Interseasonal heat storage in silica-alumina based composites

Nowadays, several researches are investigating the possibility to store thermo-chemically the heat produced by renewable energies [1] for various applications. In order to increase the stored energy density, new composite materials should be developed. Often these materials are composed of a host material and a hygroscopic salt [2]. This combination can double the heat storage properties by optimizing the water sorption–desorption cycles [3]. In this work, porous silica-alumina powders with different quantities of alumina were selected, impregnated with 10 wt. % of calcium chloride and tested as heat storage materials. Composite materials were prepared by incipient wetness impregnation of commercial silica-alumina powders (containing respectively 13, 25 or 75 wt.% of alumina) with calcium chloride and labelled as S13ACa, S25ACa, and S75ACa. The composite materials were calcined at 260°C and their surface area was measured by means of N2 adsorption at -196°C. The heats of water sorption-desorption on the silica-alumina composites were determined using a Setaram TG-DSC 111 apparatus. Moreover, successive cycles of hydration (at 20°C) / dehydration (at 150 °C) were performed to check the cyclability of the system. The hydration / dehydration cycles and the corresponding recorded heats for the three porous silica-alumina powders and their composites are reported in the figure. The S75A sample provides the lower heat per gram of material if compared to S13A and S25A. We can observe that the heat released per gram of material decreases by increasing the Al2O3/SiO2 ratio. Once the calcium chloride is deposited on the silica-alumina powders, the quantity of generated heat increased with respect to the corresponding bare silica-alumina support. The S13ACa composite shows the highest heat released during the hydration step (457 J.g-1 of material) compared to S25ACa and S75ACa composites (342 and 274 J.g-1 of material respectively). In conclusion we can observe that to obtain good thermal energy storage materials, both the Al2O3/SiO2 ratio and the salt hydrates deposition have to be optimized. More composites with tuned AlO3/SiO2 ratio and different types and concentration of salt hydrates are under study; the systematic adjustment of the composites composition is pursued starting from the feed-back data obtained by TG-DSC analysis. [1] J. Janchen, D. Ackermann, H. Stach, W. Brosicke, Solar Energy, 76 (2004) 339 [2] E.A. Levitskij, Yu.I. Aristov, M.M. Tokarev, V.N. Parmon, Solar Energy Materials and Solar Cells, 44 (1996) 235. [3] I.A. Simonova, A. Freni, G. Restuccia, Yu.I. Aristov, Microporous and Mesoporous Materials, 122 (2009) 223-228.