Off-design modeling and performance analysis of supercritical compressed air energy storage systems with packed bed cold storage

Abstract

Supercritical compressed air energy storage (SC-CAES) systems have particular merits of both high efficiency and high energy density. In SC-CAES systems, the use of packed bed cold storage has plentiful advantages of simple structure, safety and reliability. However, the previous studies of packed bed models traditionally adopted the assumption of uniform flow which is not applicable in cold storage actually. Meanwhile, the coupling relationship between the dynamic behavior of packed bed and the operation performance of other system parts is not clear. Aiming at the above issues, a detailed revised packed bed model is developed, and the calculation model of off-design characteristics of SC-CAES system is established. Besides, the evaluation indexes of system efficiency considering the changes of mass and energy in the packed bed are established. With the above models, the temperature distribution in the packed bed is deeply analyzed, and the off-design characteristics of SC-CAES system are revealed comprehensively on the conditions that the thermocline is steep and flat, respectively. Results show that the temperature difference under flat thermocline condition is much smaller than that under the steep thermocline condition, which makes the system efficiency corresponding to the flat thermocline achieve 13.4 percentage points higher than the other one. Due to the uneven mass distribution in the packed bed, the mass flow rate of liquid expander/cryopump is greater than that of compressor/expander. When the ambient temperature increases by 10 K, the system efficiency decreases by 0.5–0.75 percentage points on average. The fluctuating load has slight impact on the system efficiency.