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.

Abstract

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 10−9m2/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.