Improving the Thermal Efficiency of Flat Plate Solar Collector Using Nano-Fluids as a Working Fluids: A Review

DOI: https://doi.org/10.53523/ijoirVol8I3ID86 Abstract Solar energy is one of the most important types of renewable energy and is characterized by its availability, especially in Iraq. It can be used in many applications, including supply thermal energy by solar collectors. Improving the thermal efficiency of solar collector leads to an increase in the thermal energy supplied. Using a nano-fluid instead of base fluid (water is often used) as a working fluid is a method many used to increase the thermal efficiency of solar collectors. In this article, the latest research that used nano-fluid as a working fluid in evaluating the thermal efficiency of solar collector, type flat plate was reviewed. The thermal efficiency improvement of flat plate solar collector was reviewed based on the type of nanoparticles (metal oxides, semiconductors oxides, carbon compounds) used in the base fluid and comparison was made between these nanoparticles under the same conditions. Moreover, the effect of varying the concentration of nanoparticles in the base fluid and changing the working fluid flow rate on the thermal efficiency of flat plate solar collector was also reviewed. The results of the review showed that nano-fluids containing carbon compounds are better than other nano-fluids and that copper oxide is better than the rest of the metal oxides used in improving the thermal efficiency of flat plate solar collectors.

The thermal efficiency of flat plate solar collectors is low compared with tracking solar collectors [1]; so, by using many methods, many researchers have tended to increase the thermal efficiency of a flat plate solar collector. The most prominent and recent of these methods, which is the use of nanoparticles in base fluid (usually water) as working fluid. Using a nano-fluid (nanoparticles + base fluid) as a working fluid that has superior thermal properties improves the efficiency of a flat plate solar collector compared to using water as a working fluid.
In this article, the latest research (from 2016 to 2021) in which the thermal efficiency of flat plate solar collector was improved using nano-fluids as a working fluid will be reviewed. This research will be a review based on the nanomaterials used in the base fluid (metal oxides, semiconductor oxides, and carbon compounds), as well as the comparison between those nanomaterials. The effect of nanoparticles concentration in the base fluid and the working fluid flow rate on the thermal efficiency will also be reviewed. This can give the researcher in this field a clear vision for the best nano-fluids and/or fluid flow used in a flat plate solar collector.
It is noteworthy that nano-fluids should be used only in flat plate solar collector with a closed-loop system.

Mathematical Basis
In this section, the basic mathematical equations used in calculating the thermal efficiency of flat plate solar collector will be presented. Additionally, the specific heat and density of nano-fluid will be stated. Thermal efficiency in the flat plate solar collector is the ratio of useful thermal energy obtained from the collector to incidence solar energy on the collector [2]: The specific heat and density of nano-fluid is calculated from [3]:

The Thermo-Physical Properties of Nano-Fluids
The thermos-physical properties of the nano-fluid such as density, specific heat, thermal conductivity, and viscosity are directly affected by dispersing nanoparticles in the base fluid [5]. In this section, some thermosphysical properties of different types of nano-fluids will be presented.
Increasing the density of working fluid enhances thermal efficiency. Density greatly affects the heat transfer resistance of nano-fluids and directly affecting Reynolds number, Nusselt number, friction factor, and pressure loss. Figure (2) shows the density of some nano-fluids and the effect of nanoparticle concentration on them [6].
Specific heat plays an essential role in the heat transfer and storage processes because low specific heat of nanofluid causes more rise in temperature when exposed to solar energy. Figure (3) shows the specific heat of some nano-fluids and the effect of nanoparticle concentration on them [6].
Increasing the thermal conductivity of the working fluid enhances thermal efficiency. Many factors such as nanoparticles type, nanoparticles shape, nanoparticle size, base fluid type, temperature, and nanoparticles concentration affects the thermal conductivity of nano-fluids [7]. Figure (4) shows the thermal conductivity of some nano-fluids and the effect of nanoparticle concentration on them [6].
Increasing the viscosity of the working fluid will negatively affect the thermal efficiency because the friction factor will be increased. Figure (5) shows the viscosity of some nano-fluids and the effect of nanoparticle concentration on them [6].

The Use of Nano-Fluids Containing Metal Oxides as Working Fluids
Many types of metal oxides are used as nanoparticles in the base fluid like aluminum oxide (Al2O3), copper oxide (CuO), zinc oxide (ZnO), iron oxide (Fe3O4/water), and magnesium oxide (MgO). Janusz T. Cieśliński et al. (2016) [8] used a nano-fluid consisting of aluminum oxides in water (Al2O3/water) with volume concentrations (0.1%, 0.2%, and 0.4%) in addition to water as working fluids in a flat plate solar collector. Experimentally tested flat plate solar collector at flow rates of (1, 2, and 2.83 l/min). The results showed that the thermal efficiency of flat plate solar collector increases when using nano-fluid compared to the result when using water; the best increase was when a nano-fluid with a concentration of 0.2% and a flow rate of 2 l/min were used.
Abbas Sahi Shareef et al. (2016) [9] they used a nano-fluid consisting of aluminum oxides in water (Al2O3/water) with two a volume concentration (0.1% and 0.2%) in addition to water as working fluids in a flat plate solar collector. Experimentally tested flat plate solar collector at flow rates of (1, 1.6, and 2 l/min). The results showed a remarkable improvement in the thermal efficiency when using nano-fluid compared to water in a flat plate solar collector, especially when using nano-fluid with a 0.5% concentration and a 2 l/min flow rate. Sahil Arora and Sudhakar Subudhi (2017) [10] used a nano-fluid consisting of aluminum oxides in water (Al2O3/water) with a volume concentration (0.1%) as working fluids -in addition to water -in a flat plate solar collector. Experimentally tested flat plate solar collector at flow rates (1, 2, 3, 4, and 5 l/min). The results showed a remarkable increase in the thermal efficiency of flat plate solar collector when using nano-fluid compared to the result when usingwater. The best increase was 23.76% at a flow of 3 l/min. [11] they presented an experimental and numerical study to examine a flat plate solar collector's performance using water and aluminum oxide (Al2O3/water) with different volume concentrations as working fluids. Experimental results showed an improvement in the thermal efficiency of flat plate solar collector from 3% to 18% when using nano-fluid compared to water, while the numerical results showed that an excessive increase in the concentration of aluminum oxide in water leads to negative results.
Mohsen Mirzaei (2019) [12] used a nano-fluid consisting of copper oxides in water (CuO/water) with a volume concentration of 0.1% and water as working fluids in a flat plate solar collector. Experimentally tested flat plate solar collector on more than one flow rate in which the results showed that the thermal efficiency of flat plate solar collector has increased when a nano-fluid was used compared to using water as follows: 15.2%, 17.1%, and 25.1% at flow rates 1, 2, and 4 l/min respectively. Nitesh Singh Rajput et al. (2019) [13] used a nano-fluid consisting of aluminum oxides in water (Al2O3/water) with volume concentrations (0.1%, 0.2%, and 0.3%) in addition to water as working fluids in a flat plate solar collector. Experimentally tested flat plate solar collector at flow rates (1, 2, and 3 l/min) showed a remarkable increase in the thermal efficiency of flat plate solar collector when using nano-fluid compared to water. The best increase was 21.32% at a concentration of 0.3% and a flow of 3 l/min. Mousa M. Mohamed et al. (2020) [14] they used water and a nano-fluid consisting of zinc oxides in water (ZnO/water) at two volume concentrations (0.05% and 0.1%) as working fluids in a flat plate solar collector used with a thermal storage system. The experimental results showed an increase in the thermal efficiency of flat plate solar collector when using the nano-fluid compared to water as follows: 4.81% and 6.57% at a concentration of 0.05% and 0.1%, respectively.
Shubham Sharma et al. (2020) [15] they presented an experimental study to evaluate a flat plate solar collector performance. They used water and copper oxide in water (CuO/water) with a range of volume concentrations (0.25%-2%) as working fluids. The results compared the thermal efficiency with a nano-fluid and the thermal efficiency with water; the thermal efficiency with a nano-fluid was higher than that with water, and the highest thermal efficiency recorded was 57.1% at a concentration of 1.5% and a flow of 0.03 kg/s. Nang Khin Chaw Sint et al. (2020) [16] ran a simulation using MATLAB software to verify the perfect concentration of a nano-fluid (CuO/water) to be used in a flat plate solar collector as a working fluid. Their results showed that the maximum thermal efficiency was obtained at a volume concentration of 0.5% and a flow rate of 1 l/min. They also compared the nano-fluid to water and found that the thermal efficiency increased by 4.78% when using (CuO/water).
Suraj Choudhary et al. (2021) [17] used a nano-fluid consisting of magnesium oxides in water (MgO/water) with two volume concentrations (0.1% and 0.3%) as working fluids in a flat plate solar collector. Experimentally tested flat plate solar collector at flow rates (0.5, 1, and 1.5 l/min) showed that the best performance was when a nanofluid of 0.3% concentration and 1 l/min flow was used.

The Use of Nano-Fluids Containing Metal Oxides as Working Fluids
Many types of semiconductor oxides are used as nanoparticles in the base fluid like silicon oxide (SiO2/water), titanium oxide (TiO2/water), cerium oxide (CeO2/water), and tungsten oxide (WO3/water).  [19] presented a numerical study to investigate the performance of a flat solar collector using (SiO2/water) as a working fluid and compare its performance when using water as a working fluid. They chose volume concentrations that rangined from 1% to 10%, and with several flow rates when tested. The results showed that increasing the concentration of SiO2 nanoparticles in water improves the performance of the flat plate solar collector, especially at high flow rates. M.A. Sharafeldin and Gyula Gróf (2018) [20] used a nano-fluid consisting of cerium oxides in water (CeO2/water) with volume concentrations of (0.0167%, 0.0333%, and 0.0666%) in addition to water as working fluids in a flat plate solar collector. Experimentally tested flat plate solar collector at flow rates (0.015, 0.018 and 0.019 kg/s m2). Their results showed an increase in the thermal efficiency of flat plate solar collector when using nano-fluid compared to water, and the best increase was 10.74% at a concentration of 0.0666% and a flow of 0.019 kg/s m 2 .
J. Vinoth   [21] they used a nano-fluid consisting of tungsten oxide in water (WO3/water) with a volume concentration of (0.0167%, 0.0334% and 0.0667%) in addition to water as working fluids in a flat plate solar collector. Experimentally tested flat plate solar collector at constant flow rate results showed an improvement in the thermal efficiency of flat plate solar collector when using the nano-fluid compared to water, especially when using the nano-fluid with a concentration of 0.0667%.

The Use of Nano-Fluids Containing Carbon Compounds as Working Fluids
Omer A Alawi et al. (2020) [22] used a nano-fluid consists of pentaethylene glycol-thermally treated graphenewater with weight concentrations (0.025%, 0.05%, 0.075%, and 0.1%) in addition to water as working fluids in a flat plate solar collector. The results of experimentally tested flat plate solar collector at different flow rates showed an increase in the thermal efficiency of flat plate solar collector when using nano-fluid compared to using water, and the best increase was 10.74% at a concentration of 0.0666% and a flow of 0.019 kg/s m2. The results showed an increase in the thermal efficiency of flat plate solar collector when using nano-fluid compared to water, and the best increase at a concentration of 0.1% and as follows: 10.6%, 11.1%, and 13.1% at a flow rate of 0.00833 kg/s, 0.01667 kg/s, and 0.025 kg/s. Omar Mohammed Hamdoon (2020) [23] presented a numerical study to improving the performance of a flat plate solar collector using (Multi-Wall Carbon Nano Tube-Water) MWCNT-H2O as a working fluid and compared its performance to the performance when using water. Various concentrations ranging from 0% to 6% were selected, as well as, varying the flow rates from 0.004 kg/s to 0.03 kg/s were tested. The results showed an increase in the thermal efficiency of flat plate solar collector when using nano-fluid compared to water, and this increase ranged from 2.41% to 2.91% at 0.004 kg/s from 4.67% to 6.68% at 0.03 kg/s. L. Syam Sundar et al. (2020) [24] used a nano-fluid consisting of nanodiamond particles in water (DN/water) with volume concentrations (0.2%, 0.4%, 0.6%, 0.8%, and 1%) in addition to water as working fluids in a flat plate solar collector. The results showed that when using water, the thermal efficiency reached 53%, while when using nano-fluid at a concentration of 1%, the efficiency reached 74%, and this is the best improvement.
Omer A Alawi et al. (2021) [25] presented an experimentally and theoretically study of covalently functionalized graphene (Gr) suspended in distilled water with different weight concentrations (0.025%, 0.05%, 0.075%, and 0.1%) as working fluid inside a flat plate solar collector. The comparison was also made when using water as a working fluid. It was tested experimentally and theoretically at flow rates (0.5, 1, and 1.5 kg/min). The results showed an increase in the thermal efficiency of flat plate solar collector when using nano-fluid compared to water, and the best increase was 13% at the highest concentration and flow rate.
L. Harish   [26] presented an investigation about the effects of using eco-friendly, noncorrosive, covalently functionalized Graphene nanoplatelets with gallic acid (GGNPs) as working fluid inside a flat plate solar collector. A nano-fluid consists of (GGNPs/water) with a weight concentrations (0.025%, 0.05%, 0.075% and 0.1%). It was experimentally tested at flow rates (0.5, 1, and 1.5 kg/min), and the comparison was made when using water as a working fluid. The results they had was that the maximum enhancement in the thermal efficiency of flat plate solar collector was 24.09% compared to the one with water at a concentration of 0.1% and a flow rate of 1.5 l/min.

Comparison of Nano-Fluids Based on The Type of Nanoparticles Used
Marjan B. Nejad et al. (2017) [27] presented a numerical study to investigate in performance a flat plate solar collector used different nano-fluids including (MWCNT-H2O and Al2O3/water) in addition to water as working fluids. Different weight concentrations ranging (1-2%) were selected and tested on flow rates ranging (0.05-0.6 l/min). The results showed an increase in the thermal efficiency of flat plate solar collector when using nano-fluids compared to water, and the thermal efficiency of MWCNT-H2O was greater than the thermal efficiency of Al2O3/water. Sujit Kumar Verma et al. (2017) [6] prepared several nano-fluids consisting of carbon compounds, metal oxides, and semiconductor oxides in water as follows: (MWCNTs-H2O, Gr/water, CuO/water, Al2O3/water, TiO2/water, and SiO2/water). These nano-fluids, in addition to water, were used as working fluids in the evaluation of a flat plate solar collector. Experimental tested was with a volume concentrations (0.25%, 0.5%, 0.75%, 1%, 1.25%, 1.5%, 1.75% and 2%) and a flow rates ranging from 0.01 kg/s to 0.05 kg/s. The results showed an increase in the thermal efficiency of flat plate solar collector when using nano-fluids compared to water, especially between the concentrations 0.75%-1% and the flow rates 0.03 kg/s-0.035 kg/s. The best results were recorded for an increase in the thermal efficiency of flat plate solar collector with a concentration of 0.75% and a flow rate of 0.03 kg/s as follows: 23.47%, 16.97%, 12.64%, 8.28%, 5.07%, and 4.08% for MWCNTs-H2O, Gr/water, CuO/water, Al2O3/water, TiO2/water, and SiO2/water, respectively as shown in Figure ( [28] used different working fluids in a flat plate solar collector as follows: two types of nano-fluids (Al2O3/water) and (ZnO/water) with and without ethylene glycol (EG) in addition to water. A weight concentration in water of 0.25% oxides and 25% ethylene glycol was used, and it was experimentally tested with flow rates (0.05, 0.07, and 0.09 kg/s). The results showed an increase in the thermal efficiency of flat plate solar collector when using nano-fluids compared to water, especially when adding ethylene glycol, and the best increase was (15.13%) for (Al2O3/water) with ethylene glycol at a flow rate of 0.09 kg/s. Yijie Tong et al. (2020) [29] presented an experimental comparison between several working fluids in a flat plate solar collector, where they used water and (MWCNT/water, CuO/water, Al2O3/water, Fe3O4/water); and for each nano-fluid, they used multiple concentrations close to its ideal concentration. Tested a flat plate solar collector with different flow rates. The results showed that the maximum thermal efficiency was 87% when using MWCNT/water. Also, increasing the flow rate improves the thermal efficiency, and the best improvement was when using MWCNT/water, as shown in Figure (7).

Figure (7)
. Improvement in thermal efficiency when changing flow rates for different types of working fluids [29].
M. S. Gad and Mokhtar Said (2021) [30] used water and nanoparticles of aluminum oxides Al2O3 and titanium oxides TiO2 at a concentration of 2% by weight in water as working fluids in a flat plate solar collector. It was experimentally tested at flow rates (2, 4, and 6 l/min). The results showed that the thermal efficiency of flat plate solar collector was as follows (TiO2/water > Al2O3/water > water) at all flow rates and the maximum increase in thermal efficiency was 30% for TiO2/water and 22% for the Al2O3/water compared to water, also the best flow rate was 2 l/min. The results showed that the best performance is when using CuO /water at a concentration of 0.5% and the activity of CuO/water is better than that of Al2O3/water under the same conditions. R. M. Mostafizur et al. (2021) [32] used four types of nano-fluids, all from oxides of solid materials in water (CuO/water, Al2O3/water, TiO2/water, and MgO/water) in addition to water as working fluids in the flat plate solar collector with volume concentrations (1-4%). Theoretically tested with a flow rate (1-4 l/min), the results showed an increase in the thermal efficiency of flat plate solar collector when using nano-fluids compared to water as shown in Figure (8), and that the largest thermal efficiency values were at a volume concentration of 2% and a flow of 3.9 l/min and as follows: 38.21%, 35.32%, 34.77%, 34.17% and 31% for CuO/water, Al2O3/water, MgO/water, TiO2/water, and water, respectively.  [33] presented an experimental study to investigate in performance of a flat plate solar collector used different nano-fluids (f-GNP is carbon compound, ZnO/water, and SiO2/water) in addition to water as working fluids. The nano-fluids used were 0.1% by weight, and the experimental test was carried out at flow rates (0.8, 1.2, and 1.6 kg/min). The results showed an increase in the thermal efficiency of flat plate solar collector when using nano-fluids compared to water, and the best increase was at 1.6 kg/min and as follows: 17.45% for f-GNP, 13.05% for ZnO/water, and 12.36% for SiO2/water as shown in Figure (

Conclusions
From the articles presented, we concluded that using nano-fluids improve the the efficiency of flat plate solar collector compared to using water, and that the best nano-fluids that improve the thermal efficiency of flat plate solar collector are the ones using carbon compounds in the first place, then metal oxides, and finally the semiconductor oxides.When comparing the nano-fluids that use metal oxides, it was concluded that the best nanofluid that improves the thermal efficiency of flat plate solar collector is the copper oxide CuO/water. It was also concluded that increasing the flow rate of the working fluid within certain limits leads to an increase in thermal efficiency of the flat plate solar collector as an excessive increase brings negative results because the difference between the outlet and inlet temperature of the flat plate solar collector will decrease. It is well known that each nano-fluid has ideal concentrations that differ from any other nano-fluid; low concentrations of nanoparticles in a base fluid may not improve thermal efficiency significantly, and an excessive increase in the concentration of nanoparticles in a base fluid may lead to negative results due to the high viscosity of the nano-fluid and agglomeration of nanoparticles, which leads to a decrease in the heat transfer process. It is worth mentioning, there are nano-fluids based on other materials that have not been addressed, including metal nanoparticles like copper, aluminum, and silver [34,35] also hybrid nanoparticles (mixing two or more types of nanoparticles) [36,37]. We recommend researchers in this field to see it.