Hydration and dehydration behavior of MgSO4 hydrate-silicone composite foams for thermal heat storage application

The absorption of water vapor by means of salt hydrates is a promising and potentially suitable method for compact, loss-free and long-term seasonal thermal energy storage (TES). Reversible reactions involving salts, such as hy/dehydration reactions, can be used for storing heat through the thermochemical technology. In this context, the magnesium sulphate heptahydrate (MgSO4·7H2O) represents one of the most effective salts for compact and efficient seasonal TES. Although the thermochemical properties of this salt are relevant, its practical application is still difficult due to several issues occurring on its storage system application. The aim of this work is to evaluate the thermochemical performance of an innovative polymeric foam based on silicone and MgSO4∙7H2O and of its suitability in applications of seasonal heat storage system, for a built environment. Composite foams, at varying filler content (40wt.%- 70wt.% of MgSO4∙7H2O salt), were synthetized, according to an established composite foaming procedure [4]. Both charging (dehydration) and discharging (hydration) behaviors of the composite foams were assessed using thermo-gravimetry (TG) coupled to differential scanning calorimetry (DSC) under real operating conditions, evidencing that salt filler, embedded in the matrix, is still able to efficiently exchange water during the hydration and dehydration phases. Finally, in order to better identify the relationship between performances and microstructure of the foams and to tailor the optimal composite foam formulation, 3D optical and scanning electron microscopy was carried out showing that a well-structured porous structure favor the vapor flow through the composite bulk. The good flexibility and thermal stability of the siloxane matrix combined to the stable interconnected cellular structure of the composite foam are considered as potentially key factors to guarantee a good durability and aging stability for heat storage applications without furthermore implying a relevant reduction in the water vapor kinetic diffusion.