Characterization of the sorption process in thermochemical materials for seasonal solar heat storage application

The heat demand for dwellings in summer can be completely covered by solar heat, but in winter the heat demand exceeds the solar supply. For a low-energy house, a solution is to store the excess of solar energy in summer, and to use it to meet the heat demand in winter. Water is traditionally used for storing heat (e.g. solar boiler), but seasonal heat storage requires large water tanks (> 40 m) that are too large to be placed inside an average family house. An alternative option is to store heat by means of chemical processes using a reversible reaction in thermochemical materials (TCM) . Such thermochemical heat storage has a 5 to 10 times higher energy storage density than water, with the additional benefit that, after charging, the heat can be stored for a long time without losses. With thermochemical materials, the entire heating demand of a low-energy house could be met using a storage volume of 4 to 8 m, that is charged during summer by solar collectors. Because of the amount of material required for implementation in the built environment, safety and low cost of the TCM materials are important aspects. Previous theoretical and experimental studies at ECN identified magnesium sulfate heptahydrate (MgSO4.7H2O) and magnesium chloride hexahydrate (MgCl2.6H2O) as promising materials for seasonal heat storage in the built environment, based on their high energy density, suitable temperature range, low cost and inherent safety. Solar heat provided by solar thermal collectors with a temperature below 150°C can be used to dehydrate the hydrates during summer, while during winter the dehydrated materials can be hydrated again with low water vapor pressure (13 mbar) provided by means of the evaporation of water by a borehole at 10°C, releasing the heat again. A representation of the practical conditions used during the dehydration and hydration reactions of the salt hydrate material is presented in fig. 1.