_{2}O Nanofluid

This paper presented the improvement of the performance of the photovoltaic panels under Iraqi weather conditions. The biggest problem is the heat stored inside the PV cells during operation in summer season. A new design of an active cooling technique which consists of a small heat exchanger and water circulating pipes placed at the PV rear surface is implemented. Nanofluids (Zn-H2O) with five concentration ratios (0.1, 0.2, 0.3, 0.4, and 0.5%) are prepared and optimized. The experimental results showed that the increase in output power is achieved. It was found that, without any cooling, the measuring of the PV temperature was 76°C in 12 June 2016; therefore, the conversion efficiency does not exceed more than 5.5%. The photovoltaic/thermal system was operated under active water cooling technique. The temperature dropped from 76 to 70°C. This led to increase in the electrical efficiency of 6.5% at an optimum flow rate of 2 L/min, and the thermal efficiency was 60%. While using a nanofluid (Zn-H2O) optimum concentration ratio of 0.3% and a flow rate of 2 L/min, the temperature dropped more significantly to 58°C. This led to the increase in the electrical efficiency of 7.8%. The current innovative technique approved that the heat extracted from the PV cells contributed to the increase of the overall energy output.

Photovoltaic (PV) systems represent a solution for the problem of low carbon, nonfossil fuel used to generate electricity. Solar radiation absorbed and converted by semiconductor devices (solar cells) can provide a supply of electricity to meet energy needs. An energy source with less emissions of carbon, no dependence on fossil fuels, massive potential for developing countries, and well suited to be distributed, PV, is considered as a medium and long range energy prospect as presented by Firth [

EL-Basit et al. [

Chaji et al. [_{2} nanoparticles in water with three values of flow rates, namely, 36, 72, and 108 L/hr. They investigated four particles’ concentration ratios (0, 0.1, 0.2, and 0.3% wet). The results showed that adding nanoparticles to water increased the initial efficiency of flat plate solar collector by 3.5 to 10.5% and the index of collector total efficiency by 2.6 to 7% relative to that of the base fluid.

The major problem in PV is the accumulation of heat, which reduces the electrical performance obviously; therefore, heat must be dissipated. In Iraq, the problem becomes much serious, because of a hot weather in most of the year; this makes the electrical efficiency of PV cells to decrease with the increase of the heat inside the PV cells. The active solution for this problem can be using a water-cooling technique to decrease the heat effects by transferring the heat to the water which can be used in many applications as a hot water. Thermal conductivity enhancement can be achieved by using nanofluid applications such as Zn-H_{2}O. The originality of the current work is the use of a new design of a cooling technique including copper pipes placed on PV rear surface to absorb the heat accumulated inside the PV cells. This aim was achieved through evaluation of the performance of photovoltaic panels under different operating conditions, enhancement of the electrical and thermal performance for the photovoltaic/thermal system with water pumping system at different water mass flow rates, and studying the effect of using nanofluid (Zn) as a working fluid in water-circulating pipes at different concentration ratios (0.1, 0.2, 0.3, 0.4, and 0.5).

The equations of the nominal electrical efficiency (

The thermal efficiency is evaluated by the following equations, presented by El-Seesy et al. [

Thermophysical properties of the working fluid (Zn-H_{2}O nanofluid) are changed due to influence of the nanoparticles. These properties for conventional fluids can be found from standard tables or equations as presented by Darby [

Calculation of Reynolds, Peclet, and Prandtl numbers is as follows [

Friction factors (

Friction factor of each flow rate for nanofluid which can be found in single-phase flow cannot be used for calculating friction factor as well as Nusselt number as presented by Hussein [

The prototype of PV/T system is where the water pumping system was used for all experimental investigations of electrical and thermal effects on system performance and suggested improvements. The setup comprises PV panel, charge controller, battery, DC-DC boost converter, PMDC motor used as a pumping system load, copper pipes fixed on the backside of PV panel, and a radiator with a fan and circulation pump for cooling hot water. The experiment was performed on the site of Electromechanical Engineering Department, University of Technology, in summer and winter seasons.

The photograph of the setup as shown in Figures _{2}O nanofluid) was measured using flow rate meter.

Photograph (a) and schematic diagram (b) of the experimental setup.

In this work, a specially made serpentine flow collector has been designed. The PV/T collector comprises PV module and thermal collector which are made of copper sheet and pipe. The copper sheet and the piping are paste directly to the back side of PV panel. Copper material has been used due to its high thermal conductivity with the pipe’s inner diameter of 11 mm and thickness of 1 mm to transfer the temperature from PV panel to the working fluid. Thermal sink was used between the bottom surface of PV panel and the surface of 2 mm copper plate to increase thermal conductivity.

The copper pipes are linked using a welding machine. The storage capacity of piping system is 1.5 liters welded on the copper sheet with a height and length and then fixed on the back surface of standard PV panel. The welding method is with 40% tin and 60% silver. The oscillatory flow has at least one inlet and outlet to permit working fluid to enter and to exit from the copper pipes, respectively. Water enters the pipes with low temperature and travel as hot water. The hot water can be consumed or stored for later use. However, this work is dedicated to water pumping system and thus there is no need for hot water output from the proposed PV/T system. In this way, solar radiation energy can be fully used for solar heating applications. The dimension of the thermal collector is shown in Figure

(a) Dimensions of thermal pipes mounted on the backside of PV. (b) Steps of nanofluid preparation.

After studying the impact of a water-cooling technique on the performance of PV/T system, Zn-water nanofluid was prepared at five concentration ratios (0.1, 0.2, 0.3, 0.4, and 0.5%) by mixing the particles with 1.5 liters of ionized water. Figure

The PV Matlab model that has been developed is tested to assess the solar radiation effects and PV temperature variations. From the results, it notes that the current increases proportionally with the increase of solar radiation, but the voltage increases nonlinearly with solar radiation and then increases the level of power output as shown in Figure ^{2}, the PV open-circuit voltage is decreased from 21.8 to 18.8 volts and this leads to decrease the PV power generated from 100 to 84 Watts which represents the variation of current-voltage (

(a) Theatrical ^{2}). (d) ^{2}).

(a) Experimental ^{2}).

Figure ^{2} to 900 W/m^{2} and then falls to 700 W/m^{2} nearly. It is observed that when the increase of flow rate causes a decrease in the output temperature and the temperature difference and when the decrease of flow rate leads to increase in the output temperature and the temperature difference and then gets the best thermal gain, this is due to the fluid which takes a long time to absorb heat from the surface of PV module.

(a) Temperature variations at climatic conditions. (b) Comparison between the voltages at climatic conditions. (c) Comparison between the output powers at climatic conditions.

Figure

(a) Comparison of the electrical efficiency at climatic conditions. (b) Heat transfer rate with different mass flow rates. (c) Effect of the mass flow rates of water on the thermal efficiency.

It is observed that the electrical efficiency of the PV module increases with increasing the flow rate of fluid. The best electrical efficiency is obtained at optimum flow rate (2 L/min) because all the performance is improved at this rate. The results show that the operation of pumping system depends deeply on the performance of the photovoltaic system and the peak power of the photovoltaic system. The DC voltage influences the speed of running motor. It is observed that low voltage generated from PV module due to high operating temperature leads to a decrease in the output of DC pump, while high voltage leads to an increase in the output of DC pump. It is observed that circulating the fluid through pipes at photovoltaic cells’ rear surface strongly enhances the performance of system and subsystem, since motor pump can receive most of the power of cells by improving the performance of PV module as shown in Figure

(a) Effect of mass flow rates on the PV temperature. (b) Effect of mass flow rates on the voltage. (c) Effect of mass flow rates on the PV power.

The thermal conductivity of Zn metal is higher than the water: 112.2 W/m.k for Zn metal while for the water, 0.596 W/m.k. This feature gives an increase in the thermal conductivity of working fluid. Figure

(a) Effect of mass flow rates on the electrical efficiency. (b) Heat transfer rate at constant flow rate (2 L/min) at different nanofluid concentrations. (c) Effect of Zn-H_{2}O nanofluid at 2 L/min on thermal efficiency.

Figure _{2}O nanofluid at this ratio which led to more absorption of heat from photovoltaic surface. If the concentration ratio increases more than 0.3%, the PV temperature will increase because of the increase in density and viscosity of working fluid with the rising of concentration ratio, and this gives reverse impact of improvement. By decreasing PV temperature with the use of nanofluid, the maximum power produced from the PV module will be increased. It was noticed that the better maximum power generated is at 0.3% nanofluid concentration ratio because this volume ratio gives good cooling for PV module; also, it was observed that there is an improvement in _{max} and _{max} which leads to enhancement in PV power when using nanofluid and a good case at 0.3% concentration ratio. The electrical efficiency of PV module is improved by using nanofluid at 0.3% volume concentration ratio and reduced when it is greater than 0.3% because of the increase of PV temperature as the volume concentration ratio increases above 0.3%, as shown in Figure

(a) Effect of Zn-H_{2}O nanofluid at 2 L/min on the photovoltaic temperature. (b) Effect of Zn-H_{2}O nanofluid at 2 L/min on the voltage. (c) Effect of Zn-H_{2}O nanofluid at 2 L/min on the power.

Effect of Zn-H_{2}O nanofluid concentrations at 2 L/min on (a) electrical efficiency, (b) working fluid density, and (c) specific heat.

All the physical properties of working fluid will change depending on the concentration ratio of nanoparticles such as density, specific heat, viscosity, and thermal conductivity. It was observed from the sketch that the variation of density of nanofluid is a function of volume concentration ratios and the density of water when increasing the temperature. Figure

Effect of Zn-H_{2}O nanofluid at 2 L/min on (a) viscosity, (b) thermal conductivity, and (c) thermal diffusivity.

Effect of Zn-H_{2}O nanofluid at 2 L/min on (a) Reynolds number, (b) Prandtl number, and (c) Peclet number.

Effect of Zn-H_{2}O nanofluid at 2 L/min on (a) Nusselt number, (b) pump output at different mass flow rates, and (c) pump output at constant mass flow rate (2 L/min) with different concentration ratios.

In this work, we have tested the operation of pumping systems designed to supply water for drinking or irrigation. The results show that the operating of pumping system depends deeply on the performance of the photovoltaic system and the peak power of the photovoltaic system.

The goal achieved via this study is the investigation of solar radiation changing effects on the pumping system performances. The obtained results show that due to increasing solar radiation, the pump flow increased. The DC voltage influences the speed of running motor. It is observed that low voltage generated from PV module due to high operating temperature lead to a decrease in the output of DC pump, while high voltage lead to an increase in the output of DC pump. It is observed that circulating the fluid through pipes at the photovoltaic cells’ rear surface strongly enhances the performance of systems and subsystems, since motor pumps can receive most of the power of the cells by improving performance of the PV module as shown in Figures

(a) Theoretical and experimental results comparison of ^{2}. (b) Theoretical and experimental results comparison of ^{2}.

When comparing between the experimental results which have been measured manually as shown in Figures ^{2}). It can be noticed from these figures that the difference between experimental and theoretical results is about less than 2% which is quite acceptable.

(a) Theoretical and experimental results comparison of ^{2} and 70°C. (b) Comparison of electrical efficiency of PV/T without water, with water (2 L/min), and with Zn-H_{2}O nanofluid.

The variations in solar radiation mainly influence the output current, while the changes in temperature mainly affect the output voltage. Hybrid PV/T systems are one of the methods used to enhance the electrical efficiency of panel then improve the photovoltaic water pumping system performance. The electrical and thermal efficiencies of the hybrid system will increase with increasing mass flow rate of water. At optimum flow rate of 2 L/min, electrical efficiency was 6.5% and thermal efficiency was 60%. The results indicated that when nanofluid (Zn) is used at various concentration ratios (0.1, 0.2, 0.3, 0.4, and 0.5%) at 2 L/min flow rate, the cell temperature dropped more significantly from 76°C to 58°C at an optimum concentration ratio of 0.3% nanofluid; this led to an increase in the electrical efficiency of PV panel to 7.8%.

Area of the PV module (m^{2})

_{c}:

Area of collector (m^{2})

_{pf}:

Heat capacity of the base fluid (J/kg.c)

_{pnf}:

Heat capacity of the nanofluid (J/kg.c)

Solar radiation (W/m^{2})

_{m}:

Maximum current of PV (A)

_{sc}:

Short-circuit current of solar cell (A)

_{f}:

Thermal conductivity of base fluid (W/m.c)

_{I}:

Cell’s short-circuit current temperature coefficient (A/

_{nf}:

Thermal conductivity of the nanofluid (W/m.c)

_{ρ}

Thermal conductivity of the nanoparticle (W/m.c)

Mass flow rate (kg/s)

Volume concentration of the nanoparticles

_{in}:

Inlet temperature of the working fluid (°C)

_{0}:

Temperature of standard condition (25°C)

_{out}:

Outlet temperature of working fluid (°C)

_{m}:

Maximum voltage of PV (V)

_{PV}:

Output voltage (V)

Coefficient of silicon cell (^{−1})

_{0}:

Nominal electrical efficiency at standard conditions

_{nf}:

Nanofluid viscosity (kg/m.s)

_{w}:

Water viscosity(kg/m.s)

_{nf}:

Density of the nanofluid (kg/m^{3})

_{p}:

Density of the nanoparticles (kg/m^{3}).

The authors declare that they have no conflicts of interest.