## 5 Noble Nanofluids
### Silver Nanofluids
[56] manufactured silver nanoparticles and nanorods through chemical reduction to produce and evaluate the resulting nanofluids. The authors reported that both silver nanoparticles and nanorods exhibited a thermal conductivity peak at a volumetric concentration of 2% vol. The silver nanorods nanofluids showed better results in comparison to those attained with the nanoparticles. The corresponding enhancement peaks were nearly 53% at 313 K for the nanoparticles and of around 78% at 323 K for the nanorods. Also, it was confirmed that the thermal conductivity decreased with increasing concentration of silver nanoparticles or nanorods because of the nanoparticle clustering that occurred at higher concentrations. The thermal conductivity of the nanofluids was also found to increase with increasing temperature values. Moreover, the researchers [PERSON] and [PERSON] [57] evaluated the thermal performance of a V-basin tube solar collector with 20 nm-sized silver nanoparticles dispersed in ethylene glycol and distilled water at volumetric fractions between 1% vol. and 5% vol. The nanofluids provided an appreciable performance enhancement of the V-basin tube solar collector translated in a higher heat transfer capability and thermal conductivity as compared to that of the ethylene glycol and distilled water. The silver nanofluids obtained an improvement when it was compared to the distilled water and ethylene glycol mostly at a increased inlet temperature values. The authors concluded that the size of the nanoparticles was of relevance in the heat transfer enhancement and thermal performance of the solar collector. It should be emphasized that the authors also did a comparative analysis with nanofluids having titanium oxide nanoparticles. The researchers concluded that the silver nanofluids provided extra enhancements in the thermal solar parameters in respect to those attained with the titanium oxide nanofluids. Also, the solar collector efficiency for the silver nanofluids was larger than the one achieved when using titanium oxide nanofluids due to the smaller silver nanoparticles and corresponding higher thermal conductivity. It was also reported that the difference between the inlet and outlet temperature of the silver nanofluids was increased due to the heat rate of the solar collector and lossless heat in the cases where the concentration of the nanoparticles was increased. Additionally, the researchers [PERSON] et al. [58] inferred the effect of silver nanoparticles stabilized with surfactants on the thermal conductivity of hexane, ethylene glycol, and water. The silver nanoparticles were prepared in aqueous medium using the gum arabic surfactant and oleic acid /deylamine to the silver nanoparticles stabilization. There were achieved up to 10% enhancements in the thermal conductivity of the ethylene glycol and hexane at 2% wt. and 1% wt. of silver nanoparticles, respectively. Nonetheless, it was confirmed that a nearly 10% decrease in the effective thermal conductivity employing the 1% wt. silver aqueous nanofluids. The silver nanoparticles performed quite better in ethylene glycol in which the stabilizer did not reduce the base fluid thermal conductivity. At a temperature of 20 \({}^{\circ}\)C, the inclusion of silver nanoparticles into the hexane provided enhancements of 3% and 11% at 0.06% vol. and 0.13% vol., respectively. Also, an enhancement of nearly 10% was observed for the ethylene glycol nanofluid at 0.11% vol. The increase in the effective thermal conductivity was attributed to the thermal diffusion and Brownian motion of the nanoparticles. The thermal conductivity increase could be obtained by augmenting the concentration of the nanoparticles. Nonetheless, the thermal conductivity increasing rate weakens at higher concentrations of the nanoparticles because of the eventual nanoparticle agglomeration. When the gum arabic was solubilized in ethylene glycol, it did not change the base fluid thermal conductivity.