Effect of the dynamic humid environment in salt caverns on their performance of compressed air energy storage: A modeling study of thermo-moisture-fluid dynamics

The performance of a salt cavern compressed air energy storage (CAES) system is affected by the state of air in the cavern. Scholars have been focusing on the fluctuation of air temperature during CAES operation. However, few noticed the simultaneous humidity fluctuations. In this work, a thermo-moisture-fluid dynamics model (TMFD model) was proposed for simulating the dynamic wet environment in CAES salt caverns to estimate its impact on the air state and the system's energy storage performance. In addition to the thermo-moisture coupling, air injection and withdrawal, the water phase transitions, including condensation, evaporation, absorption, and deliquescence, were also considered. The model was applied to the cavern NK1 of the Huntorf plant, with a volume of 140,000 m3. The simulation agrees quite well with the measured temperature data, and it revealed the coupling effect between the humidity and the temperature. During charging, the temperature increases while the relative humidity (RH) decreases, activating the bottom brine to evaporate. Conversely, the temperature drops rapidly in the discharging, supersaturating the water vapor, triggering absorption, condensation, and deliquescence. The reaction heat of these phase transitions raises the salt cavern temperature level by about 0.61 °C while moderating the temperature fluctuations during air injection and withdrawal. These two effects mutually result in the unchanged storage capacity. The latent heat released from the liquefaction during discharging slightly increases the energy density of the withdrawn air and the system's round-trip efficiency. After a cycle, 646.11 kg of water vapor was liquefied, mainly at the end of the withdrawal, and retained in the salt caverns. Its accumulation reduces the cavern's available volume and energy storage capacity by 5.01 % and 6.23 %, respectively, within 30 a. Reducing the inlet humidity can effectively solve this problem. This work reveals the cyclic changes in CAES salt caverns and provides a more accurate estimation of their performance as a part of an energy storage system. The findings are potentially employed to optimize the CAES operation strategies, maximizing the storage capacity.