Design optimization and key thermo-physical properties of the NaCl-KCl-FeCl3 molten salts for low-temperature thermal energy storage

Two kinds of NaCl-KCl-FeCl3 eutectic salts were developed based on the thermodynamic calculation and experimental measurement for the low-temperature thermal energy storage. The NaCl-FeCl3 and KCl-FeCl3 systems were firstly carried out through the CALculation of PHAse Diagrams method, where the calculated values were in excellent agreement with the experimental data. A set of reliable thermodynamic parameter of the NaCl-KCl-FeCl3 system was finally obtained. The predicted two eutectic temperature (T = 138.6 °C and T = 140.0 °C) of the NaCl-KCl-FeCl3 system are in excellent agreement with the present experimental data (T = 138.2 ± 0.2 °C and T = 140.2 ± 0.3 °C). The actual eutectic points of the NaCl-KCl-FeCl3 system are placed at 138.2 ± 0.2 °C & XNaCl = 0.35 mol, XKCl = 0.17 mol, and XFeCl3 = 0.48 mol and 140.2 ± 0.3 °C & XNaCl = 0.34 mol, XKCl = 0.13 mol, and XFeCl3 = 0.53 mol. The fusion enthalpies of the NaCl-KCl-FeCl3 system are 70.05 ± 1.18 and 75.18 ± 1.27 J/g, respectively. The average specific heat capacities are 0.80 ± 0.02 J/g·°C and 0.90 ± 0.02 J/g·°C, separately. Their decomposition temperatures are 389.4 ± 0.2 °C and 399.6 ± 0.3 °C. The weight losses are still less than 6 wt% even at 400 °C for 300 h. Two kinds of NaCl-KCl-FeCl3 eutectic salts have relatively low melting temperature, good thermal stability, suitable working temperature and low cost, which can be considered as one of the promising candidate thermal energy storage materials.