Carbon Nanotube Spectrally Selective Solar Absorbers

The use of multi-walled carbon nanotubes (MWCNTs) as spectrally selective solar thermal absorbers (SSA) in solar thermal collectors is presented in this paper. CNT layers were coated by electrophoretic deposition (EPD) on aluminum substrates. Stable aqueous CNT suspensions consisted of multi-walled CNTs, DI water and surfactant was used for EPD. The deposited samples were heat treated in a tube oven before any analysis. Various thicknesses and compositions of homogenous CNT coatings were prepared by tuning the electrophoretic parameters, such as voltage, inter-electrode spacing and deposition time. EPD proved to be a simple, feasible and environment-friendly process of fabricating SSA and has low chemical consumption. In order to achieve homogenous and good quality CNT films an EPD voltage of at least 15V has to be used. The effect of thickness and heat treatment of CNT coatings on the solar absorptance and thermal emittance was investigated. With thicker CNT coating, the absorption of SSA increased in both visible and IR spectral range. The results have indicated CNTs as a promising material for SSA. Different peak temperatures and the dwell time at the peak temperature during the heat treatment were applied to understand the effect. Higher heat treatment peak temperatures resulted in a lower thermal emittance without decreasing the solar absorptance; hence the spectral selectivity of CNT absorbers was improved. The same effect has been seen for longer dwell time at the heat treatment peak temperature