Gao et al. ACS Photonics, 2019

Xinwe Li, a graduate student from Rice University (left), and Weilu Gao, postdoctoral researcher. Gao contributed to the development of a device to recycle heat lost in photovoltaic cells. This could ultimately improve the efficiency of industrial waste heat recovery. Credits: Jeff Fitlow.

Bibliography:



Macroscopically Aligned Carbon Nanotubes as a Refractory Platform for Hyperbolic Thermal Emitters

Weilu GaoChloe F. DoironXinwei LiJunichiro KonoGururaj V. Naik*

ACS Photonics2019671602-1609

https://doi.org/10.1021/acsphotonics.9b00452

Solar cells, or photovoltaic cells, which "turn" sunlight into electricity, are a brilliant element of modern technology. However, one particular aspect has proved to be an important problem: they are not very effective. Indeed, the majority of sunlight absorbed is lost, in the form of heat. As a result, the average efficiency of a commercial solar panel is between 11 and 22% only. But now, a new device could increase this figure up to 80%, which would be absolutely revolutionary.This new design is based on a set of single-walled carbon nanotubes, which recaptures "thermal" photons from the infrared radiation (heat) lost from solar panels. Then, the device emits this energy in the form of light in a different wavelength, which can in turn be recycled into electricity.Thermal photons are just photons emitted by a hot body ," said Junichiro Kono of Rice University. " If you look at something hot with an infrared camera, you will see it shine. The camera captures exactly those thermally excited photons , "Kono added.It should be known that infrared radiation is the part of the sunlight that carries heat. Of course, this is invisible to the human eye, but is on the same electromagnetic spectrum as visible light, radio waves, and X-rays.This type of infrared radiation is emitted by your stove, by a campfire or even by your cat purring on your lap. In other words, basically, anything that emits heat emits radiation. " The problem is that the thermal radiation is broadband, while the conversion of light into electricity is only effective if the emission is narrowband. The challenge was to bring broadband photons into a narrow band , "said engineer Gururaj Naik.One of the properties of nanotubes is that the electrons in them can only move in one direction. This produces an effect known as hyperbolic dispersion, in which the films are metal conductors (in one direction), but insulate perpendicular to that direction.This means that thermal photons can enter from (almost) anywhere, but they can only escape in one direction. This process converts heat into light and from there it can be converted into electricity.In the device created by the researchers, the carbon nanotube film can withstand temperatures up to 700 degrees Celsius, although the base material is able to withstand a much higher temperature, up to 1600 degrees Celsius.Then, the research team exposed their device to a heat source to confirm the narrowband output. Each of the resonator cavities in the film reduced the thermal photon band, producing light. The next research step will therefore be to collect this light using photovoltaic solar cells and to confirm the predicted efficiency.By compressing all the thermal energy wasted in a small spectral region, we can transform it into electricity very efficiently. The theoretical prediction is that we can achieve efficiency up to 80%! Naik said.