An Experimental Investigation on Thermal Efficiency of Flat Plate Tube Solar Collector using Nanofluid with Solar Tracking Mechanism

In the present work, flat-plate solar collector (FPSC) in terms of various parameters as well as in respect of lower (Area of FPSC, volume fraction concentration of nanofluids, and mass flow rate) has been studied in this work. The FPSC has been fabricated with 0.192 m, Dioxide silicon SiO2 (40nm) with the volume fraction of SiO2+Distilled water (0.05, 0.075, and 0.1%) and varying of flow rate (10, 15, 20L/h). These technological devices operate under forced circulation mode of fluid under varying climate conditions. The tracking mechanism has been used in the experiment of FPSC for tracking the sun position during the daytime. As per the ASHRAE standard. The results showed that at volume fraction 0.10 % and flow rate of 20 L/h, the highest increase in the absorbed energy parameter FR(τα) was 7.3 %, and the removed energy parameter FRUL was 11.9 % compared with distilled water. The changes in absorbed energy parameter FR(τα) they vary from 4.4% to 7.3% while in removed energy parameter FRUL, the vary from 1.3% to 11.9% as compared with the distilled water case. The maximum efficiency was about 70 % as the decreased temperature parameter [(Ti– Ta)/GT] is equal to zero at a volume fraction of 0.10 % and flow rate of 20 L/h. KeywordsFlat plate solar collector; thermal efficiency; SiO2/water Nanofluids How to cite this article: S.T. Hamidi, “An Experimental Investigation on Thermal Efficiency of Flat Plate Tube Solar Collector using Nanofluid with Solar Tracking Mechanism,” Engineering and Technology Journal, Vol. 37, No. 11, pp. 475-487, 2019.


Introduction
One of the cleanest forms of renewable energy sources is solar energy. The most common method to employ solar energy is to utilize a solar energy collector. Flat-plate collectors FPSCs are the most commonly utilized type of solar collector as to the heater, the water, or air. Those collectors have distinguished by low outlet temperature and efficiency [1][2][3][4][5][6][7][8]. Lately, many researchers tried enhancing the performance and efficiency of the FPSC by using several techniques. One of these techniques is to use nanofluids in solar collectors instead of commonly used liquids [9][10][11][12][13]. The researchers [14,15] studied the effects of environmental, thermal, and economical use of nanofluids for improving solar collector efficiency. Youssef et al. [16] who had been tested a coolant of Al 2 O 3 /H 2 O as a coolant in a FPSC. They have proven that the nanofluids have raised the temperature outlet and collector efficiency. The performance of a mini-channel solar energy collector had been evaluated with four different nanofluids by Mahian et al. [17]. The nanofluids were Al 2 O 3 /H 2 O, TiO 2 /H 2 O, SiO 2 /H 2 O, and Cu/H 2 O. They concluded that coolant Cu/H 2 Ois the best nanofluid, which provides the highest outlet-temperature and lowest entropy. Verma et al. [18] carried out an experimental investigation on a FPSC with variety of nanofluids with regarding various parameters concerning energy and energy efficiency. Experimental test results deduced that volumetric concentration about 0.75% at a rated flow of 0.025 kg/sec might cause optimal energy efficiency. Faizal et al. [19] conducted studies to save the necessary material for manufacturing solar collectors, in addition to reduce energy consumption for every collector fabrication. This study showed that it is possible to reduce the solar collector area by 21.6%, 25.6%, 21.5% & 22.1% with SiO 2 , CuO, Al 2 O 3, and TiO 2, respectively. Alim et al. [20] carried out a theoretical study for analyzing the consequences on entropy generation, the ability to enhance heat transfer in addition to pressure drop with various types of nanofluids like CuO, TiO 2 , Al 2 O 3 and SiO 2 with water as fluid at different values of rates the flow in a FPSC. The remarkable drawn results of the investigation showed that the enhanced heat transfer was 22.15% with low heat generation 4.34% comparing with pure water as working fluid.

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Vol. 37, Part A, No. 11, 2019 Arıkan et al. [21] have studied the influence of ZnO-H 2 O, Al 2 O 3 -H 2 O nanofluids, with/without ethylene glycol (EG), on the FPSC efficiency. They build two systems and at the same time, examined the nanofluids with /without EG on the PSC efficiency. The volume fraction of EG and nanoparticles were 0.25% and 25%, respectively. Three mass flow rates were used in this study: 0.05 kg/s, 0.07 kg/s & 0.09 kg/s. ASHRAE Standard 93-2010 had been utilized to determine system efficiency. The efficiency of the collector was compared with the base fluid (distilled water). The experimental results also showed that increasing the mass flow rate and use EG led to increasing the system efficiency. Moreover, there was a great increase in efficiency by (15.13%) at 0.09 kg/s when utilizing nanofluid Al 2 O 3 -H 2 O/EG as compared with the base liquid. Tiwari et al. [22] theoretically investigated the influence of utilizing Al 2 O 3 nanofluid as the medium of absorption on the performance of a FPSC. The study also addressed the impact of particle size fraction and mass flow rate on collector efficiency. The results proved that employing the optimum particle size fraction of 1.5% for Al 2 O 3 nanofluid will increases the solar collector thermal efficiency by 31.64% as compared with distilled water as the working fluid. Golden et al. [23] provided empirical results on solar collectors based on nanofluid, which includes a variety of nanoparticles (carbon nanotubes and graphite, silver). Efficiency enhancements in thermal solar collectors using nanofluids as a mechanism to absorb have reached 5%. Experimental and numerical findings showed a quick initial raise in the efficiency with volume fraction, followed when by a decline in efficiency as volume fraction continues increasing. Many scientists have employed the solar tracking system to improve the solar collector system performance further. Rhushi et al. [24] presented an experimental analysis of a FPSC and introduced a comparison of performance with or without using a tracking system. The commercial 100 litter/day flat plate water heater has been built and developed into a testing-rig to carry out the experimental setup. Experimental tests were carried out on the FPSC in almost identical weather conditions for a week, and data were collected from both cases, the use of the tracking system or without. The results showed a 40 o C average increase in the outlet temperature. It has been calculated the efficiency of two conditions, and the comparison proved that there is a rise of 21 % in the efficiency. Drago [25] had compared the energy gain of four flat-panel collectors. Two collectors were mounted: one has a single cover, and the other has double covers while the other two solar collectors have combined with full tracking system. The experimental results demonstrated that the efficiencies for the single cover with and without using tracking were 5.7 % and 10.1 %, respectively, while in double cover were 17.4 % and 21.8 %, respectively. Pavelet el al. [26] analyzed theoretically and experimentally the collected energy with and without using a tracking mechanism with bifacial and nonbifacial photovoltaic PV solar systems. The calculated and measured tracking influence proved that there was an increase in collected energy by 30 -40 % using bifacial panels without tracking action, and projecting reflector collecting solar radiation on the back had given rise in the collected solar energy around 50 -60 % with the same panel. It noted in this day that the use of nanofluid technology rather than traditional liquids is a potential area where the performance of the solar energy collectors might be enhanced. The nanofluid selection process is a very significant factor in the solar collector. The nanofluids have some limitation i.e., erosion and corrosion of components, pressure drop, high cost, pumping power problem, conventional heat transfer fluids. Figure 1 detailed photograph showing the various components involved in the experimental work. Construction of the FPSC is modified [27] to application in this experimental work. Figure  1with specifications listed in Table 1. Dioxide silicon SiO 2 (40nm) + Distilled water (DW), based nanofluid of volume fraction concentration of 0.05% , 0.075%, and 0.10%, as working fluid is fabricated to flow through the collector at different values of flow rates of 10L/h, 15L/h and 20L/h.

II. Tracking mechanism
In general, solar tracking could be represented by two modes: one-axis tracking and two-axis tracking. The one-axis tracking of the FPSC system could be orientated in the north-south direction to track the sun from the east to west [27], while in the two-axis tracking system, the collector follows both of the sun's changing altitude and azimuth. The mechanical system includes the main structure that supports the solar collector, providing one degree of freedom (the movement in one direction). A lightweight steel structure was built to provide a reliable mechanical support against high wind speed and harsh environmental conditions. The mechanical system consists of a fixed part and a moving part. The fixed part is very important to fix the FPSC system, called the base. The moving part of moving the FPSC from east to west to track the sunrays, this movement can be directed by using a manual tracking mechanism used for this experiment to track the sun position during the day is shown in Figure 2.

III Nanofluid a. Nanofluid Preparation
Before preparing the Nanofluids, it is very necessary to measure the weight of SiO 2 nanoparticles required in water for varying concentrations, byemploying the typical expression [28].

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Vol. 37, Part A, No. 11, 2019 (1) Where: -the volume fraction concentration, W npweight of nanoparticles (kg) -the density of nanoparticles (kg/m 3 ), W bfweight base fluid (ml) -the density of the base fluid (kg/m 3 ) The weight of nanoparticles is measured in grams by using a digital weighing machine shown in Figure 3a, through mixing nanoparticles of SiO 2 in 1000 ml of distilled water to make the volume concentration ( ) of 0.05%, 0.075%, and 0.10%, respectively. Now, stirring is done by putting a small amount of SiO 2 nanoparticles in 1000 ml of distilled water to make the volumetric concentration, continuously for about 30 minutes on the magnetic stirrer with hot plate system, figure 3b. In order to get more stable and more widespread nanoparticles in the water, and ultra sonicator could be used. The solution then put on the sonicator figure 3c and sonication is done for three hours, to be the nanofluid ready the applied and figure 4, show the prepared sample of 0.05%, 0.075%, and 0.10% By vol.conc.SiO 2 +H 2 O, respectively.

b. Thermophysical Properties of the Nanofluid SiO 2 -H 2 O
The physical properties of nanofluid, density ρ nf , specific heat C pnf , Thermal conductivity k nf , and dynamic viscosity μ nf [29,30]. Specifications of thermophysical properties of the nanofluid show in Table 2. According to the below equations: Where: C bf -specific heat of bas fluid (J/kg.K) , C npspectific heat of nanoparticlas (J/kg.K) K bf -thermal conductivity of base fluid (w/m.K), K np -thermal coducivity of nanoparticals (w/m.K), μ bf -viscosity of base fluid (m 2 /s)

Thermal Performance
ASHRAE Standard 93-86 [32] suggested implementing the experimental test at different inlet temperatures, after measuring the fluid temperature in the inlet, outlet, water mass flow rate, and specific heat capacity. The useful energy can be determined according to Eq.6. The useful energy could also be indicated in terms of the energy absorbed by absorbent and those lost from the absorber function of removal factor, F R , solar radiation intensity G T , cover transmittance factor τ, glass absorbance factor α, and overall heat transfer coefficient U L . As given by Eq.7. [33]: The thermal efficiency( ) of the FPSC is determined as the energy gained to the energy of fallen solar radiation intensity on the FPSC, The efficiency, combined with Eqs. (8)(9)(10)(11) provides the basis for simulation models: The collector efficiency, according to Eq. (11), maybe expressed as a straight line [33]. From the average data plotted against (T i -T a )/G T , this line intersects the efficiency axis (vertical) at a point of F R (τ α). The collector efficiency, at this point, will reach its peak value, and the collector inlet temperature will be equivalent to the ambient temperature. F R (τ α) parameter is known as the "absorber energy parameter". The slope of the straight line is equal to F R U L and expressed how to remove the energy from the FPSC. Also, it is known as the "removed energy parameter".

Experimental Setup and Testing Rig
The experiments were carried out during March-April 2019.TheFPSC's performance has experimented with a volume fraction of (0.05%, 0.075%, and 0.10%) SiO 2 + water nanofluid and varying flow rates 10 L/h, 15 L/h, and 20 L/h, and the tilt angle of the collector was 33°. The storage tank is made up of plastic, having a capability of 10 litters. The experimental test set up includes the solar collector, closed working fluid system, and appropriate measuring devices. The working fluid system includes a tank, pay pass pipes system, and simple manual valves utilized to regulate the flow rate of working fluid. The flow rate may be measured using a flow meter. Glass wool insulation is used on the storage tank to protect it from heat loss. The pump is mounted inside the storage tank in order to circulate the working fluid. Moreover, the pump is also mounted inside the storage tank to avoid settling nanoparticles. A thermometer is mounted at both ends of the absorber tube, the inlet, and outlet, for measuring the inlet and outlet fluids temperatures. A solar power meter was utilized to measure solar radiation intensity. The manual tracking mechanism from (East-West) direction has been used in the experiment to follow the position of the sun during the day.

Results and Discussion
The experimental tests were conducted to collect the results. The results include the temperature variation, useful heat, solar collector efficiency, and effect of the various mass flow rate of fluid passing through the FPSC on its performance when employing distilled water or a SiO 2 +H 2 O nanofluid. All data had been tested in the quasisteady-state condition. Collector tests have been taken place from 11 AM to 2 PM.The experimental results are presented in the form of graphs and tables describing the temperature variation, useful heat gain, and efficiency of solar collectors.

I. Variation of temperature rise with inlet temperature
The influence of nanofluid type on the outlet-inlet temperature difference (DT) of the FPSC is demonstrated in Figures (4-6)  compared to distilled water. Enhancing the heat transfer of the nanofluids is attributed to the enhanced thermophysical properties such as thermal conductivity and heat transfer coefficient caused by adding the SiO 2 nanoparticles to water. In addition to the above reason, it was noted that adding nanoparticles to the water has many advantages: (a) The heat capacity of the water is decreased so that less energy is required for the nanofluid in comparison with water; in other words, the temperature difference of the nanofluids is larger than that of water if the same amount of heat is provided. (b) The heat transfer area is increased by mixing a little amount of the nanoparticles with the base fluid (water). (c) The mass migration phenomenon of the nanoparticles in the nanofluid working media further improves the heat transfer enhancement, (d) Diffusion and relative movement of nanoparticles close to the wall of the tube still cause a quick heat transfer from the wall to the nanofluid [34]. The outletinlet temperature difference (DT) with the increase of flow rate are shown in Figure 7. It is shown that increasing the flow rate causes the DT to decrease. It is found, at low flow rates, the behavior of the pure H 2 O and SiO 2 +H 2 O nanofluid were identical; nevertheless, at a higher value of flow rate, the DT line slope of the nanofluid was less than that of the pure water. Consequently, the collector based on the SiO 2 +H 2 O nanofluid has preferable thermal behavior than with pure water.    (11)(12)(13), the efficiency of the FPSC, SiO 2 nanofluid has a large value depends on volume fraction. This conclusion can detect by finding the value of the absorbed energy parameter F R (τα) and removed energy parameter, F R U L for SiO 2 nanofluid in Table 3. As demonstrated in Figure  11 for the flow rate of 10 L/h. The absorbed energy parameter, F R (τα) values for SiO 2 +H 2 O nanofluid, is more than water by 3.6%, 4.4%, and 5.6% for %(0.05, 0.075, and 0.1%), respectively.
However, removing energy parameter, F R U L values for SiO 2 +H 2 O nanofluid rise by 2.5%, 2.6% and 3.6% for % (0.05, 0.075, and 0.1%) with respect to water. Based on Figure 12 going up in the F R (τα) values for the mass flow rate of 15 L/h is 4.3%, 5.3% and 6.7% for % (0.05, 0.075 and 0.1%), respectively, and increasing in values of F R U L is 2.3%,3.7% and 4.1% for ( %) (0.05 ,0.075 and 0.1%) respectively, comparing to water. Figure 13 showed the values of F R (τα) and F R U L for the flow rate of 20 L/h. Values of F R (τα) is raised by4.4% , 5.6% and 7.3% for % (0.05 ,0.075 and 0.1) , respectively. It is observed that the gain in values of F R U L for SiO 2 nanofluid increased by 1.3%, 3.9%, and 11.9% for %(0.05, 0.075, and 0.1%) respect to water. The study of % of nanofluid on the FPSC efficiency is complex. It has been noted that higher % at 0.1% is absorbed and more energy removed than other % of 0.05, 0.075%, and pure water as it has the peak values of F R (τα) and F R U L for all mass flow rate of nanofluids. The explanation of that is as the volume fraction increased, the thermal conductivity of fluid rose because more particles were added. However, the collector efficiency does not depend only solely on the values of F R (τα) and F R U L but also on the dropped temperature coefficient [(T i -T a )/G T ]. The The sequence was the same whatever the mass flow as, shown in Figures (11-13). The lower [(Ti-Ta)/GT] values could be due to the increase in solar radiation or by reducing the temperature difference. Hence, the higher % of nanofluid is considered useful because it absorbed more heat than others. Also, fewer particles tend to agglomerate comparing to the lower volume % nanofluid. The microwatts of heat transfer increased as the collision of particles raised. Based on these reasons, the efficiency of the solar collector increased with the rising of the volume fraction. On the other hand side, higher viscosity of nanofluids and full thickness of the boundary the layer was found when the [(Ti-Ta)/GT] values increased because of the rise in the average fluid temperature. At the same time, solar radiation will decrease, because causing a decrease in absorbent energy and increasing heat loss. Hence, less performance and lower efficiency were detected as the heat transfer rate reduced [35]. All of the above explanation clarifies the fact that solar collector efficiency has a noticeable depend on the value of [(Ti-Ta)/GT]. Since the slopes of the models are negative, it can notice that the increasing (Ti-Ta) makes the efficiency equal to zero. Diffusion and relative movement of nanoparticles near the tube wall causes fast heat transfer from the wall to nanofluid [34]. The slopes for the nanofluids became steeper as s compared with the water, which demonstrates the influence of utilizing the nanofluids in enhancing the heat removal factor (F R ) of the solar collector. Raising the mass flow rate values or employing nanofluids instead of basic fluid are the methods used to increase the efficiency factor of the collector via increasing the heat transfer coefficient inside the tube [33]. The collector efficiency is in the near range in both coolants. The SiO 2 +H 2 O nanofluid is more efficient than pure water, with a small variation, mainly since that the thermal conductivity of the SiO 2 is not so bigger than water [19,34,35]. The values of F R U L and F R (τα) of the FPSC for water and nanofluid are listed in Table  3 .

IV. Flow rate effect on the efficiency
The effect of the working fluid flow rate on the FPSC efficiency had been tested. Both water and nanofluid had been tested at various flow rates for evaluating their impact on collector performance. The variation of the FPSC efficiency versus the mass flow rates of (10, 15, and 20 L/h) for water and nanofluid, with a mass fraction of 0.05, 0.075, and 0.10 % are shown in Figure 14. It is seen that the collector efficiency increases in the flow rate for coolants. The researchers C. Cristofari et al. [36] and A. Minsta et al. [37]studies the impact factor of the value of flow rate, the number of Reynolds, and heat transfer rate on the FPSC performance. Figure 14 clarifies that the efficiency of the solar collector was greatly elevated by using the nanofluid more than distilled water. The interpretation of this trend goes back to that average velocity and number of Reynolds of coolant had been increased with the increase of rat of the flow. Therefore, the motion rate, especially the Brownian motion and motion of the particles in

V. Comparison with previous studies
Many researchers studied the performance of the FPSC using SiO 2 +water. Table 4 demonstrates the summary of the experimental studies and their results. The results obtained from these studies on the effect of SiO 2 almost identical to the results in the present study. Nevertheless, there is a difference between the SiO2 results and that in this study. The difference is resulting from utilizing different amounts of SiO2 and applying the tests at various values of flow rate. Aminreza et al. [41] SiO 2 /water 1wt % 12nm -Thermal efficiency enhanced by using nanofluid

Conclusions
The highlights of the study may be summarized as follows: -The maximum thermal efficiency was about 70 % as decreased temperature parameter, [(Ti-Ta)/GT] is equal to zero at volume fraction 0.10 % and flow rate of 20 L/h. The highest rise in the absorbed energy parameter F R (τα) was 7.3%, as compared with pure water. Note that the difference is not that big. This produces mainly due because of the fact that the thermal conductivity of SiO 2 and SiO 2 +H 2 O is not much bigger than in water.
-The maximum increase in F R U L is 11.9 at a volume fraction 0.10% and a flow rate of 20 L/h. The variation in the absorbed energy parameter F R (τα) varies from 4.4% to 7.3%, while in removed energy parameter FRUL, they vary from 1.3% to 11.9% as compared with the water case. The collector efficiency is directly proportional to both flow rate and volume fraction of nanoparticles.
-For broad operating conditions, the performance of FPSC is raised using the SiO 2 +H 2 O nanofluid compared to that using water. One is the highest temperature related to using the nanofluid in the collector can be influenced by the specific heat of the working fluid. Nanoparticles and nanofluids have lower specific heat than water. Therefore, less heat required raising the temperature of nanofluids, which makes the resulting output temperature, and thus, thermal efficiency becomes the higher.