The WEC design consist of two cylindrical floaters that are connected to a central buoy through joint connections allowing 1 degree of freedom rotation. Mechanical power due to the oscillating angular motion of each floater is about 300 watts. This power is only an initial estimation of the power absorbed from the waves. The oscillation has high torque but low velocity reflecting the nature of wave power. The mooring configuration, which maintains stability of the buoy, is also simulated. The hydraulic PTO system is designed to convert the fluctuations in wave power to a steady and regular hydraulic output. The accumulator, which is a crucial component of the hydraulic circuit, provides hydraulic fluid at high pressure to the hydraulic motor, which generates power at high speed. Simulation of the hydraulic circuit showed that the motor at a speed of 325 rpm and 25 bar hydraulic pressure produced about 350 watts of mechanical power. The PTO system performed effectively in absorbing the fluctuation in the input from the floaters and providing a stable hydraulic output to the hydraulic motor. Experimental testing on the PTO system was also carried out which indicated that the motor was able to produce 318 watts of power at a speed of 800 rpm and a hydraulic pressure of 27 bar. The accumulator was charged to 27 bar and then discharged to run the motor. A voltage output of 27 volts was measured which was appropriate for charging a 24-volt battery bank. The detail methodology and results of the study are shown in the corresponding sections of the report. The findings conclude that the WEC is able to perform reliably under the wave conditions. More importantly, the hydraulic PTO system is able to effectively absorb the fluctuations in wave power and convert it to a regular hydraulic output.