Modeling of liquid-piston based design for isothermal ocean compressed air energy storage system

•Design specifications for 2 MWh compressed air energy storage at 500 m ocean depth.•Liquid-piston based compressor/expander system design and its sizing for the desired storage pressure.•Improvement of roundtrip efficiency for the 2 MWh ocean compressed air energy storage system.•Isothermal compression/expansion can significantly improve the roundtrip efficiency. Ocean renewable energy resources are intermittent, and a large scale energy storage is needed for their optimal utilization. Ocean compressed air energy storage (OCAES) system is a promising large-scale energy storage for integration of ocean energy with the electric grid. In OCAES, energy is stored in the form of compressed air in an underwater storage device. In this paper, modeling and design of various components in the OCAES system are presented. Furthermore, design specifications and efficiencies of the various components in the 2 MWh storage system at 500 m ocean depth are evaluated. Design of compressor/expander is crucial for high efficiency and sensible component sizing. Trade-off between the polytropic index and stroke time should be addressed in the compressor/expander design. Compressor/expander volume can be divided into a large number of liquid piston cylinders to achieve high efficiency. Greater ocean depth of the air storage system decreases storage volume requirement but affects roundtrip efficiency. The roundtrip efficiency of OCAES can be significantly improved by designing compressor/expander operating under near-isothermal conditions.