Dimensionless thermal performance analysis of a closed isothermal compressed air energy storage system with spray-enhanced heat transfer

The isothermal compressed air energy storage (I-CAES) technology boasts the advantages of high theoretical round-trip efficiency and zero carbon emissions. In order to rapidly and efficiently assess the thermodynamic performance of I-CAES, and addressing the limitations of the overly complex thermodynamic model for spray heat exchanged closed-loop I-CAES (CI-CAES), this paper establishes a dimensionless evaluation model related to container size and initial state a^*, spray water quantity b^*, start or end spraying time c^*, and the ratio of liquid level to droplet velocity u^*. The expressions for evaluation indicators such as round-trip efficiency, indicated efficiency, compression exergy efficiency, and expansion exergy efficiency are summarized, revealing the impact patterns of dimensionless numbers on the thermodynamic performance of CI-CAES. The results show that the relative difference in the percentage of air enthalpy change calculated using the real gas model versus the ideal gas model during the expansion stage is 88.26 %. The indicated efficiency and round-trip efficiency of CI-CAES simulated by the real gas model were 88.18 % and 60.51 %, respectively, for the design parameters, and the variable condition operation of the hydrodynamic machinery was the main reason for the lower round-trip efficiency. The larger the values of a^* and b^*, the closer to the isothermal process, and the greater the effect of b^* on the performance of the CI-CAES system compared to the two. When b^* is 50, the polytropic indices of the compression and expansion stages reach 1.03 and 1.01, respectively. The round-trip, compression exergy efficiency and expansion exergy efficiency all vary parabolically with c^*, suggesting that there is an optimal c^* that optimizes the system performance. The change in pressure ratio has a greater effect on round-trip efficiency and on the percentage change in air enthalpy during compression and expansion, and u^* has a lesser effect on system performance. The results of the research can provide theoretical guidance for the efficient operation of CI-CAES.