Morphological evaluation of hydration/dehydration stages of an innovative MgSO4 filled composite silicone foam for thermal energy storage applications

The development of innovative Thermal Energy Storage (TES) materials is considered as a critical topic in order to further promote the desertion of fossil fuels, to encourage the use of renewable at building scale and to enhance the energy efficiency of industries. TES technologies are classified in three categories: sensible, latent and thermochemical. The thermochemical one is a very attractive solution for its high energy storage density and the possibility to keep energy almost indefinitely stored as long as the two thermochemical components are kept separated. Among the thermochemical TES, the sorption TES technology is considered as the most attractive. In fact, the interaction between the two components (sorbent and sorbate), characterized by weak bonding energy, makes the charging phase effective already at low and medium temperatures (i.e. 80-150°C). Usually, pure salt hydrates deal with agglomeration, corrosion and swelling problems during hydration/dehydration cycles, while, adsorbent materials show low heat transfer efficiency and sorption storage density. In order to overcome these limitations, sorbent composites formed by a porous structured matrix embedded by salt hydrates have been, recently, deeply studied in literature. In such a context, an innovative concept was proposed employing a polymeric macro-porous foam to host MgSO4∙7H2O salt. The flexibility characteristics of this foam can improve the cycling and mechanical stability of the composite, allowing an expansion of the salt hydrate volume during the hydration process. Additional information is required in order to evaluate the hydration/dehydration phenomena of filler salt embedded on the macro-porous silicone structure. The present work deals with the application of a characterization protocol for the cycling stability of this innovative sorbent composite concept. Particularly, a real-time in situ scanning electron microscopy (SEM) investigation was carried out in controlled temperature and humidity conditions. The specific set-up was proposed in order to evaluate the morphological evolution of the composite material during the hydrating and dehydrating stages of the salt. The focus of the present research activity is to evaluate if the embedded matrix influences the interaction between the salt and the water vapor environment. Furthermore, an investigation of mechanical stability of the foam after some hydration cycles was performed assessing the interfacial adhesion and the cracks formation of the composite silicone foams. The results evidenced an effective micro-thermal stability of the material, confirming its potential use for TES applications.