Thermochemical investigation of the water uptake behavior of MgSO4 hydrates in host materials with different pore size

• Influence of confinement in porous materials on hydration reactions is investigated.
• The heat of sorption in porous composites with MgSO4 depends on pore size.
• The heat of sorption increases with decreasing pore size due to higher water uptake.
• The final hydration product varies with pore size.
• Salt hydrates in porous host materials are promising heat storage materials.

The reversible dehydration and re-hydration of salts is considered as a promising thermochemical process for the storage of heat from a renewable energy source such as solar energy. Such materials would help to overcome the gap between demand of energy and supply from renewable sources. So far however, attempts to make full use of the very high storage densities of salt hydrates were unsuccessful, largely due to problems with the kinetics of the hydration and dehydration reactions. In the present work, in order to overcome these problems, magnesium sulfate was embedded in various porous host materials with average pore diameters (dm) ranging from 1.7 μm to 7 nm. The dehydration and water uptake behavior of these composites was investigated by thermogravimetry and isothermal calorimetry. The heat of reaction of the composites at 85% RH and 30 °C varies from 103 MJ/m3 in glass frits (dm = 1.7 μm) to 556 MJ/m3 in porous glass (dm = 7 nm) and increases with decreasing pore size. The contribution of the heat of hydration to the overall thermal effect decreases with decreasing pore size while the contribution of the heat of adsorption increases. In bulk samples and in glass frits with coarse, micron-sized pores, MgSO4·6H2O is the major hydration product. In host materials with submicron pores, water vapor uptake yields both a crystalline hydrated phase (most likely MgSO4·7H2O) and its saturated solution. Apart from pore size, the salt content of the porous substrate, i.e., the degree of pore filling, is another critical parameter affecting the overall heat of reaction. Pore size and pore filling as well as porosity are the most important parameters that need to be optimized in order to achieve high storage densities with composite materials of a porous host materials and embedded salt hydrates.