Thermal stability mechanism and operating temperature limit of molten chloride salts for thermal energy storage and concentrated solar power applications

NaCl–KCl–MgCl2 molten salt is widely recognized as a potential excellent material for high-temperature heat transfer and thermal energy storage in concentrated solar power systems. The thermal stabilities and high-temperature evaporations of NaCl–KCl–MgCl2 were studied by experiments and simulations in this work. The liquid-vapor phase transition mechanism and properties, such as microstructures, interface thickness, surface tensions, density distributions, mass losses, vapor pressures, and excess thermodynamic properties were revealed by molecular dynamics simulations. The number density distributions of various ions at interface and vapor phase were clarified, and there was an enrichment of K+ ions in the interface and vapor phase. The mass loss was only 0.25 % at 1373 K when Vliquid = Vvapor in a closed system, which starkly contrasted with the 1 % mass loss observed at 707 K in an open system. The saturated vapor pressures of 10 kPa and 1.0 atm corresponded to 1223 K and 1383 K respectively. Considering the stability of compositions and salt condensation problems caused by heat loss of storage tanks, the upper operating temperature limit can be reached to 1223 K for the NaCl–KCl–MgCl2 molten salt in a tightly sealed storage tank, the thermal energy storage density is improved to 1198.1 kJ/kg.