Effect of silica nanoparticle size on the stability and thermophysical properties of molten salts based nanofluids for thermal energy storage applications at concentrated solar power plants

Molten salts-based nanofluids are attractive candidates for thermal energy storage applications due to their enhanced thermophysical properties. However, their stability remains an open issue. In the present work, the size effect of SiO₂ nanoparticles on the stability and thermophysical properties of molten binary nitrate salt was studied. For that purpose, the effect of SiO₂ based nanofluids was systematically studied by using in-situ high-temperature observations and zeta potential experiments. From the analysis, the nanofluids having nanoparticles larger than 450 nm demonstrate superior stability compared to the ones with nanoparticles of 27 nm. Moreover, in contrast to the case of 27 nm particles increase of viscosity was shown to be negligible for particles larger than 450 nm. The absence of specific heat capacity (C_p) or thermal conductivity enhancement for the cases of larger nanoparticles suggests that the development of molten salts-based nanofluids is bounded by the compromise between the stability and improvement of thermophysical properties, depending on the particle size. These results open a pathway for the development of stable molten salt-based nanofluids with enhanced thermophysical properties where the size of the nanoparticles must be optimized.