Thermal synthesis of a thermochemical heat storage with heat exchanger optimization

Abstract Thermochemical materials, particularly salt hydrates, have a significant potential for use in thermal energy storage systems. When a salt hydrate is heated to a critical temperature, a chemical reaction is initiated to dissociate it into its anhydrous form and water vapour. The anhydrous salt stores the sensible energy that was supplied for dehydration, which can be later extracted by allowing cooler water or water vapour to flow through the salt, transforming the stored energy into higher sensible heat. This work presents a thermal synthesis modelling of a thermochemical heat storage based on the reactive porous bed of MgCl2⋅2H2O in a closed system. An analytical sharp front model is also developed to determine the required hydration time and the optimal bed size for such a system at prototype level. A sensitivity analysis helps to identify the optimal parameters that significantly affect the performance of the heat release process. Numerical heat and mass transfer through principal system components is studied using Comsol Multiphysics Software. Numerical results show good concordance with the experiment and reveal that to achieve quasi-complete hydration of the bed, mass flow rate should not be higher than 0.001 kg⋅h−1 for a bed thickness between 4 and 5 cm, considering the present system configuration. In this closed system, inlet vapour pressure into the bed has no influence on the conversion process. Optimal porosity of around 0.76 is found based on the heat and mass transfer dilemma analysis. Optimal heat exchanger design is performed for mass transfer enhancement in the reactive bed.