A microscope image shows cavities patterned into a film of carbon nanotubes which trap thermal photons and convert them into light to be converted by a PV cell.

Waste heat is a hot topic among PV researchers since harnessing it could greatly increase the output and operational lifetime of solar systems.

There is broad scope for solutions to the challenge, including sophisticated cooling systems, thermoelectric generation and hybrid systems, which take the heat away and use it for other purposes. The latest suggestion, by scientists at Rice University in Texas, involves using carbon nanotubes to convert waste heat into light which could then be converted to electricity in a solar cell.

“Any hot surface emits light as thermal radiation,” said Gururaj Naik, assistant professor of electrical and computer engineering at Rice. “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.”

Potentially huge leap in efficiency

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The team worked to create a device that could squeeze the photons emitted as heat into a narrower band that could be absorbed by a solar cell. The proof of concept device they developed is described in the paper Macroscopically Aligned Carbon Nanotubes as a Refractory Platform for Hyperbolic Thermal Emitters, published in the journal ACS Photonics.

The device uses a film of carbon nanotubes which electrons can only travel through in one direction and the researchers claim it could operate at temperatures of up to 700 degrees Celsius. The next step for the research will be to combine the ‘hyperbolic thermal emitter’ device with a solar cell.

“By squeezing all the wasted thermal energy into a small spectral region we can turn it into electricity very efficiently,” said Naik, “the theoretical prediction is that we can get 80% efficiency.”