**4. Applications**

Nanofluids can either absorb or transmit specific solar spectrum and thus making assorted nanofluids ideal candidates for various solar applications [95]. Based on the tunable optical absorption performance of plasmonic nanofluids, several

The optical properties of nanofluids also can be solved by the Monte Carlo (MC) method to obtain the solar absorption performance of nanofluids. MC technique is a flexible method for simulating light propagation in the medium. The simulation is based on the random walks that photons make as they travel, which are chosen by statistically sampling the probability distributions for step size and angular deflection per scattering event. After propagating many photons, the net distribution of all the photon paths yields an accurate approximation to reality. In this method, the scattering effect is considered by the scattering efficiency and scattering phase function. The absorptance, transmittance, and reflectance of nanofluids can be

For the plasmonic nanofluids applied in the solar thermal applications, the absorption spectral distribution is one of the most important parameters, which is proportional to the NP parameters (concentration, shape and size), Qin et al. theo-

retically optimized the spectral absorption coefficient of an ideal plasmonic nanofluid for a DASC to maximize the thermal efficiency while maintaining the magnitude of the average absorption coefficient at a certain value [62]. However, considering that the SPR frequency of metallic NPs, such as Au, Ag, and Al, is usually located in the ultraviolet to visible range. The actual plasmonic nanofluids usually have the narrow absorption band due to the SPRs. Two strategies can be

adopted to overcome this shortage in the NP theoretical design process.

scattering distributions [63]. Based on MC method and FEM, four type of Au nanoshells were blended in the base fluid to enhance the solar absorption performance of plasmonic nanofluids with an extremely low particle concentration (e.g., approximately 70% for a 0.05% particle volume fraction) [64]. By applying the customized genetic algorithm, an optimal combination for a blended nanofluid (metal nanosphere, metal@SiO2 core-shell, and metal nanorod) was designed with the desired spectral distribution of the absorption coefficient [65]. Besides the core-shell NPs, other NP shapes were also designed to expand the absorbance over the entire solar spectrum [66, 67]. Although different blended NPs were designed to broad the absorption spectrum, the comparison or enhancement is usually done based on the single-element nanofluid, which is not enough to compare with the other blend styles. The other is to design complex NP structures with multiple absorption peaks at different wavelengths or coupled with the great intrinsic absorption materials. For

example, core-shell NPs (Al@CdS [68], Ag@SiO2@CdS [69], Au@C [70],

lengths [17], which was also similar as the thorny NPs [75].

**3.2 Experimental design**

**118**

Ag@TiO2 [71], and gallium-doped zinc oxide@Cu [72, 73]) were the direct way to enhance the solar absorption performance due to the enhancement and tunable SPRs of shell and intrinsic absorption of core by optical simulations, thus broadening the absorption spectrum and improving the solar absorption performance of plasmonic nanofluids. Results also found that Ag NPs with sharp edges can induce multiple absorption peaks due to both LSPR and lightning rod effect to broad the absorption spectrum [74]. In addition, a plasmonic dimer nanofluid, consisting of the rod and sphere, was proposed to enhance the solar absorption performance by LSPR, PSPR, and gap resonance between the rod and sphere at different wave-

Although various NP structures were designed to enhance the solar absorption

performance of plasmonic nanofluids theoretically, the synthesizes of these

One is to blend NPs with different absorption peaks to form hybrid plasmonic nanofluids for full utilization of solar energy in a broad spectrum. For example, an ideal distribution of spherical metal NPs, including nanospheres and nanoshells, were designed to match the AM 1.5 solar spectrum with an determination of absorbing and

calculated by counting the fate of photons.

*Advances in Microfluidics and Nanofluids*

applications, including full spectrum absorption in direct solar absorption collector, selective absorption in solar PT/V systems, and local heating in solar evaporation or steam generation, are discussed below in **Figure 4**.

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 concentrations can be achieved [102].

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

hybrid of volumetric and surface absorption can achieve a significantly higher thermal efficiency than the previous design of a DAPTSC with a reflective coating [8]. An innovative nanofluid enabled pump-free DASC concept was presented by combining the advantages of volumetric solar harvesting and oscillating heat pipes to enhance the solar harvesting and spontaneously transfer the heat into targeted areas, providing a novel approach for efficient solar energy utilization [106]. Nanofluid-based spectral beam splitters have become dramatically popular for PV/T applications due to it can achieve tunable optical properties inexpensively [107]. For example, CoSO4-based Ag nanofluid was developed to be utilized as fluid optical filter for hybrid PV/T system with silicon concentrator solar cell [108]. Furthermore, Ag NPs suspended in hybrid CoSO4 and propylene glycol base fluids were prepared for both silicon and GaAs cells. Ag/CoSO4-PG nanofluid filters exhibited broad absorption outside solar wavelengths and showed high transmittance in wavelength range used by the two types of cells efficiently [109]. More review about the

*Solar Thermal Conversion of Plasmonic Nanofluids: Fundamentals and Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.96991*

application of nanofluids in solar PT/V systems can be found in [96, 110].

[113]. A similar experiment under a sunlight intensity of 280 sun was also conducted to investigate the steam production phenomenon using Au nanofluids [114]. To further improve the solar evaporation, bubbles were also introduced into dilute plasmonic nanofluids to enhance solar water evaporation, which acted as light scattering centers to extend the incident light pathway and provided large gas– liquid interfaces for moisture capture as well as kinetic energy from bubble bursting to improve vapor diffusion [115]. Well-controlled experiments were performed to clarify the mechanism of the solar evaporation process using plasmonic Au nanofluid, carbon black nanofluid, and micro-sized porous medium. The results showed that Au nanofluids are not feasible for solar evaporation applications due to the high cost and low absorptance. High nanofluid concentration is needed to trap the solar energy in a thin layer at the liquid-gaseous interface, resulting in a local

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

higher temperature and a higher evaporation rate [116, 117].

**5. Conclusions and challenges**

**121**

Steam generation by nanofluid under solar radiation has attracted intensive attention recently. Due to strong absorption of solar energy, NP-based solar vapor generation could have wide applications in many areas including desalination, sterilization and power generation. Steam generation of Au nanofluids under focused sunlight of 5 sun and 10 sun were performed. Results showed that localized energy trapping at the surface of nanofluid was responsible for the fast vapor generation [111]. The total efficiency reached 65% using a plasmonic Au nanofluid (178 ppm) under 10 sun, achieving a 300% enhancement in efficiency compared with the pure water [112]. Optimizing the range of nanofluid concentration and optical depth can be used for future solar vapor generator design. To further increase the sunlight intensity to 220 sun, experiment results coupled with the simulation model indicated that the initial stage of steam generation is mainly caused by localized boiling and vaporization in the superheated region due to highly non-uniform temperature and radiation energy distribution, albeit the bulk fluid is still subcooled

#### **Figure 4.**

*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 evaporation [97].*
