First-Principles Study of Chemical Mixtures of CaCl₂ and MgCl₂ Hydrates for Optimized Seasonal Heat Storage

Abstract

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 onset hydrolysis temperature with a maximum value of 79 K, thus increasing the resistance against hydrolysis with respect to the elementary salt hydrates. We also found that the chemical mixing of CaCl2 and MgCl2 hydrates widens the operating dehydration temperature range by a maximum value of 182 K (CaMg2Cl6·2H2O) and lowers the binding enthalpy with respect to the physical mixture by ≈65 kcal/mol for TCM based heat storage systems.

Molten Chloride Thermophysical Properties, Chemical Optimization, and Purification

“…This corrosive impurity is fundamentally caused by the hygroscopic nature of the MgCl2 component in the ternary salt (i.e., MgCl2 takes various forms of hydrates, such as MgCl2•H2O, MgCl2•2H2O, MgCl2•4H2O, and MgCl2•6H2O). Very similar behaviors to produce hydroxychloride during salt dehydration had been found in CaCl2 salt as well [16,17,[31][32][33][34][35]. To mitigate the risks of the corrosive hydroxychloride species, multiple methods for salt dehydration and purification have been proposed, such as thermal treatment by controlled heating [26,29,43,31,[36][37][38][39][40][41][42], reduction by elemental Mg [19,22,23,25], dehydration with ammonium chloride [14,29,42,44,45], and carbochlorination with CO, Cl2, and/or CCl4 to remove oxide species [13,14,46,47].…”

“…1-4) and hydrolysis of anhydrous MgCl2 with the released H2O (Eqs. 5 and 6) form MgOHCl which have been reported by the literature [3,15,[49][50][51]16,17,29,31,39,41,42,48]. The temperature range after each reaction is a compilation of various reported values from the literature.…”

“…Molten chloride salts have been proposed as an alternative HTF chemistry for the next generation (Gen3) CSP technology operating at higher temperatures in order to maximize the benefit of using the sCO2 power cycle [1,2]. A significant number of research efforts [3,4,[13][14][15][16][17][18][19][20][21][22]5,[23][24][25][26][27][28][6][7][8][9][10][11][12] have been devoted to investigating molten chlorides' chemistry, thermodynamics, corrosion, and properties, especially the ternary system of NaCl-KCl-MgCl2 due to its availability and low cost.…”

Study on the sorption mechanism of middle‐low temperature sorption thermal storage materials from the microscale simulation: A review

“…Smeets et al 72 75,76 Chemical mixing can promote them formed as complex, which is conducive to inhibiting hydrolysis. Figure 19 shows the structure of chemical mixture.…”

“…The structure of chemical mixtures. 76 dissociated from water would coordinate with Zn 2+ -(Figure 22). This process destroyed the structure of (Zn 4 O)(BDC) 3 , causing the following reaction: This reaction would cause the entire frame to collapse.…”