## 5 Solar Panels and Collectors  
Photovoltaic solar panels are devices capable of converting solar energy into electrical energy. These panels are formed out of photovoltaic cells that need sunlight for their operation. However, an excess of solar energy causes high temperatures on the surface of the cells, impairing the functionality of these systems. The problems with the high environmental temperatures from some regions of the globe have led to the development of alternative cooling techniques. The performance of the output power of a photovoltaic module decreases by 0.4% to 0.5% per 1 \({}^{\circ}\)C when it works above its ideal temperature, in most cases being 25 \({}^{\circ}\)C [97]. To avoid the overheating of PV modules, nanofluids are potential candidates for cooling these systems. [PERSON] et al. [97] using two types of nanofluids, Al\({}_{2}\)O\({}_{3}\) and TiO\({}_{2}\) mixed in water, for different volume flow rates and concentrations of 0.01%, 0.05%, and 0.1% by weight, the authors evaluate the performance of heat transfer for three PV panels cooled simultaneously using nanofluids, water, and natural air. Results obtained showed that nanofluids enhanced heat transfer rate much better than water or natural air. The interest in nanofluids is due to their possibility to change the transport properties of the base fluid with the presence of suspended metallic or non-metallic nanoparticles when compared to conventional heat transfer fluids [2, 98]. In the solar energy area, the applications of nanofluids can be found, for example, in photovoltaics thermal systems and thermal energy storage [99]; solar collectors, solar stills, and hydrogen conversion (fuel cells) [100]. Several researchers have shown that the thermal conductivity of nanofluids increased as a function of nanoparticles volume concentration and temperature [101, 102, 103, 104]. Regarding the increase in thermal conductivity with temperature, this characteristic of nanofluids can positively benefit the cooling of the modules. However, the increase in volumetric concentration, which on the one hand benefits the increase in thermal conductivity, on the other hand, can raise the viscosity of nanofluids to levels that impair pumping capacity and increase the pressure drop of the systems. The problem with the pressure drop was related in experiments under the laminar flow regime [105] as under a turbulent regime [106], where there was an increase in power pumping with the increase of the volume fraction of nanoparticles in base fluids. Most studies reported in the literature have evaluated the use of serpentine heat exchangers using nanofluids to cool solar photovoltaic/thermal systems (e.g., [107, 108]). These studies show that the nanoparticles added to the base fluid have enhanced the heat transfer and reduced the operating temperature of the panels. For example [108], concluded that the utilization of water/magnetic nanofluid with 2% volume concentration possesses the highest ability for being employed as a cooling fluid compared with the NF volume concentration of 0% (pure water), 0.5%, 1%, and 1.5%, respectively. [PERSON] et al. [109] conducted an experimental investigation for improving a hybrid photovoltaic/thermal system performance using a nanofluid (Al\({}_{2}\)O\({}_{3}\)-Water) by applying forced convection for different concentrations, from 0.1 to 0.5%. The results showed that for a concentration of 3% the temperature dropped significantly to 42.2 \({}^{\circ}\)C and the electrical efficiency rose to 12.1%. However, increasing the concentration for more than this value caused a raising of the temperature to 52.2 \({}^{\circ}\)C while the electrical efficiency declined to 11.3%. An example of using nanofluids in serpentines for cooling photovoltaic solar panels can be seen in Figure 3.  
[PERSON] et al. [110] conducted an experimental investigation to analyze the performance of a flat plate solar collector using Al\({}_{2}\)O\({}_{3}\)/Distilled Water Nanofluid.