**1. Introduction**

Nowadays, with the development of society and the improvement of people's living standards, environment issues, such as: greenhouse effect, acid rain, and haze, has become more serious. Developing green energy technology has attracted more researchers' attention, especially for solar energy, which is universal, harmless, huge, and sustainable. Solar utilizations can be divided into two main categories: solar-electric and solar-thermal. And both of them are needed to enhance the solar absorption performance of working media at its first step of solar conversion applications.

Solar thermal conversion is one of the most simple and direct ways of solar utilizations by heating the working mediums directly for follow-up usages, which can be widely used in the solar thermal collectors [1], solar distillation [2, 3] and so on [4]. Therefore, it's critical to improve the solar absorption performance of working mediums for the solar thermal conversion applications. For example, based on the surface absorber, various nanostructure coatings (e.g., grating, porous structure, and so on [5, 6]) were designed to achieve the selective absorption ability, which serves as solar selective absorbers by heating the surface and transferring the heat to the working fluid for the follow-up applications. The heat loss from the absorbed surface due to the high temperature and heat transferred resistance between the absorbed surface and working should be considered during the design processes, which also limits its large-scale practical application at the high or middle temperature solar thermal conversion applications.

Instead of absorbing solar energy by a surface, work fluid can be used to absorb solar energy, which serves as both the solar absorber and heat transfer medium and can avoid the local high temperature area and reduce the heat transfer resistance. However, the common working fluids such as: water, oil, and alcohol usually have the limited solar absorption ability [7]. It was found that adding nanoparticles (NPs) to these working fluid (i.e., nanofluid) can greatly improve the solar collector efficiency [8, 9]. Nanofluid is a suspension of NPs (1–100 nm) in a conventional base fluid, which was first used by Choi in 1995 [10]. Nanofluids show unique characteristics in many aspects, including the heat transfer [11, 12] and the solar absorption ability due to the interaction between the light and NPs at nanoscale [9, 13]. For example, carbon nanotube, graphite and the other black carbon NPs were added into the base fluid to achieve the great solar absorption performance [14].

Plasmonic nanofluids show great interests to improve the absorption ability by dispersing plasmonic NPs in the base fluid stability. Due to the surface plasmon resonance (SPR) around the NP surface [15], the incident electric coupled with the free electron oscillation around the NP surface at the resonance frequency can strongly enhance the absorption performance of NPs [16] in **Figure 1**. The optical absorption performance of nanofluids can be enhanced by tuning the NP shape, size, or base fluid. Using plasmonic nanofluids as the absorber and heat transfer medium in the solar thermal applications shows great potential due to the excellent optical and thermal characteristics. To choose a proper nanofluids for specific solar thermal applications (such as: solar collectors, solar PV/T systems), many researchers investigated the optical and thermal properties of various nanofluids. For example, for the direct absorption solar collectors (DASCs), nanofluids as the absorber need to absorb the solar radiation in the full solar spectrum (0.3–2.5 um). While the nanofluid only serves as a beam splitter (i.e., selective absorber) in solar PV/T systems, which absorbs the useless spectrum for the PV cell and avoids heating the PV cells to improve the overall PV/T efficiency [4]. Hence, the optical absorption performance of plasmonic nanofluids should be considered in different solar thermal applications.

In this chapter, we focus on the solar thermal conversion of plasmonic nanofluids in **Figure 2**, which consists of the following three parts: 1) plasmonic nanofluid preparation including NPs and nanofluids; 2) solar absorption of plasmonic nanofluids based on the theoretical and experimental design; 3) solar thermal applications and challenges, including direct solar absorption collectors, solar PT/V systems, solar evaporation, other applications and challenges. To increase the understanding of previous studies, related analyses and calculation techniques are illustrated. This chapter is expected to provide researchers with deep insight into the solar thermal conversion of plasmonic nanofluids and facilitate

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

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

*The main parts of this chapter for solar thermal conversion of plasmonic nanofluids.*

As discussed above, the NP parameters and dispersed base fluid have great effect on the optical absorption and solar thermal conversion performance of plasmonic nanofluids. We will first discuss the preparation methods of plasmonic nanofluids, and then the preparation methods of plasmonic NPs (**Figure 3**) are summarized due to the great interaction of NP parameters with the light. In this section, some common methods to prepare plasmonic NPs or nanofluids are listed and their

The preparation method of nanofluids can be classified into two main categories

future studies in this field.

**Figure 2.**

**111**

**2. Plasmonic nanofluid preparation**

advantages or limitation would also be discussed.

in **Figure 3a**: one-step method and two-step method [18].

**2.1 Plasmonic nanofluid preparation**

#### **Figure 1.**

*Light propagation in the nanofluid [17] and the surface plasmon resonance (SPR) around the NP surface, dividing into localized and propagating surface plasmon resonance (LSPR and PSPR).*

*Solar Thermal Conversion of Plasmonic Nanofluids: Fundamentals and Applications DOI: http://dx.doi.org/10.5772/intechopen.96991*

**Figure 2.** *The main parts of this chapter for solar thermal conversion of plasmonic nanofluids.*

In this chapter, we focus on the solar thermal conversion of plasmonic nanofluids in **Figure 2**, which consists of the following three parts: 1) plasmonic nanofluid preparation including NPs and nanofluids; 2) solar absorption of plasmonic nanofluids based on the theoretical and experimental design; 3) solar thermal applications and challenges, including direct solar absorption collectors, solar PT/V systems, solar evaporation, other applications and challenges. To increase the understanding of previous studies, related analyses and calculation techniques are illustrated. This chapter is expected to provide researchers with deep insight into the solar thermal conversion of plasmonic nanofluids and facilitate future studies in this field.
