Researchers have found a way to control the thermal emissions of high-temperature radiation in a way that could lead to the efficient harvesting of waste heat from power plants and factories.
The technique uses a thermal metamaterial – nanoscale layers of tungsten and hafnium oxide –to suppress thermal emissions in one portion of the spectrum while enhancing emissions in another. The idea is to use the metamaterial to restrict emission of thermal radiation to a specific portion of the spectrum – in other words, controlling a hot object to radiate only certain wavelengths. The wavelengths desired are those at which electrons in photovoltaic cells absorb energy.
Shining light on a photovoltaic cell causes the cell electrons to absorb energy, elevating them into the higher energy conduction band. As the electrons move to the conduction band, they leave behind holes in the valence band. The region between both bands is the band gap. Energy hitting the device that is at an energy below the band gap is generally wasted. High-efficiency thermal energy conversion comes from suppressing the thermal emission below the band gap and enhancing it above the band gap.
The new metamaterial enhances the high-energy part of the emission spectrum and suppresses the low-energy thermal photons. The result is the emission of light only within the energy spectrum above the band gap.
Metamaterials are composite media that contain features, patterns or elements such as tiny nanoantennas that enable an unprecedented control of light. Under development for about 15 years, the metamaterials owe their unusual abilities to precision design and manufacture on the scale of nanometers. So far they have been used mainly to manipulate coherent light, as in a laser. Researchers say the ability to manipulate infrared thermal radiation at 1,000°C opens up new areas of research.
The work resulted from a collaboration among researchers at Purdue University, Hamburg University of Technology in Germany, the University of Alberta in Canada, and Helmholtz-Zentrum Geesthacht Centre for Materials and Coastal Research in Germany. The technique used to get thermal suppression and enhancement harnesses a phenomenon called topological transitions.
Future research will include work to convert heat radiation from a thermal metamaterial to electron-hole pairs in a semiconducting material, a critical step in developing the technology. Researchers think the thermophotovoltaic technology might be ready for commercialization within seven years.