Thermodynamic analysis of isothermal compressed air energy storage system with droplets injection

Compressed air energy storage (CAES) is regarded as an effective long-duration energy storage technology to support the high penetration of renewable energy in the gird. Many types of CAES technologies are developed. The isothermal CAES (I-CAES) shows relatively high round-trip efficiency and energy density potentially. The isothermal processes of compression and expansion are realized by injecting water droplets into the compressor/expander cylinders. The compression heat is absorbed to abate temperature increase, and extra heat is added to abate temperature decrease during expansion. Firstly, the thermodynamic model of the I-CAES system using droplet injection method was established, and the calculation formula of droplet mass with rotation angle and air quality was given. The simulation modelling was verified by experimental results. The effects of gas-liquid mass ratio (ML) and rotation speed on thermodynamic performances including isothermal compression/expansion efficiency, isothermality, round-trip efficiency and energy density were studied. Results showed that isothermal compression/expansion efficiency, round-trip efficiency and isothermality were increased by increasing ML and reducing rotation speed, while, ML had the more obvious influence. When charging time was 6 h, discharging time was 4 h and ML equaled 10, round-trip efficiency of the one-stage I-CAES system was 83.15 % and energy density was 1.94 MJ/m3. Round-trip efficiency of the two-stage I-CAES system was 82.53 % and the energy density was 39.93 MJ/m3 under the same conditions.