Electrophoretically deposited carbon nanotube spectrally selective solar absorbers

Three types of carbon nanotubes (CNT) have been investigated regarding their suitability as spectrally selective solar thermal absorbers. The CNT coatings were electrophoretically deposited on aluminum substrates using kinetically stable CNT aqueous suspensions, of which two CNT aqueous suspensions (N-CNT and P-CNT suspensions) were prepared as part of this study and the third one (T-CNT suspension) was purchased. The CNT suspension systems are simple and consist of only CNTs, DI water and a cathodic surfactant. Heat treated CNT coatings are visibly uniform. The CNT coating thickness, surface morphology and reflectance of CNT absorbers were characterized by White Light Interferometry, Scanning Electron Microscopy and Spectrophotometry, respectively. T-CNT absorber achieved a solar absorptance α of 0.79 and a thermal emittance ε of 0.14. N- and P-CNT absorbers achieved better spectral selectivity, with α=0.90 and ε=0.14 for N-CNT absorber, α=0.90 and ε=0.13 for P-CNT absorber. The effect of CNT coating thickness, deposition parameters and peak heat treatment temperature on the spectral selectivity were studied. For these CNT suspension systems, the thickness and the heat treatment peak temperature are the key factors to achieving a good spectral selectivity.