Performance analysis of a copper sheet laminated photovoltaic thermal collector using copper oxide – water nanofluid

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ScienceDirect Solar Energy 119 (2015) 439–451 www.elsevier.com/locate/solener

Performance analysis of a copper sheet laminated photovoltaic thermal collector using copper oxide – water nanofluid Jee Joe Michael ⇑, S. Iniyan Institute for Energy Studies, Department of Mechanical Engineering, Anna University, Chennai 600 025, India Received 1 February 2015; received in revised form 3 May 2015; accepted 20 June 2015

Communicated by: Associate Editor G.N. Tiwari

Abstract Solar photovoltaic module and solar water heater are of immense benefits to the common man in terms of independent energy solutions and conventional fuel savings. However, due to the inherent drawback of lower efficiencies per unit area, these technologies are facing some reluctance from domestic consumers for widespread acceptance. A combination of the above two technologies is called the solar photovoltaic thermal hybrid technology which produces electrical and thermal output from the same unit area. But, the lower individual efficiencies of the PV/T collector compared to their individual technologies due to the low solar energy absorption and high thermal resistance between the PV cell and the cooling medium hinders the potential advantages of this hybrid technology. In this paper, a novel photovoltaic thermal collector was constructed by laminating a copper sheet directly to the silicon cell, thereby reducing the thermal resistance and its performance was improved by using copper oxide – water (CuO/H2O) nanofluid. From the experimental setup, it was observed that the nanofluid made a significant improvement in the thermal performance compared to water. Further it can be suggested that the use of a heat exchanger with higher effectiveness can improve the electrical performance as well. The in-house synthesis of CuO nanoparticle, CuO/water nanofluid preparation and nano-characterization tests were also presented. Ó 2015 Elsevier Ltd. All rights reserved.

Keywords: Photovoltaic thermal; CuO; Nanofluid; Nano-characterization

1. Introduction Solar heaters and solar PV modules have been accepted worldwide for their decentralized renewable energy solutions with minimal maintenance cost and long lifetime. For example, a separate solar collector for drying of medicinal herbs and another solar PV module to provide the power to operate the air blower is used to produce high quality dry products in Egypt (Fargali et al., 2008). These separate installations causes low effective utilization of the limited roof-space of residential and commercial ⇑ Corresponding author. Tel.: +91 9444338954.

E-mail address: [email protected] (J.J. Michael). http://dx.doi.org/10.1016/j.solener.2015.06.028 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.

buildings. Solar PV/T technology reduces this drawback by producing electrical and thermal energies from a single unit by circulating air through a small gap (Assoa and Menezo, 2014) and using it for space heating (Kim et al., 2014) or passing water underneath the PV module thereby removing the heat and using it for water heating (Dubey and Tiwari, 2008). The use of micro-channel instead of single channel underneath the PV/T module to send air has produced higher thermal and exergy improvements of 70.62% and 60.19% respectively (Agrawal and Tiwari, 2011). The need for cooling a PV cell for improved electrical performance in a low concentration solar collector (Chaabane et al., 2013) and the best cooling method to reduce the temperature based on the overall heat loss

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coefficient was analyzed using FEM software (Tamayo Vera et al., 2014). The performance optimization based on the energy balance (Rejeb et al., 2015) and the environmental performance of a PV/T collector were tested for different types of PV modules, including thin film technology (Aste et al., 2015) and found that c-Si PV module is the best for the production of higher electrical power output (Mishra and Tiwari, 2013a). Partially covered PV/T collector with PV module produced higher thermal efficiency (Dubey and Tiwari, 2008), compared to fully covered, but less than a flat plate collector (Mishra and Tiwari, 2013b). The low thermal conductivity of common heat transfer fluids like water or air which are primarily used for the removal of heat from the PV modules, propelled scientists, engineers and researchers to use nanofluids. Significant thermal performance improvement was observed in solar thermal collectors due to the use of different types of nanofluids. The widely used Al2O3/water nanofluid of 0.2% weight fraction with Triton X-100 as the surfactant produced a 28.3% increase in thermal efficiency in a flat plate solar water heater (Yousefi et al., 2012a). Similarly, Al2O3/water nanofluid was theoretically analyzed for different nano-particle sizes, volume fractions, mass flow rates (Mahian et al., 2014) and FEM analysis (Nasrin et al., 2013) in a solar collector. The addition of 4% CuO/water nanofluid improves its thermal conductivity by 20% (Amrut Lanje et al., 2010), and a 21.8% enhancement in collector efficiency was observed for 0.4% volume fraction between 1 and 3 kg/min flow rates (Moghadam et al., 2014). ## A substantial performance improvement was observed using MWCNT/water nanofluid of 0.2% weight fraction and Triton X-100 as the surfactant (Yousefi et al., 2012b), a 15.33% increase in heat transfer coefficient was observed using SWCNT/water nanofluid (Said et al., 2014). A 76.6% thermal improvement was obtained for 0.1% volume concentration TiO2/water nanofluid with PEG400 dispersant at 0.5 kg/min mass flow rate (Said et al., 2015). In a direct absorption solar collector, the performance of Cu/water nanofluid was tested using FEM analysis for various volume fractions and Reynolds number (Parvin et al., 2014), the effect of aluminum/water nanofluid was tested for various particle sizes, volume fractions and the extinction coefficients (Saidur et al., 2012) and a 50% absorption of incident solar radiation was observed for 0.000025% volume concentration graphite nanoparticles (Ladjevardi et al., 2013). The effect of different tilt angles and volume fractions using FEM analysis (Rahman et al., 2014), and the effect of pH with respect to pH of isoelectric point (Yousefi et al., 2012c) were analyzed for maximum heat transfer. The technical, economic and environmental impact of a solar collector were estimated for different nanofluids (Faizal et al., 2013) and different solar collectors (Mahian et al., 2013) and concluded that using nanofluid has a lower embodied energy and pollution reduction of approximately 9% and 3% respectively (Todd Otanicar and Jay Golden, 2009).

The solar PV/T technology has an inherent drawback of producing lower efficiencies compared to their individual units due to lower absorption coefficient and higher thermal resistance. The improved thermal performance due to the use of nanofluids in solar thermal collectors encouraged researchers to introduce these nanofluids in solar PV/T collectors as well. The surface temperature of a PV cell was reduced from 62.29 °C to 32.5 °C with a 27% electrical improvement for 0.01% weight fraction (Karami and Rahimi, 2014a) and a 24.22 °C temperature reduction with a 37.67% electrical improvement for 0.1% weight fraction Boehmite/water (AlOOH–xH2O) nanofluid (Karami and Rahimi, 2014b). A 3.6% and 7.9% improvement on using 1% and 3% weight fractions respectively were observed for silica/water nanofluid (Sardarabadi et al., 2014). The computational analysis of a concentrated PV cell was investigated using Al2O3/water nanofluid (Xu and Kleinstreuer, 2014). The performance of the solar PV panel cooled on the top surface using nanofluids was studied for different mass fractions, fluid thicknesses (Cui, 2012), and their light transmittance and thermal conductivity using Silica/water nanofluid were tested using 2D CFD model for different nanoparticle sizes, light concentrations and velocities (Jing et al., 2015). A review on the application of different nanofluids (Javadi et al., 2013) and on the experimental tests, numerical models and simulation works on a solar PV/T collector were presented (Al-Shamani et al., 2014). A numerical model and experimental validation of a single glazed PV/T solar collector, focusing on the heat transfer between the PV cells and the fluid (Dupeyrat et al., 2011); and the lamination method (Dubey and Tiwari, 2008) are presented. The use of nanofluid has produced enhanced performance in solar thermal and solar PV/T collectors. Fluids with millimeter or micron sized suspended particles are known to cause severe problems in heat transfer equipment, by quickly settling out of suspension and cause severe clogging, thereby increasing the pressured drop and causing erosion of industrial components and pipelines due to the abrasive action of the particles. In nanofluids, the size, shape, hardness, dispersion, stability and concentration of the nano-particles in the base fluid influence the extent of friction and wear properties. For e.g., CuO nanoparticles in low concentrations added to a lubricating fluid exhibited good tribological properties (Chang et al., 2014) such as reduced friction, anti-wear properties and coefficient of friction (Pisal and Chavan, 2014), but at higher flow rate conditions, the wear rate increased (Hernandez Battez et al., 2008). Also, the effect of nanofluid flow impacting metallic surfaces strongly depends on the target material and on the nano-particle material. Nanofluid gave significant erosion in aluminum (Molina et al., 2014), compared to copper and negligible erosion for stainless steel (Celata et al., 2014). As a result, it can be concluded that any given nanofluid should be particularly tested before its adoption in a particular application as it can shorten the life of the components.

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In this paper, a solar PV/T collector was fabricated by laminating a copper sheet directly to the PV cells, thereby eliminating the need for Tedlar sheet and thermal conductive adhesive. A single water channel was attached underneath the fabricated PV module for uniform cooling of the PV cells. CuO/water nanofluid of a low volume concentration of 0.05% is compared to water at a fixed mass flow rate of 0.01 kg/s. 2. Methodology The experiment was conducted by synthesizing the CuO nanoparticle, preparing the CuO/water nanofluid and comparing the performance of the prepared CuO/water nanofluid with water in a fabricated PV/T system. 2.1. Synthesis of CuO nanoparticles To perform a closed loop operation by utilizing a heat transfer fluid, the PV/T collector was attached to a thermal storage tank with an in-built heat-exchanger. The quantity of heat transfer fluid required was measured to be 8 l. The quantity of nanoparticles required for a 0.05% volume concentration of CuO/water nanofluid using water as base fluid for a total volume of 8 l capacity, was calculated to be 25.3 g. Hence, the nanoparticles were synthesised from Copper Acetate by the Aqueous Precipitation method. Analytical grade Copper Acetate Monohydrate (Cu(CH3COO)2H2O) as the precursor and glacial Acetic acid (CH3COOH) to prevent hydrolysis, are taken in a flat-bottomed glass beaker, and heated with constant stirring in a magnetic stirrer. Sodium Hydroxide (NaOH) pellets as stabilizing agents were added slowly till the pH of the solution reaches between 6 and 7, and the color of the solution slowly turns from blue to black as shown in Fig. 1. The solution is removed from the magnetic stirrer and cooled to room temperature. The black precipitate formed is centrifuged, and then washed with distilled water three times to remove the impure ions. It is then covered with aluminum foil and placed in a hot air oven to completely dry the contents. The dried precipitate is finely powdered, using Agate mortar to produce CuO solid particles in nano dimensions. 2.2. Preparing the CuO/water nanofluid The high density (6310 kg/m3) of the CuO nanoparticles compared to the basefluid, water (995 kg/m3), causes the immediate settlement of the CuO nanoparticles at the bottom of the beaker. Hence, a compatible amphiphilic surfactant, namely, Sodium Do-decyl Benzene Sulphonate (SDBS) of 10% weight of the amount of nanoparticles to be added, was first dispersed completely in double distilled water using a magnetic stirrer, and then CuO nanoparticles of the required quantity are slowly added with constant stirring for 30 min. The solution is further sonicated using an ultrasonicator for 60 min to break the agglomerated

441

particles and facilitate a homogeneous mixture of the CuO nanoparticles and water, called CuO/water nanofluid. The CuO nanoparticles’ stability was checked with the SDBS surfactant, with Triton X-100 surfactant and without any surfactant. In Fig. 2, the dispersability of nanoparticles with the SDBS surfactant in water, compared to without-surfactant water after 24 h, clearly shows the stability of the nanoparticles using the surfactant. Similarly, the CuO/water nanofluid showed better stability with the SDBS surfactant, compared to the Triton X-100 surfactant after 3 days. Therefore, it was concluded that the SDBS surfactant performed better compared to the Triton X-100 surfactant for CuO/water nanofluid. 2.3. Experimental setup of the PV/T collector The conventional method of attaching a commercially available PV module to a metal thermal absorber using a thermal conductive adhesive was not followed in this paper. A novel PV/T module was constructed by laminating a copper sheet instead of the Tedlar layer in a 37 W PV module of dimensions 0.6  0.4 m, during the module fabrication process, in a module lamination chamber. The EVA layer, due to its high tensile strength, high elongation property and high adhesive strength was used to bond the copper sheet, silicon cell and the glass layer to form a glass-to-copper PV module. The thermal resistance of this glass-to-copper PV module was reduced by 9.90% compared to the commercial PV module. The EVA layer has a high thermal expansion coefficient of 180 K1 compared to glass of 5.9 K1, silicon of 3 K1 and copper of 16.6 K1, thereby preventing bulging or bowing due to differential thermal expansion of the different layers. This modification also excludes the necessity of an additional adhesive layer, thereby improving uniform heat transfer from the PV cells to the heat transfer medium. A single water channel of depth 0.002 m was constructed using an aluminum tray to cover the entire back surface of the PV/T module, with provisions for inlet and outlet on opposite sides of the aluminum sheet, as shown in Fig. 3. The single water aluminum channel/tray holds a sheet of water between the copper sheet and aluminum channel and cools the PV cells uniformly. A 0.05 m thick glasswool thermal insulation was placed along the sides and bottom to reduce heat losses. The PV/T collector so made is shown in Fig. 4. It is connected to a 100 l per Day (LPD) thermal storage tank with a ladder type heat exchanger, connected to a rotameter and a pump for forced circulation closed-loop operation. The water (secondary fluid) inside the thermal storage tank was stagnant during the conduction of the experiments. The constructed PV/T collector was compared simultaneously with a reference PV module of the same technical specifications of Pm = 37 W, Voc = 21 V, Isc = 2.3 A, Vmp = 17 V and Imp = 2.18 A. Both the PV/T collector and PV module were tested on the building roof-top of the Institute for Energy Studies, Anna University,

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Fig. 1. (a) The copper acetate powder, (b) the dried black precipitate of CuO, (c) the black color solution after adding NaOH pellets (d) the pH strip showing pH between 6 and 7.

Fig. 2. Dispersion stability of prepared CuO/water nanofluid (a) after 24 h and (b) after 3 days.

Chennai, at a fixed tilt angle of 13° corresponding to the latitude of the location, and facing the South. The Voltage(V), Current(A) and temperature measurements at

the PV glass layer, PV Tedlar layer, PV/T glass layer, Water inlet, Water outlet, Tank top, Tank middle and Tank bottom using RTD PT-100 temperature sensors.

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Fig. 3. (a) Different layers of the PV/T collector, (b) Cross section of the PV/T collector, and (c) PV/T collector without thermal insulation.

Fig. 4. The PV/T collector connected to the storage tank and the schematic diagram of the experimental setup.

The readings were recorded on sunny days from 10:00 am to 03:00 pm continuously every 5 min, and plotted on graphs. The ambient temperature and wind speed were recorded using a Weather station (WatchDog 2000, Spectrum Technologies) installed at the location. The solar irradiation value was recorded using a Pyranometer (LP02, Hukseflux) placed at the same tilt angle. All the instruments and sensors were connected to the same and connected to the same Data Acquisition (Agilent) System. 2.4. Uncertainty analysis A solar PV/T water heating system has electrical and thermal efficiencies. The electrical efficiency of the solar PV/T water heating system is directly proportional to the voltage, current and inversely proportional to the area of the solar collector and incident solar radiation falling on

the solar collector. The thermal efficiency of the solar PV/T water heating system is directly proportional to the mass flow rate & specific heat of the heat transfer fluid, temperature difference between the outlet and inlet fluid to the solar collector and inversely proportional to the area of the solar collector and incident solar radiation falling on the solar collector. Assuming negligible uncertainty in the specific heat and area of the solar collector, the overall uncertainty equation for the electrical and thermal efficiencies of the solar PV/T water heater can be calculated from the following equations. ðgi Þelec ¼ f ðV ; I; GÞ _ G; T Þ ðgi Þtherm ¼ f ðm; Therefore the overall uncertainty equations can be expressed as:

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"  #  2  2  2 U gi 2 Uv UI UG ¼ þ þ gi V I G elec "  #  2  2  2 U gi 2 U m_ UG UT ¼ þ þ gi G T m_ therm

where U denotes uncertainty and the variables such as, m_ DT, G, V, I are the mass flow rate of water (kg/s), the temperature difference between the outlet and inlet fluid of the solar collector (°C), global solar radiation incident on the solar collector plane (W/m2), voltage (V) and current (A) respectively. The accuracy of the rotameter, RTD PT-100 temperature sensors, the pyranometer, voltmeter and ammeter were ±5%, ±0.1 °C, ±5%, ±0.25% and ±0.25% respectively. The maximum uncertainty in the mass flow rate, temperature difference, solar radiation, voltage and current were calculated as 2.14%, 5.58%, 2.56%, 2.95% and 2.73% respectively. Therefore, the maximum uncertainties in electrical efficiency and thermal efficiency, thus calculated were 4.77% and 6.27% respectively. 3. Results and discussion The properties of the synthesized CuO nanoparticle and the prepared CuO/water nanofluid were tested by different nano-characterization tests and the performance of the PV/T system with CuO/water nanofluid was compared with water. 3.1. Nano-characterization tests A large number of techniques are available to measure the properties of nanoparticles, but the measured values for a property of nanoparticles from different techniques are not identical. Since, the nanoparticles used in this study were synthesized; a few characterization tests were conducted to understand the thermo-physical properties of the prepared nanofluid. 3.1.1. X-ray Diffractometer (XRD) method The XRD method was used to determine the crystal structure, size and the purity of the CuO nanoparticle. A beam of X-rays is passed through the nanoparticle sample, and the constructive interference of the mono-chromatic X-ray as it gets scattered by the different atoms, is used to estimate the crystalline size, using Bragg’s law given by 2dSinh ¼ nk, where d is the spacing between layers of atoms (nm), h is the angle between the incident rays and the surface of the crystal (degree), n is an integer and k is the wavelength of the X-rays (nm). The average crystalline size of the CuO nanoparticle was calculated to be 0.3 nm and 0.21 nm for the peaks of 2h = 35.6648 and 2h = 38.8354 respectively, using the Debye–Scherrer equation given as, D ¼ ðkkÞ=ðbCoshÞ, where D is the mean size of the ordered crystalline domains (nm), k is a dimensionless crystalline shape factor, k is the wavelength of the X-rays (nm), b is

the line broadening at half maximum intensity (FWHM) (degree) and h is the Bragg angle (degree). The intensities and position of peaks in XRD image shown in Fig. 5, prove, that the values are in good agreement with the reported values of the ICDD file no. 96-101-1195. Also, it was observed that the sample was a single phase CuO with a monoclinic structure, and lattice constant parame˚ , b = 3.43 A ˚ , c = 5.12 A ˚ , where a, b, c ters of a = 4.67 A are the axial lengths of the crystal lattice. 3.1.2. Scanning Electron Microscopy (SEM) method The morphology of the CuO nanoparticle was studied by passing a fine beam of high energy electrons on the surface of the sample. The image formed, as shown in Fig. 6, due to the scattered electron beam and sample interaction, shows that the synthesized nanoparticles are spherical in shape. 3.1.3. Energy Dispersive X-ray Spectroscopy (EDX) method The EDX method is used to characterize the elemental composition of the sample, by analyzing the emitted X-rays from the sample after being bombarded by the SEM electron beam. From Fig. 7, it was observed that the peaks obtained correspond to the copper and oxygen elements. No other peaks were formed suggesting the absence of any impurity. 3.1.4. Transmission Electron Microscopy (TEM) method The actual size of the nanoparticles can be measured by passing a stream of very high energy electrons through a thin film of the sample. The transmitted electrons through the sample are focused on a screen to depict the image, as shown in Fig. 8. The TEM image shows that the sample is composed of spherical nanoparticles. The average size of the nanoparticle observed from the TEM image is 75 nm 3.1.5. Photon correlation spectroscopy (PCS) method This method is used to determine the average size of the nanoparticle dispersed in very low concentration in water. Based on Dynamic Light Scattering (DLS), the temporal variation of the light intensity scattered by the Brownian diffusion of nanoparticles can be related to the particle diameter by Stokes Einstein relation. As shown in Fig. 9, the mean size of the nanoparticle obtained by this method was 111.3 nm, which is larger than the size obtained through other methods, because the van der Waals attractive force between the dry nanoparticles is large, and hence, agglomerates even after sonication. The agglomeration can be reduced by adding a surfactant, thereby helping in the effective dispersion stability of the nanoparticles. 3.1.6. Differential Scanning Calorimetry (DSC) method The DSC method is used to observe the temperature and heat flow changes due to physical and chemical transitions in the sample, when subjected to temperature and time in an inert gas atmosphere. CuO/water nanofluid and water samples of weight 13 mg were heated at the rate of

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Fig. 5. XRD image of the CuO nanoparticle.

Fig. 6. SEM image of the CuO nanoparticle.

Fig. 8. TEM image of the CuO nanoparticle.

Fig. 7. The EDX graph of the sample.

Fig. 9. Particle Size Analyzer image of CuO particles dispersed in water without surfactant.

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10 °C/min in aluminum crucibles placed in nitrogen gas atmosphere in the DSC instrument (Netzsch DSC 204). From Fig. 10, it is observed that the reaction is completely endothermic, with the first peak formed is called the Heat of Fusion, and the second peak formed is called the Heat of Vaporization. It is observed that more heat energy was absorbed in the CuO/water nanofluid compared to water, due to the better thermal properties of CuO/water nanofluid compared to water. It is also observed that the reactions occurred at slightly lower temperatures compared to water.

7

Water CuO/water nanofluid

Heat Capacity (J/g ˚C)

6

5

4

3

2 15

25

35

45

55

65

75

Temperature (˚C)

Fig. 11. Specific heat graph of the CuO/water nanofluid.

3.1.7. Modulated Differential Scanning Calorimetry (MDSC) method The MDSC method (TA Instruments Q200 DSC) was used to measure the thermal capacity, which is defined as the amount of heat required to increase the temperature of the sample by 1 °C. The value was measured by imposing a sinusoidal temperature modulation to the constant heating rate of 10 °C/min and separating the total heat flow into two components, namely, sensible heat which is reversible and temperature dependent, and latent heat which is non-reversible and time dependent. The average value of the obtained result is mathematically computed using the Fourier analysis, and the heat capacity curves as shown in Fig. 11, are obtained. The specific heat of the prepared nanofluid was 3.965 kJ/kg K. The specific heat of water is also included in the graph for the same temperature range. 3.1.8. Transient hot-wire method The thermal conductivity of the nanofluid was measured by the transient hot-wire method, using a thermal properties analyzer (KD2 Pro, Decagon Devices). A value of 0.722 W/m K was observed as shown in Fig. 12, for a temperature of 34.25 °C. 3.1.9. Rheological method The viscosity of the CuO/water nanofluid was measured using a parallel plate Rheometer (Malvern Bohlin Gemini Rotonetic 2 Drive). The change in the viscosities of 25

Fig. 12. Thermal conductivity meter with CuO/water nanofluid sample.

CuO/water

CuO (30 degree celsius) Water (30 degree celsius) CuO (10 degree celsius) CuO (50 degree celsius)

0.0012

Water

0.00115

20

15

Viscosity (Pas)

DSC (mW/mg)

0.0011 0.00105

10

5

0.001 0.00095 0.0009 0.00085 0.0008

0 -10

0.00075 10

30

50

70

90

110

130

Temperature (˚C)

0.0007 0

2

4

6

Shear Stress (Pa)

Fig. 10. The DSC graph of CuO/water nanofluid and water showing the heating rate at 10 °C/min.

Fig. 13. Viscosity graph.

8

10

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447

CuO/water nanofluid due to change in shear stress at temperatures of 10 °C, 30 °C and 50 °C are shown in Fig. 13. The measured experimental thermo-physical property values of 0.05% volume concentration CuO/water nanofluid were similar to the numerical values obtained from theoretical models, which were calculated for the same volume concentration, and shown in Table 1.

to the solar cell. A power reduction of 12–28% was observed due to the inclusion of additional glazing. The lower electrical performance in the PV/T collector using nanofluid compared to water is due to the higher temperature of the PV/T collector, attained due to the higher thermal conductivity of the nanofluid as shown in Figs. 15 and 16.

3.2. Experimental analysis

3.2.2. Top glass layer temperature of the PV/T collector The top glass surface of any PV module is generally at a higher temperature than the bottom surface. The CuO/water nanofluid helps in absorbing more heat from the PV/T collector, thereby reaching higher outlet temperature, which consequently causes higher inlet temperature due to the fixed effectiveness of the heat exchanger. Also, the higher outlet temperature of CuO/water nanofluid is partly due to the slightly lower Reynolds number and higher viscosity compared to water. Due to these conditions, a higher useful temperature was attained in the thermal storage tank. A temperature reduction of 13.82% was observed without glazing, however a temperature increase of 11.11% was observed due to the presence of glazing.

The following test conditions were conducted at a constant mass flow rate of 0.01 kg/s (laminar flow), using CuO/water nanofluid and their performances were compared to water and a reference PV module of the same technical specifications tested simultaneously. (a) With and without glazing: An additional glazing of 3 mm toughened solar glass above the PV/T collector increases the thermal efficiency due to trapping of infrared radiation and reduction of convective losses, but reduces the electrical efficiency due to higher light transmission losses and higher PV cell temperature. (b) Water and CuO/water nanofluid: The electrical and thermal performances of the PV/T collector with water and 0.05% volume concentration CuO/water nanofluid were compared. This test condition analyses the effect of higher thermal conductivity nanofluid on the electrical and thermal performance of the PV/T collector. The values obtained in Table 1, were used for the calculation of heat transfer coefficient calculation of water and CuO/water nanofluid. A 10.03% and 9.91% increase in heat transfer coefficient were observed due to the use of CuO/water nanofluid compared to water with and without glazing respectively. The experiment was conducted over a period of one month. The test conditions used in this experimental analysis are discussed in Section 3.2. The list of measurements taken in the conduct of the experiment is presented in Section 2.3. The experiments were carried out during the month of April, 2014, during which the solar irradiance intensity on the PV/T collector was high without much cloud cover. The change of solar radiation and ambient temperature of one day is given in Fig. 14. The solar collectors were exposed to solar radiation of 600–1000 W/m2 and ambient temperature of 31–35 °C between 10:00 am and 03:00 pm. 3.2.1. Electrical power output of the PV/T collector As seen from Fig. 15, it is seen that the power output with glazing is lower compared to that without glazing due to the reduced transmissibility and additional reflection losses. Also, we observe that the highest power output was obtained for the reference PV module due to the presence of the Tedlar layer which helps in increasing the electrical output by reflecting some of the incident radiation back

3.2.3. Electrical efficiency of the PV/T collector The instantaneous electrical efficiency of the solar photovoltaic thermal collector was calculated using the following equation, gi ¼ P m =ðAc  GÞ ¼ ðV m  I m Þ=ðAc  GÞ, where Pm = power at maximum power point (W), Ac = area of the collector (m2), G = global solar radiation incident on the solar collector (W/m2), Vm = voltage at maximum power point (V), Im = current at maximum power point (A). The electrical efficiency of the PV/T collector with glazing was less compared to that without glazing. From Fig. 17, it was observed that the reference PV module gave the highest electrical efficiency of 8.98%. The electrical efficiency is reduced using CuO/water nanofluid, because of the higher temperature of the PV/T collector as discussed in Section 3.2.2. Since nanofluid has higher heat transfer rate compared to water, the electrical efficiency can be improved if the heat exchanger is redesigned for the nanofluid volume concentration. 3.2.4. Thermal efficiency of the PV/T collector The instantaneous thermal efficiency of the solar photovoltaic thermal collector was calculated using the following equation, gi ¼ Qu =ðAc  GÞ ¼ ðm_  C p  DT Þ=ðAc  GÞ, where Qu = useful heat (W), Ac = area of the collector (m2), G = global solar radiation incident on the solar collector (W/m2), m_ = mass flow rate of water (or) nanofluid (kg/s), Cp = specific heat of water (or) nanofluid (J/kg K). A significant improvement of 44.90% and 45.76% in thermal efficiency were found using CuO/water nanofluid compared to water with and without glazing respectively as shown in Fig. 18. As discussed in the introduction section, the addition of nanoparticles in the solar thermal collectors improves the thermal efficiency. In a solar

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Table 1 Thermo-physical properties of water and CuO/water nanofluid. Parameter

Water

CuO/water nanofluid (theoretical values)

CuO/water nanofluid (experimental values)

Density (kg/m3)

995

1260 (Pak and Cho, 1998)

1165

Dynamic Viscosity (Ns/m2)  103

0.829

0.945 (Bachelor, 1999)

0.932 (Einstein)

0.942 (Brinkman, 1952)

Thermal conductivity (W/m K)

0.6

0.672 (MaxwellEucken, 1892)

0.724 (Wasp, 1977)

0.686 (Hamilton and Crasser, 1962)

Specific heat (kJ/kg K)

4.179

3.996 (Pak and Cho, 1998)

15:50

55

15:15

15:30

15:00

14:45

14:15

14:30

14:00

13:45

13:15

13:30

13:00

12:45

Fig. 14. Solar radiation and ambient temperature.

12:30

45

12:00

Time (hours)

12:15

50

11:45

15:30

15:10

14:50

14:30

14:10

13:50

13:30

13:10

12:50

12:30

12:10

11:50

31 11:30

32

0 11:10

100

60

11:15

33

200

11:30

34

300

65

11:00

35 400

10:45

500

70

10:30

36

10:00

37

600

10:50

3.965 (MDSC method)

CuO with glazing CuO without glazing Water with glazing Water without glazing Reference PV

10:15

700

10:30

3.269 (Xuan and Roetzel)

75

Temperature (˚C)

38

Temperature (˚C)

39

800

9:50

0.722 (Hot-wire transient method)

40

900

10:10

0.716 (Yu and Choi)

80

Solar Insolaon Ambient Temperature

1000

Solar Irradiance (W/m²)

0.7 (Bruggeman)

0.947 (Ostwald Viscometer), 0.884(Rheometer)

Time (hours)

Fig. 16. Top glass layer temperature of the PV/T collector.

CuO with glazing CuO without glazing Water with glazing Water without glazing Reference PV

35

30

CuO with glazing CuO without glazing Water with glazing Water without glazing Reference PV

10

9 20

8

Efficiency (%)

Power (was)

25

15

10

7

6

10:00 10:15 10:30 10:45 11:00 11:15 11:30 11:45 12:00 12:15 12:30 12:45 13:00 13:15 13:30 13:45 14:00 14:15 14:30 14:45 15:00 15:15 15:30

5 5

4

15:30

15:15

14:45

15:00

14:30

14:15

14:00

13:45

13:30

13:15

13:00

12:45

12:30

12:15

12:00

11:45

11:30

11:15

11:00

10:30

10:45

3

10:15

Fig. 15. Electrical power output of the PV/T collector.

10:00

Time (hours)

Time (hours)

PV/T collector, the thermal efficiency increases, but electrical efficiency improvement is obtained only when the absorbed heat is transferred to the secondary fluid, which requires a redesigned heat exchanger. Thus, comparing CuO/water nanofluid and water in identical conditions in a PV/T system, it has been concluded that using nanofluid as the heat transfer fluid helps to improve the thermal efficiency.

Fig. 17. Electrical efficiency of the PV/T collector.

3.2.5. Overall efficiency of the PV/T collector Generally, a PV/T collector has a higher overall efficiency compared to the individual thermal collectors or PV modules, due to the combined efficiency of the two technologies. But, considering that electric energy is a high

J.J. Michael, S. Iniyan / Solar Energy 119 (2015) 439–451 Water with glazing Water without glazing CuO with glazing CuO without glazing

35

30

Efficiency (%)

25

20

15

10

15:15

15:30

14:45

15:00

14:15

14:30

14:00

13:45

13:30

13:00

13:15

12:45

12:30

12:15

11:45

12:00

11:15

11:30

11:00

10:30

10:45

10:15

0

10:00

5

Time (hours)

Fig. 18. Thermal efficiency of the PV/T collector.

CuO with glass CuO without glass Water with glass Water without glass Reference PV

60

Efficiency (%)

50 40 30 20

15:15

15:30

15:00

14:45

14:30

14:15

14:00

13:30

13:45

13:15

13:00

12:45

12:30

12:15

11:45

12:00

11:30

11:00

11:15

10:45

10:30

10:15

0

10:00

10

Time (hours)

Fig. 19. Overall efficiency of the PV/T collector.

grade form of energy, the overall efficiency of the PV/T collector is calculated by considering the primary energy saving efficiency, that is calculated by taking into account the electric power generation efficiency of a conventional thermal power plant of 38%. It is calculated using the following equation, gPES ¼ gthermal þ ðgelectrical =0:38Þ, where gelectrical is the electrical efficiency (%) and gthermal is the thermal

449

efficiency (%). As shown in Fig. 19, the PV/T collector overall efficiency was high for the condition in which CuO/water nanofluid is used, compared to water as the heat transfer fluid. The CuO/water nanofluid produced 19.25% and 11.94% higher overall efficiency with and without glazing respectively compared to water as heat transfer fluid. Also, without glazing produced higher overall efficiency compared to with glazing due to higher electrical efficiency in the absence of glazing. The values obtained during the experimental analysis and the percentage improvements are listed in Table 2. The nanofluid absorbs more thermal energy from the PV/T collector, but due to closed loop operation and stagnant secondary fluid (water), the inlet fluid temperature to the PV/T collector is higher. This results in higher thermal losses and lower electrical efficiency. The electrical efficiency and thermal efficiency can be enhanced further, if the inlet fluid temperature to the PV/T collector is maintained at a lower temperature. This is possible only if a pump is attached to circulate the secondary fluid or the heat exchanger is re-designed for the nanofluid. The amount of thermal energy absorbed by the nanofluid is higher compared to water, due to the higher thermal properties of nanofluid as discussed in Table 1. In the case of glazing, the absorbed energy of nanofluid is very high compared to the transferred energy through the heat exchanger. This leads to higher inlet temperature to the PV/T collector. The absorbed energy of nanofluid is 88.81 W compared to the water (primary fluid) of 58.68 W. This difference of excess energy (30.13 W), is not completely transferred through the heat exchanger. In the case of without glazing, the absorbed energy of the nanofluid is higher compared to water, while it transfers the higher thermal energy through the heat exchanger. This leads to lower inlet temperature to the PV/T collector, thereby cooling the PV/T collector better than water. The absorbed energy of nanofluid is 68.91 W compared to the water (primary fluid) of 60.66 W. The difference of excess energy (8.25 W), is much less compared to glazed condition

Table 2 Summary of experimental results. Condition

Reference PV module

Electrical power output (W) Maximum top glass layer temperature (°C) Maximum electrical efficiency (%) Electrical enhancement w.r.t. to reference PV (%) Maximum thermal efficiency (%) Thermal enhancement w.r.t. to water (%)

31.05 67 8.98

PV/T collector (water with glazing)

PV/T Collector (water without glazing)

PV/T collector (CuO/water nanofluid with glazing)

21.93

30.28

21.61

24.53

71.79

57.38

74.45

57.74

6.40

8.77

6.18

7.62

–

28.73

–

21

–

–

PV/T collector (CuO/water nanofluid without glazing)

2.3

31.18

19.36

30.43

28.22

44.90

45.76

–

15.14

450

J.J. Michael, S. Iniyan / Solar Energy 119 (2015) 439–451

discussed above. This small excess energy is effectively transferred through the heat exchanger. The effectiveness of the heat exchanger is higher in the passage of water compared to nanofluid. The lower effectiveness using nanofluid is lower due to the lower specific heat of the nanofluid as shown in Fig. 11. But, the higher thermal conductivity of the nanofluid as shown in Fig. 12, is responsible for the higher temperature difference across the outlet and inlet of the PV/T collector as well as the heat exchanger. This condition can be explained using the term thermal diffusivity. The nanofluid has a higher thermal diffusivity of 1.56  107 m2/s, compared to water of 1.44  107 m2/s. The higher thermal diffusivity of the CuO/water nanofluid enables the nanofluid to transfer the heat quickly compared to water. From this experiment, it can be concluded that under identical conditions, the electrical efficiency using nanofluid will be inferior, while thermal efficiency will be higher compared to water. 4. Conclusion A novel photovoltaic thermal collector was fabricated and its performance was tested using 0.05% volume fraction CuO/water nanofluid. The nanofluid has been proved to increase the thermal efficiency up to 45.76%. Based on the observations from the reduced electrical efficiency using the CuO/water nanofluid, the electrical and thermal efficiencies of the discussed solar photovoltaic/thermal collector can be further improved if the heat exchanger is re-designed for the new nanofluid. Acknowledgements The authors wish to express their sincere thanks to Anna University, for providing the necessary funds through the “Anna Centenary Research Fellowship”. They also wish to thank Waaree Energy Systems, Surat, for their help in the construction of the PV/T module, and Malar Engg Works Pvt. Ltd., Chennai, for the fabrication of the PV/T collector. The author gives special thanks to Anna University, Indian Institute of Technology Madras and Sathyabama University for providing the required facilities to conduct the nano-characterization tests. References Agrawal, Sanjay, Tiwari, G.N., 2011. Energy and exergy analysis of hybrid micro-channel photovoltaic thermal module. Sol. Energy 85 (2), 356–370. Al-Shamani, Ali-Najah, Yazdi, Mohammad H., Alghoul, M.A., Abed, Azher M., Ruslan, M.H., Mat, Sohif, Sopian, K., 2014. Renew. Sustain. Energy Rev. 38, 348–367. Amrut Lanje, S., Satish Sharma, J., Ramchandara Pode, B., Raghumani Ningthoujam, S., 2010. Synthesis and optical characterization of copper oxide nanoparticles. Adv. Appl. Sci. Res. 1, 36–40. Assoa, Y.B., Menezo, C., 2014. Dynamic study of a new concept of photovoltaic–thermal hybrid collector. Sol. Energy 107, 637–652.

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