Thermochemical heat storage materials: Performance of mixed salt hydrates

Thermochemical heat storage is highly promising, in particularly with a view to long-term heat storage. For the implementation of heat storage in households, thermochemical reactions in the low temperature range below 120 °C are important. Especially salt hydrates such as MgCl ₂, CaCl ₂ or MgSO ₄ were tested with micro gravimetric methods for their suitability. However, the cycle stability of consecutive charging (dehydration) and discharging (hydration) reactions of these materials was low and could be improved only by control of the water uptake (i.e. discharging time) to prevent overhydration. In contrast, mixtures of these salt hydrates showed significant improvements in cycle stability, mass and enthalpy balances. The experiments also showed that the cycleability of all investigated materials increased if hydration and dehydration reactions were performed under constant vapor pressure of 21 mbar. Contrary to other materials, the mixture of CaCl ₂ and MgCl ₂ showed good cycleability under all tested conditions. In addition, the mixture showed superior kinetic properties. Additionally, there is evidence of tachyhydrite (CaMg ₂Cl ₆⋅12H ₂O) formation during cycling of the mixture by the use of XRD after the thermal analysis. Further investigations will be performed to identify further synergies, ideal mixing ratios and formed phases.