Design optimization and thermal storage characteristics of NaNO3-NaCl-NaF molten salts with high latent heat and low cost for the thermal energy storage

Abstract

NaNO3-NaCl-NaF salts were investigated by thermodynamic calculation and experimental measurement. A set of self-consistent thermodynamic parameter for the NaNO3-NaCl-NaF salts was finally obtained, by which the predicted melting point (i.e., 288.2 °C) is in excellent agreement with the experimental value (i.e., 288.0 °C). The actual eutectic point of the NaNO3-NaCl-NaF salts is placed at 288.0 °C and XNaNO3 = 0.905 mol, XNaCl = 0.059 mol, and XNaF = 0.036 mol. The fusion enthalpy of the NaNO3-NaCl-NaF salts is 210.0 J/g, and the average liquidus specific heat capacity is 1.760 ± 0.030 J/g·°C. Weight loss is only 5.0 wt% at 650 °C, and its thermal stability is high. The fusion enthalpy is increased by 86.7% than that of NaNO3-KNO3 and 252.0% than that of NaNO3-NaNO2-KNO3. Furthermore, the cost of storage solid-liquid phase change heat of the NaNO3-NaCl-NaF salts is only 50% of that of NaNO3-KNO3 salt and 25% of that of NaNO3-NaNO2-KNO3 salts. The developed NaNO3-NaCl-NaF eutectic salts has relatively superior thermophysical properties, suitable working temperature and good economy performance, and can be used as one of the candidate thermal energy storage fluids. This work provides fundamental reference for the selection of thermal energy storage materials and enriches molten salts databases.