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.
Highlights
Due to the large number of data points, the measured/experimental results are shown as lines while the modeling results are shown as marker curves
Equation 16 is used to manipulate reference text data from the complex refractive index, kEXP, to the extinction coefficients shown in the plot
Particle size was discredited as the root of poor model predictions for metals
Summary
Nanofluids, or suspensions of nanoparticles in liquids, have been studied for at least 15 years and have shown promise to enhance a wide range of liquid properties [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]. 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]. At most, a factor of ten difference (and in many cases less than 100% change) in the real part of the refractive index between the bulk particle material and the base fluid, this approach gives rather accurate results. Keff for water is many orders of magnitude (approximately ten) less than that of metal nanoparticles Due to this large difference, the MaxwellGarnett theory is generally not an accurate approach to obtain the extinction coefficient for nanofluids. It should be noted that the volume-weighted average yields particle sizes that lie between number and intensity-weighted averages
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