Experimental study on thermal properties and microstructure of carbon nanotube molten salt nanofluids for solar thermal utilization

With the continuous increase in energy demand, the exploration and research of new energy sources are becoming increasingly important. Molten salt nanofluids as solar thermal conversion heat transfer and storage media are gradually becoming widespread. To futher enhance the influence of types and concentrations of multi-walled carbon nanotubes (MWCNTS) on the thermal properties of Solar and Hitec salt, short multi-walled carbon nanotubes (S-MWCNTS), MWCNTS, and carboxylated carbon nanotubes (COOH-MWCNTS) with the same size but different types were used as additives to prepare molten salt nanofluids. Specific heat (Cp), thermal conductivity, and other parameters were experimentally measured and analyzed. The results demonstrated that the addition of 0.5 wt% S-MWCNTS resulted in the most significant enhancement in Cp and thermal conductivity, with an increase of 13.79 % and 78.18 % compared to Solar salt, and an increase of 21.13 % and 130.69 % compared to Hitec salt. Scanning electron microscopy (SEM) observation showed that the molten salt nanofluid containing 0.5 wt% S-MWCNTS and COOH-MWCNTS exhibited a densely stacked network structure, which increased its surface area. Based on these findings, further research was conducted on the thermal properties of S-MWCNTS nanoparticles on Solar and Hitec salt at different concentrations. 0.3 wt% S-MWCNTS resulted in the most significant enhancement in Cp, which was 22.07 % higher than Solar salt and 23.95 % higher than Hitec salt; The maximum thermal conductivity improvement of adding 0.6 wt% S-MWCNTS to Solar salt is 101.14 %, and the maximum thermal conductivity improvement of adding 0.5 wt% S-MWCNTS to Hitec salt is 130.69 %.