CFD Analysis of a Subsea Liquid Piston Accumulator for Energy Storage Applications

Abstract Hydro-pneumatic energy storage uses liquid pistons and hydraulic machinery to store energy by compressing air. The technology offers significant potential for co-location with offshore wind farms, where the sea water may be used to augment the heat exchange between the air undergoing compression or expansion and the surrounding environment, hence improving the thermal efficiency by operating at isothermal conditions. A major challenge for the numerical simulations to assess the transient thermal behaviour of such a system is the lack of appropriate models for the convective heat transfer coefficients. This paper addresses this knowledge gap through the use of CFD by simulating a subsea accumulator housing a liquid piston compressing the air from atmospheric conditions up to a pressure of 100 Bar. The adopted numerical approach examines the heat exchange between the air undergoing compression and the surrounding sea water, with the internal and external heat transfer coefficients being derived concurrently. It is noted that the internal heat transfer coefficient is significantly dependent on the state of charge of the accumulator. Yet, quasi-isothermal conditions were predicted by the CFD model for the simulated conditions. The paper provides insight about the complexity of the heat transfer phenomena related to subsea liquid pistons and the need to derive correlations specifically for this application.