Effect of nanoparticle dispersion on specific heat capacity of a binary nitrate salt eutectic for concentrated solar power applications

Abstract

In this study we investigate nanoparticle dispersions in a molten binary nitrate salt eutectic. It has been recently reported that nanoparticle dispersions in molten salt mixtures can significantly enhance the specific heat capacity of the salt mixtures. These molten salt mixtures can be used as heat transfer fluid (HTF)/thermal energy storage (TES) in a concentrated solar power (CSP) plant and enhancing their specific heat capacity can significantly reduce the cost of electricity produced by CSP. However, the mechanism for the enhanced specific heat capacity is still under investigation and has not been clearly explained. In this paper, we investigate the effect of nanoparticle size on the specific heat capacity of nanoparticle/molten salt eutectic mixture. Four different sizes of nanoparticles (5 nm, 10 nm, 30 nm, and 60 nm) were dispersed in a molten nitrate salt eutectic at 1% concentration by weight. The molten nitrate salt eutectic consisted of sodium nitrate (NaNO3) and potassium nitrate (KNO3) at compositions of 60% and 40% by weight. A modulated differential scanning calorimeter (MDSC) was employed to measure the specific heat capacity of the pure molten salt eutectic and the nanomaterials (pure molten salt eutectic mixed with nanoparticles). The specific heat capacity of the nanomaterials was enhanced with increase of nanoparticle size. The observed enhancement was found to be 8% for 5 nm, 12% for 10 nm, 19% for 30 nm and 27% for the 60 nm. Material characterization analyses were carried out to investigate microstructural change of the nanomaterials. It was observed that special nanostructures were formed by molten salt mixtures in the nanomaterial samples and the amount of observed nanostructures was increased with the measured specific heat capacity. This indicates that nanostructures formed in the nanomaterials may be responsible for the enhanced specific heat capacity of the nanomaterials.