Modeling and Sizing of an Undersea Energy Storage System

This paper presents modeling and sizing of an undersea energy storage system (USS). The USS, which is placed at the seabed, consists of a concrete sphere, a reversible pump-turbine unit, a permanent magnet synchronous machine, and a steel pipe through which water flows into/out of the sphere from/to the deep ocean. A novel mathematical model is derived to describe the governing equations of the USS operation. It is assumed that the USS operates in parallel with a direct-drive wave energy converter (DDWEC) and a novel sizing algorithm is developed to obtain the required USS capacity/size for regulating the DDWEC output power fluctuations during a designated period of time. In order to determine the DDWEC output power profile, wave spectrums are required. Real significant wave height and dominant wave period data, which are measured and collected by a buoy station in the Gulf of Maine, are used to generate wave profiles based on the modified two-parameter Pierson-Moskowitz spectrum. Eventually, Monte Carlo simulations with a thousand generated wave profiles are carried out to determine the optimal size of the USS for all possible scenarios. Furthermore, through promising results obtained from hardware-in-the-loop studies, the effective role of the USS in regulating the wave energy converter output power fluctuations is demonstrated.