WEB Cement-based composite materials for water sorption heat storageWednesday (01.01.2020) 00:15 - 00:30 Part of:
Thermochemical heat storage by water sorption is very attractive due to a sufficiently large energy density, reduced losses and low environmental impact. Although salt hydrates show rather high energy density, their use as adsorbents is limited by poor transport properties and stability. Zeolites may provide a better performance at the cost of lower energy density and high prices on the market. More recently, composite materials have been suggested thanks to acceptable energy densities, low costs and possibly good hydrothermal stability.
In this work, we report the preparation and characterization of cement-based composite materials for water sorption heat storage.
Cement is a porous material, easily available and cheap. It presents the capability for thermal storage, since it is composed of nanostructured hydrated silicates and aluminates, but its performance is scarce. But when cement is coupled with an hydrated salt, for instance MgSO4-7H2O, the obtained materials demonstrate very interesting thermal storage properties.
The cement composites were prepared following two approaches. In the first, very porous cement was obtained by optimizing the water-to-cement ratio, then the cement matrix was impregnated with the salt. In the second, the composites were prepared by a new in-situ method, by making the cement hydration reaction happen in an almost saturated solution of the salt, thus obtaining in a single step the composite sorbent material.
The obtained materials were characterized in terms of their physical and thermal properties. Porosity increases with the water-to-cement ratio, but over a certain value the phenomenon of bleeding occurred, requiring the use of specific additives to obtain the most porous cement samples. The surface area of the cement samples was rather large, in the range of tens of m2/g, due to the presence of a nano-sized porosity among the hydrated silicate particles.
The thermal properties were first characterized by a preliminary calorimetric analysis on sufficiently large samples (around 20 g) where, for simplicity, the composite was initially hydrated by pure liquid water. Then, an energy density estimation was obtained by DSC, and finally, equilibrium adsorption isotherms were carried out. The performance of the composite was compared with literature values, showing that the cost for kWh of these composites lies in the range of a few €/kWh, significantly lower than literature values for pure zeolite or zeolite/salt.