**5. Conclusions and challenges**

Plasmonic nanofluids show great interest to improve the absorption ability due to the surface plasmon resonance (SPR) around the NP surface. By designing the NP parameters (material, shape, and size) or base fluid, plasmonic nanofluids can either absorb or transmit specific solar spectrum and thus making nanofluids ideal candidates for various solar applications in full spectrum absorption in direct solar absorption collectors, selective absorption in solar PT/V systems, and local heating in solar evaporation. As discussed above, some efforts have been made to improve

applications, including full spectrum absorption in direct solar absorption collector, selective absorption in solar PT/V systems, and local heating in solar evaporation or

Some efforts have been made to investigate the solar thermal conversion performance of stationary plasmonic nanofluids based on the direct solar absorption collectors (DASCs). A one-dimensional transient heat transfer analysis was carried out to analyze the effects of NP volume fraction, collector height, irradiation time, solar flux, and NP material on the collector efficiency. Results showed that the plasmonic nanofluids (e.g., Au and Ag) achieved the better collector efficiency in the stationary state [98]. Solar thermal conversion performance of Au nanofluids in a cylindrical tube under natural solar irradiation conditions was studied and a efficiency of 76.0% at a concentration of 5.8 ppm can be achieved [99]. Although Au nanofluids have high solar absorption performance, their expensive cost limits their practical use [100]. The solar thermal conversion performance of six (Ag, Cu Zn Fe, Si and Al2O3) common NPs in direct absorption solar collectors (DASC) was investigated under a focused simulated solar flux. Ag nanofluid turned out to be the best among all due its strong plasmonic resonance nature [101]. Stable silver nanofluids were prepared through a high-pressure homogenizer and the outdoor experiments were conducted under sunlight on a rooftop continuously for 10 h and the excellent photothermal conversion capability even under very low concen-

Recently, the direct-absorption parabolic-trough solar collector (DAPTSC) using the flow nanofluids has been proposed, and its thermal efficiency has been reported to be 5–10% higher than the conventional surface-based parabolic-trough solar collector. In order to reduce the cost of a collector and avoid NP agglomeration when using plasmonic nanofluids, the configuration with the lowest possible absorption coefficient but with the reasonably high temperature gain as well as efficiency was explored [103]. For the collector design, an extra glass tube inside was inserted so the nanofluid was separated into two concentric segmentations (i.e., an inner section and an outer section), and a nanofluid of lower concentration was applied in the outer section while a nanofluid of a higher concentration in the inner section. Results showed that at the same NP concentration parameter, the DAPTSCs with two concentric segmentations of nanofluids outperform those with one uniform nanofluid for all considered configurations [104]. Furthermore, the transparent DAPTSC was improved by applying a reflective coating on the upper half of the inner glass tube outer surface such that the optical path length was doubled compared to that of the transparent DAPTSC, allowing a reduction in the absorption coefficient of the nanofluid [105]. In addition, by replacing the semi-cylindrical reflective coating with a semi-cylindrical absorbing coating for exploiting both volumetric and surface absorption of the solar radiation. The DAPTSC with a

*Solar thermal applications of plasmonic nanofluids, including nanofluid based direct solar absorption collector, solar spectral beam splitter in solar PV/T systems [96], solar steam or nanobubble generation in solar*

steam generation, are discussed below in **Figure 4**.

*Advances in Microfluidics and Nanofluids*

trations can be achieved [102].

**Figure 4.**

**120**

*evaporation [97].*

the solar thermal conversion applications of plasmonic nanofluids. Some challenges are still needed to overcome for the further development of plasmonic nanofluid applications. The first one is the stability of nanofluids including the long-time and high-temperature. Currently, many works were conducted in the lab and a great solar thermal conversion performance can be achieved in a short period. The performance of nanofluids in the actual applications should be considered. The second one is that the performance evaluation standard of nanofluids should be unified. Many experiments were conducted and compared in the unique experimental conditions by oneself and a more extensive evolution method is needed for different researchers to compare the performance of different nanofluids. The last one is the cost of plasmonic nanofluids. The common plasmonic metal NPs, such as: Au, Ag, Cu, are expensive in the actual applications. More cheap NPs, including the low cost of preparation processes and materials, should be developed in the further.

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