Novel low melting point quaternary eutectic system for solar thermal energy storage

Abstract

A new low melting point quaternary eutectic mixture consisting of alkali nitrates and sodium nitrite was modeled by using thermodynamic principles. The eutectic temperature of the quaternary salt mixture was predicted to be 371.75K. The quaternary eutectic mixture was synthesized based on the predicted eutectic composition and the eutectic temperature was determined using the Differential Scanning Calorimeter (DSC). The experimental melting point was 372.17K which was in excellent agreement with that of the predicted value. The heat capacity of the eutectic mixture was determined by the DSC as function of temperature in the liquid state and heat capacity increased with increase in temperature. Density of the molten quaternary eutectic mixture decreases linearly with increase in temperature. Thermal stability of the eutectic mixture was determined by both short-term and long-term thermogravimetric analyses. Short-term thermal stability test showed negligible weight loss. However, 7.4wt.% weight loss was observed in the long–term thermal stability test at 773.15K which corresponds to thermal decomposition of the salt mixture. Upper temperature limit for long-term thermal stability of the eutectic mixture was determined to be 703.15K. Thermal energy storage capacity of the novel eutectic mixture was found to be larger than that of conventional solar salt and HITEC® salt.