Study of the reversible water vapour sorption process of MgSO4.7H2O and MgCl2.6H2O under the conditions of seasonal solar heat storage

The characterization of the structural, compositional and thermodynamic properties of MgSO4.7H2O and MgCl2.6H2O has been done using in-situ X-ray Diffraction and thermal analyses (TG/DSC) under practical conditions for seasonal heat storage (Tmax=150°C, p(H2O)=13 mbar). This study showed that these two materials release heat after a dehydration/hydration cycle with energy densities of 0.38 GJ/m3 for MgSO4.7H2O and 0.71 GJ/m3 MgCl2.6H2O. The low heat release found for MgSO4.7H2O is mainly attributed to the amorphization of the material during the dehydration performed at 13 mbar which reduces its sorption capacity during the rehydration. MgCl2.6H2O presents a high energy density which makes this material interesting for seasonal heat storage in domestic applications. This material would be able to fulfil the winter heat demand of a passive house estimated at 6 GJ with a packed bed reactor of 8.5 m3. However, a seasonal heat storage system built with this material should be carefully set with a restricted temperature at 40°C for the hydration reaction to avoid the liquefaction of the material at lower temperature which limits its performances for long term storage.