Characterization of MgSO4 Hydrate for Thermochemical Seasonal Heat Storage

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 (MgSO(4)center dot 7H(2)O). This paper describes the characterization of MgSO(4)center dot 7H(2)O 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 MgSO(4)center dot 7H(2)O can be dehydrated at temperatures below 150 degrees C, which can be reached by a medium temperature (vacuum tube) collector. Additionally, the material was able to store 2.2 GJ/m(3), 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 MgSO(4)center dot 7H(2)O at atmospheric pressure is problematic for a heat storage system where heat is released above 40 degrees 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 degrees 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 MgSO(4)center dot 7H(2)O for seasonal heat storage is possible for space heating operating at 25 degrees C and a water vapor pressure of 2.1 kPa.