Infrared: A new renewable energy source?

"We’re talking about the use of physics at the nanoscale for a completely new application," says Federico Capasso. (Image courtesy of Steven J. Byrnes.)
“We’re talking about the use of physics at the nanoscale for a completely new application,” says Federico Capasso. (Image courtesy of Steven J. Byrnes.)
Harvard physicists propose a device to capture energy from earth’s infrared emissions to outer space.

When the sun sets on a remote desert outpost and solar panels shut down, what energy source will provide power through the night? A battery, perhaps, or an old diesel generator? Perhaps something strange and new.

Physicists at the Harvard School of Engineering and Applied Sciences (SEAS) envision a device that would harvest energy from Earth’s infrared emissions into outer space.

Heated by the sun, our planet is warm compared to the frigid vacuum beyond. Thanks to recent technological advances, the researchers say, that heat imbalance could soon be transformed into direct-current (DC) power, taking advantage of a vast and untapped energy source.

Their analysis of the thermodynamics, practical concerns, and technological requirements will be published this week in the Proceedings of the National Academy of Sciences.

“It’s not at all obvious, at first, how you would generate DC power by emitting infrared light in free space toward the cold,” says principal investigator Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at Harvard SEAS. “To generate power by emitting, not by absorbing light, that’s weird. It makes sense physically once you think about it, but it’s highly counterintuitive. We’re talking about the use of physics at the nanoscale for a completely new application.”

Challenging convention

Capasso is a world-renowned expert in semiconductor physics, photonics, and solid-state electronics. He co-invented the infrared quantum-cascade laser in 1994, pioneered the field of bandgap engineering, and demonstrated an elusive quantum electrodynamical phenomenon called the repulsive Casimir force—work for which he has received the SPIE Gold Medal, the European Physical Society Prize for Quantum Electronics and Optics, and the Jan Czochralski Award for lifetime achievement. His research team seems to specialize in rigorously questioning dated assumptions about optics and electronics.

“The mid-IR has been, by and large, a neglected part of the spectrum,” says Capasso. “Even for spectroscopy, until the quantum cascade laser came about, the mid-IR was considered a very difficult area to work with. People simply had blinders on.”

Now, Capasso and his research team are proposing something akin to a photovoltaic solar panel, but instead of capturing incoming visible light, the device would generate electric power by releasing infrared light.

“Sunlight has energy, so photovoltaics make sense; you’re just collecting the energy. But it’s not really that simple, and capturing energy from emitting infrared light is even less intuitive,” says lead author Steven J. Byrnes (AB ’07), a postdoctoral fellow at SEAS. “It’s not obvious how much power you could generate this way, or whether it’s worthwhile to pursue, until you sit down and do the calculation.”

As it turns out, the power is modest but real.

As Byrnes points out, “The device could be coupled with a solar cell, for example, to get extra power at night, without extra installation cost.”

Two proposed devices—one macro, one nano

To show the range of possibilities, Capasso’s group suggests two different kinds of emissive energy harvesters: one that is analogous to a solar thermal power generator, and one that is analogous to a photovoltaic cell. Both would run in reverse.

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