Optimum design and key thermal property of NaCl–KCl–CaCl2 eutectic salt for ultra-high-temperature thermal energy storage

NaCl–KCl–CaCl2 eutectic salt was developed using the thermodynamic calculation and experimental validation for the ultra-high-temperature thermal storage. Substitutional solution model (SSM) was used to describe the liquid phase and solid solution phase, and stoichiometric compound was applied to depict the intermediate phase. The predicted eutectic temperature (481.3 °C) of NaCl–KCl–CaCl2 eutectic salt agrees well with experimental value (496.3 °C). The actual eutectic point of NaCl–KCl–CaCl2 system is placed at T = 496.3 °C, XNaCl = 0.408 mol, XKCl = 0.075 mol and XCaCl2 = 0.517 mol. The fusion enthalpy is 171.5 J/g. The weight loss is only 1.7 wt% even at 900.0 °C and 939.0 °C is determined to be the upper limit of working temperature. The average liquid specific heat capacity is 0.80 ± 0.15 (J/g⋅oC). The cost of NaCl–KCl–CaCl2 eutectic salt is only 51.6% of solar salt and 54.3% of NaCl–KCl–MgCl2 salt. SSM of thermodynamic calculation can be used as a reasonable model to optimize the chlorides salts in future. The developed NaCl–KCl–CaCl2 eutectic salts have relatively better thermo-physical properties particularly excellent ultra-high-temperature thermal stability and lower cost, and can be considered as the ultra-high-temperature thermal energy storage materials.