Off-design performance and operation strategy of expansion process in compressed air energy systems

Compressed air energy storage (CAES) systems usually operate under off-design conditions due to load fluctuations, environmental factors, and performance characteristics of the system. In order to optimize design and operation of CAES systems, it is significant to study off-design performance. The expansion process plays an important role in the whole system. The main objective of this study is to explore the off-design characteristics and optimization strategy of the multistage expansion process of CAES systems with thermal storage. Two kinds of off-design operating modes, which are equal-power-ratio (EPR) operation and optimizing variable stator vane rotation angle (OVRA) operation, are proposed and compared for the first time. Correlation between key parameters such as total output power ratio, exergy efficiency, outlet air/water temperature versus mass flow rate ratio, and inlet pressure are revealed. Furthermore, optimal operation principles are obtained. The primary optimizing operation principle is to optimize the isentropic efficiencies of low-pressure stages to improve the whole efficiency of expansion process. Lastly, the optimized regulating law for variable stator vane is expressed in a polynomial form.