A new application of carbon nanotubes nanofluid as working fluid of low-temperature direct absorption solar collector

• Functionalized CNT nanofluid is introduced as working fluid of solar collectors.
• Spectral absorbency analysis confirms the stability of nanofluids with sediment time.
• Extinction coefficient of nanofluid show remarkable improvement compared to the base fluid.
• Thermal conductivity improvement of up to 32% is obtained by adding only 150 ppm f-CNT to water.

Nanofluids are found to have good stability and useful optical and thermal properties as direct sunlight absorbers in solar collectors. The inherent hydrophobic nature of carbon nanotubes was overcome using a new dispersion procedure (treating carbon nanotubes with base media) to prepare nanofluids. To the authors' knowledge, this is the first application of aqueous suspension based on alkaline functionalized carbon nanotubes as an absorber fluid in a sunlight harvesting device. Dispersion stability and optical properties of the nanofluid were estimated. Spectral absorbance analysis confirms the relative stability of prepared nanofluids versus sediment time. The extinction coefficient of aqueous suspensions of functionalized carbon nanotubes shows remarkable improvement compared to the base fluid even at low particle loadings. We also demonstrate thermal conductivity improvements of up to 32% by adding only 150 ppm functionalized carbon nanotubes to water as the absorbing medium. Their promising optical and thermal properties, together with the appropriate stability of nanofluids, make them very interesting for increasing the overall efficiency of low-temperature direct absorption solar collectors.