## 5 Noble Nanofluids
### Palladium Nanofluids  
Figure 9 presents the main methodologies to produce palladium nanoparticles. The authors [PERSON] et al. [26] prepared and characterized palladium nanoplates dispersed in oil nanofluids and evaluated their adequacy to be used as heat transfer fluids and volumetric absorbers for concentrated solar power systems. Concentrations in the order of 0.02% vol. of these nanomaterials promoted the spectral extinction of the visible spectrum, suggesting that these palladium nanofluids were very promising choices for volumetric absorption purposes. The same nanomaterial content was enough to cause a 11.5% thermal conductivity enhancement with palladium nanoplates at 100 \({}^{\circ}\)C and to enhance the specific heat by 12.9% with palladium nanoplates at 250 \({}^{\circ}\)C, with no increases in the dynamic viscosity, which suggested that the developed nanofluids are also very suitable alternatives for heat transfer enhancement purposes. Besides, the authors reported that the palladium nanoplatenanofluid could improve the efficiency of a volumetric parabolic-trough collector for concentrated solar power stations by around 32%. Also, the nanofluid could reduce the pumping power demand by 15%, minimized the structural alterations, and ensured the structural integrity of the collectors by preventing their overheating. Moreover, the researchers [PERSON] et al. [73] discussed how the modification magnitude of the specific heat and thermal conductivity of the base fluid depended on the surface chemistry of the dispersed nanomaterials. The authors found that at 0.01% wt. of palladium nanoplates dispersed in a diphenyl oxide and biphenyl eutectic mixture, the specific heat capacity was increased by around 12.5%, and the thermal conductivity was increased by 6% at 373 K. The researchers confirmed through density functional theory and molecular dynamics simulations that the molecules of the base fluid were chemisorbed on the palladium surfaces. The authors proposed that the stronger interactions at solid-liquid interfaces, the greater the specific heat enhancements, and the lower the thermal conductivity enhancements at high temperature values. Besides, the researchers [PERSON] [74] elaborated an in-depth examination of the heat transfer and optical characteristics of palladium aromatic oil nanofluids, presenting promising results as heat transfer fluids and volumetric absorbers in the parabolic trough concentrated solar power stations. The authors found that at 0.03% wt. of concentration, the palladium nanoplates increased by 90% the sunlight extinction after a propagation length of 20 nm. The thermal conductivity was enhanced by 23.5% at a temperature of 373 K that was sufficient to increase the efficiency of the system by up to 45.3% and to decrease the pumping needs by 20%, with minimum collector length increases. The molecules chemisorbed at the interface acted as shelter boundaries that inhibited the heat transport, behaving as thermal resistance enhanced pathways, which strongly reduces the effects of the solid on the viscosity, given that it weakens the interactions between the nanoplates and the non-adsorbed molecules of the base fluid. The sunlight extinction was of 53%, 90% and 98% after 20 nm of propagation and at 0.006% wt., 0.03% wt, and 0.060% wt, suggesting that the palladium nanofluids are very suitable to be applied as volumetric absorbers. Furthermore, the effective thermal conductivity augmented by 2.6%, 11%, 17% and 23.5% at 0.03% wt. and at temperature values of 298 K, 323 K, 348 K, and 373 K. The viscosity, in the studied range of temperature values and concentrations, remained practically the same. Moreover, the researchers noted that in reference to the surface parabolic-trough collector using the common Dowthern TMA thermal fluid, the volumetric absorption with the palladium nanofluids enhanced the efficiency of the system by 45% and, simultaneously, reduced the pumping power demand by 20%.