Compressed air energy storage system with an ejector integrated in energy-release stage: Where is the optimal location of constant-pressure operation?

As one of promising large-scale energy storage technologies, compressed air energy storage (CAES) system can release stable power by expanders in constant-pressure operation (CPO) mode. An ejector integrated in the valvetrain of energy-release stage has shown a certain potential for improving CAES system efficiency by recovering part of throttling loss. In such an ejector-integrated CAES system, different CPO locations may significantly influence total energy-release work by changing the inlet air flow rate of expanders. However, the issue of optimal CPO location for maximizing total energy-release work has been overlooked so far. Then, to demonstrate the optimal CPO location, the thermodynamic model of a 10 MW thermal-storage CAES system with or without the ejector is established, in which different low-pressure air sources (namely different suction positions of the ejector), motive air pressures, and CPO locations are considered, and the maximum entrainment ratio of the ejector is self-consistently calculated by a one-dimensional semi-empirical model. The results show that different CPO locations have negligible influence on the maximum entrainment ratio of the ejector, the optimal low-pressure air source, and the total amount of entrained low-pressure air, but change the total energy-release work, round-trip efficiency and profit significantly. For example, the total energy-release work, the rise amplitude of round-trip efficiency, and the profit of the ejector corresponding to the CPO location, namely the inlet of the second expander, can be respectively 4.72 × 10⁹ J, 4.38% and 128$ bigger than those corresponding to the CPO location, namely the inlet of the first expander. As a result, the optimal CPO location is the inlet of the second expander. This paper provides a new optimal operating rule, namely the optimal CPO location, for the CAES system integrated with ejector technology.