Optical characterisation and photothermal conversion efficiency of a water-based carbon nanofluid for direct solar absorption applications

Carbon nanoparticles are very useful in solar thermal applications as they absorb much of the solar spectrum and can be inexpensive. Water-based carbon nanofluids with two different concentrations (3 and 33 mg l(-1)) were prepared with sodium dodecyl sulphate as surfactant to achieve good high-temperature stability with a constant mean particle size of 200 nm at 25 degrees C and 85 degrees C. The morphology of the nanoparticles was observed by Transmission Electron Microscopy and the particle size distribution was studied using Dynamic Light Scattering at room and high temperature. Ballistic transmittance, absorption coefficient and scattering albedo of the three fluids were measured by a spectrophotometer with and without an integrating sphere using the Kubelka-Munk theory. The average value of the absorption coefficient showed important increases when comparing water against the nanofluid with the highest concentration (from 0.1 to 3.3 cm(-1)). Finally, the temperature change achieved when lighting the samples with an artificial sunlight simulator were measured and photothermal conversion efficiencies were evaluated, with increases of up to 200% when comparing nanofluid and base fluid. The results of this study show this kind of nanofluids to be very interesting for increasing the overall efficiency of the direct absorber solar collectors. (C) 2020 Elsevier Ltd. All rights reserved.