Thermodynamic assessment of binary chloride salt material for heat transfer and storage applications in CSP system

Molten chloride salt is recognized as a promising heat transfer fluid and thermal energy storage material (TES) in concentrating solar power (CSP) in recent years, and its phase diagram as key data for material design still need further research. In this work, the phase equilibrium characteristics of solid-liquid equilibrium and vapor-liquid equilibrium for binary chloride salt system (Na/K–Ca/Mg) are investigated respectively. Based on Wilson model, the activity coefficient and non-ideal behavior of liquid salts are predicted effectively, and then the solid-liquid phase diagrams of binary salts are calculated, and the predicted eutectic temperature and component agree well with experimental data. Vapor-liquid phase diagrams of binary salts are also calculated, and azeotropic temperature can only be found in the MgCl2 based mixed salts. As the system pressure rises, the azeotropic temperature increases, while azeotropic component slightly changes. According to the variation of vapor pressure for eutectic salts, the correlations for vapor pressure with temperature and composition are developed. Thermal stability of eutectic salts is experimentally studied by Thermo-gravimetric analyzer (TGA) and operating temperature ranges of these mixed salts are revealed. It can be found that the mass loss rate at different temperature is directly proportional to the vapor pressure. In general, present binary chloride salts show wide operating temperature range and high heat storage density, and eutectic NaCl–MgCl2 with advantage of heat storage density and azeotropic point existence is recommended as TES in the CSP plant.