engineering careers  Rice device converts heat to light
engineering careers  Rice device converts heat to light

A new carbon nanotube film created by Engineers at Rice University converts waste heat into light and electricity.

The new invention could make solar panels (and anything that wastes energy through heat) much more efficient in future. Currently, 20% of our industrial energy consumption is waste heat. The invention could be a game-changer in the long term.

This electron microscope image shows submicron-scale cavities patterned into a film. This is made up of carbon nanotubes that trap thermal photons and narrow their bandwidth. This changes them into light that can then be recycled as electricity.

The team’s breakthrough invention works by a clever array of aligned single-wall carbon ‘nanotubes’ which can channel heat (aka mid-infrared radiation) into visible light. This raises the efficiency of solar energy systems.

What is a Nanotube?

Carbon nanotubes (CNT) are a class of nanomaterials that consist of a two-dimensional hexagonal lattice of carbon atoms, bent and joined in one direction so as to form a hollow cylinder.

A Carbon nanotubes (CNT)

Thanks to their nanostructure and to the strength of the bonds between atoms, these cylindrical carbon molecules have exceptional mechanical stiffness and tensile strength. They are also chemical stabile and have high electrical conductivity as well as extraordinary thermal conductivity

How does new device work?

Initially, Rice University used simulations to show how an array of cavities patterned into a film of aligned carbon nanotubes would work. When optimized, this new film absorbs thermal photons and emits light in a narrow bandwidth that can be recycled as electricity.

The team, led by Gururaj Naik and Junichiro Kono of Rice’s Brown School of Engineering, revealed the new tech at ACS Photonics. Rice University researchers, from left, Xinwei Li, Junichiro Kono, Weilu Gao and Gururaj Naik

The device is classed as a hyperbolic thermal emitter. These are materials which absorb intense heat and squeeze it into a narrow bandwidth which it can then emit as light. Light can then be turned into electricity.

To understand why the device works you first need to understand that when you look at something hot the glow you see is the result of heat being emitted as thermally excited photons – also known as light. That means that ‘heat’ is really just light.

This is even more obvious when you consider that the planets largest source of Infrared radiation is light emitted by the sun.

The problem has always been that while we have solar technology that can convert the narrow band of visible light to electricity; thermal radiation is emitted in the broadband spectrum. The challenge the team faced was how to squeeze broadband photons into a narrow band.

The nanotube films gave them the opportunity to isolate mid-infrared photons that would otherwise be wasted and convert it into visible light. This works because electrons in nanotubes can only travel in one direction. The aligned films are metallic in that direction and insulating in the perpendicular direction (angle of 90° that line). This is known as hyperbolic dispersion.

The short version is that thermal photons can hit the film from any direction, but can only leave via one.

In theory, the Nanotube film can work up in temperatures as high as 1,700 degrees Celsius. Naik’s proof-of-concept devices can currently operate at up to 700C.

Currently, waste heat in industrial processes is captured and turned into electricity using turbines, and steam or some other liquids. While the conversation rate is almost 50%, the system is expensive to implement and involve many moving parts.

The team believe that the tech could be worked into existing solar cells to boost their efficiency from the current peak of about 22%. However, theoretical technology could allow 80% efficiency.