Experimental investigation and performance evaluation of a closed three-phase absorption thermal energy storage system

Absorption thermal energy storage (TES) is a promising technology in low-grade waste heat recovery and storage, as well as for domestic heating and space cooling. It is characterized by a high energy storage density (ESD), negligible heat loss, and high flexibility. The energy storage density is related to the concentration glide of the working fluids. To fully exploit the energy storage density potential of absorption TES, this study establishes a closed three-phase absorption TES system. The three-phase absorption involves crystallization during the charging and storage processes, and dissolution during the discharging process. Charging and discharging experiments under several typical working conditions were conducted to reveal the dynamic characteristics of three-phase absorption TES. In the charging process, the concentration glides are 38.0%–50.8 % and 38.0%–54.3 % under charging temperature of 75°C and 85°C, corresponding to charging heats of 902.6 kJ/kg of solution and 993.1 kJ/kg of solution. In the discharging process, the three-phase absorption TES produces heating effects, combined heating and cooling effects, and cooling effects at the evaporation temperature of 30°C, 20°C, and 10°C, with respective concentration glides of 52.6%–41.0 %, 52.8%–40.2 %, and 52.2%–40.8 %. The three-phase processes enhance the energy storage densities by 19.3%–80.3 %. A dynamic absorption TES model is also established and validated using the experimental data. The energy performance of the three-phase absorption TES under a full range of working conditions is studied. The three-phase absorption can double the energy storage density, which further validates the energy storage density enhancement potential of three-phase absorption TES.