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Research Papers

Potential Heat Transfer Fluids (Nanofluids) for Direct Volumetric Absorption-Based Solar Thermal Systems

[+] Author and Article Information
Vikrant Khullar

Mechanical Engineering Department,
Thapar University,
Patiala 147001, Punjab, India
e-mail: vikrant.khullar@thapar.edu

Vishal Bhalla, Himanshu Tyagi

School of Mechanical, Materials and
Energy Engineering,
Indian Institute of Technology Ropar,
Rupnagar 140001, Punjab, India

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF THERMAL SCIENCE AND ENGINEERING APPLICATIONS. Manuscript received December 31, 2016; final manuscript received April 17, 2017; published online July 19, 2017. Assoc. Editor: Jingchao Zhang.

J. Thermal Sci. Eng. Appl 10(1), 011009 (Jul 19, 2017) (13 pages) Paper No: TSEA-16-1397; doi: 10.1115/1.4036795 History: Received December 31, 2016; Revised April 17, 2017

Nanoparticle dispersions or more popularly “nanofluids” have been extensively researched for their candidature as working fluid in direct-volumetric-absorption solar thermal systems. Flexibility in carving out desired thermophysical and optical properties has lend the nanofluids to be engineered for solar thermal and photovoltaic applications. The key feature which delineates nanofluid-based direct absorption volumetric systems from their surface absorption counterparts is that here the working fluid actively (directly) interacts with the solar irradiation and hence enhances the overall heat transfer of the system. In this work, a host of nanoparticle materials have been evaluated for their solar-weighted absorptivity and heat transfer enhancements relative to the basefluid. It has been found that solar-weighted absorptivity is the key feature that makes nanoparticle dispersions suitable for solar thermal applications (maximum enhancement being for the case of amorphous carbon nanoparticles). Subsequently, thermal conductivity measurements reveal that enhancements on the order of 1–5% could only be achieved through addition of nanoparticles into the basefluid. Furthermore, dynamic light scattering (DLS) and optical measurements (carried out for as prepared, 5 h old and 24 h old samples) reveal that nanoclustering and hence soft agglomeration does happen but it does not have significant impact on optical properties of the nanoparticles. Finally, as a proof-of-concept experiment, a parabolic trough collector employing the amorphous carbon-based nanofluid and distilled water has been tested under the sun. These experiments have been carried out at no flow condition so that appreciable temperatures could be reached in less time. It was found that for the same exposure time, increase in the temperature of amorphous carbon based nanofluid is approximately three times higher as compared to that in the case of distilled water.

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References

Figures

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Fig. 1

Schematic showing the heat transfer mechanism in (a) surface absorption-based solar collector, and (b) nanofluid-based volumetric absorption solar thermal collectors

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Fig. 2

Spectral transmittance in the short wavelength region (0.2–2.5 μm) for (a) distilled water, (b) ethylene glycol, (c) perfluorodecalin, (d) silicone oil, (e) paraffin oil (light), (f) propylene glycol, and (g) paraffin oil (heavy). Sample thickness is 10 mm.

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Fig. 3

Spectral transmittance in the long wavelength region (2.5–25 μm) for (a) distilled water, (b) ethylene glycol, (c) perfluorodecalin, (d) silicone oil, (e) paraffin oil (light), (f) propylene glycol, and (g) paraffin oil (heavy). Sample thickness is 10 mm.

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Fig. 4

Spectral index of absorption (κ) in the long wavelength region (2.5–25 μm) for (a) distilled water and (b) silicone oil: comparison of the calculated values with those available in the literature [33,34]

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Fig. 5

Calculated values of spectral transmittance for (a) distilled water and (b) silicone oil. Sample thickness of 10 mm.

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Fig. 6

Spectral transmittance of various concentrations for (a) aluminum nanoparticles, (b) aluminum oxide nanoparticles, (c) amorphous carbon nanoparticles, (d) carbon-coated cobalt nanoparticles, (e) cobalt oxide nanoparticles, (f) copper nanoparticles, (g) MWCNTs, (h) nickel nanoparticles, and (i) silver nanoparticles in ethylene glycol. Sample thickness is 10 mm.

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Fig. 7

Solar-weighted absorptivity (a) as a function of nanoparticle concentration for nanoparticle dispersions employing aluminum, aluminum oxide, amorphous carbon, carbon-coated cobalt, cobalt oxide, copper, MWCNTs, nickel, and silver nanoparticles, and (b) for basefluids such as distilled water, ethylene glycol, silicone oil, perfluorodecalin, propylene glycol, paraffin oil (light), and paraffin oil (heavy)

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Fig. 8

Transmission electron microscope (TEM) images of nanoparticles

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Fig. 9

Ratio of thermal conductivity values for nanoparticle dispersion (kND) to that of the ethylene glycol (kEG) as a function of nanoparticle concentration for various nanoparticle dispersions employing aluminum, aluminum oxide, amorphous carbon, carbon-coated cobalt, cobalt oxide, copper, MWCNTs, nickel, and silver nanoparticles

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Fig. 10

DLS measurements to assess nanoparticle size distribution as a function of time: measurements made for the cases namely—as prepared (average particle size: 79.15218 ± 0.0783 nm), 5 h old (average particle size: 142.50866 ± 0.19082 nm), and 24 h old sample (average particle size 240.8 ± 53.48676 nm)

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Fig. 11

(a) Optical properties of nanofluids as a function of time: measurements made for the cases namely—as prepared, 5 h old, and 24 h old sample, photograph of the (b) as prepared, (c) 5 h old, and (d) 24 h old nanofluid samples

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Fig. 12

Schematic of the direct volumetric absorption solar thermal system employed in the study

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Fig. 13

(a) Schematic diagram of the volumetric receivers employing distilled water and amorphous carbon-based nanofluid, and (b) temperature rise as a function of solar irradiance exposure time for the two working fluids

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