Thermal Conductivity and Heat Capacity of Anhydrous MgCl₂ and Its Hydrates for Efficient Thermal Energy Storage in Solar and Nuclear Applications

Using phase change materials (PCMs) in thermal energy storage (TES) systems, particularly for applications in solar panels and nuclear power plants, offers significant improvements in efficiency and energy management. Magnesium chloride (MgCl₂) is a promising TES candidate due to its high energy density and high melting point temperature (&gt;700 °C). However, moisture leakage in TES facilities can initiate hydrolysis reactions, leading to the corrosion of metal storage containers. Since MgCl₂ is hygroscopic, it absorbs moisture, altering its hydration state. These hydrates, formed under varying moisture conditions, can act as impurities, potentially affecting MgCl₂’s storage capacity. While the performance of pure anhydrous MgCl₂ is well documented, the effects of different hydrates on its thermal properties are not well understood.<br/>In this study, we perform a computational analysis on anhydrous MgCl₂ and its various hydrates; monohydrate (MgCl₂.H₂O), dihydrate (MgCl₂.2H₂O), tetrahydrate (MgCl₂.4H₂O), and hexahydrate (MgCl₂.6H₂O); to evaluate changes in their thermal properties across a range of temperatures. Using Density Functional Theory (DFT), we assess their heat capacity (C_p) and thermal conductivity (K), with detailed phonon density of states (DOS) calculations providing insight into the subtleties of their thermal behavior.<br/>Our results show significant variations in thermal conductivity and heat capacity among the different hydrates. The anhydrous form exhibits high thermal conductivity, making it suitable for rapid heat exchange. In contrast, the hydrated forms, which increase in number of water molecules, demonstrate enhanced heat storage capabilities due to their larger C_p values. This study uncovers the stability and formation propensity of these materials under various thermal conditions, often overlooked in conventional studies. By integrating fundamental science, including first-principles calculations and phonon DOS analysis, we provide a deeper understanding of how moisture as an impurity influences the thermophysical properties of MgCl₂. These findings guide the application of MgCl₂ in the efficient design of TES systems, emphasizing temperature dependence and the role of hydration states in heat storage and thermal transport mechanisms.