Thermodynamic analysis of a hybrid system combining compressed air energy storage and pressurized water thermal energy storage

Large-scale electrical energy storage is an urgent requirement currently. This paper presents a hybrid system integrating compressed air energy storage (CAES) with pressurized water thermal energy storage (PWTES). The open type isothermal compressed air energy storage (OI-CAES) device is applied to the CAES subsystem to achieve near-isothermal compression of air. Meanwhile, the heat storage capacity of liquid water is improved by pressurization in the PWTES subsystem. Electricity is converted directly to heat through joule-resistive heating to improve the temperature of pressurized water. Moreover, the thermodynamic model of the hybrid system and the transient mathematical model of the OI-CAES in the four stages of intake, compression, exhaust and expansion are developed in this paper. The effects of design parameters on the working process of OI-CAES with and without spraying are the focus of the study. Furthermore, three different cases of the hybrid system are analyzed. The results show that the increase in tank height or tank volume is beneficial to restrain the variation of air temperature in the operation of OI-CAES when there is spraying, and the fluctuation range of the air temperature with spraying can be controlled within 25.7 K. When the tank volume is 5 m³ and the flow rate of the pump unit is 10 m³/h, the maximum temperature rise of the air in the compression process of OI-CAES can be managed within 30.5 K and 11.5 K respectively when there is no spraying and spraying. When the water in the PWTES subsystem supplies thermal energy, the efficiency of the CAES subsystem of the hybrid system can achieve 91.9 % with the energy storage density being 2.28 kWh/m³. When the hybrid system provides thermal energy through heat transfer, the energy efficiency of the hybrid system is expected to reach 65.6 %. With the supply of thermal energy, the flow rate of heating water can be flexibly adjusted and the heating water supply temperature is able to meet 363.16 K. Without the supply of thermal energy, the electrical efficiency of the hybrid system can reach 53.7 %.