Analysis of Wave Energy Converters and Impacts of Mechanical Energy Storage on Power Characteristic and System Efficiency

Wave energy is largely untapped, and together with the offshore wind farms, has a great renewable energy potential globally. However, due to the fluctuating nature of the wave power, designing a robust wave energy converter (WEC) without overloading both power take-off (PTO) system and electric generator is highly challenging. Although battery storage systems can be considered on the electrical side, mechanical energy storage is also required to reduce the peak power that has an impact on the rotating shaft design and the ratings of the generator hence the inverter. This paper provides a dynamic model of a hydraulic PTO system in WECs and analyses the mechanical output power characteristics and subsystem efficiencies under various wave conditions. Two mechanical energy storage options are considered in the analysis by adding inertia and changing the size of gas accumulator. A fully submerged point absorbed WEC with hydraulic PTO system is proposed and simulated using these mechanical energy storage methods under four distinct levels of wave conditions (sea states). It was found that, both storage methods can effectively reduce power variation, and their size has a significant impact on the efficiency of the hydrodynamic system and the performance of PTO system. Moreover, if the size of the tuning gas accumulator is selected appropriately, the resonance impact of added inertia can be eliminated without impacting power absorption bandwidth under most sea states.