**1. Introduction**

Exceeding energy demands and swiftly eliminating conventional energy resources have compelled the researchers to find the alternative means of power generation. To date, only 14% of the world's power demands are being met via renewable energy means. Sun is the most

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

vital source of energy, almost 1.8x 1011 MW energy from the sun intercepts the earth's surface [1]. According to the estimate of International Energy Agency (IEA), quarter of world's power demands could be fulfilled by solar energy by 2050 [2]. Silicon-based photovoltaic cells are used to convert the solar radiations into electricity. But the issue with these PV solar cells is that almost 85% of the solar energy reaching the surface of the PV unit is either reflected or absorbed as heat energy [3]. Al-shamani et al. [4] reviewed that only 5–20% of the solar radiation reaching to the PV cell surface is converted into electrical energy. Whereas, rest of the radiations are either reflected back or absorbed by the cell in the form of heat. Absorbed heat can increase its temperature up to 70° *C*. Oruc et al. [5] found that the electrical efficiency of PV module drops by 0.5% with every unit degree increment in the temperature of the module above 25°*C* due to the contraction of the band gap and increased number of carriers. Increased number of carriers cause the saturation current to increase whereas the open circuit voltage to decrease thus lowering the electrical power output. Cooling of PV units depicts electrical efficiency enhancement as per the experimental results obtained by the researchers. Underdeveloped countries like Pakistan, with hot and sunny days throughout the year, are well suitable for power production via solar energy. According to research, during summer, temperature of the module can elevate in a devastating way (about 20° *C* higher), in turns destructing the conversion efficiency of PV modules [6, 7]. Bashir et al. [8] reported that cooling of PV modules via water minimized heat losses and module's temperature elevation, thus, improving the efficiency by 13% and 6.2% for monocrystalline and polycrystalline PV modules respectively. Ali et al. [9] experimentally showed that cooling of PV modules by using micro-channels increased the efficiency of PV modules by 3%.

flow rate of nanofluid, size of nanoparticles and geometry of micro-channels. Impact of other factors such as the type of nanoparticles and base fluid on the system efficiency are discussed.

Application of Nanofluids for Thermal Management of Photovoltaic Modules: A Review

http://dx.doi.org/10.5772/intechopen.74967

37

There are several methods of extracting heat from the PV units via nanofluids. The most common ways are, employing heat collector at the rear end of the panel and using nanofluid as a liquid in spectral splitting filter joined on the front surface of PV module. Sometimes, both

In rear-end cooling a thermal collector is coupled at the back end of the PV module to extract the heat. Nanofluid is set to flow through the collector thus taking up the heat of the cells and increasing its own temperature. Nanofluid gets warmed and its heat is further employed for useful purposes. Nanofluid is able to extract major part of the heat energy because of its improved thermophysical properties. The most important thermophysical property is the thermal conductivity. A schematic display of such an arrangement is depicted in the **Figure 1**.

Energy balance of such PV/T systems is evaluated by the following equation by [19].

*in* <sup>=</sup> *<sup>E</sup>*̇

*el* "

*loss* "

Overall efficiency of the system is found by the following formula.

**Figure 1.** Schematic setup of rear end cooling of PV panel via Nanofluid. (a) [18], (b) [16].

*ηpvt* ≅ *E*˙ *el* <sup>+</sup> *<sup>E</sup>*˙ \_\_\_\_\_\_*th E*˙ *th*

*el* <sup>+</sup> *<sup>E</sup>*̇

*th* <sup>+</sup> *<sup>E</sup>*̇

is output electrical power, " *E* ̇

*loss* (1)

*th* "

presents the energy losses from the control volume.

⇒ *ηpvt* = *ηpvt* = *ηth* + *r* × *ηel* (2)

is useful thermal

Eventually, economic and environmental advantages are described.

**2. Methods of cooling of PV systems via nanofluids**

methods are used simultaneously in order to increase the efficiency.

**2.1. Rear end cooling**

Here, " *E*̇

*in* "

*E*̇

energy obtained by the collector and " *E*̇

is the incident irradiation, " *E*̇

There are several methods of PV cooling such as, air cooling (natural air circulation and forced air circulation), water cooling, heat pipe cooling, cooling with Phase Change Materials (PCMs) and cooling via nanofluids [10, 11]. A PV/T system consists of PV module coupled with a heat absorbing unit in which a liquid (water or nanofluid) is circulated to absorb the heat of PV unit to improve the efficiency. The researches show that a PV/T system performs way better than conventional PV systems [12, 13]. Lelea et al. [14] investigated the effect of cooling via *Al*<sup>2</sup> *O*<sup>3</sup> on the performance of concentrated PV/T system. The results showed a decrement in the temperature of module, when cooled by nanofluid and water.

Mixture of solid particles (metallic oxides, metals or carbon nanotubes) of less than 100 nm size at least in one dimension (nanoparticles) disseminated in the liquid fluids like water and polyethylene glycol etcetera (base fluid), is known as nanofluid. Nanofluids can be employed as a coolant as well as optical filters within PV/T systems [15]. PV/T system using nanofluid as coolant can produce far better results than the water cooled system. Al-Waeli et al. [16] conducted an experimental study and they found that cooling of PV module via SiC increased the electrical efficiency by 24.1%, thermal efficiency by 100.19% and overall efficiency by 88.9% as compared to the water-cooled PV/T system. Xu and Kleinstreuer [17] suggested nanofluid based silicon PV/T systems as a useful option for domestic applications as its overall efficiency reached up to 70% (11% electrical efficiency and 59% thermal efficiency).

This chapter reviews the efficiency of PV systems being cooled by various nanofluids. The common ways of cooling PV system via nanofluids are stated in detail along with the parameters influencing the efficiency of the PV/T systems such as irradiance, concentration and flow rate of nanofluid, size of nanoparticles and geometry of micro-channels. Impact of other factors such as the type of nanoparticles and base fluid on the system efficiency are discussed. Eventually, economic and environmental advantages are described.
