**2.1. Rear end cooling**

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%.

decrement in the temperature of module, when cooled by nanofluid and water.

reached up to 70% (11% electrical efficiency and 59% thermal efficiency).

cooling via *Al*<sup>2</sup> *O*<sup>3</sup>

36 Microfluidics and Nanofluidics

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

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

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

on the performance of concentrated PV/T system. The results showed a

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].

$$
\dot{E}\_{\text{in}} = \dot{E}\_{el} + \dot{E}\_{th} + \dot{E}\_{\text{loss}} \tag{1}
$$

Here, " *E*̇ *in* " is the incident irradiation, " *E*̇ *el* " is output electrical power, " *E* ̇ *th* " is useful thermal energy obtained by the collector and " *E*̇ *loss* " presents the energy losses from the control volume. Overall efficiency of the system is found by the following formula.

$$
\eta\_{pvt} \cong \frac{\mathcal{E}\_{el} + \mathcal{E}\_{th}}{\mathcal{E}\_{th}} \implies \eta\_{pvt} = \eta\_{pvt} = \eta\_{th} + r \times \eta\_{el} \tag{2}
$$

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

Here,"r" is the packing factor.

$$r = \frac{A\_{pv}}{A\_e} \tag{3}$$

the spectrum of radiation. Nanofluid-based optical filters separate the part of solar radiation for the PV cells from the radiation part that is more useful for heat generation. There are two

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

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

39

In single pipe system, there are two sections of pipe; primary section and secondary section. Primary section is set underneath the rear surface of the Photovoltaic module having aluminum sheet in between. Primary section further elongates above the upper surface of PV module. Nanofluid enters from the inlet of primary section, thus, absorbing heat of the module. Heated nanofluid further passes over the PV's upper surface, in turns filtering the solar radiation. Part of radiation having wavelength equal to silicon bandgap is filtered and rest of the section is absorbed by the nanofluid flowing in the secondary channel which gets out of the secondary pipe at secondary outlet. Air exists between upper surface of PV module and secondary channel section. As the air gets hot, it flows in upward direction and the cool air still remains in contact with PV surface. It is assumed that no convection current is produced in the air. The results indicated, 83% and 80% overall and 76.5% and 74% thermal efficiency for Ag/water and Cu/water nanofluid respectively for above configuration [23]. Schematic

Wei An et al. [24] designed a spectral splitting Polypyrrole nanofluid based PV/T system in order to impede thermal losses and escalate the system's efficiency. Nanofluid used in spectral splitting filter is capable to absorb the part of solar radiations that cannot be utilized by PV cell and converts it into medium temperature thermal energy. The efficiency of PV/T system was found to be 25.2% for nanofluid based spectral splitting filter whereas, its value was 13.3% when there was no filter employed. Hjerrild et al. [25] worked on the cooling of PV system by the help of optical filters, they used Silver as nanoparticle (50 nm diameter) with coating of Silica. The results showed that, base fluid absorbed the ultraviolet part of solar radiation thus decreasing the heat losses whereas, nanoparticles absorbed visible portion of radiation, in turns increasing the overall efficiency of the system. Water showed highest electrical efficiency

) showed highest

(85% higher than unfiltered PV) whereas highly diluted nanofluid (*Ag* <sup>−</sup> *SiO*<sup>2</sup>

**Figure 2.** Schematic diagram of Nanofluid based spectral splitting filter PV/T system (reproduced) [23].

kinds of proposed configurations of these systems.

diagram of such system is shown in the following **Figure 2**.

**1.** Single Pipe System **2.** Two Pipe System

Here, " *Ac* " is the collector area and " *Apv* " is area of PV cells.

Area of PV to produce a certain amount of electrical power is calculated by the following formula.

$$A\_p = \frac{R \ E\_{out,max}}{E\_{out,tm^3}} \tag{4}$$

Here, " <sup>E</sup>out,1m<sup>2</sup> " is output electrical power per unit area and " *<sup>R</sup> Eout*,*max* " is the required output power. Thermal output energy is found by the following equation.

$$\dot{\bar{E}}\_{th} = \; m\_f \times \mathbb{C}\_{\eta f} \times \left(T\_{fo} - T\_{in}\right) \tag{5}$$

Here, " *mf* " is the mass flowrate of the fluid through the collector, " *Cpf* " is the fluid's specific heat and " *Tin* " and " *To* " depicts the fluid's inlet and outlet temperature respectively. The formulas to determine " *Cpf* " are given in Ref [20].

Electrical efficiency is found by the following formula.

$$\eta\_{el} = \frac{\dot{E}\_{el}}{\dot{E}\_{in}} = \frac{V\_{oc} \times I\_{sc} \times FF}{G\_{el}} \tag{6}$$

Here, " *Voc* " is the open circuit voltage, " *I sc* " is the short circuit current, and " *<sup>G</sup>eff* " is the effective absorbed solar irradiation by the PV module. "FF" represents the fill factor and it is defined as the maximum power conversion efficiency.

$$FF = f \times \left(\frac{V\_w}{T}\right) \tag{7}$$

Using the aforementioned formulas, the efficiency of a PV/T system is determined.

Radwan et al. [20] examined the cooling effect of *Al*<sup>2</sup> *<sup>O</sup>*<sup>3</sup> , SiC nanoparticles and water on the performance of concentrated PV system. Pertaining to the results, SiC-water nanofluid produced better impact as compared to *Al*<sup>2</sup> *<sup>O</sup>*<sup>3</sup> and water. It was observed that at higher concentration ratio (area of aperture/area of cell) and smaller Re, higher electrical efficiency was found. Using the pure water at CR = 40, the cell temperature reached a maximum of 68° *C*. Were as, for 4vol% SiC, the maximum temperature of the cell was found to be 60° *C*.

#### **2.2. Optical filter cooling**

Extensive work has been carried out on efficiency improvement by using nanofluid flowing through optical filters [21, 22]. Silicon-based Photovoltaic cells can generate electricity by absorbing the part of solar radiation with 400 nm to 1200 nm wavelength. Rest of the solar radiation's part is either reflected back or absorbed by the PV cells as heat. In optical filter cooling, an optical filter containing nanofluid is held above the front surface of cells to split the spectrum of radiation. Nanofluid-based optical filters separate the part of solar radiation for the PV cells from the radiation part that is more useful for heat generation. There are two kinds of proposed configurations of these systems.


(3)

(4)

(6)

is the effective

is the required output power.

is the fluid's specific heat

"

*<sup>T</sup>* ) (7)

, SiC nanoparticles and water on the per-

and water. It was observed that at higher concentration

Here,"r" is the packing factor.

38 Microfluidics and Nanofluidics

Here, " *Ac* "

formula.

Here, " <sup>E</sup>out,1m<sup>2</sup>

Here, " *mf* "

Here, " *Voc*

"

determine " *Cpf*

and " *Tin* " and " *To* "

"

"

*E*̇

*<sup>r</sup>* <sup>=</sup> *Apv* \_\_\_

*Ap* <sup>=</sup> *<sup>R</sup> <sup>E</sup>* \_\_\_\_\_\_\_ *out*,*max*

are given in Ref [20].

is the open circuit voltage, " *I*

*FF* = *f* × (

Radwan et al. [20] examined the cooling effect of *Al*<sup>2</sup> *<sup>O</sup>*<sup>3</sup>

better impact as compared to *Al*<sup>2</sup> *<sup>O</sup>*<sup>3</sup>

**2.2. Optical filter cooling**

as the maximum power conversion efficiency.

*<sup>η</sup>el* <sup>=</sup> *<sup>E</sup>*̇ \_\_\_*el*

Electrical efficiency is found by the following formula.

Thermal output energy is found by the following equation.

is the collector area and " *Apv*

*Ac*

*Eout*,1*m*<sup>2</sup>

depicts the fluid's inlet and outlet temperature respectively. The formulas to

is the short circuit current, and " *<sup>G</sup>eff*

"

*th* = *mf* × *Cpf* × (*Tfo* − *Tin*) (5)

"

 is area of PV cells. Area of PV to produce a certain amount of electrical power is calculated by the following

"

is output electrical power per unit area and " *<sup>R</sup> Eout*,*max*

is the mass flowrate of the fluid through the collector, " *Cpf*

*E*̇ *in*

*sc* "

Using the aforementioned formulas, the efficiency of a PV/T system is determined.

for 4vol% SiC, the maximum temperature of the cell was found to be 60° *C*.

<sup>=</sup> *Voc* <sup>×</sup> *<sup>I</sup> sc* × *FF* \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ *Geff*

absorbed solar irradiation by the PV module. "FF" represents the fill factor and it is defined

formance of concentrated PV system. Pertaining to the results, SiC-water nanofluid produced

ratio (area of aperture/area of cell) and smaller Re, higher electrical efficiency was found. Using the pure water at CR = 40, the cell temperature reached a maximum of 68° *C*. Were as,

Extensive work has been carried out on efficiency improvement by using nanofluid flowing through optical filters [21, 22]. Silicon-based Photovoltaic cells can generate electricity by absorbing the part of solar radiation with 400 nm to 1200 nm wavelength. Rest of the solar radiation's part is either reflected back or absorbed by the PV cells as heat. In optical filter cooling, an optical filter containing nanofluid is held above the front surface of cells to split

*V*\_\_\_*oc*

In single pipe system, there are two sections of pipe; primary section and secondary section. Primary section is set underneath the rear surface of the Photovoltaic module having aluminum sheet in between. Primary section further elongates above the upper surface of PV module. Nanofluid enters from the inlet of primary section, thus, absorbing heat of the module. Heated nanofluid further passes over the PV's upper surface, in turns filtering the solar radiation. Part of radiation having wavelength equal to silicon bandgap is filtered and rest of the section is absorbed by the nanofluid flowing in the secondary channel which gets out of the secondary pipe at secondary outlet. Air exists between upper surface of PV module and secondary channel section. As the air gets hot, it flows in upward direction and the cool air still remains in contact with PV surface. It is assumed that no convection current is produced in the air. The results indicated, 83% and 80% overall and 76.5% and 74% thermal efficiency for Ag/water and Cu/water nanofluid respectively for above configuration [23]. Schematic diagram of such system is shown in the following **Figure 2**.

Wei An et al. [24] designed a spectral splitting Polypyrrole nanofluid based PV/T system in order to impede thermal losses and escalate the system's efficiency. Nanofluid used in spectral splitting filter is capable to absorb the part of solar radiations that cannot be utilized by PV cell and converts it into medium temperature thermal energy. The efficiency of PV/T system was found to be 25.2% for nanofluid based spectral splitting filter whereas, its value was 13.3% when there was no filter employed. Hjerrild et al. [25] worked on the cooling of PV system by the help of optical filters, they used Silver as nanoparticle (50 nm diameter) with coating of Silica. The results showed that, base fluid absorbed the ultraviolet part of solar radiation thus decreasing the heat losses whereas, nanoparticles absorbed visible portion of radiation, in turns increasing the overall efficiency of the system. Water showed highest electrical efficiency (85% higher than unfiltered PV) whereas highly diluted nanofluid (*Ag* <sup>−</sup> *SiO*<sup>2</sup> ) showed highest

**Figure 2.** Schematic diagram of Nanofluid based spectral splitting filter PV/T system (reproduced) [23].

overall efficiency as well as greatest merit function. Hassani et al. [26] numerically investigated the effect of cooling on PV performance. The results revealed that PV system with optical filter (containing Ag-Water nanofluid) held above the PV surface along with thermal receiver (containing CNTs) at the rear end of PV, performed best in terms of high-grade energy as compared to conventional PV, PV being cooled by water only, PV being cooled by CNTs and PV being cooled by CNTs at rear end and optical filter containing water held at upper surface of the panel. Optical filter containing nanofluid was able to absorb both UV and IR spectrum and it only allowed the radiation in range of PV absorptivity spectrum (400-1200 nm). Whereas, optical filter containing water could only absorb IR spectrum. Saroha et al. [27] tested the effect of silver and gold based nanofluid working as optical filters in PV/T system. The results revealed that unwanted wavelengths were more absorbed by silver as compared to gold based nanofluid. Silver/water nanofluid based PV/T system approached 9.6% electrical, 67.8% thermal and 78.4% overall efficiency. Whereas, gold/water nanofluid based PV/T system achieved 9% electrical, 67.6% thermal and 76.6% overall efficiency. Jin et al. [28] investigated the effect of liquid optical filter based on magnetic electrolyte nanofluid for PV/T system. Electrolyte nanofluid is prepared by dispersing Fe<sup>3</sup> O4 nanoparticle in 50% water and 50% EG solutions containing methylene blue or copper sulfate, in this way they obtained two stable ENF filters. By adjusting the volume fraction of nanoparticles and molar fraction, more optimized ENF is produced. This ENF presents more better results compared to the simple liquid filters. Merit function of this newly developed ENF is found to be much more than the conventional liquid optical filter.

using nanofluid. Ebaid et al. [33] used *TiO*<sup>2</sup>

compared to the water cooling (0.82% for *TiO*<sup>2</sup>

process. Pertaining to the results, *Al*<sup>2</sup> *<sup>O</sup>*<sup>3</sup>

mance was witnessed in case of *TiO*<sup>2</sup>

*TiO*<sup>2</sup>

water-polyethylene glycol nanofluid and *Al*<sup>2</sup> *<sup>O</sup>*<sup>3</sup>

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

nanofluid decreased the cell temperature by 13.83% and

and 0.48% for water cooling compared with no

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

nanofluid cooling, it produced 50% more average efficiency

cetyltrimethylammonium bromide nanofluid (with 0.01, 0.05, and 0.1 wt% concentration at a flowrate of 500–5000 ml/min) to test the efficiency enhancement of PV module via the cooling

reduced the temperature by 11.2% at 5000 ml/min relative to water cooling. The best perfor-

cooling). Karami and Rahimi [34] performed experiments to investigate the enhancement in the efficiency of PV module being cooled by the Boehmite (AlOOOH-x*H*<sup>2</sup> *<sup>O</sup>*) based nanofluid flowing inside microchannel at the rear end of the PV module. The results showed that the maximum increase in the electrical efficiency due to cooling as compared to the without cooling power output was found to be 27.12% at a concentration ratio of 0.01 wt.% and 300 ml/min flowrate. Similarly, Sardarabadi et al. [64] observed as much as 9.75% electrical efficiency increment for silica/water nanofluid based PV/T system as compared to uncooled system. **Figures 3** and **4**

depict the maximum efficiencies of PV/T systems obtained by different researchers.

**Figure 3.** Maximum efficiency for obtained by researchers with Nanofluid cooling.

**Figure 4.** Maximum efficiency improvement by Nanofluid cooling compared to conventional PV.

water

41

An arrangement in which nanofluids flows in separated channels outperforms the single channel through which the nanofluid is set to flow. In this arrangement a channel is placed underneath the rear surface of PV panel whereas, a separate channel is held above the front surface of the module. Upper channel nanofluid is made to achieve high liquid filter performance whereas the nanofluid flowing beneath the surface achieves higher thermal performance (working as a coolant). This technique achieved 8.5% higher electrical efficiency as compared to the double pass channel in which fluid flows in a single channel [29].
