CNT-based metamaterial turns heat into light
The refractory hyperbolic material described in the ACS Photonics journal under the title “Macroscopically Aligned Carbon Nanotubes as a Refractory Platform for Hyperbolic Thermal Emitters”, consists of stacks of highly aligned and closely packed single-wall carbon nanotubes in deeply subwavelength-sized cavities. The macroscopic alignement and stacking of the nanotubes is obtained through a simple vacuum filtration method the group reported back in 2016.
Due to the strong quantum confinement at play in the nanometer-scale features of the one-dimensional material, the researcher were able to exploit its extremely anisotropic electronic, optical and thermal properties. In fact, each stack of aligned CNTs behaves as an uniaxial anisotropic medium, highly conductive along the nanotubes’s main axis but insulating in the perpendicular plane. It was observed that when thermal photons hit the stacks (from any direction), they can only leave via one, turning IR frequencies from a broad spectrum into a narrow bandwidth IR radiation that would be more efficiently converted into electricity.
“Any hot surface emits light as thermal radiation,” explains corresponding author Gururaj Naik. “The problem is that thermal radiation is broadband, while the conversion of light to electricity is efficient only if the emission is in a narrow band. The challenge was to squeeze broadband photons into a narrow band”.
Today, waste heat accounts for 67% of all energy used in direct energy production in the United States, note the authors. And today, the most efficient way to turn heat back into electricity is to use turbines, and steam or some other liquid to drive them, with a conversion efficiency under 50%. “Instead of going from heat directly to electricity, we go from heat to light to electricity,” explains Naik. “It seems like two stages would be more efficient than three, but here, that’s not the case.”
The new metamaterial could prove to be also much more cost effective than today’s large heat-transfer and turbines installations. Designed on a thin-film substrate, they could also be used to harvest extra energy from untapped industrial heat. Naik estimates that adding the so-called hyperbolic thermal emitters to standard solar cells could boost their efficiency from the current peak of about 22%. “By squeezing all the wasted thermal energy into a small spectral region, we can turn it into electricity very efficiently,” he said. “The theoretical prediction is that we can get 80% efficiency.”
A proof-of-concept CNT-based device designed at the lab was shown to operate at up to 700ºC and output narrow-band IR light.
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