Experimental investigation of CuO nanofluid-based Direct Absorption Solar Collector for residential applications

Renewable and Sustainable Energy Reviews 52 (2015) 793–801

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Experimental investigation of CuO nanofluid-based Direct Absorption Solar Collector for residential applications M. Karami a, M.A. Akhavan-Bahabadi a,n, S. Delfani b, M. Raisee a a Center of Excellence in Design and Optimization of Energy Systems, School of Mechanical Engineering, College of Engineering, University of Tehran, P.O.Box. 14395-515, Tehran, Iran b Department of Building Installations, Road, Housing and Urban Development Research Center (BHRC), P.O.Box.13145-1696, Tehran, Iran

art ic l e i nf o

a b s t r a c t

Article history: Received 9 January 2015 Received in revised form 15 May 2015 Accepted 28 July 2015

Solar water heating systems are the most economical and large scale application of solar energy in residential buildings. In order to enhance the efficiency of these systems, Direct Absorption Solar Collector (DASC) which used nanofluids with appropriate optical and heat transfer properties as absorbing medium, has been recently proposed. In this study, a prototype of this new type of collector was built with applicability for domestic solar water heater. Different volume fractions of copper oxide nanoparticles in water and ethylene glycol mixture (70%:30% in volume) as the base fluid were prepared and their thermo-physical and optical properties were presented. The procedure of EN 12975-2 standard was used for testing the thermal performance of the collector. The tests were performed in different flowrates from 54 to 90 l/h (0.015–0.025 kg/s) and two different internal surfaces (black and reflective) of bottom wall. The efficiency of the collector with black internal surface is about 11.4% more than that of with reflective internal surface using the base fluid at 90 l/h flowrate. The collector efficiency is increased by increasing nanofluid volume fraction and flowrates. The nanofluids improved the collector efficiency by 9–17% than the base fluid. Based on the results, the performance of this new kind of collector as the main part of solar water heater appears promising, leading to considerably higher efficiencies. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Solar water heating Direct Absorption Solar Collector Nanofluid Residential applications

Contents 1. 2. 3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DASC design features. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Nanofluid preparation and optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Test setup and procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Test period . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Efficiency calculation and error analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Results and discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Solar water heating systems, as one of the renewable energy technologies used mostly in residential buildings, can reduce the

n

Corresponding author. Tel.: þ 98 21 88005677; fax: þ 98 21 88013029. E-mail address: [email protected] (M.A. Akhavan-Bahabadi).

http://dx.doi.org/10.1016/j.rser.2015.07.131 1364-0321/& 2015 Elsevier Ltd. All rights reserved.

793 795 795 795 796 797 797 798 800 800 801

use of fossil fuels as well as associated environmental problems. In order to enhance the efficiency of these systems, attempts have been made to absorb more heat from solar radiation by solar collectors whose low efficiency and high cost compared with the conventional devices convince scientists and engineers to make effort to increase performance of solar collectors [1]. Attention to the concept of direct absorption of incident solar radiation caused introducing Direct Absorption Solar Collector (DASC) in the mid

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Nomenclature a1 a2 A cp fv GT _ m Q_ R2 T amb T in T nin T out

heat loss coefficient (W m  2 K  1 ) temperature dependence of the heat loss coefficient (W m  2 K  2 ) area (m2) 1 specific heat (J kg K  1) particle volume fraction (ppm) incident solar flux (W m  2 ) mass flowrate (kg=s) heat flux (W m  2 ) correlation coefficient ambient temperature (1C) inlet fluid temperature (1C) reduced temperature difference (m2 K W  1 ) outlet fluid temperature (1C)

1970's [2,3]. In this type of solar collector, the working fluid absorbs the solar radiation directly and converts the energy to heat. By comparison DASC with flat plate solar collector (FPSC), it is found out that DASC has the lower thermal resistance, and hence, higher efficiency. The first generation of DASCs demonstrated several shortcomings, such as inappropriate optical properties of common solar fluids [4] (water, ethylene glycol, propylene glycol, etc.) and the clogging of the pump and pipes due to rapid settling of small (millimeter or micro-sized particles) particles which added to working fluid to improve absorption ability. Both of these problems can be solved using nanofluids made from proper nanoparticles in the base fluid as the working fluid of DASC. Studies in this field indicate that exploiting nanofluid in solar systems, offers unique advantages over conventional fluids [5]. Therefore, remarkable attention has been given to the optical properties of various nanofluids in the recent years [6–16]. Sani et al. [6,7] found that nanohorn spectral features are far more favorable than those of amorphous carbon for the specific application. This result shows that carbon nanohorn-based nanofluids can be useful for increasing the efficiency and compactness of thermal solar devices, reducing both environmental impact and costs. Taylor et al. [9] found that over 95% of incoming sunlight can be absorbed (in a nanofluid (Graphite, Al, Cu, Ag, Au/TherminolVP-1, Water) thickness Z 10 cm) with extremely low nanoparticle volume fractions (0.001 vol%). Saidur et al. [12] presented that the particle size has little influence on the optical properties of nanofluids, while the loadings of nanoparticles is linearly related to the extinction coefficient of the nanofluids. Karami et al. [13] presented that the determined spectral transmission confirmed that carbon nanoballs have a considerable role in raising the optical properties of the fluid, due to improvement of the light extinction level even at low concentrations. By exploring the potential of nanofluids, a new type of Direct Absorption Solar Collector developed as nanofluid-based Direct Absorption Solar Collector, firstly investigated numerically by Tyagi et al. [17]. They used aluminum nanoparticle suspensions in water as the working fluid and show the efficiency enhancement of 10% in comparison with a conventional flat plate collector. In general, researches about nanofluid-based DASC in recent years could be categorized into low- and high-temperature DASC. Otanicar et al. [19] have numerically evaluated the performance of low-temperature DASC based on the work of Tyagi et al. [17]. They also report on the experimental results on microsolar direct absorption collector based on nanofluids made from a variety of nanoparticles (carbon nanotubes, graphite, and silver) and

Ui U m_ U GT U ΔT U ηi U i;f U i;r V_

combined fixed and random uncertainty (%) _ (%) uncertainty of m Uncertainty of GT (%) uncertainty of ΔT (%) uncertainty of ηi (%) fixed uncertainty of ith component (%) random uncertainty of ith component(%) volumetric flowrate (l/h)

Greek symbols

η η0 ηi ρ

collector efficiency zero-loss collector efficiency instantaneous collector efficiency density ðkg m  3 Þ

demonstrate efficiency improvements of up to 5% by utilizing nanofluids as the absorption mechanism. They also investigated the impact of size and scattering mode on the optimal solar absorbing nanofluid [18] and of the extinction coefficient on the collector efficiency [20]. The comparative environmental and economic analysis of conventional and nanofluid solar hot water technologies by Otanicar and Golden [21] show that the nanofluid based collector has a lower embodied energy (  9%) and approximately 3% higher levels of pollution offsets than a conventional collector. Veeraragavan et al. [22] presents an analytical model that investigated the effect of heat loss, particle loading, solar concentration and channel height on temperature profiles. The obtained temperature profiles showed that at locations downstream of the inlet, the surface temperature becomes lower than the bulk temperature which suggests the advantage of volumetric absorption. Ladjevardi et al. [23] have also numericaaly studied lowtemperature solar receivers and show that by using graphite nanofluids having volume fraction around 0.000025%, it will be possible to absorb more than 50% of incident irradiation energy by just about 0.0045$/L increase in cost, while pure water solar collector can only absorb around 27% of incident irradiation energy. Most recently, Parvin et al. [24] investigated the heat transfer performance and entropy generation of forced convection through a low-temperature Direct Absorption Solar Collector numerically. The results show that both the mean Nusselt number and entropy generation increase as the volume fraction of Cu nanoparticles and Reynolds number increase. Taylor et al. extended the application of nanofluid-based DASC to high-temperature concentrated solar power systems [26]. They built a laboratory-scale nanofluid dish receiver that measures 2 cmn2 cm, with a fluid depth of 1 mm. Their results show that the use of a nanofluid in the receiver can improve the efficiency by 10%. Lenert and Wang [27] presented a combined theoretical and experimental work to optimize the efficiency of high temperature, high flux direct solar receivers seeded with carbon-coated cobalt nanoparticles. They concluded that the efficiency of nanofluid volumetric receivers increases with increasing solar concentration and nanofluid height. The receiver of Luo et al. [28] was an aluminum channel of 10 cm width and 2.5 cm depth which has been tested using a solar radiation simulator. Their results show that the nanofluids improved the outlet temperature and the efficiency by 30–100 K and by 2–25% than the base fluid. Khullar et al. [29] introduced the idea of harvesting solar radiant energy through usage of nanofluid-based concentrating parabolic solar collectors (NCPSC). They observed that the NCPSC has about 5–10%

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Table 1 Summary of the studies about nanofluid-based Direct Absorption Solar Collectors (DASC). Category

Author (year)

Nanofluid

Investiagtion type

References

Nanofluid Properties

Sani et al. (2010, 2011) Kameya and Hanamura (2011) Taylor et al. (2011) Mercatelli et al. (2011) Han et al. (2011) Saidur et al. (2012) Karami et al. (2013) Zhang et al. (2014) Hordy et al. (2014) Karami et al. (2014) Tyagi et al. (2009) Otanicar et al. (2009, 2011) Otanicar et al. (2010) Veeraragavan et al. (2012) Ladjevardi et al. (2013) Parvin et al. (2014) Karami et al. (2014) Taylor et al. (2011) Lenert et al. (2012) Lou et al. (2014) Kullar et al. (2014) Liu et al. (2014)

SWCNH/water/glycol Ni/alkyl naphthalene Al–Ag–Au–Cu–TiO2–graphite/water/therminol VP-1 SWCNH/water/glycol Carbon black/water Al/water Carbon nanoball/water/glycol Carbon-coated Ni/[HMIM][NTf2] MWCNT/water/EG/PG/therminol VP-1 MWCNT/water Al/water Au–Ag–graphite/water Ag–graphite–MWCNT/water Graphite/therminol VP-1 Graphite/water Cu/water SWCNH/water Al–Ag–Cu–graphite/therminol VP-1 Carbon-coated cobalt/therminol VP-1 C, Ag, Cu, TiO2, SiO2, Al2O3, LWCNT, SWCNT/Texatherm oil Al/therminol VP-1 Graphene/[HMIM]BF4

Experimental Experimental/theoretical Experimental/theoretical Experimental Experimental Numerical Experimental Experimental/theoretical Experimental Experimental Numerical Numerical Experimental/numerical Analalytical Numerical Numerical Numerical Experimental/numerical Experimental/numerical Experimental/numerical Numerical Experimental/numerical

[6,7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18,20] [19] [22] [23] [24] [25] [26] [27] [28] [29] [30]

Low-temperature DASC

High-temperature DASC

higher efficiency as compared to the conventional parabolic solar collector while maintaining the same external conditions. Liu et al. [30] presented a combined analytical and experimental study on high temperature direct solar thermal collectors using grapheme/ ionic liquid nanofluids as the absorbers. They found that the receiver efficiency increases with the solar concentration and receiver height, but decreases with the grapheme concentration. Table 1 provides a summary of the studies about nanofluidbased Direct Absorption Solar Collectors (DASC) considered in this work and outlines the nanofluids applied in them. It should be mentioned that all test setups that was built around nanofluidbased DASC were laboratory-scale (indoor test). They have shown that in laboratory-scale solar collector efficiency can be improved by using nanofluids. This paper will extend that concept to a fullscale low-temperature nanofluid solar thermal collector. To the best of author's knowledge, this type of solar collector have not been experimentally studied to date in the perspective to use it as the collecting device in a domestic solar water heater. A review of the mentioned literature shows that different nanofluids have been investigated as working fluid of nanofluidbased DASC, such as metallic nanoparticle [17–19,22,24,26–29], carbon nanostructures [18,19,22,23,25,26,28,30], metal oxide nanoparticles [28] into various base fluids such as water [17– 19,23–25], oil [28], therminol VP-1 [22,26,27,29]. However, there is no investigation on the nanofluid-based DASC performance using copper oxide nanoparticles into mixture of distilled water and ethylene glycol (EG) (70%:30% by volume). Taking this into consideration, it has been decided to use CuO nanofluid, as working fluid of full-scale low-temperature nanofluid-based DASC.

2. DASC design features A prototype of Direct Absorption Solar Collector (DASC) was built that measures 60  60 cm2, with a channel depth of 1 cm with applicability for domestic solar water heater. Fig. 1(a) shows the experimental collector schematic. The main body of the collector was made of aluminum. A manifold with pinholes is used to uniformly entry of working fluid into the channel from the bottom of the collector. Fig. 1(b) demonstrates the detail of working fluid entering the collector. Instead of manifold, three holes are considered at the top of the collector to exit the working

fluid to avoid increasing the pressure drop. The working fluid flows from the bottom to the top of the collector. The collector glazing of the toughened glass with 4 mm thickness is selected due to prevent cracking caused by water pressure. The possible leakage of working fluid is restrained by mounting a seal gasket before installing the glazing which is tightened by means of aluminum frame. Under this frame, a seal tape is also mounted to more sealing. The whole collector is insulated within a Polyurethane block of 10 mm thickness to limit heat loss from the back and sides of the collector. The Polyurethane block was shielded from incident radiation with aluminum foil so as to not absorb any of the sunlight. The internal surface of the collector bottom wall is reflective aluminum for all experiments with nanofluids and one experiment with the base fluid. The internal surface was also black painted to another experiment with the base fluid. All experiments with nanofluids were performed with the reflective internal surface to evaluate only nanofluid absorption ability; whereas the base fluid is tested with both black and reflective internal surfaces.

3. Experimental investigation 3.1. Nanofluid preparation and optical properties In the present work, CuO nanoparticles with an average diameter less than of 40 nm and density of 6.3 g/cm3 were used. The SEM Photography of CuO nanoparticles is shown in Fig. 2(a). As the dispersant, PVP (Polyvinylpyrrolidone) with the weight ratio of 0.25:1 (PVP:CuO) was used to prepare the CuO nanofluid [31]. Nanoparticles with different volume fractions (C3: 25, C2: 50, C1: 100 ppm) were dispersed in a 70%:30% (in volume) water and EG mixture as the base fluid. Fig. 2(b) shows the comparative image of nanofluid samples. The nanofluid mixture was then stirred and agitated thoroughly for 60 min at 50% amplitude using a 130 W, 20 kHz probe (Hielscher, UP400S, Inc., USA). This ensures uniform dispersion of nanoparticles in the base fluid. Additionally, the mixture is ultrasonicated intermittently (once every 15 min) to avoid overheating. The breaks duration is typically about 2 min which provides the

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Fig. 1. Direct absorption solar collector (a) Experimental schematic (b) Detail of working fluid entry.

opportunity for the energized Cuo nanoparticles to dissipate the energy. Fig. 2(c) shows the transmittance spectra of the CuO nanofluid samples. Based on this figure, in the volume fractions more than 100 ppm of CuO nanoparticles in base fluid, color of the solution becomes completely black and light is not able to pass through it, so the samples have synthesized at the volume fractions up to 100 ppm. Optical transmittance spectra have been measured using a double-beam spectrophotometer (Perkin-ElmerLambda1050). A quartz cuvette of 1 cm was used which has the same path length with collector depth. The characterization method is similar to Karami et al. [13]. 3.2. Test setup and procedure The DAS collector was experimentally investigated at the Building and Housing Research Center of Tehran, Iran (latitude is 35.69611N and longitude is 51.42311E). The schematic of the test loop and the photo of the test setup based on EN-12975-2 [32] are shown in Fig. 3. DAS collector is mounted by tilt angle of 351 to receive maximum solar energy (regarding Tehran latitude). An electrical pump and a flow control valve (connected to the water pipe after the electric pump) are used to maintain the flowrate

through the collector stable to within 1%. An expansion tank with about 5 l capacity is used which has been insulated due to reduce heat loss. For primary temperature control, the working fluid was heated or cooled using a heat exchanger to remain inlet temperature constant. Measuring instruments include a flowmeter which connected to the water pipe before the electric pump with the 71% accuracy of the measuring span, two PT100 temperature sensors to measure fluid temperatures in the inlet and outlet of collector with the accuracy of 70.1 1C, another temperature sensor to measure the air temperature, Kipp&Zonen-CMP6 pyranometer to measure total solar radiation which its sensor is mounted coplanar, within a tolerance of 711 with the plane of the collector aperture and TESTO 425 anemometer which recorded air speed by accuracy 70:03 m=s. A data acquisition system was used to record all measurements. Calibration of measuring instruments was performed using calibrated references. Temperature sensors were calibrated using a calibrated thermometer to give an uncertainty less than 70.05 1C; flowmeter using drawing off water from the system into a container and measuring the volume and time with accuracy scales. The uncertainty was about 72%. Pyranometer and anemometer have a valid calibration certificates with uncertainty of about 73.5% and 72.5%, respectively.

M. Karami et al. / Renewable and Sustainable Energy Reviews 52 (2015) 793–801

C1

C3

Base Fluid

797

limits given in Table 2. To establish that a steady state exists, average values of each parameter taken were compared with the mean value over the measurement period. The collector time constant testing for the base fluid is presented in Fig. 4. According to this figure, the resulted time constant was 5.7 min. For the collector system tested the time constants ranged from 5 to 6 min for all fluids and internal surfaces tested. Therefore, each test period was about 40–50 min. 3.4. Efficiency calculation and error analysis

100 Base Fluid C3 C2 C1

90 80

_ p ΔT Q_ ¼ mc

ð1Þ

The instantaneous collector efficiency (ηi ) relates the useful energy to the total radiation incident on the collector surface (AGT ) by Eq. (2):

70

Transmittance, %

The useful energy extracted can be calculated after the inlet and outlet fluid temperatures and the flowrate of working fluid were measured, using Eq. (1):

60 50

ηi ¼

40 30 20 10 0 200

400

600

800

1000

1200

1400

Wavelength, nm Fig. 2. (a) SEM image of CuO nanoparticles, (b) comparative image of nanofluid samples, and (c) transmittance spectra of the CuO nanofluid samples.

The collector is tested for at least four fluid inlet temperatures over its operating temperature range under clear sky conditions. At least four independent data points is obtained for each fluid inlet temperature, to give a total of 16 data points. The data for each test period were averaged and applied as a single point whereas other data were rejected. During a test, hemispherical solar irradiance, diffuse solar irradiance, air speed, surrounding air temperature, temperature of the heat transfer fluid at the collector inlet and outlet and flowrate of the heat transfer fluid are measured and recorded over successive periods of 30 s. Experimental data have been recorded for different inlet temperatures between 30 1C and 50 1C based on the ambient temperature at three different flowrates 54, 72 and 90 l/hr. 3.3. Test period The test period for a steady state data point include a preconditioning period and a steady state measurement period which both of them are at least 4 times the time constant of the collector. The time constant of the collector is defined as the elapsed time in which the fluid outlet temperature arrives 63.2% of its final increase. For the purpose of this test, the aperture of the collector was shielded from the solar radiation by means of a solarreflecting cover, and the temperature of the heat transfer fluid at the collector inlet was set approximately equal to the ambient air temperature. When a steady state has been reached, the cover was removed and measurements continued until steady-state conditions have been achieved again. A collector is considered to have been operating in steady-state conditions over a given measurement period if none of the experimental parameters deviate from their mean values over the measurement period by more than the

_ p ðT out  T in Þ ρVc Q_ ¼ AGT AG

ð2Þ

where V_ is the volumetric flowrate, ρ and cp is the density and heat capacity of working fluid. Table 3 shows the corresponding experimentally determined thermophysical properties of the CuO nanofluids. The density of the nanofluids was obtained using the Pycnometer method (ASTM D-1217) [33], whereas, the ASTM standard test method (E1269-05) [34] was followed to obtain the specific heat (cp ) using a differential scanning calorimeter (DSC) (SETARAM, DSC131). As expected, the thermophysical properties are not significantly affected by the addition of nanoparticles, as expected for such low volume fractions. An instantaneous efficiency curve is obtained by statistical curve fitting of the experimental data, using the least squares method:

η ¼ η0  a1 T nin a2 GT ðT nin Þ2

ð3Þ

n

where T in is the reduced temperature difference and calculated as: T nin ¼

T in  T amb GT

ð4Þ

The intersection of the line with the vertical efficiency axis equals to η0 which called zero-loss efficiency. At this point the temperature of the fluid entering the collector equals to the ambient temperature and collector efficiency is maximum. The slope of the line (a1 ) indicates removed energy from the solar collector which called heat loss coefficient. The coefficient a2 shows temperature dependence of the heat loss coefficient [32]. If the value deduced for a2 is negative, a second-order fit shall not be used. Error analysis for experimental results presented in this study (“steady-state” collector efficiency) has been performed using the method proposed by Abernethy et al. [35]: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi U i ¼ U 2m_ þU 2GT þ U 2ΔT ð6Þ _ GT and ΔT, where U m_ , U GT and U ΔT are the uncertainties of m, respectively, all of which are expressed in percent (i.e., relative error with respect to the average values) and each consists of fixed error which is caused by error of the measuring equipments and random error which is caused by data scattering due to random fluctuation of the process: Ui ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi U 2i;f þ U 2i;r

ð7Þ

798

M. Karami et al. / Renewable and Sustainable Energy Reviews 52 (2015) 793–801

1-DAS collector 2-Insulated pipe 3-Temperature sensor 4-Air vent 5-Flowmeter 6-Pump 7-Flow control valve 8-Heater/Cooler for primary temperature control 9-Expansion Tank 10-Insulated pipe 11-Temperature sensor 12-Pyranometer 13- Anemometer 14- Surrounding air temperature sensor

Fig. 3. DASC outdoor performance test (a) schematic of the standard test loop (b) the photo of the experimental setup. 10

Table 2 Permitted deviation of measured parameters during a measurement period [32]. Permitted deviation from the mean value

Global test solar irradiance Surrounding air temperature Fluid mass flow rate Fluid temperature at the collector inlet

7 50 W=m2 7 1:5 K 7 1% 7 0:1 K

U i;f represents the fixed error of the ith component; U i;r , represents the random error of the ith component. The maximum uncertainty obtained in the present study in determining the collector efficiency, ηi (Eq. (2)), at various tests was around 5.4% (including both fixed and random errors).

9 8 7

(Tout-Tamb)

Parameter

6 5 4 3 2 1 0 11:50:30

4. Results and discussion

11:56:30

12:02:30

Time Fig. 4. Collector time constant test for base fluid at 72 l/h flowrate.

The tests have performed around solar noon when the hemispherical solar irradiance is greater than 700 W m  2, diffuse solar irradiance is less than 30% and the average value of air speed is 2–4 m/s. Each test was repeated in several days and the best experimental data has been chosen. The experimental results are presented in the form of graphs that describe the collector efficiency against the reduced temperature difference ððT in  T amb Þ=GT Þ. Based on Eq. (2), the operating parameters affect the DASC efficiency are the working fluid flowrate and the temperature

difference between fluid inlet and outlet. Therefore, the effect of variations in flow (working fluid flowrate and inlet temperature) and radiative parameters (internal surface emissivity and working fluid absorption coefficient) which impact on the outlet temperature, on the efficiency of this new type of collector is experimentally investigated and discussed in this study. Fig. 5 presents the variations of collector efficiency for the base fluid in case of the black internal surface. The test was performed

M. Karami et al. / Renewable and Sustainable Energy Reviews 52 (2015) 793–801

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1

Table 3 Thermophysical properties of CuO nanofluid.

DASC-Reflective Back DASC-Black Back DASC-C1

0.9

Sample

Volume fraction (ppm) Density (kg=m3 ) Heat capacity (J=kg K) 0.8

1043.78 1043.95 1044.05 1044.55

3674.29 3674.42 3674.65 3674.83

0.7

Efficiency

Base fluid 0 C3 25 C2 50 C1 100

1

54 l/hr (0.015 kg/s) 72 l/hr (0.020 kg/s) 90 l/hr (0.025 kg/s)

Efficiency

0.9

0.6 0.5 0.4 0.3

0.8

0.2

0.7

0.1

0.6

0 0

0.01

0.02

0.03

0.04

0.05

2

0.5

(Tin-Tamb)/GT (m K/W) Fig. 7. Comparison of DASC efficiency with different working fluids at 72 l/h flowrate.

0.4 0.3 0.2

1 0.1

Base Fluid C3 C2 C1

0.9 0 0

0.01

0.02

0.03

0.04

0.05

0.8

(Tin-Tamb)/GT (m2 K/W)

1

54 l/hr (0.015 kg/s) 72 l/hr (0.020 kg/s) 90 l/hr (0.025 kg/s

0.9

Efficiency

0.7

Fig. 5. Efficiency of DASC with the water/EG mixture as the working fluid in case of black internal surface.

0.6 0.5 0.4 0.3

0.8

0.2 0.7

Efficiency

0.1 0.6

0 0.5

0

0.01

0.02

0.03

0.04

0.05

2

(Tin-Tamb)/GT (m K/W)

0.4

Fig. 8. Effect of nanofluid volume fraction on collector efficiency at 72 l/h flowrate.

0.3 0.2 0.1 0 0

0.01

0.02

0.03

0.04

0.05

2

(Tin-Tamb)/GT (m K/W) Fig. 6. Efficiency of DASC with the CuO nanpfluid (C1 sample) as working fluid.

for various volumetric flowrates of 54, 72, and 90 l/hr. Since the reduced temperature difference is the ratio of heat loss to solar energy intercepted by the collector; the collector efficiency reduced by increasing the reduced temperature difference (i.e. increasing the inlet temperature), as can be seen in Fig. 5. In other words, fluid entering the collector at a lower temperature (lower reduced temperature difference) emitted less total radiation than that entering the collector at higher temperature, because heat emission is proportional to the fourth power of absolute

temperature. Since the collector efficiency was lower in the higher inlet temperatures. It is also found from Fig. 5 that the efficiency increased with flowrate. At low flowrates, the time delay between the entry and exit of working fluid into and out of the collector, i.e. the fluid residence time, is high, allowing for the fluid temperature to rise more. Since heat loss to the ambient including convective and radiative loss increased by temperature (especially, radiation heat loss from the fluid scales with the fourth power of temperature), the fluid suffered higher losses at lower flowrates, which resulted in smaller collector efficiencies. At higher flowrates, the temperature rise in the fluid is small. This resulted in a progressively weaker effect of heat losses described above, and hence, collector efficiencies were seen to be larger at higher flowrates. Fig. 6 presents the variations of collector efficiency with the CuO nanofluid (C1 sample) as the working fluid. It is found that at all flowrates, the collector efficiency is higher than that of with the base fluid as the working fluid. The variation of the efficiency by increasing the flowrate is similar to the base fluid, shown in Fig. 5. It can be concluded from Fig. 6 that the maximum efficiency

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M. Karami et al. / Renewable and Sustainable Energy Reviews 52 (2015) 793–801

Table 4 Values of zero-loss efficiency (η0 ) (%) and heat loss coefficient (a1 ) (W=m2 K) for various working fluids at different flowrates. Working fluid

Flow rate (l/hr)/(kg/s) 54 (0.015)

Base fluid (Reflective internal surface) Base fluid (Black internal surface) CuO nanofluid (C3) CuO nanofluid (C2) CuO nanofluid (C1)

72 (0.02)

90 (0.025)

η0

a1

R2

η0

a1

R2

η0

a1

R2

50.2 58.0 59.1 61.6 64.7

20.18 18.59 18.33 18.66 19.36

0.984 0.998 0.972 0.964 0.970

56.9 66.0 67.8 70.7 74.3

19.98 17.89 17.93 18.24 18.98

0.998 0.999 0.976 0.977 0.994

60.3 71.7 71.4 74.9 77.3

19.85 17.21 17.17 17.82 18.06

0.996 0.976 0.996 0.996 0.997

enhancement with flowrate variation from 54 l/h to 72 l/h (10%) is larger than the enhancement with flowrate variation from 72 l/h to 90 l/h (2.6%). This shows that efficiency enhancement has asymptotic trend with increasing of flowrate. The effect of the emissivity of the bottom wall on the efficiency of the collector with the base fluid as working fluid is indicated in Fig. 7. The bottom wall emissivity is seen to have a significant effect on DAS collector efficiency only at low values of fluid absorption coefficient, i.e. when a significant fraction of the incident solar radiation reached the bottom wall. When the bottom wall is perfect reflector (reflective internal surface), solar radiation incident on the top wall was partly absorbed by the fluid before it reached the bottom, where it got reflected back into the fluid. This reflected radiation was again absorbed partly by the fluid before being transmitted out of the domain through the semi-transparent top wall. However, when the bottom wall is perfect absorber or non-reflective (black internal surface), any radiation reaching there was absorbed by it. This caused the temperature of the bottom wall to rise significantly, and that of the fluid in its vicinity. This resulted in a higher mean fluid temperature at the collector exit, and hence a higher collector efficiency. Comparison of the collector efficiency with both working fluids (the base fluid and CuO nanofluid) at 72 l/h is also illustrated in Fig. 7. The efficiency of the base fluid is shown with both reflective and black internal surfaces. Improvement of zero-loss efficiency (η0 ) of up to 17.8% and 8.7% by utilizing nanofluids than to the base fluid is demonstrated with reflective and black internal surfaces, respectively. The collector efficiency at various volume fractions (i.e. absorption coefficient) for a 72 l/h flowrate is shown in Fig. 8. The fraction of radiation incident on a fluid that is absorbed by it is proportional to the fluid absorption coefficient. It can be resulted from this figure that at constant flowrate, the absorption of solar energy within the nanofluid is increased by increasing nanofluid volume fraction and thus, the collector efficiency is enhanced. The efficiency increased considerably by the addition of small amounts of nanoparticles (25 ppm for C3 sample) than to the base fluid case because of more solar energy absorption within the working fluid, but after that efficiency improvement is marginally by increasing nanofluid volume fraction. This was the consequence of two competing effects at play with increasing volume fraction. For low volume fraction, most of the radiation incident on the top wall penetrated all the way through the fluid layer to the bottom wall. This leads to more uniform temperature profile, which limits the amount of heat loss at the boundaries. For high volume fraction, although the solar energy absorption is increased, but fluid adjacent to the top wall absorbed most of the radiation incident on it, allowing little radiation to penetrate the fluid layer and reach the bottom wall. This resulted in a high temperature region near the top wall, with the attendant heat losses and

reduction in collector efficiency. The trade-off between the two effects (higher absorption and higher heat loss at higher volume fraction) described above accounts for the trend shown in Fig. 8. The zero-loss efficiency,η0 and heat loss coefficient, a1 at each flowrates for all test cases are expressed in Table 4. Since the value deduced for a2 is negative, a first-order fit is used. As can be seen, η0 value of the collector for 90 l/h and C1 nanofluid sample is highest (77.3%), and a1 value in this flowrate and volume fraction is lowest (18.06 W/m2 K), whereas the lowest η0 (50.2%) and the highest a1 (20.18 W/m2 K) are obtained for the base fluid with reflective internal surface at 54 l/h flowrate. From the current study, the calculated collector efficiency (η) _ and volume fraction are correlated with volumetric flowrate (V) (f v ) with 0 r f v r 100 ppm and 54 l=h r V_ r90 l=h for reflective internal surface through the DASC. This correlation can be written as:   η ¼ 0:281V_ þ 35:6 ðf v þ 1Þ0:0565       0:0186 T in  T amb —  0:01V_ þ20:66 f v þ 1 GT where the confidence coefficients are R2 ¼ 98.17% for η0 and R2 ¼98.57% for a1 . 5. Conclusion A prototype of CuO nanofluid-based Direct Absorption Solar Collector (DASC) was built with applicability for domestic solar water heater. Based on the transmittance spectra of the nanofluid samples, light is not able to pass through the sample in the volume fractions more than 100 ppm. The effect of collector internal surface of bottom wall, fluid inlet temperature and flowrate and CuO nanoparticles volume fraction on the efficiency of the collector is experimentally studied. The collector efficiency increased by increasing nanofluid volume fraction and flowrate; however, this increase has asymptotic trend. The results show that zero-loss (maximum) efficiency of the collector for 90 l/h and nanofluid with 100 ppm volume fraction is highest; so that, the collector efficiency is about 17% more than that of using the base fluid as working fluid at similar flowrate. Acknowledgments The authors would like to express their thanks to the Department of Building Installations, Road, Housing and Urban Development Research Center (BHRC) (No. 2012-1313) for the financial supports through the set-up construction and research implementation and also the Center of Excellence in Design and Optimization of Energy Systems, College of Engineering, University of Tehran.

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