Dual use of carbon nanotube selective coatings in evacuated tube solar collectors

Continuous carbon nanotube sheets spun from the side of vertically aligned forests were investigated as solar selective coatings in evacuated tube collectors for solar thermal applications and for Joule heating in the absence of solar radiation. Forests grown by chemical vapor deposition at various synthesis temperatures produced sheets with tunable thermal and optical properties. The sheets were laminated on miniature and regular size evacuated tube collectors and tested at 100 mW/cm2 AM 1.5 solar illumination under high vacuum. It was found that 10 layers of carbon nanotube sheets with medium sheet areal density produced the best solar selective coating. Interestingly, carbon nanotube sheet densification with organic solvents slightly increased the reflectance and significantly decreased the emissivity; improving the thermal transport even further. By taking advantage of the Joule heating effect, carbon nanotube sheets provide fast and efficient heating rates on demand, which can be useful in solar water heating systems when solar radiation is insufficient. Compared with commercial coatings, carbon nanotube sheets were found to have higher absorption and lower reflection in the visible and near UV region of the solar spectrum, making them ideal solar selective coatings for evacuated tube collectors even in overcast weather.

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