Energy synergy and optimization in a novel insulated near-isothermal compressed air energy storage and freshwater co-generation system

Coastal and island regions commonly face the coupled challenges of unstable energy supply and freshwater scarcity. To address this issue, a novel adiabatic–near-isothermal compressed air energy storage (CAES) system coupled with seawater desalination is proposed for coordinated energy–water supply. Existing energy storage–desalination systems generally suffer from a trade-off between round-trip efficiency (RTE) and desalination performance, while their system-level energy synergy remains insufficiently exploited. In this study, a full-process thermodynamic model is established to investigate the energy synergy mechanisms and parameter-matching characteristics of the proposed system. The results show that the variable exhaust pressure mode of the liquid piston provides better overall performance than the constant exhaust pressure mode. The pressure ratio allocation between adiabatic and near-isothermal compression, together with the liquid-to-gas mass ratio, is identified as the dominant factor governing the trade-off between electricity storage and freshwater production. When electricity supply is prioritized, the system achieves a maximum RTE of 56.52% and an exergy efficiency of 50.18%; when freshwater production is prioritized, the maximum recovery rate reaches 11.61%. Multi-objective optimization under ambient temperatures of 283–308 K further shows that the system exhibits better energy storage performance at lower ambient temperatures and relatively better freshwater-oriented performance at higher temperatures. At 293 K, the TOPSIS-selected compromise solution yields an RTE of 56.07%, an exergy efficiency of 50.47%, and a recovery rate of 10.98%. demonstrate that the proposed system can realize flexible and efficient electricity–freshwater cogeneration through rational parameter matching and operation strategy adjustment, providing theoretical support for integrated energy–water systems in coastal and island regions.