Web Date: November 3, 2011
Turning Heat Into Electricity
A thin-film device turns an annoying optical effect into a means to convert infrared radiation directly into electricity (Nano Lett., DOI:10.1021/nl203196z). The device is still in its early stages, but researchers hope it will lead to inexpensive, flexible films that could recoup wasted energy when they are wrapped around hot machines such as industrial generators or car engines.
The hotter an object is the more infrared radiation it produces. Harvesting energy from this radiation could help make machinery run more efficiently. But infrared radiation is difficult to take advantage of, says Nicholas Melosh, a materials scientist at Stanford University. The materials used in most conventional solar cells, which also convert light into electricity, don’t work with infrared light.
To tap into that energy source, Melosh and his colleague, Fuming Wang, had to turn to different materials—metals, instead of the semiconductors used in solar cells. Their energy harvester relies on the physics of a photon hitting a metal surface: The photon’s energy can produce a wave called a plasmon. But plasmon waves don’t last long before they transfer their energy to electrons in the metal. This decay process causes optics researchers, who want to exploit plasmonic effects, to wring their hands.
To make use of the high-energy electrons produced by the plasmon decay, the researchers designed a simple energy-harvesting device: two thin-film metal electrodes sandwiching an insulating layer. When light hits the top electrode, it generates plasmons, which in turn decay and excite electrons in the metal. These so-called hot electrons jump across the insulating layer to the other electrode, generating a current. The Stanford researchers showed that both infrared and visible light can generate electricity.
“The beauty of this is that it takes an effect that’s usually considered an undesirable byproduct, and uses a simple device structure to turn it into a usable current,” says Pieter Kik, a physicist at the University of Central Florida.
The Stanford group’s first devices have “terrible” efficiency, Melosh admits, converting less than 1% of the light energy into electrical power. However, he has found ways to reach 4 to 5% efficiency for converting infrared light. At that level of efficiency, he says, the devices could pay for themselves by harvesting lost energy generated by industrial machines.
This story was updated on November 4, 2011, to change the headline.
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