Design and thermal properties of a novel ternary chloride eutectics for high-temperature solar energy storage

A new ternary chloride salt mixture, named SYSU-4, was developed composed of NaCl, CaCl2 and MgCl2 to meet the needs of solar energy storage over 550 °C. The eutectic point and composition of SYSU-4 was predicted from the calculated phase diagram and also validated by differential scanning calorimetry (DSC). SYSU-4 was prepared experimentally by static melting method and its thermo-physical properties including heat capacity, density and viscosity at high temperature were measured, which were determined by DSC, Archimedes and vibration-rotation methods, respectively. According to the calculated ternary phase diagram, the lowest eutectic temperature was 424.05 °C, in good agreement with experimental results of 424 °C measured by DSC. Heat capacity of SYSU-C4 was averaged as 0.83 and 1.19 J/g K for solid and liquid phase, respectively referred to sapphire standard material. Density of SYSU-C4 decreased linearly from 2.5 g/cm3 to 1.9 g/cm3 with the increase of temperature from 500 to 750 °C. Viscosity of SYSU-C4 also decreased from 4.0 cp to 3.0 cp as the temperature increased. Negligible changes on the melting and freezing points of SYSU-C4 in 50 thermal cycles at 370-550 °C indicates excellent thermal cycling stability. Moreover, the thermal stability of SYSU-C4 was excellent under 700 °C. The developed ternary chloride salt mixture SYSU-4 can be a promising eutectic mixture for high-temperature solar thermal energy storage.