Thermal energy storage based salt hydrates: Heat and mass transfer during hydration

Thermochemical heat storage is an actual and interesting technology for future storage of solar heat and waste energy. Thermochemical materials, particularly salt hydrates, have therefore significant potential of use. When a salt hydrate is heated to a critical temperature (lower than the melting temperature), a chemical reaction is initiated to dissociate it into its anhydrous form and water vapor. The anhydrous salt stores the sensible energy that was supplied for dehydration, which can be later extracted by allowing cooler water or water vapor to flow through the salt, transforming the stored energy into higher sensible heat. This work presents the 3D modelling during the thermochemical hydration reaction in the reactive porous bed based MgCl2•6H2O in a closed system. Numerical result is compared to the lab-scale experiment. An analytical sharp front model is also developed in order to determine the required hydration time. Through a sensitivity analysis, we identify parameters (reaction kinetics, heat exchanger dimensions) that more significantly influence the performance of the heat release process. Numerical heat and mass transfer through principal system components are studied using Comsol Multiphysics Software. Results show that a kinetic parameter of the magnitude of x10-4 allows an optimal chemical conversion neither too fast nor too slow. In this closed system, the inlet vapor pressure into the bed has no influence on the process conversion. The optimal porosity of around 0.6 has been taken based on the heat and mass transfer dilemma analysis. The difference from simulated and experimental temperature is around 3 K, leading to the validation of the model. Concerning the heat exchanger, here cylindrical fin plate, results reveal it should be made by a big plate’s radius and a large distance between the plates to solve the mass transfer issue in the reactive bed.