Carbon Nanotube Spectrally Selective Solar Thermal Absorbers

Abstract

The main objectives of this work were to investigate a) the feasibility of depositing multi-walled carbon nanotubes (MWCNTs) homogeneously on metal substrates by electrophoresis and b) the performance of MWCNT coatings as spectrally selective absorbers. Stable aqueous MWCNT suspensions were prepared and employed in electrophoretic deposition (EPD) of MWCNT coatings on aluminum substrates. The surface morphologies of MWCNT coatings were examined. Spectral reflectance of the MWCNT absorbers was measured to assess their spectral selectivity. Accelerated ageing tests on the MWCNT absorbers were carried out in order to evaluate long-term durability. The deposited MWCNT coatings are homogeneous and exhibit good spectral selectivity combined with the underlying aluminum substrate. The best MWCNT absorber spectral selectivity achieved was a solar absorptance of 0.90 and a thermal emittance of 0.13 which are slightly under par with the performance of currently available commercial absorber products. Accelerated ageing tests revealed that MWCNT absorbers had an excellent thermal stability however it was not resistant to damage from condensation. Therefore, silica/silica-titania thin films were coated on top of the MWCNT coating as a protective layer. The protective films had little impact on the spectral selectivity of MWCNT absorbers and improved the long-term durability significantly. After 600 hours of testing, the performance criterion (PC) value was lower than 0.015, which confirms that the MWCNT absorbers coated with protective films can be qualified according to ISO 22975-3.