Analysis of utilizing Graphene nanoplatelets to enhance thermal performance of flat plate solar collectors

Energy Conversion and Management 126 (2016) 1–11

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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Analysis of utilizing Graphene nanoplatelets to enhance thermal performance of flat plate solar collectors Alireza Ahmadi a, Davood Domiri Ganji a,⇑, Farzad Jafarkazemi b a b

Department of Mechanical Engineering, Sari Branch, Islamic Azad University, Sari, Iran Department of Mechanical Engineering, South Tehran Branch, Islamic Azad University, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 27 April 2016 Received in revised form 18 July 2016 Accepted 23 July 2016

Keywords: Flat plate solar collector Nanofluid Thermal efficiency Colloidal stability

a b s t r a c t It is the aim of the present paper to investigate the effect of Graphene nanofluid on thermal performance of flat plate solar collectors. Therefore, the influence of Graphene Nanoplatelets on thermal efficiency of a closed-cycle solar system consisted of a flat plate solar collector has been investigated and its hot water production capability has also been tested. To start, the structure of Graphene has been evaluated by Field Emission Scanning Electron Microscopy imaging and UV–vis spectroscopy. Afterward, by utilizing deionized water as the base fluid, Graphene/Water nanofluid has been prepared by a two-step method with different mass fractions (0.01 and 0.02 wt%). To prevent from sedimentation, the colloidal stability of the nanofluid has been tested in different pHs and its best level has been obtained. Moreover, the thermal conductivity, kinematic viscosity and the influence of Graphene/Water nanofluid on the efficiency of the flat plate solar collector have been studied thoroughly. The results indicate that dispersing Graphene in the base fluid can increase thermal efficiency of the solar collector up to 18.87%. As an obvious result, the outlet temperature and absorbed energy of water heater in various states (deionized water, 0.01 and 0.02 wt% nanofluid) has been measured and the temperature of water in the solar heater reached up to 71 °C. Eventually, the theoretical thermal efficiency of the solar collector has been calculated and compared with the experimental one, and the results have been closed to each other. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Limitation of using fossil fuel resources and the pernicious effects of their wrong usage by human beings for our nature made researchers in scientific and industrial communities concentrate on renewable energies such as wind, solar, and geothermal like the research done by Chiari and Zecca [1]. Also Pfenninger and Keirstead [2] studied the above items in order to find a suitable scenario for the power systems of Britain. Due to the cleanness and especially its availability, solar energy has been known as a suitable case among other sources and many studies have been done in this area such as the research done by Aydin et al. [3]. With regard to its accessible property in every place of our world, solar energy is used in various applications, for instance, in production of hot water, electricity, air conditioning and so on. Enough knowledge about solar radiation can be helpful to make systems for heating water in industry or for household consumptions and valuable case studies have been done in the recent years like Halawa et al. [4]. Therefore, heating water by solar water hea⇑ Corresponding author. E-mail address: [email protected] (D.D. Ganji). http://dx.doi.org/10.1016/j.enconman.2016.07.061 0196-8904/Ó 2016 Elsevier Ltd. All rights reserved.

ters can be considered as the most economical and easiest approach. It is necessary to mention that collectors are the most important part of each solar system which their main role is absorbing solar radiation, converting it to the heat and then transferring the converted energy to the working fluids which flow in pipes or channels. Detailed information about collectors were mentioned by Visa et al. [5]. One of the problems of such systems is how to boost their thermal performance. Stanciu and Stanciu [6] determined the optimized angle for installing collectors but their results indicate that better solutions are needed. Li et al. [7] studied nanofluids which are suspensions prepared by dispersing nanoparticles, rods or tubes in the base fluids. Also the high stability of nanofluids has been investigated by Farbod et al. [8]. Furthermore, Zhu et al. [9] studied the effects of nanoparticles on thermal conductivities of Iron(II,III) oxide [Fe3O4] nanofluids. Moreover, Soleimani et al. [10] analyzed heat transfer capability of nanofluids in semi-annulus enclosure. In addition, Sheikholeslami and Ganji [11] investigated heat transfer of nanofluid flow between parallel plates. Moreover, the effect of electric field on the behavior of nanofluid was studied by Sheikholeslami et al. [12]. Finally, the influence of magnetic field on the heat transfer of nanofluid has been investigated by Sheikholeslami et al. [13].

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A. Ahmadi et al. / Energy Conversion and Management 126 (2016) 1–11

Nomenclature A Gnp FESEM DLS K _ m FR UL Re T G wt N t Cb D W Tpm F0 hfi

gross collector area (m2) Graphene Nanoplatelet Field Emission Scanning Electron Microscopy dynamic light scattering thermal conductivity (W/m K) mass flow rate (kg/s) heat removal factor overall loss coefficient (W/m2 K) Reynolds number temperature (°C) solar radiation (W/m2) mass fraction number of glass covers insulator thickness (m) bond conductance (W/m °C) diameter (m) distance between the tubes (m) mean plate temperature (K) collector efficiency factor average convection coefficient (W/m2 °C)

q c /

l g d

density (kg m
3) specific heat capacity (J/kg K) volumetric fraction viscosity thermal efficiency plate thickness

Subscripts bf base fluid nf nanofluid p particle, constant pressure i inlet, instantaneous, inside o outlet a ambient T tilted w wind h hydraulic t from top b from bottom e from edge

Greek symbols sa absorbance-transmittance product

Improvement of heat transfer capabilities of solar systems seems to be vital since, up to now, human beings have not been successful to broadly utilize such systems economically. To overcome this shortage, many researchers take for example Sarsam et al. [14] investigated many solutions such as applying nanofluids. And nowadays by developing nanotechnology this manner can be applicable for most of the researchers. They studied techniques of increasing outlet temperature of a small collector and compared it by a real size one. They concluded that nanofluid is the only way that results more thermal efficiency and Copper(II) oxide nanoparticles [CuO] have the highest performance in comparison with Silicon dioxide [SiO2], Titanium dioxide [TiO2] and Aluminium oxide [Al2O3]. The influence of Al2O3, Zinc oxide [ZnO] and Magnesium oxide [MgO] on the efficiency of cylindrical solar collectors has been showed by Li et al. [15] that ZnO/H2O by / = 0.2% is the best choice among others. Liu et al. [16] studied the thermal performance of a small thermosiphon system experimentally by dispersing Carbon nanotubes [CNT] in water and they obtained its optimized mass fraction. The effect of Al2O3 with water as a base fluid on flat plate solar collectors have been analyzed by Yousefi et al. [17]. Also, Yousefi et al. [18] studied the influence of Multi walled carbon nanotubes [MWCNT] with water as a base fluid on flat plate solar collectors. In the first case when particle diameter (dp) was 15 nm with 0.2 wt% and 0.4 wt%, collector efficiency for 0.2 wt% was higher than 0.4 wt%. In the second experiment which was done for MWCN, 0.4 wt% nanofluid without any surfactants had the highest thermal performance in comparison to other states. Moghadam et al. [19] tested the effect of CuO/H2O nanofluids on flat plate solar collector with / = 0.4% and dp = 40 nm by dif_ = 0.016 kg/s, ferent ranges of mass flows. They concluded that by m the system performance can be raised up to 21.8%. Among all of the experimental researches, the effect of Al2O3/H2O nanofluid with / = 5% on glazed collectors by sinusoidal absorbers was numerically investigated by Nasrin et al. [20] and they could increase convectional heat transfer up to 19% for nanofluid and 12% for the base fluid. Moreover, Tiwari et al. studied the effect of Al2O3/H2O nanofluid with / = 2% and dp = 0.5 nm on flat plate solar collectors theoretically [21] and their results showed that the system performance can be increased up to 31.64% in comparison with water as

the base fluid. Experimental scrutiny which was carried out by various researchers such as Said et al. on CNTs [22] indicates that carbon nanostructures and especially grapheme can be considered as suitable choices in order to be utilized in nanofluids. Subsequently, Zhu et al. investigated thermal conductivity of graphite nanoparticles [23]. Last but not least, heat transfer performance of Graphene nanofluids for fully developed turbulent flow has been investigated by Yarmand et al. [24]. For their higher thermal conductivity compared with other nanoparticles and also because their density is lower than metals or metal oxides like the afore-mentioned done experiments, carbon nanostructures are our main priority. All in all, the main goal of this study is interpreted as creating a high quality solar water heating system with acceptable performance for household consumption that can be used even in adverse climate conditions. Therefore, attempts have been made to enhance thermal performance of a flat plate solar collector and a solar water heater by using Graphene Nanoplatelets [Gnp] which can be considered as a suitable choice among other nanomaterials based on the achieved results explained in the subsequent parts. And finally in order to check the obtained experimental data, theoretical thermal efficiency have been calculated and compared to experimental results.

2. Preparation of Gnp/H2O nanofluid The two-step method is known as one of the standard and ordinary techniques for preparing nanofluids like the research done by Haddad et al. [25]. After making nanomaterials by every existed method such as exfoliation, and Chemical vapor deposition (CVD), they should be dispersed in the base fluid. There are some ways for dispersion such as ultrasound method selected for production of Gnp/H2O nanofluids. As a prototype, 10
2 g of Graphene nanoplatelets has been added and dispersed into 5  10
5 m3 deionized water by ultrasound method. It is noteworthy to mention that the dispersion process was lasted 1 h. The dispersion of Gnps in the deionized water which was set in 25 °C and time duration of 1 h can be investigated by Dynamic light scattering test (DLS). Also, this approach (ultrasound) is reliable and makes the

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probability of sedimentation which is a vital factor in nanofluids properties decrease significantly. 2.1. Field Emission Scanning Electron Microscopy (FESEM) To start making every kind of nanofluids, the originality of the nanomaterials should be checked first. Various tests have been existed in this field such as Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), Raman Spectroscopy and FESEM which have been used by a broad range of researchers for instance Argast and Tennis [26]. Then, the results of FESEM test is shown in Fig. 1 as follows: 2.2. UV–vis spectroscopy On the basis of Kataura et al. [27], analysis of UV–vis spectroscopy is a suitable technique for identifying carbon allotropes. Therefore in this case study, a JAS.co V-630 spectrophotometer (made in Japan) has been utilized in order to test the amount of absorbency of different samples. All of the prepared samples were diluted to some extent and while pouring the nanofluid in the cuvette, cares have been taken to prevent bubbles like the research done by Nasiri et al. [28]. All the experiments have been done three times in order to get acceptable accuracy. In Fig. 2, a linear relation between the amount of Graphene dispersion and absorbance is observed and it is concluded that the amount of Graphene dispersion (colloidal stability) is related to the intensity of the corresponding absorption spectrum. Finally, based on the peak values of all samples (265 nm), the Gnp structure has been observed. 2.3. Colloidal stability Zeta potential test is a useful approach to evaluate the colloidal stability or stable dispersion of nanomaterials. With regard to the above subject, pH can be considered as a very important item

Fig. 2. UV–vis spectra of Gnp dispersed in deionized water as a function of sonication time.

which is related to the electrostatic charge on the particles’ surface and this can be interpreted and assessed by the zeta potential analysis implemented by many researchers such as Lee et al. [29]. In order to preserve the stability of nanofluids, pH should be kept far from the isoelectric point (IEP) which is interpreted as a state that the surface of the particle does not carry any net electrical charge (zero zeta potential). Therefore, in a colloidal dispersion like Gnp/H2O, IEP can cause precipitation and agglomeration of particles and its reason can be defined as follows: in this state, there isn’t any effective repulsive force among particles (nanosheets in this research). Also when the amount of pH recedes into the IEP, the absolute value of zeta potential surface of the particle increases so that the interaction among particles due to electrical double layer will be sufficient until preventing absorption and collision among particles. To start, samples with various pHs should be prepared and Sodium hydroxide [NaOH] and Hydrochloric acid [HCl] are suitable candidates. After sonication of Gnps in deionized water for 1 h and preparing 0.02 wt% nanofluids, the product is divided into eleven pieces to make five acidic and five alkaline samples as explained in Fig. 3. While synthesizing acidic and alkaline samples, the concentration of NaOH and HCl should be decreased remarkably (0.1 mol). Inasmuch as the molarity of NaOH is 40 g and the volume of the solution is considered to be 10
4 m3, 0.4 g NaOH is needed. And also for production of diluted HCl (100 mol/m3), the following formula is used where a and d are 0.37 and 1.19 kg/m3, respectively.


N1 V 1 ¼ N2 V 2 ! N1 V 1 ¼ V 2

Fig. 1. FESEM image of Graphene nanoplatelets.

 10ad M

ð1Þ

As a result, the final required volume of HCl will be 82.89  10
6 m3. After preparation of acidic and alkaline samples based on Fig. 3, zeta potential test can be done in the following form: It is concluded that the level of acidity of the total working fluid should be changed exactly like the obtained results of Fig. 4 which is at pH = 11.6 and this level is considered as the most stable mode of Gnp/H2O. On the whole, a summary of the prepared nanofluids’ characteristics has been written in Table 1 as follows:

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A. Ahmadi et al. / Energy Conversion and Management 126 (2016) 1–11 Table 1 Specifications of the prepared nanofluids. Item

Explanation

Nanomaterials Appearance Size

Graphene nanoplatelets Black granules At least one dimension of each platelet (typical thickness) is less than 100 nm Deionized water Sonication

Base fluid Method of preparation Time of preparation Structure check

Modification Number of prepared samples

1h Based on the identified peak values of the absorption spectra took place in 265 nm, it is concluded that our nanomaterial is Graphene belonged to carbon allotropes Enhancement of nanofluid’s colloidal stability by zeta potential test in pH = 11.6 0.01 wt% and 0.02 wt% for being used as working fluids in solar system

2.4. Thermal conductivity coefficient

Fig. 3. Levels of sample preparation for colloidal stability test. (a) Sonication procedure of Gnps in deionized water. (b) Diluted acid and alkaline (100 mol/m3). (c) Gnp/H2O with 9:68 6 pH 6 12:7. (d) Gnp/H2O with 2:77 6 pH 6 7:34.

In order to evaluate thermal conductivity of nanofluids from 15 °C to 45 °C, a transient short hot-wire method was implemented by a KD2 Decagon, Inc., USA. The instrument is consisted of a probe, a thermo-resistor and a microprocessor to control and compute conductivity of the prepared samples in the probe. Besides, a bath for controlling temperature was utilized to keep the temperature of samples steady during the measuring procedure. Samples were kept for one hour in the bath after reaching the bath temperature in order to ensure the temperature equilibrium. Also, the number of data collecting for each temperature of the prepared samples was done three times and only the mean values were considered. To assess the influence of Gnps on the enhancement of thermal conductivity of water, different mass fractions consisted of 0.01, 0.02, 0.04, 0.1 and 0.2 wt% have been prepared and variation of nanofluids’ thermal conductivity as a function of temperature and Gnp dispersed into deionized water has been depicted in Fig. 5. Experiments done by Jana et al. [30] showed that thermal

Fig. 4. Variation of zeta potential in terms of different pHs for 0.02 wt% Gnp/H2O.

Fig. 5. Variation of thermal conductivity (W m
1 K
1) in terms of different temperatures ( C).

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A. Ahmadi et al. / Energy Conversion and Management 126 (2016) 1–11

conductivity of nanofluid is increased with the mass fraction of nanomaterials similar to the amount of Gnp in this test. For instance, the thermal conductivity of nanofluid with 0.01 wt% Gnp showed 11.37% increase in comparison with water and by enhancing Gnp mass fraction up to 0.2 wt%, thermal conductivity is raised up to 38.63%. Table 2 shows thermal conductivity enhancement of the prepared samples in various temperatures and mass fractions. In accordance with the acquired results of Fig. 5 and Table 2, thermal conductivity of nanofluids is improved by rising the temperature. 2.5. Gnp’s influence on viscosity One of the problems of applying nanofluids in mechanical systems is their influence on pump performance for circulation of working fluid. To explain more, the amount of viscosity enhancement affects dramatically on the required power of pumps and even makes the technicians prepare more powerful ones in order to circulate viscos working fluids. To resolve this issue, the test of nanofluids viscosity of the prepared samples has been done. The kinematic viscosity of nanofluid has been gained by Herzog viscometer (Carlowitz & co Hamberg, Inc., Germany) and all of the test procedures have been done according to the American Standard for Testing Materials (ASTM) D445. It is noteworthy that the Reynolds number of the working fluid in the flat plate solar collector is calculated 902.7 which shows a laminar flow. In Fig. 6, the kinematic viscosity of water and 0.01 wt% Gnp/H2O at four different temperatures has been depicted and compared graphically. It is concluded that the maximum kinematic viscosity growth is about 7.52% in the introduced temperature domain (30–60) °C which demonstrates the fact that Gnp has negligible influence on the pump power. Table 2 Augmentation percent of prepared nanofluids’ thermal conductivity in terms of different temperatures. Temperature (°C)

0.01 wt%

0.02 wt%

0.04 wt%

0.1 wt%

0.2 wt%

15 25 35 45

3.5 7.82 9.38 11.37

7.01 13.91 13.06 13.7

9.64 17.39 16.41 15.26

14.03 22.6 21.44 23.83

19.29 30.43 32.32 38.63

3. Experimental setup In this step, four components of the considered solar system consisted of a flat plate solar collector, a water heater, an inlet temperature regulator system and a thermometer have been manipulated and also two other instruments are introduced on the basis of Fig. 7 as follows: After preparation of flat plate solar collector, a temperature regulator system is required in order to prepare different inlet temperatures of the working fluid into the collector and also a water heater is needed to save the absorbed solar energy by collector. The introduced water heater in Fig. 7 (part g) also acts as a cooler to chill the outlet working fluid from solar collector and has an important role to calculate its thermal efficiency. Finally, the prepared solar system with all of its components is presented in Fig. 8 as follows: 3.1. Experimental method All the experimental solar tests were done at South Tehran Branch of Azad University at longitude 35.415 E and latitude 51.201 N in sunny days of spring season, 2015 between 9:00 am and 4:00 pm for different working fluids consisted of deionized water and two Gnp/H2O nanofluids (0.01 and 0.02 wt%) with the flow rate of 2.7  10
6 m3/s which is related to the surface area of flat plate solar collector according to Jafarkazemi and Abdi [31]. All of the tests were done according to ISO Standard with inclination angle of 35° (ISO 9806). The useful energy and the thermal efficiency of a flat plate solar collector (g) can be obtained based on Duffle and Beckman [32] in the following form:

Q u ¼ Ac F R ½GT ðsaÞ
U L ðT i
T a Þ

gi ¼


 _ p ðT o
T i Þ Qu Ti
Ta mC ¼ ¼ F R ðsaÞ
F R U L Ac GT Ac G T GT

ð3Þ

where GT represents the incident solar radiation on solar collectors (W/m2) and Ac is called gross area of the collector (m2). The heat capacity of nanofluids can be evaluated by Eq. (4) with regard to Bergman [33] as:

C p;nf ¼ C p;p u þ ð1
uÞC p;bf

ð4Þ

In Eq. (4), cp,nf, cp,p and cp,bf are heat capacity of nanofluids, particle and base fluid, respectively (J/kg K). By depicting the chart of thermal efficiency of the system in terms of (Ti
Ta)/GT and by considering Y as the vertical axis, the Y intercept is interpreted by FR(sa) which indicates the absorbed energy and also the removed energy is defined by FRUL. Finally by plotting the graph of efficiency versus (Ti
Ta)/GT, a straight line is obtained. In order to clarify the experimental method, it is better to indicate that after collecting all the required data which is broadly explained in Section 3.2. Test procedure, only the right part of Eq. (3) is _ p ðT o
T i Þ=Ac GT Þ for calculating the experimental accounted ðmC thermal efficiency of the flat plate solar collectors. Uncertainty analysis can be considered as a useful approach for handling the data and reporting logical results of a certain experimental test. On the basis of Shen et al. [34], the concept of uncertainty is used to quantify the goodness of experimental results and it generally has been accepted as the best estimation of experimental errors which clarifies the deviation between the experimental results and the true values. Therefore, in this case study, the uncertainty of instantaneous efficiency of the flat plate solar collector has been evaluated based on Moffat [35] as follows: @ gi

Fig. 6. Changes of kinematic viscosity (mm2 s
1) in terms of different temperatures ( C).

ð2Þ

gi

"
¼

@q

q

2


þ

@Q Q




2 þ

@C p Cp

2


þ

@GT GT

2


þ

@Ac Ac

2


þ

@ðT o
T i Þ To
Ti

2 #0:5

ð5Þ

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A. Ahmadi et al. / Energy Conversion and Management 126 (2016) 1–11

Fig. 7. Manufacturing process of solar water heating system. (a) A copper plate with the dimensions 0.47 m  0.27 m  0.001 m. (b) Copper pipe with D = 6.3  10
3 m. (c) Prepared serpentine copper pipe. (d) Welding procedure. (e) Prepared glazed flat plate solar collector. (f) Interior part of water heater. (g) Insulated solar water heater by wool stone. (h) Compact fabricated copper coil. (i) Inlet temperature regulation system of working fluid consisted of a water tank with a cooper coil and two electrical heater connected to a digital controller. (j) Manufactured thermometer in order to use in solar system. (k) Pyranometer to measure the solar radiation flux density. (l) Data logger (HOBO U12 4-Channel External Data Logger).

In this special case, gi has six different independent variables and in the first step, the uncertainty of each item can be specified as follows:

@q

q

@Q @C p @GT @Ac ¼ 2%; ¼ 0%; ¼ 2:5%; Q Cp GT Ac "
2
2 #0:5 @ðT o
T i Þ @T o @T i ¼ 0:15%; 6 þ To
Ti To Ti "
 #0:5
 2 2 0:5 0:5 ¼ þ ¼ 1:89% 40 35 ¼ 0%;

ð6Þ

As a result, @ gi =gi will be ±3.72% and the achieved uncertainty can be depicted graphically in Fig. 9 as follows:

neous efficiency in terms of (Ti
Ta)/GT. In order to calculate the first point from the left side of the chart (highest point), Tset which is the considered temperature for water tank should be close to the ambient temperature. It is very vital to reach to the steady state and then record the required data such as inlet and outlet temperatures of the flat plate solar collector (Tin and Tout). To obtain the efficiency of the flat plate solar collector by various working fluids, the procedure of data collecting is presented in Table 3 in the following form: The presented data in Table 3 was recorded on Tuesday, May 12, 2015. Afterward, all of the recorded data have been processed by computer and the result is a straight line which indicates the collector thermal performance. 4. Result and discussion

3.2. Test procedure At first, deionized water is pumped as the working fluid to the introduced solar system. Then, the exact amount of ambient temperature and received solar irradiation to the collector surface should be noted for obtaining the chart of the collector instanta-

It is necessary to mention that the period of collecting experimental solar data lasted only two months with the exception of weekends and the process of designing and manufacturing the introduced solar system was done during six months. Moreover, the required tests of nanomaterial and nanofluids were carried

A. Ahmadi et al. / Energy Conversion and Management 126 (2016) 1–11

7

Fig. 8. The schematic diagram of the experimental set up. (1) Glazed flat plate solar collector. (2) Pyranometer. (3, 7, 9, 19) Thermometer. (4) Ambient temperature sensor. (5) Anemometer. (6, 10, 12) Valve. (8) Water heater. (11) Electro-pump (12 V/120 W). (13) Ventilation valve. (14) Flow meter. (15, 16) Electrical heater. (17) Controller. (18) Water tank. (20) Thermal insulator.

4.1. The effect of environmental factors Based on the above explanations, the afore-mentioned variations are presented in Figs. 10 and 11 as: Based on the afore-mentioned figures, the ambient temperature differed from 27 °C to 33 °C and the variation of wind speed can be considered uniformly with the exception of a remarkable rise in a period between 12:20 pm and 1:00 pm. In addition, solar radiation in the specified period reached to the approximate peak of 940 W/m2 at 1:05 pm. 4.2. Thermal efficiency Characteristic graphs of manufactured flat plate solar collector with three different working fluids consisted of deionized water and two different nanofluids (0.01 and 0.02 wt%) at a flow rate of 2.7  10
6 m3/s are shown in Fig. 12. It is observed that the Fig. 9. The calculated uncertainty for the instantaneous efficiency of the flat plate solar collector.

Table 3 Presented data for investigation of collector instantaneous thermal efficiency with 0.01 wt% Gnp/H2O as the working fluid.

1 2 3 4 5 6

Time (pm)

Tin (°C)

Tout (°C)

Q (m3/s)

Tset (°C)

1:20 1:39 1:58 2:17 2:45 2:59

37 41 44.5 48 52 56

45.5 49 51.5 53 56.5 60

2.7  10
6 2.7  10
6 2.7  10
6 2.7  10
6 2.7  10
6 2.7  10
6

35.7 40.7 45.5 50.2 55 60.2

out for over four months. In order to investigate the experimental and even theoretical performance of solar systems, having enough knowledge about variation of environmental factors such as ambient temperature, solar radiation and wind speed (a key parameter for determination of UL in theoretical thermal performance) in the period of data collecting is very vital.

Fig. 10. Variation of solar radiation and ambient temperature between 9:00 am and 4:00 pm.

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A. Ahmadi et al. / Energy Conversion and Management 126 (2016) 1–11

fraction of Gnp to decrease instability, sedimentation and nanofluids preparation costs. Last but not least, the main reason of enhancing the efficiency of flat plate solar collector with adding Gnp to the base fluid can be interpreted as follows: The heat capacity of nanofluids (cp,nf) which can be acquired from Eq. (4) is a little smaller than cp,bf but the created temperature difference by nanofluid (T o
T i ) is much higher than the base fluid. Then, the product of the above two items is resulted a significant increase in experimental thermal efficiency. As a result, goodness of data fitting is determined by some parameters such as R2, SSE (Sum of Square Error) and RMSE (Root Mean Square Error) which are calculated for this especial case in Table 4 as the following: 4.3. The effect of Gnp on the absorbed energy and temperature of the water heater

Fig. 11. Changes of wind speed between 9:00 am and 4:00 pm.

thermal efficiency of solar collectors is improved generally by adding nanoparticles (Gnps in this especial case) to the base fluid on the basis of the current case study and different researches for instance Said et al. [36]. By increasing the amount of nanofluid concentration from 0 (deionized water) to 0.01 wt%, the thermal efficiency is increased 12.19% where the heat loss parameter is approached to zero. Furthermore, approximately 6% raise in efficiency has been seen while increasing the amount of mass fraction of nanofluids from 0.01% to 0.02%. All of the above comparisons have been done in zero (Ti
Ta)/GT and in this situation, instantaneous efficiency is acquired 0.83 which indicates the fabrication procedure of flat plate solar collector and nanofluid preparation followed standard methods. With regard to Fig. 12, the difference between efficiency of nanofluid and the base fluid is raised when the heat loss parameter is increased. Also it is concluded that the efficiency of nanofluids with mass fraction of 0.01% and 0.02% is very close to each other and other tests can be done in order to compute the optimized mass

Fig. 12. Thermal efficiency of flat plate solar collector for deionized water and Gnp/ H2O with different mass fractions.

Temperature variation of the water heater which is considered to be utilized for household consumption (in larger scale) has been shown graphically in Fig. 13. This comparison has been done at 10:30 am to 2:30 pm for different working fluids which are deionized water, 0.01 and 0.02 wt% Gnp/H2O. Based on the obtained results from Fig. 13, a rapid change in water temperature while using nanofluids as the working fluid for the prepared solar water heater is observed. It is noteworthy that between 11:40 am and 1:10 pm (from the minute 70 to the minute 160), the temperature of the water tank is dramatically increased which is compatible with the variation of solar radiation demonstrated in Fig. 10. Eventually, temperature of the considered water for household consumption is reached its maximum value at 2:30 pm which are 57 °C, 67.5 °C and 71 °C related to the base fluid, 0.01 and 0.02 wt%, Table 4 Fitting parameters for thermal efficiency of flat plate solar collector with different working fluids. Working fluid

F R ðsaÞ

F R UL

R2

RMSE

SSE

Gnp/H2O (0.01 wt%) Gnp/H2O (0.02 wt%) Water

0.7828 0.8294 0.6977

15.03 16.01 22.21

0.913 0.927 0.993

0.04615 0.04551 0.01864

0.00852 0.00828 0.00139

Fig. 13. Temperature variation of utilized water for household consumption in different working fluids.

A. Ahmadi et al. / Energy Conversion and Management 126 (2016) 1–11

9

Then, Ub can be calculated from dividing thermal conductivity by the thickness of insulation (K/L) and the edge heat loss is calculated as follows:

Ue ¼

Fig. 14. Absorbed energy by water with various working fluids consisted of deionized water, 0.01 and 0.02 wt% Gnp.

respectively. Moreover, Fig. 14 demonstrates the absorbed energy by the water heater. The maximum absorbed energy of water is happened when 0.02 wt% nanofluid is used as the working fluid and also the tendency of the absorbed energy by water is similar to Fig. 13. 4.4. Theoretical thermal efficiency After doing the required tests on the introduced solar system, it is better to calculate the theoretical efficiency of the flat plate solar collector which is placed in the northern hemisphere toward south between 1 pm and 2 pm in order to compare the obtained experimental results with theoretical ones. To complete this procedure on the basis of John A. Duffie and William A. Beckman in Solar Engineering of Thermal Process, five items should be calculated precisely which are collector overall loos coefficient (UL), average convection coefficient (hfi), collector efficiency factor (F 0 ), collector heat removal factor (FR) and transmittance-absorbance product (sa). Due to the large amount of computational operations, only the final results are presented as follows:

UL ¼ Ut þ Ub þ Ue

ð7Þ

In which Ut, Ub and Ue stand for heat loss from top, bottom and edge parts of the collector. An empirical equation for Ut was developed by Klein in 1979 as:

0 B Ut ¼ @

1
1 C

T pm



N T pm
T a Nþf

e þ

1C A hw

þ

rðT pm þ T a ÞðT 2pm þ T 2a Þ 1 ðep þ0:00591Nhw Þ

þ

h

2Nþf
1þ0:1334ep

eg

i


N ð8Þ

where N, hw, b, Ta, Tpm, eg and ep are the number of glass covers, wind heat transfer coefficient, collector tilt in terms of degree (°), ambient temperature (K), mean plate temperature, emittance of glass and emittance of the plate, respectively. Also the parameters f, e and C are achieved as:

f ¼ ð1 þ 0:089hw
0:1166hw ep Þð1 þ 0:07866NÞ C ¼ 520ð1
0:000051b2 Þ for   100 e ¼ 0:430 1
T pm

0 6 b 6 70

K wood t

ðedge circumfrenceÞðcollector thicknessÞ Ac

where t is the insulator thickness in the edges of the collector and Ac can be interpreted as the collector surface area. Based on the above explanations, the overall heat loss coefficient of the collector will be 14.313 W/m2 °C. According to the evaluated Re number in this vicinity, the fluid flow is laminar and the related procedure for estimation of convection coefficient between the tube and working fluid in the flat plate solar collector can be presented as follows: To start, the Re number for 0.01 wt% nanofluid in 40 °C is defined. After computing specific heat, thermal conductivity can be evaluated in accordance with Xuan and Roetzel [37]. Afterwards, kinematic viscosity of the related nanofluid can be computed based on Brinkman [38]. As a result, the Pandtl number will be 4.34. Also, thermal conductivity of nanofluids can be calculated from KKL (Koo–Kleinstreuer–Li) correlation which was implemented by Sheikholeslami and Ganji [39]. In accordance with Goldberg in 1958 that presented average Nusselt numbers in terms of RePrDh/L wherein Dh is hydraulic diameter, Nuave will be 5.3. As a result, convection coefficient between the tube and working fluid is calculated 554.733 W/m2 °C. In order to obtain the collector efficiency factor, it is assumed that the temperature gradient in the flow direction is negligible. Therefore, the tube-sheet configuration can be simulated as a classical fin problem by using energy balance while the two boundary conditions to solve the obtained second order differential equation are symmetry at the centerline and the known base temperature. By utilizing the concept of fin efficiency, the standard fin efficiency for straight fins can be expressed as:

F¼2

  tanh mðW
DÞ 2 mðW
DÞ

rffiffiffiffiffiffi UL where m ¼ kd

ð11Þ

In this step, the useful energy gained from sun to the working fluid in terms of the bond and the tube-to-fluid resistance can be achieved which enables us to calculate the collector efficiency factor as:

F0 ¼

1 UL   1 w UL ½Dþðw
DÞF þ C1 þ pD1h b i fi

ð12Þ

where Cb, w, D, Di, k and d are bond conductance, distance between the tubes, outside tube diameter, inside tube diameter, plate thermal conductivity and plate thickness, respectively. As a result, m = 6.097 (1/m), F = 0.996 and finally the collector efficiency factor will be 0.939. To define a quantity that relates the actual useful energy acquired from a collector to the useful gained if the whole collector surface is at the fluid inlet temperature, it is possible to have:

FR ¼


 _ p mC Ac U L F 0 1
exp
_ p mC Ac U L

ð13Þ

_ = 1/360 kg/s, Ac = 0.15 m2 and Cp = 4111.45 J/ By considering m kg K, heat removal factor will be 0.861. When an = 0.97 and sn = 0.86, the transmittance-absorbance product for beam radiation can be calculated in the following form:

ðsaÞn ¼ 1:01an sn ! ðsaÞb ¼ 0:99ðsaÞn ð9Þ

ð10Þ

ð14Þ

The transmittance-absorbance product for beam radiation will be 0.834.

10

A. Ahmadi et al. / Energy Conversion and Management 126 (2016) 1–11

gible. Eventually, theoretical thermal efficiency of the flat plate solar collector has been calculated and compared with experimental one and the results reveal that Gnp can effectively improve the performance of solar water heating systems.

References

Fig. 15. Comparison between the calculated theoretical and experimental efficiency for flat plate solar collector in 0.01 wt% Gnp/H2O.

Eventually, thermal efficiency of the flat plate solar collector can be obtained from the following formula:

gi ¼


 Qu Ti
Ta ¼ F R ðsaÞav e
F R U L Ac GT GT

ð15Þ

In Eq. (15), it is usual to omit the subscript ave and consider the phrase sa as a representative of beam radiation as:


 Ti
Ta þ 0:718 GT

gi ¼
12:327

ð16Þ

In this level, the obtained thermal efficiency of the flat plate solar collector with the experimental one for 0.01 wt% Gnp/H2O can be compared in Fig. 15 as follows: 5. Conclusion In the present paper, the effect of Graphene nanoplatelets on the performance of flat plate solar collectors has been investigated experimentally and theoretically. The ultrasound method has been chosen to disperse Gnps in deionized water and the results indicate that Gnps can increase thermal efficiency of solar collectors. The experiments show that the max UV–vis absorbance of the solution relates to the dispersion of Gnp in the base fluid. Also, the colloidal stability test in terms of different pHs has been done in order to prevent aggregation and sedimentation to obtain maximum performance. Adding 0.01 and 0.02 wt% Gnp can soar zero heat loss efficiency of the collector up to 12.19% and 18.87%, respectively. Also, the effect of different Gnp mass fractions on the thermal conductivity of working fluid has been investigated and it is concluded that in 0.02 wt%, approximately 13% increase has been observed in comparison with the base fluid. Moreover, adding Gnp to the deionized water can enhance the temperature and absorbed energy of the water heater remarkably. For instance, the outlet temperature of the water heater reaches 67.5 °C and 71 °C for 0.01 and 0.02 wt% Gnp, respectively which shows the system is suitable for household consumption. Since photovoltaic panels have been used for preparation of required electricity of pump (in order to use a complete green system with no pollution), the kinematic viscosity test has been done and the results demonstrate that the influence of Gnp on the frictional coefficient is very negli-

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