Thermal properties and thermal stability of the ternary eutectic salt NaCl-CaCl₂-MgCl₂ used in high-temperature thermal energy storage process

Highlights: •The upper temperature limit was determined to be 650 °C for the ternary salt. •The stability and recyclability were verified in the short-term cycling stability. •The mechanism of thermal instability was studied by the X-ray diffraction analysis. •The eutectic salt was used as high-temperature thermal energy storage materials. -- Abstract: The ternary eutectic chloride salt (NaCl-CaCl₂-MgCl₂) was designed and prepared for thermal energy storage over 500 °C in a concentrated solar power system. The thermal properties of the eutectic salt were measured experimentally using the Differential Scanning Calorimeter (DSC) technique, and the thermal stability including the long-term and short-term thermal stability of the ternary system was investigated by the Thermo-gravimetric Analyzer (TGA) technique and the DSC technique. The results indicated that the melting temperature and fusion enthalpy were 420.83 °C and 201.50 J/g, respectively, and the specific heat capacity was 1.49 J/(g·°C). In the long-term isothermal stability study, the total weight loss of the eutectic salt was less than 5% below 650 °C, and the melting temperature and fusion enthalpy changed slightly as the temperature increased. The ternary eutectic system demonstrated excellent thermal stability below 650 °C, which was defined as the upper temperature limit of the ternary system. During the short-term thermal cycling test, the stability and recyclability of the ternary salt were verified in future during the working temperature, generally 50 °C lower than the upper temperature limit. The eutectic composition before and after thermal treatment was characterized by the X-ray Diffraction (XRD) technique to analyze the mechanism of thermal instability. The mechanism of thermal instability turned out to be the thermal decomposition of the generated magnesium chloride hexahydrate at high temperature. It was proved that the ternary eutectic chloride salt was a potential candidate for high-temperature thermal energy storage applications up to 650 °C.