First-Principles Study of Chemical Mixtures of CaCl2 andMgCl2 Hydrates for Optimized Seasonal HeatStorage

摘要

Chloride-based salt hydrates form a promising class of thermochemical materials (TCMs), having high storage capacity and fast kinetics. In the charging cycles of these hydrates however hydrolysis might appear along with dehydration. The HCl produced during the hydrolysis degrades and corrodes the storage system. Our GGA-DFT results show that the enthalpy charge during proton formation (an important step in hydrolysis) is much higher for CaCl2·2H2O (33.75 kcal/mol) than for MgCl2·2H2O (19.55 kcal/mol). This is a strong indicator that hydrolysis can be minimized by appropriate chemical mixing of CaCl2 and Mg Cl2 hydrates, which is also confirmed by recent experimental studies. GGA-DFT calculations were performed to obtain and analyze the optimized structures, charge distributions, bonding indicators and harmonic frequencies of various chemical mixtures hydrates and compared them to their elementary salts hydrates. We have further assessed the equilibrium products concentration of dehydration/hydrolysis of the chemical mixtures under a wide range of operating conditions.We observed that chemical mixing leads to an increase of the onsethydrolysis temperature with a maximum value of 79 K, thus increasingthe resistance against hydrolysis with respect to the elementary salthydrates. We also found that the chemical mixing of CaCl2 and MgCl2 hydrates widens the operating dehydration temperaturerange by a maximum value of 182 K (CaMg2Cl6·2H2O) and lowers the binding enthalpy with respect to the physicalmixture by ≈65 kcal/mol for TCM based heat storage systems.