Analysis of energy storage rate in compressed air energy storage caverns under different air injection processes

Underground storage caverns are key components of compressed air energy storage (CAES) systems for compressed air storage and energy conversion. In this study, a wellbore-cavern integrated non-isothermal numerical model considering turbulent flow, gas-solid conjugate heat transfer between the air and cavern wall, heat conduction in the surrounding rock, and real air thermophysical properties was established. The effects of injection strategy on the energy storage rate of the cavern during the injection stage were investigated using CFD simulations. A spherical storage cavern with a volume of 268,000 m 3 was selected as the research object. Under a total injected air mass of 2.304 × 10 6 kg, an injection duration of 8 h, and an initial pressure of 4 MPa, four injection strategies were compared, including a constant mass flow rate mode, a linearly increasing mass flow rate mode, a stepwise decreasing mass flow rate mode, and an intermittent stepwise increasing mass flow rate mode. An energy storage rate evaluation index for the storage reservoir during the injection stage was also developed to quantify the energy retained in the wellbore-cavern storage system. The results show that the cavern has the highest energy storage rate under the linearly increasing mass flow rate injection mode, reaching 87.5%. Compared with the constant mass flow rate injection mode, this mode increases the average air temperature by approximately 1 K, the average pressure by 0.13 MPa, the pressure potential energy by 12.7%, and the wall heat loss by approximately 11%. The effects of wellbore size and surrounding-rock thermal conductivity on the energy storage rate during the injection stage were further investigated. When the wellbore diameter increases from 0.5 m to 2.0 m, the wellbore wall heat loss decreases from 8.5 × 10 8 J to 2.5 × 10 8 J, indicating that increasing the wellbore diameter helps improve the energy retained by compressed air in the cavern. The thermal conductivity of the surrounding rock also affects the energy storage performance of the storage reservoir. When it increases from 2.17 W/(m·K) to 5.53 W/(m·K), conductive heat loss increases by approximately 7%. The results provide theoretical guidance for injection strategy optimization and wellbore size design in underground CAES storage reservoirs.