Heat recovery device can boost efficiency of solar energy systems

Wasted heat is a problem in many industrial sectors. For some, generating heat costs money, and if it’s allowed to leak out of systems then that impacts on profitability. For other sectors, it's simply a lost opportunity. In solar energy, for example, heat lost to the environment is energy that has not been converted into electricity – the sole purpose of the equipment.

Engineers at Rice University in Texas have used the results of some previous research into producing close-packed arrays of precisely aligned carbon nanotubes to address this issue. In a paper in the journal ACS Photonics, they describe how such arrays have the potential to boost the efficiency of photovoltaic panels from around 22 per cent to a theoretical maximum of 80 per cent.

A group of researchers led by Junichiro Koro of the Brown School of Engineering at Rice discovered a simple method to create such nanotube arrays on the scale of a semiconductor wafer in 2016. Koro then recruited a newcomer to Rice, Gururaj Naik, to work on a project to see whether this discovery could be put to use to direct thermal photons. "Thermal photons are just photons emitted from a hot body," Kono said. "If you look at something hot with an infrared camera, you see it glow. The camera is capturing these thermally excited photons."

The issue with thermal photons is that they occur over a wide spread of frequencies in the infrared part of the electromagnetic spectrum, and conversion to electricity is only efficient when it focuses on a very narrow spread of frequencies. Naik determined that the nanotube arrays could isolate mid-infrared photons that would otherwise be wasted. "That's the motivation," Naik said. "A study by (co-lead author and Rice graduate student) Chloe Doiron found that about 20% of our industrial energy consumption is waste heat. That's about three years of electricity just for the state of Texas. That's a lot of energy being wasted.”

The nanotube arrays work by redirecting the energy of the infrared photons. Perpendicular to their axis, they are insulators – an effect Naik calls hyperbolic dispersion. Whichever direction a thermal photon hits the nanotubes, it can only leave via one, and by travelling down the nanotube the frequency shifts, converting infrared  into a narrow band of visible light which can be converted into electricity by photovoltaics. "Instead of going from heat directly to electricity, we go from heat to light to electricity," Naik said. "It seems like two stages would be more efficient than three, but here, that's not the case."

Carbon nanotubes are particularly suited to this task because of their thermal stability. The Rice team has built proof-of-concept devices capable of operating at temperatures up to 700°C. Naik claims that these could be added to solar cells to boost their efficiency. "The most efficient way to turn heat into electricity now is to use turbines, and steam or some other liquid to drive them," he said. "They can give you nearly 50% conversion efficiency. Nothing else gets us close to that, but those systems are not easy to implement. 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."