Optimal working-parameter analysis of an ejector integrated into the energy-release stage of a thermal-storage compressed air energy storage system under constant-pressure operation: A case study

•The compressed air energy storage systems with or without ejector are considered.•The working parameters of ejector influence discharging performances dramatically.•The rise amplitude of round-trip efficiency can reach around 10%.•The profit caused by ejector for an energy-release process can reach around 275 $.•Methods for determining the optimal working parameters of ejector are proposed. Compressed air energy storage is a promising large-scale energy storage technology. Integrating ejectors in the energy-release stage of compressed air energy storage systems is widely recognized as an effective way to improving system efficiency; however, there is a lack of detailed modelling and analysis regarding the optimal working parameters of ejectors. In this study, the thermodynamic models of a 10 MW thermal-storage compressed air energy storage system with or without an ejector (system I and system II, respectively) are established under constant-pressure operation. A one-dimensional semi-empirical model is used to determine the maximum entrainment ratio of the ejector under specific conditions of motive air pressure, a low-pressure air source, and constant-pressure operation. The results show that (1) in system I, the maximum entrainment ratio is positively correlated with motive air pressure, while the total energy-release time, the total amount of entrained low-pressure air, and the total energy-release work present parabolic-like variations as motive air pressure increases; (2) different low-pressure air sources change these performance parameters significantly; (3) compared between systems I and II, the rise amplitude of round-trip efficiency and the profit are positively correlated with motive air pressure for most constant-pressure operations; (4) the optimal rise amplitude of round-trip efficiency and profit of the ejector can reach around 10% and 275$, respectively. In addition, two methods to determine the optimal low-pressure air source for the ejector are proposed for real and design-stage compressed air energy storage systems, while three principles to determine the optimal motive air pressure of the ejector are proposed through a comprehensive analysis of performance parameters.