Numerical Modelling of the Discharge Cycle of a Subsea Hydro-Pneumatic Energy Storage System

This paper presents a numerical model of the discharge cycle of a subsea open-cycle hydro-pneumatic energy storage system intended for offshore long-duration energy storage. During discharge at high pressure ratios, air expansion can lead to significant cooling, penalising system performance. The modelled system comprises a subsea pipeline which stores compressed air coupled to a pair of reciprocating liquid pistons that expand the air to drive a hydraulic motor. The study focuses on the transient thermal behaviour of the system during air expansion at high pressure ratios, starting from an initial pressure of 200 bar in the subsea air receiver pipeline down to a target pre-charge pressure of 80 bar. A parametric study investigates the influence of the output hydraulic power and the convective heat transfer coefficients, assessing the ability of the system to approach ideal isothermal expansion. The results indicate that for the high pressure ratios considered and using currently available heat transfer coefficient correlations, significant cooling occurs within the subsea liquid piston pipeline. For a baseline output hydraulic power of 500 kW, a polytropic index of 1.23 and a work ratio just below 64% were obtained. However, the results also show that by reducing the output hydraulic power and integrating internal heat transfer mechanisms, this cooling can be substantially mitigated, resulting in quasi-isothermal conditions with work ratios higher than 86%.