Calorimetric Investigation of Magnesium Sulfate Hydration in Porous Glasses
Abstract
The sorption behavior of dehydrated magnesium sulfate in porous glasses with pore diameters from 1.7 µm to 7 nm was investigated by using isothermal calorimetry. The kinetic hindrance of the formation of the thermodynamically stable product MgSO4·7H2O was not overcome at 85 % RH and 30 °C in the material with the largest pores. In contrast, in pores below 173 nm the high water uptake clearly indicates the formation of a solution. The released heat (Qr) can be calculated as the sum of the heat of hydration (Qhyd), the heat of condensation (Qc) and the heat of solution (Qsol). The water uptake and the overall released heat both increase with decreasing pore size. Although high energy densities up to 0.95 GJ/m³ were found for the material composed of Vycor glass (dm = 7 nm) and MgSO4, this composite does not seem to be suitable as heat storage material due to the formation of a solution and the high humidity of 85 % RH required for the release of heat. Porous carbon was used as an alternative host material for MgSO4. Isotherms of the water vapor uptake of a porous carbon composite with three different loads of MgSO4·H2O were determined and are discussed in this paper. An energy density of 0.74 GJ/m3 can be achieved without formation of a solution at 72 % RH.
Keywords
Thermochemical heat storage, Salt hydration, Magnesium sulfate, Influence of pore size