Development of an innovative material for sorption thermal energy storage

The development of thermal energy storage is essential to enable the full potential of renewables sources. Sorption thermal energy storage (STES) seems to be the most interesting way to stock thermal energy, due to the highest values of stored energy density and because of its intrinsically inability to generate losses. The ideal materials for STES must have low-cost and a high thermal performance, some examples of this typology of materials being hydrated salts and zeolite. However, either they have a prohibitive cost like in the case of zeolite (≈2.50 €/kg) or they suffer from problems of deliquescence or agglomeration, like in the case of hydrated salts. To avoid those problems, current scientific research focuses its attention on the development of salt-containing composites. Composite materials are usually composed of a porous matrix and a salt, where the salt is well dispersed inside the matrix. The aim of this thesis is a development of a new cost-effective matrix, made of cement and vermiculite, to disperse a highly performant salt. Cement prepared with a high water-to-cement ratio creates a noteworthy porous matrix and vermiculite is added both to increase the capacity of adsorbing water and to reduce cost. Two different salts were investigated: magnesium sulfate and calcium chloride. Unfortunately, the intrinsic deliquescence behaviour of calcium chloride created several issues in the testing phase, so that only magnesium sulfate resulted as a proper STES material. Different samples were investigated, characterized by a different preparation route, and the best sample presents a material cost of 0.14 €/kg for a storage capacity of 0.16 kWh/kg, meaning a cost of stored kWh of 0.90 €/kWh, with a dehydration temperature of 140 °C. The storage capacity is obtained by the calculation of isosteric heat via measurement of absorption isotherm, and is in line with the best literature on the subject.