Mass diffusivity and thermal conductivity estimation of chloride-based salt hydrates for thermo-chemical heat storage: a molecular dynamics study using the reactive force field.

Mixed salt hydrates recently proved to be promising potential candidates for long-term heat storage. Among them, MgCl₂ and CaCl₂ are two widely used salts able to store energy via a reversible hydration/dehydration cycle. The hydration/dehydration of the salts is influenced by thermal and structural material characteristics. To be able to study the complete behavior of the hydration/dehydration cycle including material transformation and degradation, molecular scale modeling is essential. Reliable reactive force fields transferable to different levels of system hydration/dehydration are needed in order to reproduce the material characteristics. Two new transferable force field for MgCl₂ and CaCl₂ are proposed and used to investigate the heat and mass transport for the salt hydrates. Using these new force fields, the diffusion coefficient of water through MgCl₂.nH₂O (n =1 to 6) is found to be in the range 10⁻¹¹ to 10⁻⁹ m²/s and comparable to experimental values. The surface effects were found to play a negligible role for MgCl₂.6H₂O while for the other hydrates surface effects play a noticeable role in the dehydration reaction. The thermal conductivities showed an increase with hydration state from 0.3-0.9 W/mK for all MgCl₂ hydrates. A strong anisotropy for thermal conduction for MgCl₂.6H₂O is observed. The thermal conductivities of these two salts and their hydrates show that mixing will not impair the thermal conductivity of the storage system but it will have a strong effect on the competing hydrolysis reaction.