Operational flexibility enhancement for the compressed air energy storage system integrated with molten salt thermal energy storage

Compressed air energy storage (CAES) is a promising technology for large-scale energy storage, where operational flexibility is critical for accommodating high penetration of intermittent renewable energy in modern power grids. This study systematically investigates a CAES system integrated with molten salt thermal storage, with particular emphasis on flexibility enhancement and efficiency improvement under partial-load and off-design conditions. An intrinsically flexible CAES configuration featuring four operation modes—corresponding to high, upper-middle, lower-middle, and low power output—is proposed, and key design parameters are optimized to improve energy efficiency during partial-load operation. When the pressure of the air storage cavern drops below the design value, rated output power is maintained through air injection. This enables full-load operation and high round-trip efficiency (RTE) even under low cavern pressure conditions, significantly enhancing operational flexibility and extending the effective operating pressure range of the cavern. Off-design thermodynamic models are developed, and air consumption together with exergy loss distribution are employed to evaluate system performance. The results demonstrate that the air injection location plays a critical role in the upper-middle power operation mode. When the injection point is located at the outlet of expander E1, the system can sustain rated output power at 50% of the design cavern pressure, achieving a minimum average air consumption of 48.34 kg/s and a maximum RTE of 59.23%. At this condition, the exergy loss of the combustion chamber is also minimized to 32.01 MW, and the lowest air consumption coincides with the highest RTE. By contrast, the air injection location has a less pronounced influence on the lower-middle power operation mode. The system remains capable of full-load operation at 60% of the design cavern pressure. The minimum average air consumption of 30.45 kg/s occurs when the injection point is located at 8 MPa, whereas the maximum RTE of 78.74% is achieved at the outlet of expander E4. In this case, the minimum air consumption does not correspond to the maximum RTE.