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Nanofluid optical property characterization: towards efficient direct absorption solar collectors.

Taylor RA, Phelan PE, Otanicar TP, Adrian R, Prasher R - Nanoscale Res Lett (2011)

Bottom Line: To determine the effectiveness of nanofluids in solar applications, their ability to convert light energy to thermal energy must be known.A simple addition of the base fluid and nanoparticle extinction coefficients is applied as an approximation of the effective nanofluid extinction coefficient.Thus, nanofluids could be used to absorb sunlight with a negligible amount of viscosity and/or density (read: pumping power) increase.

View Article: PubMed Central - HTML - PubMed

Affiliation: Arizona State University, Tempe, AZ, USA. Rataylo2@asu.edu.

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.

No MeSH data available.


Scattering regime map showing the boundary between dependent and independent scattering [30].
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Figure 1: Scattering regime map showing the boundary between dependent and independent scattering [30].

Mentions: In general, for cost-effective absorption, particles must be made from low-cost, highly absorbing materials - such as graphite and metals. Resultant properties of these fluids will be modeled in this section. As a first step in determining optical properties of these nanofluids, we must find the optical properties of the bulk materials used to create the nanofluid. That is, we need to know the complex refractive index (or dielectric constant) of the base fluid and of the bulk nanoparticle material. These can be found for many pure substances in an optical properties handbook, such as Palik [29]. Given this information, it is usually possible to calculate the optical properties of the nanofluid mixture. However, this can be very difficult if the nanofluid is a strongly scattering medium. At higher particle concentrations (typically more than 0.6 vol.%), dependent and multiple scattering phenomena can play a role since the particles are closely packed [30]. However, it turns out for any solar collection with sizable absorption path lengths (anything thicker than 1 mm), an effective solar collector can be achieved at very low volume fractions. Figure 1 is a scattering regime map which helps visualize how 'solar nanofluids' compare to other common fluids. (The figure is modified from Tien [30].) Note that the particle size parameter, α, in Figure 1 is defined as [30]:(2)


Nanofluid optical property characterization: towards efficient direct absorption solar collectors.

Taylor RA, Phelan PE, Otanicar TP, Adrian R, Prasher R - Nanoscale Res Lett (2011)

Scattering regime map showing the boundary between dependent and independent scattering [30].
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211283&req=5

Figure 1: Scattering regime map showing the boundary between dependent and independent scattering [30].
Mentions: In general, for cost-effective absorption, particles must be made from low-cost, highly absorbing materials - such as graphite and metals. Resultant properties of these fluids will be modeled in this section. As a first step in determining optical properties of these nanofluids, we must find the optical properties of the bulk materials used to create the nanofluid. That is, we need to know the complex refractive index (or dielectric constant) of the base fluid and of the bulk nanoparticle material. These can be found for many pure substances in an optical properties handbook, such as Palik [29]. Given this information, it is usually possible to calculate the optical properties of the nanofluid mixture. However, this can be very difficult if the nanofluid is a strongly scattering medium. At higher particle concentrations (typically more than 0.6 vol.%), dependent and multiple scattering phenomena can play a role since the particles are closely packed [30]. However, it turns out for any solar collection with sizable absorption path lengths (anything thicker than 1 mm), an effective solar collector can be achieved at very low volume fractions. Figure 1 is a scattering regime map which helps visualize how 'solar nanofluids' compare to other common fluids. (The figure is modified from Tien [30].) Note that the particle size parameter, α, in Figure 1 is defined as [30]:(2)

Bottom Line: To determine the effectiveness of nanofluids in solar applications, their ability to convert light energy to thermal energy must be known.A simple addition of the base fluid and nanoparticle extinction coefficients is applied as an approximation of the effective nanofluid extinction coefficient.Thus, nanofluids could be used to absorb sunlight with a negligible amount of viscosity and/or density (read: pumping power) increase.

View Article: PubMed Central - HTML - PubMed

Affiliation: Arizona State University, Tempe, AZ, USA. Rataylo2@asu.edu.

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.

No MeSH data available.