Rheological analysis of nitrate molten salts with suspended particles for solar thermal plants

Abstract

Binary alkali nitrate molten salts are currently the heat transfer fluid (HTF) and thermal energy storage (TES) media of choice in commercial solar thermal energy (STE) power plants. This paper studies the rheological properties of binary alkali nitrate molten salts containing two distinct classes of suspended particles with unique practical applications and dynamics. Phase change salt slurries (liquid/solid suspensions) can appear during plant start-up after local freezing in pipes. Molten salt-nanoparticle suspensions are being proposed in the literature as an advanced HTF with enhanced properties. The phase change slurries were analyzed by rheological measurement of off-eutectic salt mixtures at sub-liquidus temperatures combined with determination of solid fraction through differential scanning calorimeter (DSC) testing in a novel approach. Molten salts with a range of silica nanoparticle loadings were also studied in the rheometer. Both suspension types exhibit non-Newtonian, shear thinning behavior, with a dramatic increase in viscosity compared to the base fluid. Additionally, both suspension types show that measured viscosity depends on the shear history, but the trends in viscosity with shear history differs between the two systems. The increase in viscosity with solid volume fraction of the nanofluid is significantly higher than that of a partially-crystallized salt slurry at a given volume fraction. Particle aggregation, particle/cluster aspect ratio, cluster breakage and restructuring are considered as determining factors in the observed rheological behavior. Deviations from classical effective medium theories of well-dispersed systems are discussed. Differing trends suggest that unique physical phenomena govern the rheological behaviors of the molten salt mixtures with different suspended solid particles. Thus, engineers and scientists must exercise caution when extrapolating rheological results to unique applications.