Effect of metallic magnesium on enhanced specific heat capacity of chloride molten salts for solar thermal storage applications

Abstract

The NaCl–CaCl 2 based composite is a novel high-temperature heat transfer and solar thermal storage material with enhanced specific heat capacity by dissolving metallic magnesium in the molten salt. However, the enhancement mechanism and the interactions between Mg atoms and various ions in the molten NaCl–CaCl 2 are not clear. In this work, the interfacial effects of local coordination structures around the nano Mg particle and the bonding characteristics between Mg atoms and molten salt ions were studied from both the chemical and physical points of view by using the ab-initio molecular dynamics simulations. The results indicate that [MgNa m ] n + and [MgCa x ] y + clusters are formed at the interface according to the analysis of calculated Mayer bond orders. A compressed ion layer with higer ion densities of Na + and Cl − ions is formed around the nano Mg particle. Moreover, a calculated specific heat capacity of 1.387 J g −1 K −1 is obtained with an enhancement of 25.98% for the Mg/NaCl–CaCl 2 system, which is mainly resulted from the formed compressed ion layer and decreased potential energy due to the interfacial effect. And a theoretical thermal energy storage density with an enhancement of 29.59% is achieved for the Mg/NaCl–CaCl 2 composite at 873 K compared to the pure molten salt. ● Bonding interactions and microstructure evolutions at the interface are revealed. ● The doped Mg atoms can react with Na + and Ca 2+ ions instead of Cl − ions. ● A compressed ion layer of Na + and Cl − ions is formed around the nano Mg particle. ● Specific heat capacity of 1.387 J g −1 K −1 of the composite is enhanced by 25.98%.