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Comparative study of different nanofluids applied in a trough collector with glass-glass absorber tube
Accepted Manuscript Comparative study of different nanofluids applied in a trough collector with glass-glass absorber tube
Alibakhsh Kasaeian, Reza Daneshazarian, Fathollah Pourfayaz PII: DOI: Reference:
S0167-7322(17)30418-X doi: 10.1016/j.molliq.2017.03.096 MOLLIQ 7131
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
3 February 2017 19 March 2017 26 March 2017
Please cite this article as: Alibakhsh Kasaeian, Reza Daneshazarian, Fathollah Pourfayaz , Comparative study of different nanofluids applied in a trough collector with glass-glass absorber tube. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Molliq(2017), doi: 10.1016/j.molliq.2017.03.096
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ACCEPTED MANUSCRIPT
Comparative study of different nanofluids applied in a trough collector with glass-glass absorber tube Alibakhsh Kasaeiana,* Reza Daneshazariana, Fathollah Pourfayaza Department of Renewable Energies, Faculty of New Sciences & Technologies, University of Tehrah, Tehran, Iran
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Correspondence Author: [email protected], Tel: +9891219475 Fax: +982188497324
Abstract
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New forms of solar collectors are presented as direct absorption solar collectors which have higher performance versus the conventional solar collector. In this work, parabolic trough collector is utilized with three different receivers: a bare glass tube, non-evacuated glass-glass tube, and a vacuumed absorber tube. Nanofluids including 0.2% and 0.3% silica and carbon nanotubes in ethylene glycol as base-fluid are used as working fluid. The outlet temperature and thermal efficiency for five types of working fluids are investigated. The results show that carbon nanotubes have higher temperature and efficiency which are equal to 74.9% and 338.3 K in the vacuumed glass-glass absorber tube. Also, the results indicate that the efficiency of the vacuumed glass-glass tube is averagely 20% higher than bare glass tube. An optimum volume fraction should be achieved due to the agglomeration of the nanoparticles. The optimum point has the volume fraction and thermal efficiency for carbon nanotubes are 0.5, and 80.7%, respectively, and for nanosilica are 0.4 and 70.9%, respectively.
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Keywords: Direct absorption; nanofluid; thermal efficiency; trough collector.
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Nomenclature ๐ด area (m2) ๐ถ concentration ratio (-)
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๐ถ๐ heat capacity (J/kg.K)
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๐ focal distance (m)
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๐ thermal conductivity (W/m.K) ๐ mass (kg)
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๐ค width (m) ๐ Temperature (K)
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๐ volume (m3)
Greek symbols
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๐ viscosity (cP)
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๐ rim angle
Subscripts
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๐ density (kg/m3)
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๐ volume fraction
ap aperture
BF base-fluid ๐๐ nanofluid
p nanoparticle re reflector BF base-fluid 2
ACCEPTED MANUSCRIPT 1. Introduction
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Solar collectorโs enhancement is one of the main parameters in the performance of solar energy systems. The parameters for the enhancement are such as collectorโs geometry optimization, changing the working fluid, and changing material of the absorber tube. One of the well-performed solar collectors is parabolic trough collector which contains three main part, the reflector, the absorber tube, and the working fluid. The reflector reflects the solar beams on its focal line, then the working fluid pumps into the absorber tube and absorbs the heat gained from the reflected solar radiation. Adding solid nanoparticles in the working fluid could improve the efficiency of the collector. Nanoparticles such as multi-wall carbon nanotube, nanosilica, Al2O3, and CuO, in the base-fluid like therminol VP-1, water, and ethylene glycol [1,2]. Coccia et al. [3] investigated the using of nanofluids in a parabolic trough collector numerically. Six water based nanofluid with different weight concentrations were adopted in the system and adding TiO2, Al2O3, Au, and ZnO improved the thermal efficiency. Bellos et al. [4] simulated a commercial trough collector with Solidworks software. They used three different nanofluids including thermal oil, nanoparticles with thermal oil as the base-fluid, and water. The thermal efficiency enhanced about 4.25% by using of nanoparticles in the base-fluid.
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Kasaeian et al. [5] evaluated the performance of the parabolic trough collector with four various absorber tubes by using of nanofluid which contains MWCNT nanoparticles in the therminol VP-1 as base-fluid. Their results showed that 0.3% volume fraction of MWCNT/therminol could enhance the thermal efficiency of the collector about 11% and use vacuumed chrome coated copper-glass absorber tube. A two-phase model for the nanofood application in the trough collector was done numerically by Kaloudis et al. [6]. They used Al 2O3 nanoparticles with 4% concentration in thermal oil. Their results were in good agreement with experimental experiences, and the thermal efficiency has 10% amelioration by using of nanofluid. Sokhansefat et al. [7] investigated the heat transfer improvement by using of Al2O3/synthetic oil in trough collector. According to their results, the thermal efficiency is an ascending function of volume fraction of the nanoparticles.
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Mwesigye et al. [8] used copper nanoparticles in the therminol VP-1 for their research on the parabolic trough collector. The thermal efficiency has 12.5% growth by adding nanoparticles into the base-fluid. Shirvan et al. [9] studied the heat transfer and the pressure drop in a heatdriven heat exchanger which is filled with nanofluid numerically. They simulation was two phase, and sensitivity of the effective parameters was analyzed. The Nusselt number was an ascending parameter of nanoparticles volume fraction and a descending function of Richardson number. Ghasemi and Ranjbar [10] studied the performance of a parabolic trough collector by using of nanofluids. They used computational fluid dynamic for the investigation and two types of nanoparticles โ Al2O3 and CuO โ were employed in the water as base-fluid. Using the nanofluid increased the thermal efficiency, and the Nusselt number was improved around 35% by using 3% volume fraction of CuO in the water. Atashrouz et al. [11] evaluated the thermal conductivity of 22 ionic liquids and the heat source in their model was gained from a parabolic trough collector. Their model has the ability for showing the real physical trends for the various working fluids in the parabolic trough collectors. 3
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Also, considering the direct absorption option for the solar collectors could increase the thermal efficiency along with using nanofluids. For this case, the transparent surface should be prepared in the absorption section of the collectors. Milanese et al. [12] studied the optical properties of nanofluids and the absorptance for the using in the direct absorption parabolic trough collector. Water was used as base-fluid, and six different oxide nanoparticles and their findings show that the transmittance coefficient was increased by passing from the visible range to IR region. Chen et al. [13] evaluated the using of silver nanoparticles for direct absorption purposes. Gold and silver nanoparticle has better photothermal properties of titanium oxide nanoparticles, and this happens because of the profound difference between the absorption spectra of these nanoparticles with the solar radiation spectrum. Gorji and Ranjbar [14] studied the effect of silver, magnetite, and graphite nanofluids experimentally and numerically. The thermal and exergy efficiency were enhanced around 33-57% and 13-20%, respectively comparing with the values of base-fluid (distilled water).
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Vakili et al. [15] studied the graphene nanoplatelets nanofluidโs photothermal properties for low-enthalpy direct absorption collector. The chosen base-fluid was deionized water. The photothermal properties of the working fluid were enhanced in the range of 250-300 nm because of adding nanoparticles. Delfani et al. [16] studied the performance parameters by using MWCNT in water-ethylene glycol mixture in the parabolic trough collector and the nanofluids enhanced the thermal efficiency by 10-29%. The thermal efficiency was ascending function of flow rate and volume fraction of nanoparticles. Shende and Ramaprabhu [17] performed a study on the thermos-optical characters of partially unzipped MWCNT for direct absorption purposes. The thermal conductivity of the working fluid improved due to the nanoparticles dispersion and the amount of amelioration for the DI water was 27% and for ethylene glycol was 20.97%. These nanofluids enhanced the thermal efficiency of flat direct absorption collector. Gupta et al. [18] investigated the performance of a low-temperature direct absorption collector using Al2O3/water nanofluid experimentally, and the efficiency was enhanced by 18.75%-39.6% but there in an optimum point for every nanoparticle.
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Liu et al. [19] performed an experimental and numerical research on the efficiency of the direct absorption collector by using of graphene/ionic water nanofluid. The volume fraction of the nanoparticles increased the thermal efficiency, and the thermal efficiency was around 70% with nanoparticles in the working fluid. Karami et al. [20] investigated the photothermal attributes of CuO nanofluid in the solar direct absorption collectors; thermal conductivity was improved by 13.7% using 100 ppm copper oxide nanofluid and the energy absorption was increased four times more than base-fluid. The capability study using of ionic liquid-based nanofluids in the medium direct absorption solar collectors by Zhang et al. [21]. The hybrid combination of Ni/C was performed well in the solar direct absorption collector. Hatami and Jing [22] evaluated the performance of wavy walled and flat plate solar direct absorption collector by using of nanofluids. TiO2, Al2O3, and CuO nanoparticles were dispersed in the water, and titanium oxide nanofluid had the largest Nusselt number so that the thermal efficiency would improve better by considering the other nanoparticles.
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ACCEPTED MANUSCRIPT According to the literature adding nanoparticles would improve the thermal efficiency and outlet temperature of the solar direct absorption collectors. In this paper, two different nanoparticles are dispersed in the ethylene glycol as base-fluid. Nanosilica and multi-wall carbon nanotubes with 0.2% and 0.3% volume fraction are added to the working fluid. Also, three different absorber tubes are studied including bare glass tube, non-evacuated glass-glass tube and vacuumed glass-glass tube. The optimum volume fraction is achieved for both nanoparticles.
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2. Mathematical modeling
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The design of the collector had to be done before the studying the performance of the collector with various nanofluids. The cross section of the parabolic trough collector and the dimensions of the system are shown in Fig. 1.
Fig. 1. (a) The cross section of the collector (b) The dimensions of the reflector and the receiver in X-Y coordination 5
ACCEPTED MANUSCRIPT 2.1. Reflector In the designed collector, the reflector part is considered to be made of steel mirror sheets with high reflectance coefficient. The length of the reflector is 2m, and its aperture width is 0.7 m. Many researchers suggested the rim angles between 80-90 ฬ, and the 90ฬ rim angle was reported by Kasaeian et al. [5] to be more efficient. The reflector has a parabola shape and the focal line should be obtained as follow: ๐ค๐๐ 2
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By consideration of ๏ฆ = 90 ฬ and wap = 70 cm, the focal distance would be equal to 17.5 cm. The absorber tube should be located precisely in the focal line to absorb all the solar beams. The dimensions of the parabola are given in Table 1.
2.2. The absorber tube
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Table 1. The parabola dimensions Parameter Dimension Focal distance 17.5 cm Rim angle 90 ฬ Reflectorโs length 2m Aperture area 1.4 m2
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The absorber tube is the most important and complicated section in the designing of a parabolic trough collector because of its significant impact on the thermal efficiency and heat transfer of the collector. In this paper, the tubes are made of borosilicate glasses which are located concentrically, and they are shown in the Fig. 2.
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ACCEPTED MANUSCRIPT Fig. 2. The simulated geometry of the concentric glass-glass absorber tube In this absorber tube, the inner tube is chosen to be made of glass, and it was due to the direct absorption of the photons. The parameters of the absorber tube are given in Table 2.
Table 2. The parameters of the absorber tube
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Amount 26 mm 2mm 57mm 3mm 1.5m 0.15m2 0.90
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Parameter Inner glass-inner diameter Thickness of the inner diameter Outer glass-inner diameter Thickness of the outer diameter Tube length The outer area of the absorber tube Transmissivity
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2.3. Nanofluid
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For prevention of the heat losses, the second tube is used, and this would increase the performance of the collector. This absorber tube has two incombustible Teflon in both ends for the couplings and O-rings are used for controlling the thermal tensions on the both glass tubes. The gap between the two tubes should be at the optimum point for preventing the convection heat losses, but it should be mentioned that vacuuming the gap would improve the performance of the collector, and in this method, the heat losses would be dependent on the gap length. The concentration ratio for the collector is calculated as follows:
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Two kind of nanoparticles are used in the ethylene glycol as base-fluid. Properties of the base-fluid are given in Table 3.
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ACCEPTED MANUSCRIPT Table 3. Properties of the ethylene glycol Parameter Amount (Unit) Density at 25 ยฐC 1132 (kg/m3) Thermal conductivity at 25 ยฐC 0.253(W/mk) Viscosity at 25 ยฐC 14.51(cSt) Specific gravity 1.13 Specific heat capacity 2426 (J/kg.K)
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The nanoparticles are selected according to their thermal properties. The thermal conductivity and specific heat capacity of the multi-wall carbon nanotubes are high, and these parameters are vital for direct absorption collector. The dimensions of the nanoparticles are given in Table 4.
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Table 4. Nanoparticleโs dimensions Dimension Inner diameter 4nm, outer diameter 10nm Inner diameter 6nm, outer diameter 15nm
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Nanoparticles Multi-wall carbon nanotube
Cylindrical
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Nanosilica
Geometry Cylindrical multi wall
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The nanofluid could be assumed single phase by neglecting the sliding motion between the nanoparticle and the base-fluid. The radial heat flux was applied one-dimensionally [23]. The properties of the nanofluid are calculated by the mixing law [5].
๐๐ +๐๐ต๐น ) ๐๐
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๐๐๐ (๐) = (1 โ ๐)๐๐ต๐น (๐) + ๐๐๐ = (1 โ ๐)(โ0.8647๐ + 869) + ๐๐๐ (5) โ๐ = โ๐๐ต๐น + 13.427๐ โ (6)
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2.4.Geometry
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The absorber tube was designed horizontally with the given dimension in Table 2. The working fluid passes through the inner glass tube. The entrance velocity (Vin) and the inlet temperature (Tin) were assumed steady, and the conditions for the outlet working fluid was thermally fully developed. The number of meshes in the circumference nodes number (Nc), the Z-axial nodes number (Nz), the radial nodes number (Nr) are equal to 124, 510, and 34, respectively. Increasing the nodes did not enhance the accuracy of the results, and the node numbers were optimized. The iteration calculations were converged after 812 repetitions, and the convergence curve is shown in Fig. 3(a); while the convergences are for the energy, the mass, and the momentum equations. Node numbers were changed, and the independence of the mean convection heat transfer coefficient is tabulated in Table 5, and the mesh is shown in Fig. 3(b).
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Table 5. The mean convection heat transfer coefficient (hc,ave) at 320 K in three different mesh Mesh size (NcรNrรNz) hc,ave for basehc,ave for 0.1% hc,ave for 0.2% fluid (EG) MWCNT/EG MWCNT/EG 124ร34ร510 253.21 286.72 303.83 154ร46ร690 255.35 288.37 304.58 166ร54ร780 256.62 289.91 306.77
Fig. 3. (a) The equations convergence (b) meshing schematic
3. Results and discussions In this section, the results of the simulation research are presented, and temperature contours are shown for three different absorber tubes, bare glass tube, non-evacuated glass-glass tube, and vacuumed glass-glass tube. The temperature distribution of the collector according to the 9
ACCEPTED MANUSCRIPT ASHRAE Standard 93-2010 is evaluated and this standard is about the determining the thermal performance of the solar collectors.
3.1. Solar radiation
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The solar radiation amount should be calculated for Tehran-Iran according to its longitude and latitude. In the mid-part of the graph โ when the end of the spring happens- the sun has its highest radiation and the period between August up October is the best time for energy harvesting. The annually solar radiation is shown in Fig. 4.
Fig. 4. Annual solar radiation in Tehran on a horizontal surface and the highest radiation period
The 3D image of the heat flux on the absorber tube area and length are shown in Fig. 5 (a). In Fig. 5(b)., the dimensions of the reflector and absorber tubes are shown with the same position of Fig. 5(a). The heat flux on the absorber tube is nonuniform, and the maximum amount of the heat flux occurs at the length of 0.4-1.2 m of the absorber tube which has the lowest radiation heat losses. In the reflector aperture area, which is equal to 0.7 m2, there are four patterns; first, the heat flux increased then it decreased before the mid-section, it increased aging after the midsection of the aperture area and at the end the heat flux decreased along the second half of the 10
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aperture area. The decreasing pattern in the mid-section because of the shadow effect on the reflector surface which intercepted the complete reflection.
Fig.5. (a) 3D view of the heat flux on the absorber tube (b) 3D view of the reflector and the absorber tube 3.2. Temperature contours Three types of absorber tubes are studied in this section which are a bare glass tube, nonevacuated glass-glass tube, and the vacuumed glass-glass absorber tube. Two types of nanofluid with 0.2% and 0.3% volume fraction of the nanoparticles in the base-fluid are investigated. The ambient temperature is set on 300K. The working fluids for each contour are different and they are mentioned in the captions of the Figs. 6-8. 11
ACCEPTED MANUSCRIPT ๏ท
Bare glass tube
Non-evacuated glass-glass tube
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In Fig. 6 (a-e)., the temperature contours are for the bare tube. In Fig. 6 (a) with EG as working fluid the temperature is 12.9 K and the thermal efficiency was 52.2%. By adding nanoparticles, the temperature for 0.2% and 0.3% silica nanofluid are increased to 317.1 and 318.7 K with thermal efficiencies of 55.9% and 57.3%, respectively. MWCNT/EG nanofluid is added by 0.2% and 0.3% volume fraction, and the outlet temperature is 322.6 K and 327.2 K and the thermal efficiencies 61% and 65.6%, respectively. The thermal efficiency for the bare glass tube and 0.3% MWCNT increased up to 13.4%.
Vacuumed glass-glass tube
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The temperature contours for non-evacuated glass-glass absorber tube are shown in Fig. 7 (ae). The outlet temperature for the base-fluid is 317.2 K and efficiency of 56.1%, but this amounts for 0.2% silica is 321.6 K and 59.6% and for 0.3% silica is 325.2 K and 63.3%. The temperature growth is observed for the MWCNT/EG. The 0.2% MWCNT gains to 325.9 K, and 64.2% for outlet temperature and thermal efficiency, respectively; for 0.3% MWCNT nanofluid obtains to 331.3 K and the thermal efficiency get to 71.9%. The last nanofluid shows 14.1 K amelioration for the outlet temperature and 15.6% for the thermal efficiency.
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Fig. 8 (a-e) shows the temperature contours for the vacuumed glass-glass absorber tube. This absorber has the highest values for both outlet temperature and thermal efficiency. The enhancement is because of the emission of the convection heat losses around the absorber tube. The thermal efficiency for the base-fluid is 59.5% and the outlet temperature is 321 K. The thermal efficiencies of 0.2% and 0.3% volume fraction of nanosilica reach to 64.1 % and 66.5%. the outlet temperature for these working fluids 325.7 K and 327.7 K, respectively; which show growth of 22.3% and 31.9% in the amount of outlet temperature. For the 0.2% and 0.3% volume fraction of the MWCNT in the ethylene glycol the thermal efficiencies are reached to 68.5% and 74.9%. The outlet temperature is enhanced versus the base-fluid and the outlet temperature for the 0.2% MECNT/EG is achieved to 329.3 K and 333.8 K for the volume fraction of 0.3%.
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Fig. 6. The temperature contours for the bare glass tube with various working fluids (a) ethylene glycol (EG) as the base-fluid (b) 0.2% nanosilica with EG (c) 0.3% nanosilica with EG (d) 0.2% MWCNT with EG (e) 0.3% MWCNT with EG 13
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Fig. 7. The temperature contours for the non-evacuated glass-glass tube with various working fluids (a) ethylene glycol (EG) as the base-fluid (b) 0.2% nanosilica with EG (c) 0.3% nanosilica with EG (d) 0.2% MWCNT with EG (e) 0.3% MWCNT with EG 14
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Fig. 8. The temperature contours for the vacuumed glass-glass tube with various working fluids (a) ethylene glycol (EG) as the base-fluid (b) 0.2% nanosilica with EG (c) 0.3% nanosilica with EG (d) 0.2% MWCNT with EG (e) 0.3% MWCNT with EG 15
ACCEPTED MANUSCRIPT 3.3.Thermally fully developed length
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Fully developed and its length should be studied for complete understanding the absorber conditions. The fully development option is important when a tube is under constant wall temperature or constant heat flux. The solar radiation is a sinusoidal function so it should be changed to an average radiation parameter which was 700 w/m2. The length of thermally fully developed is 191.66 cm and it is shown in Fig. 9.
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Fig. 9. The length of fully developed flow for vacuumed glass-glass absorber tube
3.4. The optimum volume fraction
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In Fig. 10., the thermal efficiency is an ascending function of the volume fraction and the increasing pattern is displayed. Nanofluid with 0.4% volume fraction of the nanoparticles has the thermal efficiency equal to 70.9% but it does not increased sensibley after this volume fraction.
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Fig. 10. The optimum volume fraction for nanosilica in the vacuumed glass-glass absorber tube MWCNT
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The optimum volume fraction for the MWCNT is obtaind at 0.5% and in this point the thermal efficiency is equal to 80.7% which are shown in Fig. 11. The ascending slope of the curve continues but its value deacreased by adding the nanoparticles, so it would not be reaspnable adding additional nanoparticles.
Fig. 11. The optimum volume fraction for MWCNT in the vacuumed glass-glass absorber tube 17
ACCEPTED MANUSCRIPT 3.5. Verification
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The verification of this numerical work shows good agreement with previous work [24]. Menbari et al. [24] investigated the thermal performance of the binary nanofluid which contains CuO/Al2O3 nanoparticles in the ethylene glycol-water mixture. The results of the simulation for the binary nanofluid CuO/Al2O3 are verified with the experimental work, and the results are shown in Figs. 12. The average difference between the results was 1.53% and the error was 4%.
Present Work Menbari et al. [24]
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Fig. 12. The verified results of nanosilica with experimental data
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In this study, the effect of nanofluids on the performance of a solar direct absorption parabolic trough collector is presented. Two main parts which have significant effect on the performance of a trough collectors are the working fluid and the properties of the absorber tube. Longwave radiations such as infrared are neglected in the conventional trough collectors because of its metal-made absorber tube. But in a direct absorption trough collector, the absorber tube is a glass-made to absorb the longwave radiations. For this purpose, three different receivers are studied in this study including a bare glass tube, non-evacuated glass-glass tube, and vacuumed glass-glass tube. The glass tube is made of borosilicate, and it could tolerate against the thermal shocks. Two nanoparticles which are multi-wall carbon nanotube (MWCNT) and nanosilica, are dispersed in the ethylene glycol as base-fluid. The results show that adding the solid nanoparticles enhances the thermal efficiency. The solar beams are trapped in the working fluid due to adding of the nanoparticles in the base-fluid. The presence of the nanoparticles increases the scattering coefficient. 0.3% MWCNT/EG has the highest outlet temperature and thermal 18
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efficiency in three absorber tubes. The outlet temperature and the thermal efficiency of 0.3% MWCNT/EG in the vacuumed glass-glass absorber tube are 338.3 K and 74.9%, respectively; the thermal efficiency of this tube is averagely 20% more than the bare glass tube. Adding the nanoparticles has the disadvantage of clustering in base-fluid, so the optimum volume fraction should be achieved, and the optimum volume fraction for nanosilica and MWCNT is obtained 0.4% and 0.5%, respectively.
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ACCEPTED MANUSCRIPT References [1] Kasaeian A, Eshghi AT, Sameti M. A review on the applications of nanofluids in solar energy systems. Renewable and Sustainable Energy Reviews. 2015; 43: 584-98. [2] Mahian O, Kianifar A, Kalogirou SA, Pop I, Wongwises S. A review of the applications of nanofluids in solar energy. International Journal of Heat and Mass Transfer. 2013;57(2):582-94.
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[3] Coccia G, Di Nicola G, Colla L, Fedele L, Scattolini M. Adoption of nanofluids in low-enthalpy parabolic trough solar collectors: Numerical simulation of the yearly yield. Energy Conversion and Management. 2016;118:306-19.
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[4] Bellos E, Tzivanidis C, Antonopoulos KA, Gkinis G. Thermal enhancement of solar parabolic trough collectors by using nanofluids and converging-diverging absorber tube. Renewable Energy. 2016;94:21322.
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[5] Kasaeian A, Daviran S, Azarian RD, Rashidi A. Performance evaluation and nanofluid using capability study of a solar parabolic trough collector. Energy Conversion and Management. 2015;89:36875.
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[6] Kaloudis E, Papanicolaou E, Belessiotis V. Numerical simulations of a parabolic trough solar collector with nanofluid using a two-phase model. Renewable Energy. 2016;97:218-29.
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[7] Sokhansefat T, Kasaeian AB, Kowsary F. Heat transfer enhancement in parabolic trough collector tube using Al 2 O 3/synthetic oil nanofluid. Renewable and Sustainable Energy Reviews. 2014;33:63644.
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[8] Mwesigye A, Huan Z, Meyer JP. Thermal performance and entropy generation analysis of a high concentration ratio parabolic trough solar collector with Cu-Therminolยฎ VP-1 nanofluid. Energy Conversion and Management. 2016;120:449-65.
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[9] Shirvan K.M., Mamourian M, Mirzakhanlari S, Ellahi R. Two phae simulation and sensitivity analysis of effective parameters on combined heat and pressure drop in a solar heat exchanger filled with nanofluid by RSM. Journal of Molecular Liquids, 2016; 220; 888-901
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[10] Ghasemi S.E., Ranjbar A.A. Thermal performance analysis of solar parabolic trough collector using nanofluid as working fluid: A CFD modelling study. Journal of Molecular Liquids, 2016; 222; 159-166. [11] Saeid Atashrouz S., Hemmati-Sarapardeh A., Mirshekar H., Nasernejad B., Moraveji M.K. On the evaluation of thermal conductivity of ionic liquids: Modeling and data assessment. Journal of Molecular Liquids, 2016; 224; 648-656. [12] Milanese M, Colangelo G, Cretรฌ A, Lomascolo M, Iacobazzi F, de Risi A. Optical absorption measurements of oxide nanoparticles for application as nanofluid in dir absorption solar power systemsโ Part I: Water-based nanofluids behavior. Solar Energy Materials and Solar Cells. 2016;147:315-20. [13] Chen M, He Y, Zhu J, Wen D. Investigating the collector efficiency of silver nanofluids based direct absorption solar collectors. Applied Energy. 2016;181:65-74. 20
ACCEPTED MANUSCRIPT [14] Gorji TB, Ranjbar AA. A numerical and experimental investigation on the performance of a low-flux direct absorption solar collector (DASC) using graphite, magnetite and silver nanofluids. Solar Energy. 2016;135:493-505. [15] Vakili M, Hosseinalipour SM, Delfani S, Khosrojerdi S. Photothermal properties of graphene nanoplatelets nanofluid for low-temperature direct absorption solar collectors. Solar Energy Materials and Solar Cells. 2016;152:187-91.
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[16] Delfani S, Karami M, Akhavan-Behabadi MA. Performance characteristics of a residential-type direct absorption solar collector using MWCNT nanofluid. Renewable Energy. 2016;87:754-64.
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[17] Shende RC, Ramaprabhu S. Thermo-optical properties of partially unzipped multiwalled carbon nanotubes dispersed nanofluids for direct absorption solar thermal energy systems. Solar Energy Materials and Solar Cells. 2016;157:117-25.
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[18] Gupta HK, Agrawal GD, Mathur J. An experimental investigation of a low temperature Al 2O3-H2O nanofluid based direct absorption solar collector. Solar Energy. 2015;118:390-6.
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[19] Liu J, Ye Z, Zhang L, Fang X, Zhang Z. A combined numerical and experimental study on graphene/ionic liquid nanofluid based direct absorption solar collector. Solar Energy Materials and Solar Cells. 2015;136:177-86.
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[20] Karami M, Akhavan-Behabadi MA, Dehkordi MR, Delfani S. Thermo-optical properties of copper oxide nanofluids for direct absorption of solar radiation. Solar Energy Materials and Solar Cells. 2016;144:136-42.
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[21] Zhang L, Liu J, He G, Ye Z, Fang X, Zhang Z. Radiative properties of ionic liquid-based nanofluids for medium-to-high-temperature direct absorption solar collectors. Solar Energy Materials and Solar Cells. 2014;130:521-8.
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[22] Hatami M, Jing D, Evaluation of wavy direct absorption solar collector (DASC) performance using di๏ฌ erent nano๏ฌuids, Journal of Molecular Liquids 2016, doi: 10.1016/j.molliq.2016.12.072 [23] Forristall R. Heat transfer analysis and modeling of a parabolic trough solar receiver implemented in engineering equation solver. National Renewable Energy Laboratory; 2003 Oct 1.
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[24] Menbari A, Alemrajabi AA, Rezaei A, Experimental investigation of thermal performance for direct absorption solar parabolic trough collector (DASPTC) based on binary nanofluids. Experimental Thermal and Fluid Science 2017;80:218-227.
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ACCEPTED MANUSCRIPT Highlights:
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A parabolic trough collector with direct absorption is chosen for this study. Effects of nanofluids on the performance of trough collector is studied. Effect of the direct solar absorption is investigated on the thermal efficiency. Three different kinds of absorber tubes are compared. Effect of increasing the concentration of CNT and nanosilica is investigated.
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Alibakhsh Kasaeian, Reza Daneshazarian, Fathollah Pourfayaz PII: DOI: Reference:
S0167-7322(17)30418-X doi: 10.1016/j.molliq.2017.03.096 MOLLIQ 7131
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
3 February 2017 19 March 2017 26 March 2017
Please cite this article as: Alibakhsh Kasaeian, Reza Daneshazarian, Fathollah Pourfayaz , Comparative study of different nanofluids applied in a trough collector with glass-glass absorber tube. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Molliq(2017), doi: 10.1016/j.molliq.2017.03.096
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Comparative study of different nanofluids applied in a trough collector with glass-glass absorber tube Alibakhsh Kasaeiana,* Reza Daneshazariana, Fathollah Pourfayaza Department of Renewable Energies, Faculty of New Sciences & Technologies, University of Tehrah, Tehran, Iran
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Correspondence Author: [email protected], Tel: +9891219475 Fax: +982188497324
Abstract
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New forms of solar collectors are presented as direct absorption solar collectors which have higher performance versus the conventional solar collector. In this work, parabolic trough collector is utilized with three different receivers: a bare glass tube, non-evacuated glass-glass tube, and a vacuumed absorber tube. Nanofluids including 0.2% and 0.3% silica and carbon nanotubes in ethylene glycol as base-fluid are used as working fluid. The outlet temperature and thermal efficiency for five types of working fluids are investigated. The results show that carbon nanotubes have higher temperature and efficiency which are equal to 74.9% and 338.3 K in the vacuumed glass-glass absorber tube. Also, the results indicate that the efficiency of the vacuumed glass-glass tube is averagely 20% higher than bare glass tube. An optimum volume fraction should be achieved due to the agglomeration of the nanoparticles. The optimum point has the volume fraction and thermal efficiency for carbon nanotubes are 0.5, and 80.7%, respectively, and for nanosilica are 0.4 and 70.9%, respectively.
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Keywords: Direct absorption; nanofluid; thermal efficiency; trough collector.
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Nomenclature ๐ด area (m2) ๐ถ concentration ratio (-)
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๐ถ๐ heat capacity (J/kg.K)
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๐ focal distance (m)
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๐ thermal conductivity (W/m.K) ๐ mass (kg)
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๐ค width (m) ๐ Temperature (K)
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๐ volume (m3)
Greek symbols
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๐ viscosity (cP)
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๐ rim angle
Subscripts
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๐ density (kg/m3)
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๐ volume fraction
ap aperture
BF base-fluid ๐๐ nanofluid
p nanoparticle re reflector BF base-fluid 2
ACCEPTED MANUSCRIPT 1. Introduction
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Solar collectorโs enhancement is one of the main parameters in the performance of solar energy systems. The parameters for the enhancement are such as collectorโs geometry optimization, changing the working fluid, and changing material of the absorber tube. One of the well-performed solar collectors is parabolic trough collector which contains three main part, the reflector, the absorber tube, and the working fluid. The reflector reflects the solar beams on its focal line, then the working fluid pumps into the absorber tube and absorbs the heat gained from the reflected solar radiation. Adding solid nanoparticles in the working fluid could improve the efficiency of the collector. Nanoparticles such as multi-wall carbon nanotube, nanosilica, Al2O3, and CuO, in the base-fluid like therminol VP-1, water, and ethylene glycol [1,2]. Coccia et al. [3] investigated the using of nanofluids in a parabolic trough collector numerically. Six water based nanofluid with different weight concentrations were adopted in the system and adding TiO2, Al2O3, Au, and ZnO improved the thermal efficiency. Bellos et al. [4] simulated a commercial trough collector with Solidworks software. They used three different nanofluids including thermal oil, nanoparticles with thermal oil as the base-fluid, and water. The thermal efficiency enhanced about 4.25% by using of nanoparticles in the base-fluid.
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Kasaeian et al. [5] evaluated the performance of the parabolic trough collector with four various absorber tubes by using of nanofluid which contains MWCNT nanoparticles in the therminol VP-1 as base-fluid. Their results showed that 0.3% volume fraction of MWCNT/therminol could enhance the thermal efficiency of the collector about 11% and use vacuumed chrome coated copper-glass absorber tube. A two-phase model for the nanofood application in the trough collector was done numerically by Kaloudis et al. [6]. They used Al 2O3 nanoparticles with 4% concentration in thermal oil. Their results were in good agreement with experimental experiences, and the thermal efficiency has 10% amelioration by using of nanofluid. Sokhansefat et al. [7] investigated the heat transfer improvement by using of Al2O3/synthetic oil in trough collector. According to their results, the thermal efficiency is an ascending function of volume fraction of the nanoparticles.
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Mwesigye et al. [8] used copper nanoparticles in the therminol VP-1 for their research on the parabolic trough collector. The thermal efficiency has 12.5% growth by adding nanoparticles into the base-fluid. Shirvan et al. [9] studied the heat transfer and the pressure drop in a heatdriven heat exchanger which is filled with nanofluid numerically. They simulation was two phase, and sensitivity of the effective parameters was analyzed. The Nusselt number was an ascending parameter of nanoparticles volume fraction and a descending function of Richardson number. Ghasemi and Ranjbar [10] studied the performance of a parabolic trough collector by using of nanofluids. They used computational fluid dynamic for the investigation and two types of nanoparticles โ Al2O3 and CuO โ were employed in the water as base-fluid. Using the nanofluid increased the thermal efficiency, and the Nusselt number was improved around 35% by using 3% volume fraction of CuO in the water. Atashrouz et al. [11] evaluated the thermal conductivity of 22 ionic liquids and the heat source in their model was gained from a parabolic trough collector. Their model has the ability for showing the real physical trends for the various working fluids in the parabolic trough collectors. 3
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Also, considering the direct absorption option for the solar collectors could increase the thermal efficiency along with using nanofluids. For this case, the transparent surface should be prepared in the absorption section of the collectors. Milanese et al. [12] studied the optical properties of nanofluids and the absorptance for the using in the direct absorption parabolic trough collector. Water was used as base-fluid, and six different oxide nanoparticles and their findings show that the transmittance coefficient was increased by passing from the visible range to IR region. Chen et al. [13] evaluated the using of silver nanoparticles for direct absorption purposes. Gold and silver nanoparticle has better photothermal properties of titanium oxide nanoparticles, and this happens because of the profound difference between the absorption spectra of these nanoparticles with the solar radiation spectrum. Gorji and Ranjbar [14] studied the effect of silver, magnetite, and graphite nanofluids experimentally and numerically. The thermal and exergy efficiency were enhanced around 33-57% and 13-20%, respectively comparing with the values of base-fluid (distilled water).
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Vakili et al. [15] studied the graphene nanoplatelets nanofluidโs photothermal properties for low-enthalpy direct absorption collector. The chosen base-fluid was deionized water. The photothermal properties of the working fluid were enhanced in the range of 250-300 nm because of adding nanoparticles. Delfani et al. [16] studied the performance parameters by using MWCNT in water-ethylene glycol mixture in the parabolic trough collector and the nanofluids enhanced the thermal efficiency by 10-29%. The thermal efficiency was ascending function of flow rate and volume fraction of nanoparticles. Shende and Ramaprabhu [17] performed a study on the thermos-optical characters of partially unzipped MWCNT for direct absorption purposes. The thermal conductivity of the working fluid improved due to the nanoparticles dispersion and the amount of amelioration for the DI water was 27% and for ethylene glycol was 20.97%. These nanofluids enhanced the thermal efficiency of flat direct absorption collector. Gupta et al. [18] investigated the performance of a low-temperature direct absorption collector using Al2O3/water nanofluid experimentally, and the efficiency was enhanced by 18.75%-39.6% but there in an optimum point for every nanoparticle.
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Liu et al. [19] performed an experimental and numerical research on the efficiency of the direct absorption collector by using of graphene/ionic water nanofluid. The volume fraction of the nanoparticles increased the thermal efficiency, and the thermal efficiency was around 70% with nanoparticles in the working fluid. Karami et al. [20] investigated the photothermal attributes of CuO nanofluid in the solar direct absorption collectors; thermal conductivity was improved by 13.7% using 100 ppm copper oxide nanofluid and the energy absorption was increased four times more than base-fluid. The capability study using of ionic liquid-based nanofluids in the medium direct absorption solar collectors by Zhang et al. [21]. The hybrid combination of Ni/C was performed well in the solar direct absorption collector. Hatami and Jing [22] evaluated the performance of wavy walled and flat plate solar direct absorption collector by using of nanofluids. TiO2, Al2O3, and CuO nanoparticles were dispersed in the water, and titanium oxide nanofluid had the largest Nusselt number so that the thermal efficiency would improve better by considering the other nanoparticles.
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ACCEPTED MANUSCRIPT According to the literature adding nanoparticles would improve the thermal efficiency and outlet temperature of the solar direct absorption collectors. In this paper, two different nanoparticles are dispersed in the ethylene glycol as base-fluid. Nanosilica and multi-wall carbon nanotubes with 0.2% and 0.3% volume fraction are added to the working fluid. Also, three different absorber tubes are studied including bare glass tube, non-evacuated glass-glass tube and vacuumed glass-glass tube. The optimum volume fraction is achieved for both nanoparticles.
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2. Mathematical modeling
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The design of the collector had to be done before the studying the performance of the collector with various nanofluids. The cross section of the parabolic trough collector and the dimensions of the system are shown in Fig. 1.
Fig. 1. (a) The cross section of the collector (b) The dimensions of the reflector and the receiver in X-Y coordination 5
ACCEPTED MANUSCRIPT 2.1. Reflector In the designed collector, the reflector part is considered to be made of steel mirror sheets with high reflectance coefficient. The length of the reflector is 2m, and its aperture width is 0.7 m. Many researchers suggested the rim angles between 80-90 ฬ, and the 90ฬ rim angle was reported by Kasaeian et al. [5] to be more efficient. The reflector has a parabola shape and the focal line should be obtained as follow: ๐ค๐๐ 2
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By consideration of ๏ฆ = 90 ฬ and wap = 70 cm, the focal distance would be equal to 17.5 cm. The absorber tube should be located precisely in the focal line to absorb all the solar beams. The dimensions of the parabola are given in Table 1.
2.2. The absorber tube
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Table 1. The parabola dimensions Parameter Dimension Focal distance 17.5 cm Rim angle 90 ฬ Reflectorโs length 2m Aperture area 1.4 m2
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The absorber tube is the most important and complicated section in the designing of a parabolic trough collector because of its significant impact on the thermal efficiency and heat transfer of the collector. In this paper, the tubes are made of borosilicate glasses which are located concentrically, and they are shown in the Fig. 2.
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ACCEPTED MANUSCRIPT Fig. 2. The simulated geometry of the concentric glass-glass absorber tube In this absorber tube, the inner tube is chosen to be made of glass, and it was due to the direct absorption of the photons. The parameters of the absorber tube are given in Table 2.
Table 2. The parameters of the absorber tube
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Amount 26 mm 2mm 57mm 3mm 1.5m 0.15m2 0.90
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Parameter Inner glass-inner diameter Thickness of the inner diameter Outer glass-inner diameter Thickness of the outer diameter Tube length The outer area of the absorber tube Transmissivity
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2.3. Nanofluid
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For prevention of the heat losses, the second tube is used, and this would increase the performance of the collector. This absorber tube has two incombustible Teflon in both ends for the couplings and O-rings are used for controlling the thermal tensions on the both glass tubes. The gap between the two tubes should be at the optimum point for preventing the convection heat losses, but it should be mentioned that vacuuming the gap would improve the performance of the collector, and in this method, the heat losses would be dependent on the gap length. The concentration ratio for the collector is calculated as follows:
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Two kind of nanoparticles are used in the ethylene glycol as base-fluid. Properties of the base-fluid are given in Table 3.
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The nanoparticles are selected according to their thermal properties. The thermal conductivity and specific heat capacity of the multi-wall carbon nanotubes are high, and these parameters are vital for direct absorption collector. The dimensions of the nanoparticles are given in Table 4.
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Table 4. Nanoparticleโs dimensions Dimension Inner diameter 4nm, outer diameter 10nm Inner diameter 6nm, outer diameter 15nm
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Nanoparticles Multi-wall carbon nanotube
Cylindrical
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Nanosilica
Geometry Cylindrical multi wall
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The nanofluid could be assumed single phase by neglecting the sliding motion between the nanoparticle and the base-fluid. The radial heat flux was applied one-dimensionally [23]. The properties of the nanofluid are calculated by the mixing law [5].
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The absorber tube was designed horizontally with the given dimension in Table 2. The working fluid passes through the inner glass tube. The entrance velocity (Vin) and the inlet temperature (Tin) were assumed steady, and the conditions for the outlet working fluid was thermally fully developed. The number of meshes in the circumference nodes number (Nc), the Z-axial nodes number (Nz), the radial nodes number (Nr) are equal to 124, 510, and 34, respectively. Increasing the nodes did not enhance the accuracy of the results, and the node numbers were optimized. The iteration calculations were converged after 812 repetitions, and the convergence curve is shown in Fig. 3(a); while the convergences are for the energy, the mass, and the momentum equations. Node numbers were changed, and the independence of the mean convection heat transfer coefficient is tabulated in Table 5, and the mesh is shown in Fig. 3(b).
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Table 5. The mean convection heat transfer coefficient (hc,ave) at 320 K in three different mesh Mesh size (NcรNrรNz) hc,ave for basehc,ave for 0.1% hc,ave for 0.2% fluid (EG) MWCNT/EG MWCNT/EG 124ร34ร510 253.21 286.72 303.83 154ร46ร690 255.35 288.37 304.58 166ร54ร780 256.62 289.91 306.77
Fig. 3. (a) The equations convergence (b) meshing schematic
3. Results and discussions In this section, the results of the simulation research are presented, and temperature contours are shown for three different absorber tubes, bare glass tube, non-evacuated glass-glass tube, and vacuumed glass-glass tube. The temperature distribution of the collector according to the 9
ACCEPTED MANUSCRIPT ASHRAE Standard 93-2010 is evaluated and this standard is about the determining the thermal performance of the solar collectors.
3.1. Solar radiation
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The solar radiation amount should be calculated for Tehran-Iran according to its longitude and latitude. In the mid-part of the graph โ when the end of the spring happens- the sun has its highest radiation and the period between August up October is the best time for energy harvesting. The annually solar radiation is shown in Fig. 4.
Fig. 4. Annual solar radiation in Tehran on a horizontal surface and the highest radiation period
The 3D image of the heat flux on the absorber tube area and length are shown in Fig. 5 (a). In Fig. 5(b)., the dimensions of the reflector and absorber tubes are shown with the same position of Fig. 5(a). The heat flux on the absorber tube is nonuniform, and the maximum amount of the heat flux occurs at the length of 0.4-1.2 m of the absorber tube which has the lowest radiation heat losses. In the reflector aperture area, which is equal to 0.7 m2, there are four patterns; first, the heat flux increased then it decreased before the mid-section, it increased aging after the midsection of the aperture area and at the end the heat flux decreased along the second half of the 10
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aperture area. The decreasing pattern in the mid-section because of the shadow effect on the reflector surface which intercepted the complete reflection.
Fig.5. (a) 3D view of the heat flux on the absorber tube (b) 3D view of the reflector and the absorber tube 3.2. Temperature contours Three types of absorber tubes are studied in this section which are a bare glass tube, nonevacuated glass-glass tube, and the vacuumed glass-glass absorber tube. Two types of nanofluid with 0.2% and 0.3% volume fraction of the nanoparticles in the base-fluid are investigated. The ambient temperature is set on 300K. The working fluids for each contour are different and they are mentioned in the captions of the Figs. 6-8. 11
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Bare glass tube
Non-evacuated glass-glass tube
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In Fig. 6 (a-e)., the temperature contours are for the bare tube. In Fig. 6 (a) with EG as working fluid the temperature is 12.9 K and the thermal efficiency was 52.2%. By adding nanoparticles, the temperature for 0.2% and 0.3% silica nanofluid are increased to 317.1 and 318.7 K with thermal efficiencies of 55.9% and 57.3%, respectively. MWCNT/EG nanofluid is added by 0.2% and 0.3% volume fraction, and the outlet temperature is 322.6 K and 327.2 K and the thermal efficiencies 61% and 65.6%, respectively. The thermal efficiency for the bare glass tube and 0.3% MWCNT increased up to 13.4%.
Vacuumed glass-glass tube
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The temperature contours for non-evacuated glass-glass absorber tube are shown in Fig. 7 (ae). The outlet temperature for the base-fluid is 317.2 K and efficiency of 56.1%, but this amounts for 0.2% silica is 321.6 K and 59.6% and for 0.3% silica is 325.2 K and 63.3%. The temperature growth is observed for the MWCNT/EG. The 0.2% MWCNT gains to 325.9 K, and 64.2% for outlet temperature and thermal efficiency, respectively; for 0.3% MWCNT nanofluid obtains to 331.3 K and the thermal efficiency get to 71.9%. The last nanofluid shows 14.1 K amelioration for the outlet temperature and 15.6% for the thermal efficiency.
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Fig. 8 (a-e) shows the temperature contours for the vacuumed glass-glass absorber tube. This absorber has the highest values for both outlet temperature and thermal efficiency. The enhancement is because of the emission of the convection heat losses around the absorber tube. The thermal efficiency for the base-fluid is 59.5% and the outlet temperature is 321 K. The thermal efficiencies of 0.2% and 0.3% volume fraction of nanosilica reach to 64.1 % and 66.5%. the outlet temperature for these working fluids 325.7 K and 327.7 K, respectively; which show growth of 22.3% and 31.9% in the amount of outlet temperature. For the 0.2% and 0.3% volume fraction of the MWCNT in the ethylene glycol the thermal efficiencies are reached to 68.5% and 74.9%. The outlet temperature is enhanced versus the base-fluid and the outlet temperature for the 0.2% MECNT/EG is achieved to 329.3 K and 333.8 K for the volume fraction of 0.3%.
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Fig. 6. The temperature contours for the bare glass tube with various working fluids (a) ethylene glycol (EG) as the base-fluid (b) 0.2% nanosilica with EG (c) 0.3% nanosilica with EG (d) 0.2% MWCNT with EG (e) 0.3% MWCNT with EG 13
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Fig. 7. The temperature contours for the non-evacuated glass-glass tube with various working fluids (a) ethylene glycol (EG) as the base-fluid (b) 0.2% nanosilica with EG (c) 0.3% nanosilica with EG (d) 0.2% MWCNT with EG (e) 0.3% MWCNT with EG 14
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Fig. 8. The temperature contours for the vacuumed glass-glass tube with various working fluids (a) ethylene glycol (EG) as the base-fluid (b) 0.2% nanosilica with EG (c) 0.3% nanosilica with EG (d) 0.2% MWCNT with EG (e) 0.3% MWCNT with EG 15
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Fully developed and its length should be studied for complete understanding the absorber conditions. The fully development option is important when a tube is under constant wall temperature or constant heat flux. The solar radiation is a sinusoidal function so it should be changed to an average radiation parameter which was 700 w/m2. The length of thermally fully developed is 191.66 cm and it is shown in Fig. 9.
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Fig. 9. The length of fully developed flow for vacuumed glass-glass absorber tube
3.4. The optimum volume fraction
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In Fig. 10., the thermal efficiency is an ascending function of the volume fraction and the increasing pattern is displayed. Nanofluid with 0.4% volume fraction of the nanoparticles has the thermal efficiency equal to 70.9% but it does not increased sensibley after this volume fraction.
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Fig. 10. The optimum volume fraction for nanosilica in the vacuumed glass-glass absorber tube MWCNT
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The optimum volume fraction for the MWCNT is obtaind at 0.5% and in this point the thermal efficiency is equal to 80.7% which are shown in Fig. 11. The ascending slope of the curve continues but its value deacreased by adding the nanoparticles, so it would not be reaspnable adding additional nanoparticles.
Fig. 11. The optimum volume fraction for MWCNT in the vacuumed glass-glass absorber tube 17
ACCEPTED MANUSCRIPT 3.5. Verification
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The verification of this numerical work shows good agreement with previous work [24]. Menbari et al. [24] investigated the thermal performance of the binary nanofluid which contains CuO/Al2O3 nanoparticles in the ethylene glycol-water mixture. The results of the simulation for the binary nanofluid CuO/Al2O3 are verified with the experimental work, and the results are shown in Figs. 12. The average difference between the results was 1.53% and the error was 4%.
Present Work Menbari et al. [24]
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Fig. 12. The verified results of nanosilica with experimental data
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In this study, the effect of nanofluids on the performance of a solar direct absorption parabolic trough collector is presented. Two main parts which have significant effect on the performance of a trough collectors are the working fluid and the properties of the absorber tube. Longwave radiations such as infrared are neglected in the conventional trough collectors because of its metal-made absorber tube. But in a direct absorption trough collector, the absorber tube is a glass-made to absorb the longwave radiations. For this purpose, three different receivers are studied in this study including a bare glass tube, non-evacuated glass-glass tube, and vacuumed glass-glass tube. The glass tube is made of borosilicate, and it could tolerate against the thermal shocks. Two nanoparticles which are multi-wall carbon nanotube (MWCNT) and nanosilica, are dispersed in the ethylene glycol as base-fluid. The results show that adding the solid nanoparticles enhances the thermal efficiency. The solar beams are trapped in the working fluid due to adding of the nanoparticles in the base-fluid. The presence of the nanoparticles increases the scattering coefficient. 0.3% MWCNT/EG has the highest outlet temperature and thermal 18
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efficiency in three absorber tubes. The outlet temperature and the thermal efficiency of 0.3% MWCNT/EG in the vacuumed glass-glass absorber tube are 338.3 K and 74.9%, respectively; the thermal efficiency of this tube is averagely 20% more than the bare glass tube. Adding the nanoparticles has the disadvantage of clustering in base-fluid, so the optimum volume fraction should be achieved, and the optimum volume fraction for nanosilica and MWCNT is obtained 0.4% and 0.5%, respectively.
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ACCEPTED MANUSCRIPT References [1] Kasaeian A, Eshghi AT, Sameti M. A review on the applications of nanofluids in solar energy systems. Renewable and Sustainable Energy Reviews. 2015; 43: 584-98. [2] Mahian O, Kianifar A, Kalogirou SA, Pop I, Wongwises S. A review of the applications of nanofluids in solar energy. International Journal of Heat and Mass Transfer. 2013;57(2):582-94.
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[3] Coccia G, Di Nicola G, Colla L, Fedele L, Scattolini M. Adoption of nanofluids in low-enthalpy parabolic trough solar collectors: Numerical simulation of the yearly yield. Energy Conversion and Management. 2016;118:306-19.
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[4] Bellos E, Tzivanidis C, Antonopoulos KA, Gkinis G. Thermal enhancement of solar parabolic trough collectors by using nanofluids and converging-diverging absorber tube. Renewable Energy. 2016;94:21322.
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[5] Kasaeian A, Daviran S, Azarian RD, Rashidi A. Performance evaluation and nanofluid using capability study of a solar parabolic trough collector. Energy Conversion and Management. 2015;89:36875.
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[6] Kaloudis E, Papanicolaou E, Belessiotis V. Numerical simulations of a parabolic trough solar collector with nanofluid using a two-phase model. Renewable Energy. 2016;97:218-29.
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[7] Sokhansefat T, Kasaeian AB, Kowsary F. Heat transfer enhancement in parabolic trough collector tube using Al 2 O 3/synthetic oil nanofluid. Renewable and Sustainable Energy Reviews. 2014;33:63644.
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[8] Mwesigye A, Huan Z, Meyer JP. Thermal performance and entropy generation analysis of a high concentration ratio parabolic trough solar collector with Cu-Therminolยฎ VP-1 nanofluid. Energy Conversion and Management. 2016;120:449-65.
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[9] Shirvan K.M., Mamourian M, Mirzakhanlari S, Ellahi R. Two phae simulation and sensitivity analysis of effective parameters on combined heat and pressure drop in a solar heat exchanger filled with nanofluid by RSM. Journal of Molecular Liquids, 2016; 220; 888-901
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[10] Ghasemi S.E., Ranjbar A.A. Thermal performance analysis of solar parabolic trough collector using nanofluid as working fluid: A CFD modelling study. Journal of Molecular Liquids, 2016; 222; 159-166. [11] Saeid Atashrouz S., Hemmati-Sarapardeh A., Mirshekar H., Nasernejad B., Moraveji M.K. On the evaluation of thermal conductivity of ionic liquids: Modeling and data assessment. Journal of Molecular Liquids, 2016; 224; 648-656. [12] Milanese M, Colangelo G, Cretรฌ A, Lomascolo M, Iacobazzi F, de Risi A. Optical absorption measurements of oxide nanoparticles for application as nanofluid in dir absorption solar power systemsโ Part I: Water-based nanofluids behavior. Solar Energy Materials and Solar Cells. 2016;147:315-20. [13] Chen M, He Y, Zhu J, Wen D. Investigating the collector efficiency of silver nanofluids based direct absorption solar collectors. Applied Energy. 2016;181:65-74. 20
ACCEPTED MANUSCRIPT [14] Gorji TB, Ranjbar AA. A numerical and experimental investigation on the performance of a low-flux direct absorption solar collector (DASC) using graphite, magnetite and silver nanofluids. Solar Energy. 2016;135:493-505. [15] Vakili M, Hosseinalipour SM, Delfani S, Khosrojerdi S. Photothermal properties of graphene nanoplatelets nanofluid for low-temperature direct absorption solar collectors. Solar Energy Materials and Solar Cells. 2016;152:187-91.
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[16] Delfani S, Karami M, Akhavan-Behabadi MA. Performance characteristics of a residential-type direct absorption solar collector using MWCNT nanofluid. Renewable Energy. 2016;87:754-64.
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[17] Shende RC, Ramaprabhu S. Thermo-optical properties of partially unzipped multiwalled carbon nanotubes dispersed nanofluids for direct absorption solar thermal energy systems. Solar Energy Materials and Solar Cells. 2016;157:117-25.
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[18] Gupta HK, Agrawal GD, Mathur J. An experimental investigation of a low temperature Al 2O3-H2O nanofluid based direct absorption solar collector. Solar Energy. 2015;118:390-6.
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[19] Liu J, Ye Z, Zhang L, Fang X, Zhang Z. A combined numerical and experimental study on graphene/ionic liquid nanofluid based direct absorption solar collector. Solar Energy Materials and Solar Cells. 2015;136:177-86.
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[20] Karami M, Akhavan-Behabadi MA, Dehkordi MR, Delfani S. Thermo-optical properties of copper oxide nanofluids for direct absorption of solar radiation. Solar Energy Materials and Solar Cells. 2016;144:136-42.
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[21] Zhang L, Liu J, He G, Ye Z, Fang X, Zhang Z. Radiative properties of ionic liquid-based nanofluids for medium-to-high-temperature direct absorption solar collectors. Solar Energy Materials and Solar Cells. 2014;130:521-8.
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[22] Hatami M, Jing D, Evaluation of wavy direct absorption solar collector (DASC) performance using di๏ฌ erent nano๏ฌuids, Journal of Molecular Liquids 2016, doi: 10.1016/j.molliq.2016.12.072 [23] Forristall R. Heat transfer analysis and modeling of a parabolic trough solar receiver implemented in engineering equation solver. National Renewable Energy Laboratory; 2003 Oct 1.
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[24] Menbari A, Alemrajabi AA, Rezaei A, Experimental investigation of thermal performance for direct absorption solar parabolic trough collector (DASPTC) based on binary nanofluids. Experimental Thermal and Fluid Science 2017;80:218-227.
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ACCEPTED MANUSCRIPT Highlights:
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A parabolic trough collector with direct absorption is chosen for this study. Effects of nanofluids on the performance of trough collector is studied. Effect of the direct solar absorption is investigated on the thermal efficiency. Three different kinds of absorber tubes are compared. Effect of increasing the concentration of CNT and nanosilica is investigated.
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