Thermal efficiency of a flat-plate solar collector filled with Pentaethylene Glycol-Treated Graphene Nanoplatelets: An experimental analysis

Solar Energy 191 (2019) 360–370

Contents lists available at ScienceDirect

Solar Energy journal homepage: www.elsevier.com/locate/solener

Thermal efficiency of a flat-plate solar collector filled with Pentaethylene Glycol-Treated Graphene Nanoplatelets: An experimental analysis

T

Omer A. Alawia, , Haslinda Mohamed Kamara, , A.R. Mallahb, S.N. Kazib, Nor Azwadi Che Sidikc ⁎

⁎

a

Department of Thermofluids, School of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor Bahru, Malaysia Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia c Department of Mechanical Precision Engineering, Malaysia-Japan International Institute of Technology (MJIIT), Universiti Teknologi Malaysia, Jalan Semarak, 54100 Kuala Lumpur, Malaysia b

ARTICLE INFO

ABSTRACT

Keywords: Graphene nanoplatelets Pentaethylene glycol Thermophysical properties Flat-plate solar collector Thermal performance

The effects of using aqueous nanofluids with the presence of Pentaethylene Glycol-Treated Graphene Nanoplatelets as the working fluids on the thermal performance of flat-plate solar collectors (FPSCs) were investigated experimentally. Water-based nanofluids were prepared with the concentrations of 0.025, 0.05, 0.075, and 0.1 wt%. Then the thermophysical properties were measured. Experimental setup and a MATLAB program were used to study the thermal performance of FPSCs-based nanofluids. The fluid inlet temperatures used in this study were 303, 313, and 323 K, and the flow rates were 0.00833, 0.01667, and 0.025 kg/s. Meanwhile, 500, 750 and 1000 W/m2 were set for the heat flux intensities. As the weight concentration was increased, thermal conductivity, dynamic viscosity, and density also improved, while specific heat decreased. The efficiency of FPSC increased as the flow rate and heat flux intensity were increased. However, the efficiency of FPSC decreased when the temperature of the fluid inlet was increased. Compared to water, the FPSC efficiency recorded an increase of up to 10.7%, 11.1%, and 13.3% for PEG-GNP nanofluids at different mass flow rates. Finally, the regression model was developed through MATLAB to predict the thermal efficiency coefficients of FPSC.

1. Introduction Flat plate solar collector is an example of the major common domestic and industrial applications of solar energy. FPSC absorbs solar energy. The energy is then converted to heat and transmitted via an appropriate base fluid including water, oil or ethylene glycol (Kong et al., 2015). The FPSC system consists of an aluminium or copper absorber plate. The absorber plate is painted with a perfectly selected coating for optimum solar energy absorption (Kabeel et al., 2016). Furthermore, the header and riser tubes are well welded to the surface of the absorber plate to ensure an easy heat transfer to the flowing fluid. The hot fluid from the collector to the heat exchanger heats the domestic water in an isolated storage tank for this collector model. In addition, to produce a greenhouse effect, the glass plate is used to cover the absorber. It also reduces heat loss. However, because of its low thermal efficiency, FPSC has weaknesses. The coefficient of convective heat transfer from the absorber to the base fluid is also low (Li et al., 2017). In this regard, the application of nanofluids as a preferable working fluid of traditional fluids has become recent trends in improving the

⁎

FPSC performance. Nanofluid is described as a base fluid colloidal mixture with nanoparticles of < 100 nm size. The concept of nanofluid was introduced by Choi and Eastman (Choi and Eastman, 1995). Nanofluids exhibit superior thermophysical properties compared to standard fluids. Therefore, the use of nanofluids makes heat transfer and heat absorption more efficient (Azmi et al., 2016). There are various forms of solid metal nanoparticles. These include nanoparticles such as aluminium (Al) (Zayed et al., 2019), copper (Cu) (He et al., 2015) and silver (Ag) (Tomy et al., 2016), whereas nanofluids such as aluminium oxide (Al2O3) (Sundar et al., 2018), copper oxide (CuO) (Sint et al., 2017), magnesium oxide (MgO) and iron oxide (Fe3O4) (Kianifar and Amini, 2016) were used as the working fluids in FPSCs (Suganthi and Rajan, 2017). Sedimentation problems decreased due to the relatively low density of solid metallic and metallic oxides nanomaterials when these nanofluids are used as the working fluids in FPSCs (Sharma and Gupta, 2016). Many structures of formed semiconductor oxides nanofluids have also been applied in FPSC as the working fluid. These include nanofluids of titanium oxide (TiO2) (Kiliç et al., 2018), trioxide of tungsten (WO3) (Sharafeldin et al., 2017), and cerium dioxide (CeO2)

Corresponding authors. E-mail addresses: [email protected] (O.A. Alawi), [email protected] (H. Mohamed Kamar).

https://doi.org/10.1016/j.solener.2019.09.011 Received 23 April 2019; Received in revised form 28 August 2019; Accepted 4 September 2019 Available online 10 September 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

Solar Energy 191 (2019) 360–370

O.A. Alawi, et al.

Nomenclature Ac Ag Al Al2O3 AlCl3 CeO2 Cp Cu CuO DMF FESEM Fe3O4 FPSC FR GNP GO GT HCl K m MgO

MWCNT PEG Qu RTD SiO2 SWCNT Ta TEM THF Ti TiO2 To UL WO3

Surface area of solar collector, m2 Silver Aluminum Aluminium oxide Aluminium chloride Cerium dioxide Specific heat capacity, kJ/kg K Copper Copper oxide Dimethylformamide Field Emission Scanning Electron Microscopy Iron oxide Flat plate solar collector Heat removal factor Graphene nanoplatelets Graphene oxide Global solar radiation, W/m2 Hydrochloric acid Thermal conductivity, W/m·K Mass flow rate, kg/s Magnesium oxide

Multi-Walled Carbon Nanotubes Pentaethylene Glycol Rate of useful energy gained, W Resistance Temperature Detectors Silicon dioxide Single-Wall Carbon Nanotubes Ambient temperature, K Transmission electron microscopy Tetrahydrofuran Inlet fluid temperature of solar collector, K Titanium oxide Outlet fluid temperature of solar collector, K Overall heat losses coefficient, W/m2 K Tungsten Trioxide

Greek symbol µ ηth ρ τα φ

Dynamic viscosity, mPa s Thermal efficiency of FPSC Density of fluid, kg/m3 Absorptance-transmittance product Weight concentration, wt%

To study the thermal efficiency of the solar collectors, hybrid nanofluid (a combination of two or more different nanomaterials) effects were examined. Verma et al. (2018) recently performed investigations to evaluate the energy performance of CuO/MWCNTs and MgO/ MWCNTs suspended in the base fluid to improve the effectiveness of FPSC. With different nanoparticles concentrations ranging from 0.25 to 2 wt%, different flow rates from 0.5 to 2 kg/min were applied. As a result, FPSC thermal performance increased by 20.5% for nanofluid hybrid MgO/MWCNTs and 18% for nanofluid hybrid CuO/MWCNT compared to water at 0.75 wt% and 1.5 kg/min for mass flow rates of nanofluid concentration. This study is an attempt to investigate the effects of using a nanofluid based on Pentaethylene Glycol-Treated Graphene Nanoplatelets as the heat transfer fluid on the thermal performance of flat-plate solar collectors (FPSC). Different concentrations of the nanofluid, inlet temperatures, flow rates, and heat flux intensities were experimentally tested. Based on the experimental results, a regression model was developed to predict the thermal performance of the FPSC.

(Sharafeldin and Gróf, 2018). Although SiO2 nanoparticles have low thermal conductivity associated with other nanoparticles, some investigators also reviewed the application of SiO2 nanofluids in FPSCs. Noghrehabadi et al. (2016) experimentally examined the laminar and turbulent regimes of a square flat plate collector with SiO2/water nanofluid. To conclude, with the use of 1 wt% SiO2/water nanofluid at 2.8 and 0.5 kg/min of mass flow rates, the thermal efficiency increased by 2.5% and 1% respectively. Recently, when carbon nanostructure nanofluids were applied as the working fluids for FPSC, substantial developments were achieved. These include single-wall carbon nanotubes (SWCNT) (Said et al., 2015), multi-wall carbon nanotubes (MWCNT) (Sadripour, 2017), graphene nanoplatelets (GNP) (Vakili et al., 2016), graphene oxide (GO) (Vincely and Natarajan, 2016) and graphene (Ahmadi et al., 2016). These significant advances increase thermal conductivity of carbon nanoparticles compared to other nanoparticles. In addition, density of carbon nanoparticles is lower than those of nanoparticles of metal and metal oxides (Hajjar et al., 2014).

Fig. 1. Schematic illustration of the covalent functionalization of hydroxylated GNPs. 361

Solar Energy 191 (2019) 360–370

O.A. Alawi, et al.

2. Experimentation and methodological approach

Table 1 Specifications of the flat plate solar collector and components of the experimental setup.

2.1. Materials and functionalization procedure Graphene nanoplatelets particles (GNPs) were obtained from Vira Carbon Nano Materials Co., Ltd. (VCN Materials) while Pentaethylene glycol (PEG), AlCl3, Hydrochloric acid (HCl), N,N-dimethylformamide (DMF), and tetrahydrofuran (THF) were purchased from Sigma-Aldrich (M) Sdn Bhd. For PEG, the purity was 90% with an average of 250 Mn and CAS number of (4792-15-8). Pristine graphene nanoplatelet particles (GNP) cannot be dispersed in polar solvents such as water because of its naturally hydrophobic characteristic. In the process to make PEG-GNPs hydrophilic, covalent functionalization by acid treatment was applied. The functionalization procedures introduced functional groups such as carboxyl and hydroxyl groups on the surface of GNPs. In the typical experiment (Alawi et al., 2019), the acid treatments were conducted as per Fig. 1. One gram of GNPs was dispersed in 18.54 g of AlCl3 and 10 mL of HCl, and then was radiated for 1 h with a microwave. Subsequently, the product was centrifuged at 11,500 rpm and passed by a 0.45 µm polycarbonate membrane. To separate the unreacted PEG and AlCl3, the product after filtration was washed by N, N-dimethylformamide (DMF), THF, diluted hydrochloric acid, and sufficient deionized water. Next, it was dried under vacuum at a temperature of 60 °C for 24 h.

Specifications

Dimension

FPSC

Length = 1.135 m Width = 0.6 m Thickness = 0.005 m Absorptance = 0.95 Material = Copper Collector occupied = 0.681 m2 Absorption = 0.464 m2 Do = 0.022 m Di = 0.0196 m Length = 0.6 m Material = Copper do = 0.0127 m di = 0.0105 m Length = 1.02 m Spacing = 0.128 m Material = Copper Thickness = 0.005 m Transmittance = 0.83 Emissivity = 0.88 30°

Area Header tube

Riser tube

Glass cover Tilt angle

2.3. Experimental setup Fig. 2 shows the schematic setup used in this investigation to study the nanofluid-based thermal efficiency of FPSC. The experimental system includes solar collector, flow loop, cooled water bath circulator, data logger, and control and measuring devices. An electrical centrifugal pump was used to circulate the working fluid in the forced convection system. Table 1 describes the details of the FPSC section used in this research. As shown in Fig. 3, the copper absorber plate was soldered directly to the copper riser tubes along the contact length. Ceramic fibre Isowool blankets with 0.07 W/m·K and 400 °C was used for thermal insulation. A flexible adhesive heater for constant heat flux intensity comparable to solar radiation was connected to a variable voltage transformer. Also, the self-adhesive T-type SA1XL-T-72 thermocouples from Omega, USA, were used in this work for measuring the surface temperatures of the absorber plate and the two riser tubes. At

2.2. Experimental procedure To discuss the thermal efficiency, data were collected from the experimental test runs and comprehensively presented in the graphics format. Distilled water and PEG-GNP aqueous colloidal dispersions with various weight concentration were employed as the working fluids. The weight concentrations used in this work were 0.025%, 0.05%, 0.075% and 0.1%. An experimental study was conducted to study the influences of fluid inlet temperature, heat flux intensity, and mass flow rate on the FPSC thermal performance. In this study, the fluid inlet temperatures applied were 303, 313 and 323 K. Meanwhile the heat flux intensities were 500, 750, 1000 W/m2, and the mass flow rates were 0.00833, 0.01667, and 0.025 kg/s.

Fig. 2. The schematic diagram of FPSC setup. 362

Solar Energy 191 (2019) 360–370

O.A. Alawi, et al.

Fig. 3. The components of flat plate solar collector.

Furthermore, in the FPSC inlet and outlet tubes, two calibrated temperature sensors (RTDs) of type PT100 were installed to record bulk working fluid temperatures. These thermocouples and RTDs were connected to the data logger (EC18, 18-channel Ecolog paperless recorder).

Start Input Type and specifications of fluid, flow rate, heat flux, inlet fluid temperature (Tin), and ambient temperature.

2.4. Mathematical model and MATLAB program In this experiment, the mathematical model for simulating the energy efficiency of an FPSC utilizing nanofluids as its heat transfer fluids was based on the Hottel and Whillier (HW) model introduced by Duffie and Beckman (Duffie and Beckman, 2013) with some modifications. The model was developed based on some assumptions to simplify the problem without changing the fundamental values. A MATLAB program was developed for solving the mathematical model and simulating the nanofluid-based FPSC. The flowchart for this MATLAB program is shown in Fig. 4. In the experimental setup of this study, the absorber plate of the FPSC was in contact with four riser tubes. Only one tube was considered in the mathematical model with the assumption that fluid flows uniformly through all the riser tubes of the collector working in a parallel channel arrangement.

Calculations of Optical Losses Calculations of Top Heat Loss

Calculations of Back Heat Loss

Calculations of Edge Heat Loss

No

Calculations of Overall Heat Loss

Read Mean Absorber Plate Temperature (AP)

2.5. Testing methods and uncertainty analysis The following equation is used to calculate the useful energy Qu (Hawwash et al., 2018):

Qu = mCp (To

(1)

Ti )

where Cp represents the specific heat capacity of the nanofluid. Another indication for rate of useful energy gained by the difference between the collector's absorbed energy and the collector's heat losses is as follows (Hawwash et al., 2018):

Comparison Values of AP

Qu = FR AC [GT (

Yes

)

UL (Ti

Ta )]

(2)

where UL is the overall loss coefficient of solar collector, and FR is the heat removal factor. Surface area of solar collector is AC while global solar radiation is represented by GT. The product of absorptancetransmittance is τα and Ta represents the ambient temperature. The thermal performance of solar collectors (ηth) is commonly described in terms of the Hottel–Whillier–Bliss equation (Hawwash et al., 2018):

Display Collector Efficiency

End Fig. 4. MATLAB program flowchart.

four different axial positions from the edge of the absorber plate, 12 Ttype thermocouples were installed on the back side of the absorber plate. These thermocouples were calibrated before installation.

mCp (To Ti ) Qu = AC GT AC GT

th

=

th

= FR (

)

FR UL

Ti

Ta GT

(3) (4)

Uncertainty analysis is an essential part of experimental studies 363

Solar Energy 191 (2019) 360–370

O.A. Alawi, et al.

Fig. 5. (a) FE-SEM of PEG-GNP flakes with different diameters; (b) TEM microscopy of PEG-GNP at different magnifications (Alawi et al., 2019). 90

303

313

323

Concentration of nanoparticles (%)

Thermal conductivity (W/m K)

Viscosity (mPa s)

DW 0.025 wt% 0.05 wt% 0.075 wt% 0.1 wt% DW 0.025 wt% 0.05 wt% 0.075 wt% 0.1 wt% DW 0.025 wt% 0.05 wt% 0.075 wt% 0.1 wt%

0.6103 0.6893 0.7123 0.7277 0.7456 0.6304 0.7263 0.7623 0.7777 0.7986 0.6434 0.7633 0.8123 0.8277 0.8516

0.85384 0.92736 0.98264 1.03142 1.08345 0.65481 0.77397 0.84226 0.89105 0.95934 0.54821 0.68563 0.73115 0.78318 0.84497

Density (kg/m3) 996.51 996.98 997.17 997.42 997.84 992.23 992.41 992.70 993.26 993.75 988.06 988.53 988.72 988.97 989.39

Specific heat (kJ/ kg K) 4.1809 4.0694 4.0667 4.0665 4.0644 4.1796 4.0653 4.0642 4.0645 4.0617 4.1815 4.0634 4.0621 4.062 4.0594

50

0

0.005

0.01

0.02

0.025

0.03

0.035

0.04

DW 0.025% PEG-GNP 0.05% PEG-GNP 0.075% PEG-GNP 0.1% PEG-GNP

80 70 60 50 40

60

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

[(Tin-Ta)/GT] (m2K/W)

55

(b)

90

50 45 40

0.015

(a)

0

Thermal Efficiency (%)

Thermal Efficiency (%)

60

90

0.00833 kg/s 0.01667 kg/s 0.025 kg/s

65

70

[(Tin-Ta)/GT] (m2K/W)

75 70

DW 0.025% PEG-GNP 0.05% PEG-GNP 0.075% PEG-GNP 0.1% PEG-GNP

80

40

Thermal Efficiency (%)

Temp. (K)

Thermal Efficiency (%)

Table 2 Thermophysical properties (dynamic viscosity, thermal conductivity, density, and specific heat capacity) of PEG-GNPs at various mass fractions (Alawi et al., 2019).

0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

[(Tin-Ta)/GT] (m2K/W) Fig. 6. The experimental values of FPSC’s efficiency versus reduced temperature parameter at different mass flow rates during water run.

70 60 50 40

Table 3 Heat absorbed and heat removal factors at a different flow rate for distilled water. Mass flow rate (kg/s)

FR (τα)

FRUL

R2

0.00833 0.01667 0.025

0.671 0.693 0.702

11.518 11.16 10.187

0.989 0.9966 0.992

DW 0.025% PEG-GNP 0.05% PEG-GNP 0.075% PEG-GNP 0.1% PEG-GNP

80

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

[(Tin-Ta)/GT] (m2K/W)

(c) Fig. 7. Experimentally calculated values of collector’s efficiency for water and water-based PEG-GNPs nanofluids with different weight concentrations; (a) 0.00833 kg/s, (b) 0.01667 kg/s, (c) 0.025 kg/s.

364

Solar Energy 191 (2019) 360–370

O.A. Alawi, et al.

90

0.00833 kg/s 0.01667 kg/s 0.025 kg/s

80

Thermal Efficiency (%)

Thermal Efficiency (%)

90

70 60 50 40

0

70 60 50 40

0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

0.00833 kg/s 0.01667 kg/s 0.025 kg/s

80

0

0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

[(Tin-Ta)/GT] (m2K/W)

[(Tin-Ta)/GT] (m2K/W)

(b)

(a) 90 0.00833 kg/s 0.01667 kg/s 0.025 kg/s

80

Thermal Efficiency (%)

Thermal Efficiency (%)

90

70 60 50 40

70 60 50 40

0

0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

[(Tin-Ta)/GT]

0.00833 kg/s 0.01667 kg/s 0.025 kg/s

80

0

0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

[(Tin-Ta)/GT] (m2K/W)

(m2K/W)

(d)

(c)

Fig. 8. Thermal efficiency of FPSC-based nanofluids with different mass flow rates; (a) 0.025 wt%, (b) 0.05 wt%, (c) 0.075 wt%, (d) 0.1 wt%.

since it reveals the accuracy of measurements. Based on Eq. (5), the uncertainty in efficiency depends on mass flow rate, heat capacity, inlet and the outlet temperature of working fluid, the surface area of the solar collector, and solar radiation. Therefore, the uncertainty in the efficiency of the solar collector can be determined by (Ahmadi et al., 2016): th

th

=

m

m

2

+

Cp

Cp

2

2

+

GT

GT

+

Ac

Ac

2

+

(To Ti )

(To

Ti )

2

transmission electron microscope (TEM, HT 7700, Hitachi). 3. Results and discussions 3.1. Characterization and thermophysical properties The FE-SEM image for the PEG-GNPs is shown in Fig. 5(a). Obviously, the image shows many GNP flakes with multiple diameters. It was evident that the samples lack impurities. Most of the flakes were also transparent vis-à-vis the electron beam, which confirms its limited layers. Although FE-SEM images could not measure the precise thickness of flakes and defects, the planar morphology of GNP layers could be clearly seen from this image. The TEM image for the PEG-GNPs is shown in Fig. 5(b). The image shows multi-layered graphene flakes measuring ~2 µm. The flakes lacked smooth layer surface and edge. Although the TEM images of the PEG-GNP did not allow us to tell the functional groups apart, the surface degradation and wrinkles of the GNP due to covalent functionalization with PEG were evident. The incidence of many lines and wrinkles within the PEG-GNPs flakes was due to the intrinsic instability of two-dimensional (2D) structures and enhanced flexibility of the GNP flakes post-treatment. Also, the tendency to wrinkle implies increased wettability of the GNP’s surface, attributed to covalent functionalization with PEG. The easily-miscible PEG functionalities could be responsible for the increased wettability of PEG-GNP, which resulted in higher dispersion stability of the nanofluids (Alawi et al., 2019). Table 2 presents the thermophysical properties measurements of distilled water and PEG-GNP nanofluids (Alawi et al., 2019). Dynamic viscosity, thermal conductivity, density, and specific heat were

0.5

(5)

Therefore, after the calculation process, the maximum uncertainty in the efficiency becomes about 3.365%. Nanofluids were produced when a specific weight of PEG-GNP nanoparticles were dispersed in the base fluid, as explained in the preceding paragraph (Alawi et al., 2019). The thermal conductivity of nano coolants was estimated using KD-2 pro device from Decagon, USA. The KS-1 sensor utilized was 6 cm long and 1.3 mm in diameter. For each temperature and weight concentration, an average of 16 measurements was recorded to assure nanofluid stability. Also, a Physica MCR rheometer (Anton-Paar, Austria) was used to measure the viscosity of distilled water and PEG-GNP nanofluids. A density meter of (Mettler Toledo, DE-40, USA) with ± 10−4 g/cm3 accuracy was used to measure the densities of base fluid and nanofluids. For each samples and temperatures, the values were reported three times. The specific heat capacity of base and nanofluids with ± 1.0% accuracy was assessed using a differential scanning calorimeter (DSC 8000, Perkin Elmer, USA) with ± 1.0% accuracy. Morphological characteristics of the functionalized synthesized powders were conducted by field emission scanning electron microscopy (FE-SEM, SU8000, Hitachi) and 365

Solar Energy 191 (2019) 360–370

H2O H2O Rajput et al. (2017) Current study

Absorbed energy parameter

0.8 0.76 0.72 0.1 wt% 0.075 wt% 0.05 wt%

0.68 0.64 0.6 0.00833

0.025 wt% 0.01667

Mass flow rate (kg/s)

DW 0.025

Removed energy parameter

(a)

/ 0.464 m2 Triton X-100 / 0.1–0.3 vol% 0.025–0.1 wt%

0.375 m2 Triton 100-X

20, 7, 42, 45, 44, 10 / / H2O

Gr, MWCNTs, CuO, Al2O3, TiO2, SiO2 MWCNT PEG-GNP

0.25–2 vol%

L = 0.51 m, t = 0.001 m / / / 10–12 10–12 H2O H2O

Chougule et al. (2014) Chougule and Sahu (2015) Verma et al. (2017)

MWCNT MWCNT

0.15–1 vol% /

0.47 m × 0.27 m × 0.001 m 1.51 m2 1.51 m2 / Triton X-100 Triton X-100 0.01–0.2 wt% 0.2 wt% 0.2& 0.4 wt% H2O H2O H2O Ahmadi et al. (2016) Yousefi et al. (2012) Yousefi et al. (2012)

GNP MWCNT MWCNT

/ 10–30 10–30

SDS 0.1&0.3 vol% 1–2 SWCNT H2O Said et al. (2015)

Type

Nanoparticles Basefluid Ref.

Table 4 Experimental studies on the use of nanofluids in FPSCs.

Size (nm)

Concentration

Surfactant

1.84 m2

Solar collector

Remarks

Energy efficiency improved by 95.12% for 0.3% volume fraction and 0.5 kg/ min. Thermal efficiency of the solar collector up to 18.87%. At high temperature differences, FRUL parameter was more dominant. The efficiency of collector for 0.4 wt% MWCNT nanofluid was higher than that for water. Maximum instantaneous efficiency was found to be 73% for 0.60 vol%. The effects of various filling ratio (50%, 60%, and 70%) were discussed in this study. Maximum exergetic and energetic efficiency of MWCNTs were 29.32% and 23.47%. A maximum 30.58% growth in collector efficiency. The maximum efficiency increase was approximately 13.3% at 0.025 kg/s.

O.A. Alawi, et al.

13 12 11 0.1 wt% 0.075 wt%

10 9

0.05 wt%

8 0.00833

0.025 wt% 0.01667

Mass flow rate (kg/s)

DW 0.025

(b) Fig. 9. (a) The absorbed energy parameter FR (τα); (b) the removed energy parameter (FRUL) for distilled water and nanofluids as a function of mass flow rate. Table 5 Heat absorbed and heat removal factors at a different flow rate for PEG-GNPs. Fluid

Mass flow rate (kg/s)

FR (τα)

FRUL

R2

DW

0.00833 0.01667 0.025 0.00833 0.01667 0.025 0.00833 0.01667 0.025 0.00833 0.01667 0.025 0.00833 0.01667 0.025

0.671 0.693 0.702 0.718 0.741 0.760 0.733 0.756 0.776 0.744 0.768 0.788 0.747 0.771 0.792

11.518 11.16 10.187 11.82 11.163 10.333 11.831 11.168 10.816 12.211 11.456 10.821 12.532 11.462 10.839

0.989 0.9966 0.992 0.9919 0.9919 0.9919 0.9944 0.9944 0.9944 0.9915 0.9915 0.9915 0.9954 0.9954 0.9954

0.025 wt.% 0.05 wt.% 0.075 wt.% 0.1 wt.%

measured at various temperatures of 303, 313 and 323 K. When the weight concentration of PEG-GNP was increased, thermal conductivity, dynamic viscosity, and density increased, and specific heat capacity decreased. Compared to water, the highest increase observed in thermal conductivity was 32.4% at 323 K. Meanwhile, for the viscosity of 0.1 wt %, an increase of up to 26.9% was reported. The maximum enhancement for the measured density values of 0.135% was observed. However, at 0.1 wt%, the nanofluids specific heat dropped to 3%.

366

Solar Energy 191 (2019) 360–370

O.A. Alawi, et al.

Fig. 10. Thermal efficiency contours versus reduced temperature parameter at different mass flow rates; (a) 0.00833 kg/s, (b) 0.01667 kg/s, (c) 0.025 kg/s.

3.2. Thermal performance during water run

rates of 0.00833, 0.01667 and 0.025 kg/s of distilled water and PEGGNP nanofluids at various weight concentrations. Fig. 7(a-c) indicates that the thermal performance improved by adding any amount of PEGGNP in the distilled water. Compared to base fluid, with the presence of PEG-GNP nanomaterials, FPSC efficiency increased higher. The increase was 10.7%, 11.1%, and 13.3% for flow rates of 0.00833, 0.01667, and 0.025 kg/s, respectively. Fig. 8(a-d) shows the experimental values of the collector thermal efficiency for water and water-based PEG-GNP nanofluids at different mass flow rates of 0.00833, 0.01667 and 0.025 kg/s against the reduced temperature parameter for different PEG-GNP weight concentrations (0.025, 0.05, 0.075 and 0.1 wt%). Solar collector thermal efficiency increased when the mass flow rate was increased from 0.00833 to 0.025 kg/s at each concentration. The maximum increase in solar collector thermal performance was 13.3% at a weight concentration of 0.1 wt% and the flow rate was maintained at 0.025 kg/s. The results are compatible with the work of Vakili et al. (2016). As can be seen, more weight fraction leads to more absorption of input energy and thus, more enhancement of the efficiency. For low weight fraction, the heat was absorbed uniformly through the fluid layer. Therefore, the amount of heat loss at the boundaries is lower than that for high weight fraction, that most of the heat absorption occurred in the top layer of the fluid. This caused a high-temperature region near the top wall, which enhances the heat losses and thus, reduces the collector efficiency. Table 4 shows a brief comparison of the current results with the most important previous experimental studies using

Initially, the test runs for the FPSC were carried out using distilled water as the working fluid to examine the validity, repeatability, and accuracy of the recorded data. Fig. 6 shows the calculated values for water-based FPSC thermal efficiency in comparison with the reduced temperature parameter [(Tin−Ta)/GT] at different mass flow rates. As the mass flow rate was increased, the thermal efficiency for 0.01667 and 0.025 kg/s increased by about 2.75% and 3.44%. Therefore, it was clear that the increased flow rate improved the collector efficiency, thereby lowering the flat plate temperature as shown in Fig. 6. This contributed to a lower heat loss from the collector, thus, improving the collector efficiency. The experimental data is best fitted with linear relations to provide the performance characteristic parameters of the collector for different flow rates; these parameters, FR (τα) and FRUL, are presented in Table 3 for each flow rate. Table 3 indicates that FR (τα) value of the collector for 0.025 kg/s was highest, and the FRUL value for this mass flow rate was lowest. Therefore, based on Eq. (4) the efficiency of the solar collector for this mass flow rate was highest. 3.3. Thermal performance using Pentaethylene Glycol-Treated Graphene Nanoplatelets The experimental values of the thermal efficiency were presented in Fig. 7(a-c) versus the reduced temperature parameter for mass flow 367

Solar Energy 191 (2019) 360–370

O.A. Alawi, et al.

Fig. 11. Plots the surface of the thermal efficiency versus reduced temperature parameter and weight concentrations at different mass flow rates; (a) 0.00833 kg/s, (b) 0.01667 kg/s, (c) 0.025 kg/s.

carbon based nanofluids. The main reason of enhancing the efficiency of flat plate solar collector with adding PEG-GNP to the base fluid can be interpreted as follows: The heat capacity of nanofluids (Cpnf ) which can be measured is a little smaller than (Cpbf ) but the created temperature difference by nanofluid (To Ti ) is much higher than the base fluid. Then, the product of the above two items is resulted a significant increase in experimental thermal efficiency (Ahmadi et al., 2016; Akram et al., 2019). Fig. 9(a-b) and Table 5 show the heat absorbed parameter (FR [τα]), and the heat removed parameter (FRUL) for water-based nanofluids. At the mass flow rate of 0.00833 kg/s and concentration of 0.025 to 0.1 wt %, the value of FR (τα) improved by 6.94%, 9.2%, 10.82%, and 11.38%, respectively in comparison with the base fluid data. The heat absorption factors increased for the mass flow rate of 0.01667 kg/s at the

weight concentration of 0.025, 0.05, 0.075, and 0.1 wt% by 6.84%, 9.09%, 10.71, 11.27%, respectively. Meanwhile, the FR (τα) enhancement by 8.29%, 10.57%, 12.21% and 12.79% at the flow rate of 0.025 kg/s at different concentrations of 0.025, 0.05, 0.075 and 0.1 wt %, respectively. At lower mass flow rate, the corresponding value of FRUL increased by 2.62%, 2.72%, 6.02%, and 8.80% at the mass flow rate of 0.00833 kg/s and concentration of 0.025 to 0.1 wt%. Meanwhile, at higher mass flow rate, Table 5 shows the FRUL increments by 1.43%, 6.17%, 6.22% and 6.40% at the flow rate of 0.025 kg/s at different concentrations of 0.025, 0.05, 0.075 and 0.1 wt%, respectively. 3.4. Performance optimization of flat plate solar collector using MATLAB A MATLAB code was developed based on the experimental data to 368

Solar Energy 191 (2019) 360–370

O.A. Alawi, et al.

Fig. 12. Plots the Residuals of the thermal efficiency versus reduced temperature parameter and weight concentrations at different mass flow rates; (a) 0.00833 kg/s, (b) 0.01667 kg/s, (c) 0.025 kg/s. Table 6 Coefficients (with 95% confidence bounds) of Eq. (6) for different mass flow rates. Coefficient

a b c m w SSE R-square Adjusted R-square RMSE

Mass flow rate (kg/s) 0.00833

0.01667

0.025

42.38 (38.96, 45.79) −7.018 (-8.642, −5.394) 25.94 (22.71, 29.17) −222.1 (−267, −177.3) 42.56 (36.88, 48.24) 94.14 0.9526 0.9479 1.534

43.54 (39.86, 47.22) 7.19 (5.491, 8.89) 26.89 (23.41, 30.37) 221.5 (175.5, 267.4) 42.04 (36.25, 47.82) 102.8 0.9511 0.9462 1.603

44.38 (40.25, 48.51) −7.283 (-9.103, −5.464) 27.77 (23.86, 31.68) −220.6 (−269.3, −171.9) 41.26 (35.17, 47.34) 117.3 0.9465 0.9412 1.713

369

Solar Energy 191 (2019) 360–370

O.A. Alawi, et al.

analyze the thermal performance optimization of flat plate solar collector. Thermal efficiency contours versus reduced temperature parameter [(Tin-Ta)/GT] at different mass flow rates is shown in Fig. 10. Meanwhile, Figs. 11 and 12 present surface fitting and residual plots. For the prediction of the FPSC thermal performance under certain circumstances, the general model was developed with 95% confidence limits. Eq. (6) was developed as a general model for the thermal efficiency while Table 6 lists the coefficients.

Alawi, O.A., Sidik, N.A.C., Kazi, S.N., Najafi, G., 2019. Graphene nanoplatelets and fewlayer graphene studies in thermo-physical properties and particle characterization. J. Therm. Anal. Calorim. 135 (2), 1081–1093. Azmi, W.H., Sharma, K.V., Mamat, R., Najafi, G., Mohamad, M.S., 2016. The enhancement of effective thermal conductivity and effective dynamic viscosity of nanofluids A review. Renew. Sustain Energy Rev. Choi, S.U.S., Eastman, J.A., 1995. Enhancing thermal conductivity of fluids with nanoparticles. ASME Int. Mech. Eng. Congr. Expo. 66, 99–105. Chougule, S.S., Sahu, S.K., 2015. Performance of carbon nanotubes-water nanofluid charged wickless heat pipe flat plate solar collectors having different filling ratio. J. Sol. Energy Eng. 137, 24501. Chougule, S.S., Sahu, S.K., Pise, A.T., 2014. Thermal performance of two phase thermosyphon flat-plate solar collectors using nanofluid. J. Sol. Energy Eng. 136, 14503. Duffie, J.A., Beckman, W.A., 2013. Solar Engineering of Thermal Processes. John Wiley & Sons. Hajjar, Z., Morad Rashidi, A., Ghozatloo, A., 2014. Enhanced thermal conductivities of graphene oxide nanofluids. Int. Commun. Heat Mass Transf. 57, 128–131. Hawwash, A.A., Abdel Rahman, A.K., Nada, S.A., Ookawara, S., 2018. Numerical investigation and experimental verification of performance enhancement of flat plate solar collector using nanofluids. Appl. Therm. Eng. 130, 363–374. He, Q., Zeng, S., Wang, S., 2015. Experimental investigation on the efficiency of flat-plate solar collectors with nanofluids. Appl. Therm. Eng. 88, 165–171. Kabeel, A.E., Khalil, A., Shalaby, S.M., Zayed, M.E., 2016. Investigation of the thermal performances of flat, finned, and v-corrugated plate solar air heaters. J. Sol. Energy Eng. 138, 51004. Kianifar, A., Amini, M., 2016. An analytical study on energy and exergy of a minichannelbased solar collector using Fe3O4 and MgO/water nanofluids. In: International Conference on researches in Science and Engineering 28 July 2016, Istanbul University, Turkey. Kiliç, F., Menlik, T., Sözen, A., 2018. Effect of titanium dioxide/water nanofluid use on thermal performance of the flat plate solar collector. Sol. Energy. Kong, W., Perers, B., Fan, J., Furbo, S., Bava, F., 2015. A new Laplace transformation method for dynamic testing of solar collectors. Renew. Energy 75, 448–458. Li, S., Wang, H., Meng, X., Wei, X., 2017. Comparative study on the performance of a new solar air collector with different surface shapes. Appl. Therm. Eng. 114, 639–644. Noghrehabadi, A., Hajidavaloo, E., Moravej, M., 2016. Experimental investigation of efficiency of square flat-plate solar collector using SiO2/water nanofluid. Case Stud. Therm. Eng. 8, 378–386. Rajput, N.S., Shukla, D.D.B., Sharma, S.K., 2017. Thermal analysis of MWCNT/distilled water nanofluid on the efficiency of flat plate solar collector. Int. J. Mech. Eng. Technol. 8, 233–240. Sadripour, S., 2017. First and second laws analysis and optimization of a solar absorber; using insulator mixers and MWCNTs nanoparticles. Glob. J. Res. Eng. 17 (5), 36–48. Said, Z., Saidur, R., Sabiha, M.A., Rahim, N.A., Anisur, M.R., 2015. Thermophysical properties of Single Wall Carbon Nanotubes and its effect on exergy efficiency of a flat plate solar collector. Sol. Energy 115, 757–769. Sharafeldin, M.A., Gróf, G., 2018. Experimental investigation of flat plate solar collector using CeO2-water nanofluid. Energy Convers. Manag. 155, 32–41. Sharafeldin, M.A., Gróf, G., Mahian, O., 2017. Experimental study on the performance of a flat-plate collector using WO3/Water nanofluids. Energy 141, 2436–2444. Sharma, S.K., Gupta, S.M., 2016. Preparation and evaluation of stable nanofluids for heat transfer application: a review. Exp. Therm. Fluid Sci. 79, 202–212. Sint, N.K.C., Choudhury, I.A., Masjuki, H.H., Aoyama, H., 2017. Theoretical analysis to determine the efficiency of a CuO-water nanofluid based-flat plate solar collector for domestic solar water heating system in Myanmar. Sol. Energy 155, 608–619. Suganthi, K.S., Rajan, K.S., 2017. Metal oxide nanofluids: Review of formulation, thermophysical properties, mechanisms, and heat transfer performance. Renew. Sustain. Energy Rev. 76, 226–255. Sundar, L.S., Singh, M.K., Punnaiah, V., Sousa, A.C.M., 2018. Experimental investigation of Al2O3/water nanofluids on the effectiveness of solar flat-plate collectors with and without twisted tape inserts. Renew. Energy 119, 820–833. Tomy, A.M., Ahammed, N., Subathra, M.S.P., Asirvatham, L.G., 2016. Analysing the performance of a flat plate solar collector with silver/water nanofluid using artificial neural network. Procedia Comput. Sci. 93, 33–40. Vakili, M., Hosseinalipour, S.M., Delfani, S., Khosrojerdi, S., Karami, M., 2016. Experimental investigation of graphene nanoplatelets nanofluid-based volumetric solar collector for domestic hot water systems. Sol. Energy 131, 119–130. Verma, S.K., Tiwari, A.K., Chauhan, D.S., 2017. Experimental evaluation of flat plate solar collector using nanofluids. Energy Convers. Manag. 134, 103–115. Verma, S.K., Tiwari, A.K., Tiwari, S., Chauhan, D.S., 2018. Performance analysis of hybrid nanofluids in flat plate solar collector as an advanced working fluid. Sol. Energy 167, 231–241. Vincely, D.A., Natarajan, E., 2016. Experimental investigation of the solar FPC performance using graphene oxide nanofluid under forced circulation. Energy Convers. Manag. 117, 1–11. Yousefi, T., Shojaeizadeh, E., Veysi, F., Zinadini, S., 2012. An experimental investigation on the effect of pH variation of MWCNT-H2O nanofluid on the efficiency of a flatplate solar collector. Sol. Energy 86, 771–779. Yousefi, T., Veisy, F., Shojaeizadeh, E., Zinadini, S., 2012. An experimental investigation on the effect of MWCNT-H2O nanofluid on the efficiency of flat-plate solar collectors. Exp. Therm. Fluid Sci. 39, 207–212. Zayed, M.E., Zhao, J., Du, Y., Kabeel, A.E., Shalaby, S.M., 2019. Factors affecting the thermal performance of the flat plate solar collector using nanofluids: A review. Sol. Energy 182, 382–396.

Regression

= a + b × sin m

[

(Tin Ta) ] + c × exp[ GT

2

w

[

(Tin Ta ) ] ] GT (6)

4. Conclusions The objective of this research was to experimentally investigate the effects on FPSC thermal performance as a result of the usage of aqueous colloidal dispersions of carbon-based nanostructures as alternative working fluids at various weight concentrations. The conclusions that can be achieved based on the findings and discussion presented in this study are as follows: 1- When the weight concentration of PEG-GNP in the base fluid was increased, the thermal conductivity, viscosity and density also increased while the specific heat decreased. Compared to water, the highest increment in thermal conductivity observed was 32.4% at 323 K. Meanwhile, enhancement of up to 26.9% at 303 K was reported for the viscosity of 0.1% weight concentration of water-based PEG-GNP. A maximum enhancement of 0.135% was observed for the measured densities. However, the specific heat nanofluids dropped to 3% at 0.1% weight concentration of water-based PEGGNP. 2- The energy performance enhanced as the mass flow rate and heat flux intensity raised, and the inlet fluid temperatures decreased. The measured efficiency increased to 10.7%, 11.1%, and 13.3% for 0.1 wt% PEG-GNP at various mass flow rates against water as the mass concentration increased. 3- The maximum increase in the FRUL and FR(τα) was 8.80% and 12.79% for the nanofluid weight fraction of 0.1 wt% at the mass flow rate of 0.00833 and 0.025 kg/s, respectively . 4- For the prediction of general model coefficients of the collector's efficiency, a MATLAB code was developed. The customizing contours, surface fitting, and thermal efficiency residuals relative to the reduced temperature parameter were presented at different mass flow rates and weight concentrations. Acknowledgments The authors are grateful to the Universiti Teknologi Malaysia for providing the funding for this study, under the Research University Grant vote number 04E76, managed by the Research Management Centre (RMC), Universiti Teknologi Malaysia. References Ahmadi, A., Ganji, D.D., Jafarkazemi, F., 2016. Analysis of utilizing Graphene nanoplatelets to enhance thermal performance of flat plate solar collectors. Energy Convers. Manag. 126, 1–11. Akram, N., Sadri, R., Kazi, S.N., Ahmed, S.M., Zubir, M.N.M., Ridha, M., et al., 2019. An experimental investigation on the performance of a flat-plate solar collector using eco-friendly treated graphene nanoplatelets–water nanofluids. J. Therm. Anal. Calorim. 1–13. Alawi, O.A., Mallah, A.R., Kazi, S.N., Sidik, N.A.C., Najafi, G., 2019. Thermophysical properties and stability of carbon nanostructures and metallic oxides nanofluids: Experimental approach. J. Therm. Anal. Calorim. 135 (2), 1545–1562.

370