Analysis and optimization of a quasi-isothermal compression and expansion cycle for ocean compressed air energy storage (OCAES)

A numerical analysis of a quasi-isothermal thermodynamic cycle was undertaken for its application in an underwater energy storage system. The conceptual basis for the quasi-isothermal process is firstly a use of water pistons, as opposed to air or other gas medium, which improve heat transfer rate and minimize the temperature variation on both compression and expansion sides of the cycle and secondly a use of mechanical design that maximizes a surface area of heat transfer. Numerical analysis of the heat transfer cycle confirms the validity of the quasi-isothermal nature of the water pistons. Design factors such as surface area, stroke displacement, and frequency of piston action can be analyzed for optimality. For a case study, a recent commercial design of the quasi-isothermal process is introduced and partially analyzed for its effectiveness. Impact of varying several design factors have been analyzed numerically for further understanding of optimality and for validating the quasi-isothermal nature of the design.