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

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

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