Nanofluid optical property characterization: towards efficient direct absorption solar collectors |
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Authors: | Robert A Taylor Patrick E Phelan Todd P Otanicar Ronald Adrian Ravi Prasher |
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Affiliation: | (1) Arizona State University, Tempe, AZ, USA;(2) Loyola Marymount University, Los Angeles, CA, USA |
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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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