Size effect of nanoparticle on specific heat in a ternary nitrate (LiNO₃-NaNO₃-KNO₃) salt eutectic for thermal energy storage

In this study we investigate the effect of nanoparticles on the specific heat of ternary nitrate salt eutectic doped with nanoparticles. Four different sizes of SiO₂ nanoparticles were tested: 5 nm, 10 nm, 30 nm, and 60 nm. They were doped into ternary nitrate salt eutectic (LiNO₃-NaNO₃-KNO₃) at 1% concentration by weight. Ternary nitrate salt eutectic has been considered as advanced thermal energy storage material due to its lower melting point and high thermal stability. Enhancing the specific heat of ternary nitrate salt eutectic can greatly increase its thermal storage density. This can not only reduce the material cost but also the size of pipe and storage tanks and, therefore, energy storage cost can be significantly reduced. A modulated differential scanning calorimeter was employed to measure the specific heat of ternary nitrate salt eutectic before and after doping with nanoparticles. According to the conventional specific heat model (density weighted rule), the specific heat of ternary nitrate salt eutectic should slightly decrease after doping with nanoparticles since the concentration of nanoparticles is very small (∼1% by weight) and the specific heat of nanoparticles is lower than that of ternary nitrate salt eutectic. However, the specific heat of the mixture was measured to be enhanced by 13-16% and no significant variation in specific heat was observed with nanoparticle size. From subsequent material characterization study, we observed a large amount of nanometer-sized structure formed by the salt compound around nanoparticles. Nanostructure has extremely large specific surface area. This can amplify the effect of surface energy on the effective specific heat (which was often negligible on a macroscale) and can be primarily responsible for the enhanced specific heat with doping nanoparticles.