MgCl2·6H2O-Mg(NO3)2·6H2O eutectic/SiO2 composite phase change material with improved thermal reliability and enhanced thermal conductivity

•Composition of MgCl2·6H2O-Mg(NO3)2·6H2O eutectic is determined.•No liquid leaks from eutectic/SiO2 composite even after melting.•SiO2 improves thermal and cycle stability of the eutectic salt hydrate.•SiO2 enhances thermal conductivity of the eutectic salt hydrate.

MgCl2·6H2O-Mg(NO3)2·6H2O eutectic has a proper melting point and high latent heat for solar water heating systems with latent heat storage. In this paper, the exact composition of the eutectic and the corresponding phase change temperature are determined by measuring the phase change characteristics of mixtures of MgCl2·6H2O and Mg(NO3)2·6H2O in various ratios. MgCl2·6H2O-Mg(NO3)2·6H2O eutectics, the melting temperature of which is around 58 °C, can be obtained with the mass fraction of MgCl2·6H2O ranging from 35% to 50%. The eutectics are composited with porous fumed silica to improve its thermo-physical properties. The eutectic/SiO2 composite retains a similar melting point of the pure eutectic, is of no liquid leakage even after melting while the SiO2 weighs more than 15 wt%, and the specific enthalpy reaches 88.13 kJ kg−1. The eutectic/SiO2 composite has a thermal conductivity 5% higher than pure eutectics and better thermal stability and cycle stability. The thermogravimetric analysis shows its weight loss is 8.03% less than pure eutectic at 300 °C. And the eutectics suffers a decrease in phase change enthalpy after 100 thermal cycles by 9.2%, much lower compared with the 37.23% for pure eutectics due to the phase separation during the thermal cycles. Thermo-physical properties and stability of MgCl2·6H2O-Mg(NO3)2·6H2O eutectic have been enhanced by fumed silica.