Colloidal stability of molten salt –based nanofluids: Dynamic Light Scattering tests at high temperature conditions

Abstract

The use of molten salt-based nanofluids as heat transfer fluids or thermal energy storage materials to increase the efficiency of Concentrated Solar Power plants has gained attention due to the use of the renewable energies against Global Warming. One of the issues of interest is the colloidal stability of the nanoparticles dispersed in ionic media like molten salts. In this work a new experimental set-up to measure the particle size distribution of molten salt-based nanofluids by means of Dynamic Light Scattering was developed. The colloidal stability of silica and Al/Cu nanoparticles dispersed in solar salt (NaNO3-KNO3) was experimentally measured for the first time. Silica nanoparticles were dispersed in water, calcium nitrate tetrahydrate and solar salt, and the formation of micrometrical agglomerates was observed when molten salts were used as base fluid due to the high ionic strength of the medium and the reduced Debye length. The influence of the nanoparticle composition was proved to be also important. For the Al/Cu metal alloy nanoparticles the agglomerates formed were smaller than for silica. Besides, even though both nanoparticles settle after 4 h in static conditions, only Al/Cu nanoparticles recover the initial particle size distribution when they are mechanically redispersed