CAES-SC hybrid energy storage: Dynamic characteristics and control via discharge process

Energy storage is an effective means to increase renewable energy utilization. Compressed air energy storage (CAES) and super-capacitor (SC) hybrid energy storage system is one of the potential options for renewable energy microgrids due to its low-cost and flexible response capacity. In this paper, we thoroughly investigate the dynamical characteristics of the CAES system and the dynamical interactions between different subsystems in the CAES-SC hybrid energy storage via the discharge process. Dynamical models have been developed for the CAES discharge subsystem and the SC subsystem. Studies of the start-up process of CAES discharge subsystems have shown that the no-load start-up mode is more efficient than the variable-load start-up mode if the equivalent power generation for the same compressed air mass consumption is measured. In terms of the interaction characteristics between the CAES subsystem and the SC subsystem, the capacity demand of the SC subsystem in the no-load start-up mode is about 8 % higher than that in the variable-load start-up mode. An optimal control strategy based on the constraint of source-load dynamic balance has also been proposed to achieve power allocation in a CAES-SC hybrid energy storage system. Case studies show that the cycle efficiency of the hybrid energy storage system corresponding to the proposed optimal control strategy is 57.91 %, which is 1.84 % higher than that of the conventional control strategy. Moreover, compared with the conventional design that takes twice the maximum energy storage capacity demand during the starting process of the CAES discharge subsystem (about 1.6 MWh), a higher designed energy storage capacity (about 3 MWh) of the SC subsystem is required to satisfy the complete response to the source-load deviation under the proposed optimal control strategy. Moreover, for every 0.5 MWh increase in the energy storage capacity of the SC subsystem, the cycle efficiency of the hybrid energy storage system can be increased by approximately 0.3 % to 0.5 %.