Preliminary proposal of high-temperature compressed air energy storage with reuse of depleted gas reservoirs

Depleted gas reservoirs present a compelling geological medium for large-scale compressed air energy storage, leveraging their inherent large storage capacity, negligible development costs and spatial compatibility with renewable energy deployment zones. While this application is gaining increasingly attraction as a key enabler of grid-scale energy transition, critical research gaps persist such as the underexplored potential of ultra-deep depleted reservoirs (>3000 m depth), the scarcity of comprehensive thermoeconomic assessments and the absence of holistic system-level design frameworks. This paper conceptually presents a high-temperature compressed air energy storage system that leverages ultra-deep depleted gas reservoirs (5000 m depth) as the primary air storage medium. The system is configured with a four-stage compression and three-stage expansion architecture, deliberately aligned with the current technological maturity of industrial compressors and turbines and the high pressure ratio inherent to deep geological storage. The first three stages are main compression processes corresponding to each stage expansion process according to the correspondence point theory. A thermo-economic analyzing model for the system is established. It is found that the system is provided with excellent performance of a 73.12% round-trip efficiency and a 9.33 kWh/m 3 energy density benefiting from the correspondence design and the additional geothermal energy. Simultaneously the levelized cost of storage is 0.6735 ¥/kWh at the off-peak electricity price of 0.3 ¥/kWh. This study is valuable to provide a reference for future practically deployable engineering design of depleted gas reservoir energy storage applications.