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.2011 Mar 15;6(1):225.
doi: 10.1186/1556-276X-6-225.

Nanofluid optical property characterization: towards efficient direct absorption solar collectors

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Nanofluid optical property characterization: towards efficient direct absorption solar collectors

Robert A Taylor et al. Nanoscale Res Lett..

Abstract

Suspensions of nanoparticles (i.e., particles with diameters < 100 nm) in liquids, termed nanofluids, show remarkable thermal and optical property changes from the base liquid at low particle loadings. Recent studies also indicate that selected nanofluids may improve the efficiency of direct absorption solar thermal collectors. To determine the effectiveness of nanofluids in solar applications, their ability to convert light energy to thermal energy must be known. That is, their absorption of the solar spectrum must be established. Accordingly, this study compares model predictions to spectroscopic measurements of extinction coefficients over wavelengths that are important for solar energy (0.25 to 2.5 μm). A simple addition of the base fluid and nanoparticle extinction coefficients is applied as an approximation of the effective nanofluid extinction coefficient. Comparisons with measured extinction coefficients reveal that the approximation works well with water-based nanofluids containing graphite nanoparticles but less well with metallic nanoparticles and/or oil-based fluids. For the materials used in this study, over 95% of incoming sunlight can be absorbed (in a nanofluid thickness ≥10 cm) with extremely low nanoparticle volume fractions - less than 1 × 10-5, or 10 parts per million. Thus, nanofluids could be used to absorb sunlight with a negligible amount of viscosity and/or density (read: pumping power) increase.

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Figures

Figure 1
Figure 1
Scattering regime map showing the boundary between dependent and independent scattering[30].
Figure 2
Figure 2
Maxwell-Garnett approximation of the real part of the refractive index for water-based nanofluids. The numbers in the legend represent the volume fractions of the specified nanofluids with 30 nm of average particle size.
Figure 3
Figure 3
Maxwell-Garnett modeling of the extinction coefficient for water-based nanofluids. Where "MG" is the calculated value based on the Maxwell-Garnett model (Equation 10) and "EXP" are measured values.
Figure 4
Figure 4
Diagram of the three-slab system representation for a spectrometry measurement of a nanofluid-filled quartz cuvette.
Figure 5
Figure 5
Modeled and experimental extinction coefficients for several concentrations of aqueous graphite nanofluids. Experimental results for pure water and water with 5 % surfactant are also plotted for comparison.
Figure 6
Figure 6
Extinction coefficients - measurements versus modeling for promising water-based "solar nanofluids". The curve which is the lowest on the right part of the graph represents the irradiance directly hitting a normal surface for a mid-latitude summer location in the United States.
Figure 7
Figure 7
Extinction coefficients for Therminol VP-1-based "solar nanofluids". Bottom curve shows experimental results for the pure base fluid, Therminol VP-1.
Figure 8
Figure 8
Extinction for different particle diameters and the absorption of water in a 0.004-vol.% silver nanofluid. "EXP" = experimental results for silver with manufacturer-quoted 40 nm of average particle size.
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References

    1. Choi S, Siginer DA, Wang HP. Enhancing thermal conductivity of fluids with nanoparticles. Developments and applications of non-Newtonian flows. ASME. 1995;231(MD 66):99–105.
    1. Prasher RS, Bhattacharya P, Phelan PE. Thermal conductivity of nanoscale colloidal solutions (nanofluids) Phys Rev Letters. 2005;94:025901. doi: 10.1103/PhysRevLett.94.025901. - DOI - PubMed
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    1. Taylor RA, Phelan PE, Otanicar T, Adrian RJ, Prasher RS. Vapor generation in a nanoparticle liquid suspension using a focused, continuous laser. Appl Phys Lett. p. 161907. - DOI
    1. Otanicar T, Taylor RA, Phelan PE, Prasher R. Impact of size and scattering mode on the optimal solar absorbing nanofluid. San Francisco: ASME ES; 2009. p. 90066.

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