Abstract
Chloride based salt hydrates are promising materials for seasonal heat storage. However, hydrolysis, a side reaction, deteriorates, their cycle stability. To improve the kinetics and durability, we have investigated the optimum operating conditions of a chemical mixture of CaCl2 and MgCl2 hydrates. In this study, we apply a GGA-DFT to gain insight into the various hydrates of CaMg2Cl6. We have obtained the structural properties, atomic charges and vibrational frequencies of CaMg2Cl6 hydrates. The entropic contribution and the enthalpy change are quantified from ground state energy and harmonic frequencies. Subsequently, the change in the Gibbs free energy of thermolysis was obtained under a wide range of temperature and pressure. The equilibrium product concentration of thermolysis can be used to design the seasonal heat storage system under different operating conditions.
Highlights
The variation of solar radiation with geographical basis, daily and seasonal basis is the major concern in its large scale utilizations
To improve the kinetics and durability, we have investigated the optimum operating conditions of a chemical mixture of CaCl2 and MgCl2 hydrates
In the present study, all the gaseous molecule of CaMg2Cl6 hydrates are optimized in DFT using Perdew-Wang exchange and correlation functional (PW91) [15] under generalized gradient approximation (GGA) [13] implemented in Amsterdam Density Functional (ADF) program [12]
Summary
The variation of solar radiation with geographical basis, daily and seasonal basis is the major concern in its large scale utilizations. Tachyhydrite (CaMg2Cl6·12H2O) is a chemical mixture of CaCl2 and MgCl2 hydrates It is a naturally occurring hygroscopic material, present in evaporite deposits and cretaceous potash formation [9, 10]. The salt hydrates of CaCl2, MgCl2, MgSO4 have been explored using DFT-GGA [16, 17, 18] In this present study, we will obtain the thermolysis of various hydrates of CaMg2Cl6 under a wide range of temperature and pressure. The equilibrium product concentrations of dehydration and hydrolysis reaction are obtained under various temperature and pressure regimes These equilibrium curves can be used to predict the operating range and the onset temperature of HCl formation (hydrolysis) of various hydrates of CaMg2Cl6. The equilibrium thermodynamic study of thermolysis of CaMg2Cl6 hydrates under gas assumption will be considered as the safety limits of these reactions in seasonal heat storage systems
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