Optimal selection of air expansion machine in Compressed Air Energy Storage: A review

Electrical energy storage has been recognised as an underpinning technology to meet the challenges in the power network arisen from the rapidly increasing penetration of renewable energy. Compressed Air Energy Storage (CAES) has gained substantial worldwide attention in recent years due to its low-cost and high-reliability in the large-scale energy storage systems. Air expander is one of the key components in a CAES system because its operational characteristics determine the power conversion efficiency and the power generation during the discharge period. The performance of the expander contributes heavily to the round trip efficiency of the whole system. This paper presents an up-to-date review of the CAES technology, and methods for modelling and selecting expanders for CAES systems. The focuses of selecting the appropriate expansion machines are identifying and analysing the characteristics of both CAES systems and expansion machines, and finding the matched expanders for the CAES system formulation (i.e. diabatic, adiabatic and isothermal CAES) and operational conditions (i.e. air pressure, temperature and flow rate). After all, recommendations and guidelines in selecting appropriate expanders and expansion stage numbers are formulated and discussed; this laid a step stone for choosing suitable expansion machines to achieve an overall CAES system high efficiency.