An Experimental Study of the Effect of SiO2 Nanoparticles on the Phase Change Characteristics of KNO3-NaNO3 Mixtures for Thermal Energy Storage

Combining latent and sensible heat storage within a single material has lead researchers to propose off-eutectic salt mixtures for solar thermal energy storage. A binary salt mixture with an off-eutectic composition presents a melting temperature range as opposed to a melting point, providing additional storage capacity through the sensible heat present during the melting/solidification. However, some drawbacks of off-eutectic mixtures include a higher propensity towards incongruous melting, phase segregation of the mixture upon solidification, and thermal cycling issues. Here the solid-liquid transitions of a series of eutectic and off-eutectic KNO3-NaNO3 mixtures are evaluated to determine if the addition of nanoparticles can limit phase segregation yet still present good thermal properties. Measurements performed with a Differential Scanning Calorimeter show that the effect of nanoparticles is unimportant compared to the difference between eutectic and off-eutectic phase change behavior. Onset temperatures, latent heats, and the width of the phase transition temperature trace during melting/solidification are compared for the eutectic and off-eutectic salt mixtures with and without silica nanoparticles. A reduction in the latent heat is observed and explained through classical mixing theory. The modification of the phase change properties of eutectic and off-eutectic nanofluids are discussed.