Synthesis and Characterization of Nanofluids Useful in Concentrated Solar Power Plants Produced by New Mixing Methodologies for Large-Scale Production

In this study, different nanofluids (NFs) were developed by mixing a molten salt mixture (60% NaNO₃–40% KNO₃) with 1.0 wt % of silica–alumina nanoparticles using different methods. These NFs can be used as thermal energy storage materials in concentrating solar plants with a reduction of storage material if the thermal properties of the base fluid are increased. New mixing procedures without sonication were introduced with the aim to avoid the sonication step and to allow the production of a greater amount of NF with a procedure potentially more suitable for large-scale productions. For this purpose, two mechanical mixers and a magnetic stirrer were used. Each NF was prepared in aqueous solution with a concentration of 100 g/l. The effect of different concentrations (300 g/l and 500 g/l) was also studied with the most effective mixer. Specific heat, melting temperature, and latent heat were measured by means of differential scanning calorimeter. Thermal conductivity and diffusivity in the solid state were also evaluated. The results show that the highest increase of the specific heat was obtained with 100 g/l both in solid (up to 31%) and in liquid phase (up to 14%) with the two mechanical mixers. The same NFs also showed higher amount of stored heat. An increase in thermal conductivity and diffusivity was also detected for high solution concentrations with a maximum of 25% and 47%, respectively. Scanning electron microscopy (SEM) and energy-dispersive X-ray analyses revealed that the grain size in the NFs is much smaller than in the salt mixture, especially for the NF showing the highest thermal properties increase, and a better nanoparticles distribution is achieved with the lowest concentration. NFs with enhanced thermal properties can be synthesized in a cost-effective form in high concentrated aqueous solutions by using mechanical mixers.