Study of Specific Heat Capacity Enhancement of Molten Salt Nanomaterials for Solar Thermal Energy Storage (TES)

Abstract

The overall thermal efficiency of solar power plants is highly sensitive to the operating characteristics of the Thermal Energy Storage (TES) devices. Enhancing the operating temperature of TES is imperative for enhancing the thermal efficacy of solar power plants. However, material property limitations for high temperature operation severely limit the choice of materials for TES. Molten salts and their eutectics are promising candidates for high temperature operation of TES. To enhance the thermal and operational efficiency of TES, the thermo-physical properties such as the specific heat capacity and thermal conductivity of the materials need to be maximized. The specific heat capacity (Cp) of molten salt is relatively smaller than other conventional TES materials. Recent studies have shown that addition of nanoparticles to molten salts can significantly enhance their specific heat capacity. Several transport and energy storage mechanisms have been proposed to account for these enhancements. Primarily, the layering of solvent molecules due to inter-molecular forces (due to competition between adhesive and cohesive forces) is observed at solid-liquid interface, leading to the formation of a more dense or “compressed layer” of solvent molecules on the dispersed nanoparticles. The formation and existence of the compressed layer has been demonstrated experimentally and from numerical predictions (e.g., Molecular Dynamics/ MD models). To verify the enhancement of specific heat capacity of molten salt nanofluids, the influence of compressed layer has been explored in this study. This implies that for the same amount (or concentration) of nanoparticle, the ratio of surface/volume of the individual nanoparticles can change significantly depending on the nanoparticles size and shape — which in turn can affect the mass fraction of the compressed layer formed on the surface of the nanoparticles. In this study, the specific heat capacity of the molten salt nanomaterials was investigated for: (a) silica nanoparticles in eutectic mixture of alkali chloride salt eutectics, and (b) silica nanoparticles in an eutectic mixture of alkali carbonate salts eutectics. The effect of the particle size distribution was considered in this study and it was observed that smaller nanoparticles contribute a larger proportion to the observed specific heat capacity enhancements. The size of distribution of the nanoparticles in the molten salt mixture/ nanomaterial (nanocomposites and nanofluids) was measured by using Scanning Electron Microscopy (SEM), and subsequently the actual number of nanoparticles (as a function of size) that were dispersed in molten salt fluid was calculated. The specific heat capacity of molten salt nanomaterial was calculated using a classical mixing model and by accounting for the contribution from the compressed layer in the mixture.