## 5 Solar Panels and Collectors
The Al\({}_{2}\)O\({}_{3}\) concentration varied from 0.1, 0.2, and 0.3% by volume, and the dispersion quality of nanoparticles in the fluid was enhanced using 0.8% wt. of surfactant sodium dodecyl sulfate (SDS). [PERSON] et al. [110] report that the nanofluid at a volumetric concentration of 0.3% presented a maximum increase in collector efficiency of 21.32%. [PERSON] and [PERSON] [111], using CuO/Water nanofluid prepared at a low volume concentration of 0.05%, obtained an increase in the thermal performance of the solar water heater by 6.3% considering a flow rate of 0.1 kg/s. [PERSON] et al. [112] performed an experimental investigation to evaluate the problem of sedimentation particles through a flat plate solar  
Figure 3: Scheme illustrating the use of nanofluids in serpentines for cooling photovoltaic solar panels. collector operated with nanofluids. The NFs tested were Al\({}_{2}\)O\({}_{3}\), ZnO, and Fe\({}_{2}\)O\({}_{3}\) with different shapes and concentrations of the 1 vol%, 2 vol%, and 3 vol%. They concluded that flow velocity is the principal factor affecting particle sedimentation. Furthermore, they concluded that the thermal conductivity enhancement up to 6.7% at a concentration of 3 vol% of Al\({}_{2}\)O\({}_{3}\), while the convective heat transfer coefficient increased up to 25%. Phase Change Materials (PCM) are another kind of materials used for solar thermal applications, which can help to solve part of the intermittency problem of solar energy and increase the daily thermal energy storage efficiency of solar thermal applications, such as solar power plants [113]. One of the authors' conclusions is that the dispersion effect of nanoparticles is the basic starting point of thermal conductivity enhancement of the PCMs, and is responsible for ensuring that energy is stored and released well during the flow. The thermal absorption performance of nanofluids is another important characteristic of this colloidal mixture. Recently [114], an experimental study was performed to evaluate the thermal absorption performance of nanofluids of carbon nanotubes, Cu, and Al\({}_{2}\)O\({}_{3}\) using water with base fluid. The authors show that all nanofluids increased the absorbance and electrical conductivity with increasing nanoparticle concentration, which has improved the thermal absorption performance of nanofluids. Depending on the amount of absorption and the energy transferred by the fluid during the process of capturing solar energy, the solar energy efficiency can vary widely [115]. Furthermore, according to [PERSON] et al. [116], due to their ability to change the working fluid properties, such as light absorption and thermal conductivity, nanofluids are considered the best option for solar collector applications. Proposed by [PERSON] and [PERSON] in the 1970s [117], direct absorption solar collectors (DASCs) emerged as a promising way to reach high thermal efficiency. In these devices, the solar irradiation is absorbed by the fluid, where it works as an absorber and a heat transfer medium at the same time [118]. In these collectors, heat loss to the environment is reduced as high temperature occurs in the fluid. Hence, the efficiency of DASCs will be greatly dependent on the thermal characteristics of the working fluids. Fluids having high thermal conductivity, excellent optical absorption, and good photo-thermal conversion performance are highly desirable for developing high-efficiency DASCs [118]. [PERSON] et al. [118] have tested reduced graphene oxide (RGO) nanofluid prepared by irradiating a graphene oxide (GO)/water one under UV light for different times. They have experimentally evaluated the photo-thermal conversion performance of one DASC. The results have shown that the RGO/water nanofluid prepared from the GO/water one shows great potential to be used as a working fluid in low-temperature direct absorption solar collectors due to the good stability along with the high optical absorption and thermal conductivity presented.