A DFT-based comparative equilibrium study of thermal dehydration and hydrolysis of CaCl2 hydrates and MgCl2 hydrates for seasonal heat storage

Abstract

Salt hydrates store solar energy in chemical form via a reversible dehydration–hydration reaction. However, as a side reaction to dehydration, hydrolysis (HCl formation) may occur in chloride based salt hydrates (specially in MgCl₂ hydrates), affecting the durability of the storage system. The mixture of CaCl₂ and MgCl₂ hydrates has been shown experimentally to have exceptional cycle stability and improved kinetics. However, the optimal operating conditions for the mixture are unknown. To understand the appropriate balance between dehydration and hydrolysis kinetics in the mixtures, it is essential to gain in-depth insight into the mixture components. We present a GGA-DFT level study to investigate the various gaseous structures of CaCl₂ hydrates and to understand the relative stability of their conformers. The hydration strength and relative stability of conformers are dominated by electrostatic interactions. A wide network of intramolecular homonuclear and heteronuclear hydrogen bonds is observed in CaCl₂ hydrates. Equilibrium product concentrations are obtained during dehydration and hydrolysis reactions under various temperature and pressure conditions. The trend of the dehydration curve with temperature in CaCl₂ hydrates is similar to the experiments. Comparing these results to those of MgCl₂ hydrates, we find that CaCl₂ hydrates are more resistant towards hydrolysis in the temperature range of 273–800 K. Specifically, the present study reveals that the onset temperatures of HCl formation, a crucial design parameter for MgCl₂ hydrates, are lower than for CaCl₂ hydrates except for the mono-hydrate.