Experimental study on a closed absorption thermal energy storage system using modified LiCl solution for enhancing energy storage density

Absorption thermal energy storage (TES) is recognized as a promising technology utilizing renewable energy and balancing supply and demand. Thermodynamic analysis of the absorption TES cycle indicates that extending the concentration glide can enhance energy storage density (ESD). However, in real operation, a large concentration glide can increase the risk of crystallization at high concentrations. Additionally, inefficient heat and mass transfer can limit its high ESD potential. In this paper, a closed absorption TES system with novel heat exchangers was established. The plate heat exchangers with narrow and wide channels were used as the generator/absorber and condenser/evaporator. LiCl solution was used as the base working fluid with ethylene glycol (EG) as an additive to extend the concentration glide. The mass ratios of LiCl solutions to EG were 100:0, 100:2.5 and 100:5, respectively. Charging temperatures ranging from 65 to 85 degrees C were used to represent the solar heat with different temperature levels. The discharging tests were conducted in three modes: heating at 50 degrees C, combined heating at 40 degrees C and cooling at 20 degrees C, and cooling at 10 degrees C. The experimental results revealed the dynamic characteristics and energy storage performance of the charging and discharging processes. With the addition of EG, the maximum concentration of LiCl solution was increased from 46.7 % to 49 %. The ESD was significantly improved by 18 %-48 %, reaching 123 kWh/m3, 177 kWh/m3, and 108 kWh/m3 in heating, combined heating and cooling, and cooling modes, respectively. These are the highest values recorded for the closed single-stage absorption TES systems to date. The experiments confirmed the effectiveness of EG as an additive for LiCl solution and provided insights for further research.