Two novel pressure recovery devices for compressed air energy storage: A comparative study

To mitigate throttling losses during the discharge process of constant-volume compressed air energy storage systems, this paper proposes the integration of two novel pressure recovery devices: the liquid piston expansion module and the stepped ejector group. To investigate their operational characteristics, a dynamic simulation model is established for the liquid piston module, while a one-dimensional semi-empirical model is developed for the stepped ejector group. Results show that integrating the liquid piston module increases the compressed air energy storage subsystem’s round-trip efficiency by 2.47%, with overall system round-trip efficiency and exergy efficiency improving by 1.42% and 1.49%, respectively. The hybrid system’s CO₂ emission per unit energy output and levelized cost of energy also decrease by 2.66 kg/MWh and 4.35 $/MWh. Integrating the stepped ejector group boosts the gas turbine’s additional power output by 7.42% relative to the baseline. Under design conditions, the liquid piston module reaches a specific work of 27.06 kJ/kg and discharge efficiency of 82.87%, while the ejector group entrains 60.17 tons of gas. Analysis of the liquid piston module reveals that cavern air storage pressure exerts a pronounced influence on the working chamber’s minimum water temperature, with an 80%–120% variation from design pressure reducing it by 5.19 K. An oversized chamber volume adversely affects performance, as a 10%–20% upscaling diminishes discharge efficiency by 4.08%. Enhancing sliding-pressure operation efficiency is achievable through longer cycles, a greater number of nozzles, or an elevated nozzle pressure differential. Regarding the ejector group, a cavern pressure range of 80%–120% of the design value augments the entrained gas mass by 84.54 tons. While medium-pressure gas (1.7–2.3 MPa) prolongs the first discharge stage by 757 s, it curtails the fifth stage to a mere 23 s. Low-pressure gas (0.8–1.2 MPa), on the other hand, boosts the entrained mass by 68.81 tons. It is noteworthy that the marginal performance gains from adding stages diminish in high-stage configurations.These findings provide theoretical guidance for optimizing compressed air energy storage systems.