Microencapsulation of nitrate salt for solar thermal energy storage- synthesis, characterisation and heat transfer study

Due to their enhanced thermophysical properties, encapsulated phase change materials have gained attention in solar thermal energy storage applications. In this work, we report the encapsulation of an optimized concentration of solar salt (60:40 of NaNO3: KNO3) in silica shell through the sol-gel technique. In this process, the size and quality of capsules are governed by the stirring rate and duration as well as the pH level of the solution. We investigated the effect of each parameter separately and successfully encapsulated the salt with 24 h of stirring at 600 rpm and pH level between 2.6 and 3.0. We showed that encapsulation increases the thermal conductivity of this material by 51% and does not affect its phase change temperature. However, its latent heat of melting and solidification reduced by 25.38 kJ/kg (19%) and 17.29 kJ/kg (16.6%) respectively at an encapsulation level of 80.94%. The subcooling temperature also decreased by 10.5% after encapsulating the salt. The thermophysical property studies indicate the encapsulated eutectic nitrate salt as possible PCM for solar energy storage applications.