## 6 Limitations and Challenges  
This section will briefly present the points regarding of the limitations and challenges of the noble metals nanofluids. The chose, criterium is that to one limitation will correspond a challenge that can be translated in the recommendation of further and innovative experimental and numerical works on the subject. The limitations of the points herein highlighted derive mainly from non-consistent or controversial findings or results and from insufficient published data about the underlying mechanisms or influencing factors. In this sense, the following points summarize the main limitations and challenges of the noble metals nanofluids:  
* Further analysis should be performed with the noble metals nanofluids to address the impact of the type and morphology of the nanoparticles on the thermal conductivity and specific heat. Such works will bring some more light on the mechanistic understanding of the thermal conductivity influencing factors. * There should be conducted more works on the morphology and rheological characteristics of silver nanorods nanofluids to evaluate their ability as heat transfer fluids in large-scale applications. * Given that it can be one of the main benefits of the noble metals nanofluids in general and of the silver nanofluids in particular, it is highly recommended to carry out long-term pool boiling experiments using silver nanofluids to assess the absence of fouling and nanoparticle deposition onto the boiling surface. The reason behind this fact has been appointed to the diamagnetic character of silver nanoparticles. * Further research works on the use of gold nanofluids for the heat transfer of thermal management equipment to evaluate their performance in laminar and turbulent regions at high heat fluxes. * There should be synthesized more gold-ionic liquids nanofluids to infer their stability, thermophysical features and application as heat transfer fluids. * It is highly recommended to perform preliminary tests with noble metals nanofluids in direct absorption solar collectors to avert the great dispersion of results and advance on their potential application. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Authors** & **Nanoparticles** & \begin{tabular}{c} **Concentration of** \\ **the Nanoparticles** \\ \end{tabular} & **Base Fluid** & **Main Findings** & **Observations** & **Reference** \\ \hline \hline \multirow{9}{*}{\begin{tabular}{c} [PERSON] et al. \\ \end{tabular} } & Gold-Titanium & \multirow{9}{*}{\
\begin{tabular}{c} Gold 5 ppm and \\ oval with 12-15 nm \\ \end{tabular} } & & &
\begin{tabular}{c} The light-to-heat \\ conversion characteristics \\ of hybrid nanofluids are \\ not necessarily stronger \\ than that of \\ single nanofluids \\ \end{tabular} & [124] \\  & Gold & 5 ppm and & 10 ppm & Water & 60.8\% in the absorption & The ratio of different \\  & & & & concentrations and the \\  & & & & content of different \\  & & & & & components affected \\  & & & & & the results \\ \hline \hline \end{tabular}
\end{table} 
Table 4: _Cont._Further research works on the optical and photothermal conversion efficiencies of gold-palladium bimetallic nanofluids with different ratios are most welcome to better understand their suitability for solar thermal conversion processes. * There should be performed more tests of prolonged storage, continuous and cycling heating, flowing, and solar irradiation tests to evaluate the stability of the gold nanofluids with surface modification. * Since the plasmonic nanofluid filtering has become increasingly more common in solar energy systems because of their adjustable thermophysical and optical properties, there should be further conducted more works to optimize the features of the filters of the plasmonic nanofluids.