Rice University scientists are designing arrays of aligned single-wall carbon nanotubes to channel mid-infrared radiation (heat) and significantly raise the efficiency of solar energy systems. According to the research, carbon nanotube can be just the device to make solar panels that lose energy through heat far more efficiently.

Gururaj Naik and Junichiro Kono of Rice’s Brown School of Engineering introduced their technology in ACS Photonics. Their invention is a hyperbolic thermal emitter that can absorb intense heat that would otherwise be spewed into the atmosphere, squeeze it into a narrow bandwidth and emit it as light that can be turned into electricity. The discovery rests on another by Kono’s group in 2016 when it found a simple method to make highly aligned, wafer-scale films of closely packed nanotubes.

Infrared radiation is a component of sunlight that delivers heat to the planet, but it’s only a small part of the electromagnetic spectrum. The nanotube films have presented an opportunity to isolate mid-infrared photons that would otherwise be wasted.

The aligned nanotube films are conduits that absorb waste heat and turn it into narrow-bandwidth photons. Because electrons in nanotubes can only travel in one direction, the aligned films are metallic in that direction while insulating in the perpendicular direction, an effect Gururaj Naik called ‘hyperbolic dispersion’.

Thermal photons can strike the film from any direction but can only leave via one. Instead of going from heat directly to electricity, it goes from heat to light to electricity. It seems like two stages would be more efficient than three, but here, that’s not the case, according to Naik.

Naik also added that the 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. The theoretical prediction is that we can get 80% efficiency.

Nanotube films suit the task because they stand up to temperatures as high as 1,700 degrees Celsius (3,092 degrees Fahrenheit). Naik’s team built proof-of-concept devices that allowed them to operate at up to 700 C (1,292 F) and confirm their narrow-band output. To make them, the team patterned arrays of submicron-scale cavities into the chip-sized films.

Carbon nanotubes can change the entire economics of solar photovoltaic (PV) project by exponentially increasing the efficiency which will lead to more power generated. In a Mercom report it was pointed out that the efficiency of normal mono and PERC cells are typically 20.3% and 22.2% respectively, while a multi cell has typical efficiency of 18.9%.

Image credit: Rice University