Water vapor sorption in salt hydrates is one of the most promising means for compact, low loss, and long-term storage of solar heat in the built environment. One of the most interesting salt hydrates for compact seasonal heat storage is magnesium sulfate heptahydrate $(MgSO4⋅7H2O)$. This paper describes the characterization of $MgSO4⋅7H2O$ to examine its suitability for application in a seasonal heat storage system for the built environment. Both charging (dehydration) and discharging (hydration) behaviors of the material were studied using thermogravimetric differential scanning calorimetry, X-ray diffraction, particle distribution measurements, and scanning electron microscope. The experimental results show that $MgSO4⋅7H2O$ can be dehydrated at temperatures below $150°C$, which can be reached by a medium temperature (vacuum tube) collector. Additionally, the material was able to store $2.2 GJ/m3$, almost nine times more energy than can be stored in water as sensible heat. On the other hand, the experimental results indicate that the release of the stored heat is more difficult. The amount of water taken up and the energy released by the material turned out to be strongly dependent on the water vapor pressure, temperature, and the total system pressure. The results of this study indicate that the application of $MgSO4⋅7H2O$ at atmospheric pressure is problematic for a heat storage system where heat is released above $40°C$ using a water vapor pressure of 1.3 kPa. However, first experiments performed in a closed system at low pressure indicate that a small amount of heat can be released at $50°C$ and a water vapor pressure of 1.3 kPa. If a heat storage system has to operate at atmospheric pressure, then the application of $MgSO4⋅7H2O$ for seasonal heat storage is possible for space heating operating at $25°C$ and a water vapor pressure of 2.1 kPa.

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