At any given moment, our planet bleeds 100 million gigawatts of infrared radiation back into space. Needless to say, converting this wasted heat to a renewable energy source would be a sure-fire game changer. Physicists from Harvard University may have just figured out how to do it.
Our Earth, which is heated by the sun, is much warmer than the frozen space that surrounds it. This creates a heat imbalance that could be transformed into direct-current (DC) power. But making this happen is considerably easier said than done.
"It's not at all obvious, at first, how you would generate DC power by emitting infrared light in free space toward the cold," noted principal investigator Federico Capasso in a statement. "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."
Capasso, along with his colleagues Steven Byrnes and Romain Blanchard, have proposed two designs for a device they call an emissive energy harvester (EEH). Once developed, they would convert infrared radiation (IR) into usable power. And somewhat paradoxically, they would run in reverse.
Earth's light differences: Image: NASA/JPL.
The first type of EEH is comparable to a solar thermal generator. It would consist of a "hot" plate at the same temperature as the Earth and air, with a "cold" plate on top of it. Facing up towards the sky, the cold plate would feature a material that cools by very efficiently radiating heat. The resulting heat difference between the plates could generate a few watts per square meter — day or night. And in fact, the researchers are basing these measurements on a case study conducted at a location in Larmont, Oklahoma. The researchers admit that keeping the cold plate cooler than the ambient temperature will be a challenge, but it illustrates an important principle — that difference in temperature can generate work.
"Solar panels for heating and cooking are already used in much of the world," he says. "You could easily couple that to the (infrared) harvester."
As for the second device, it would rely on temperature differences between nanoscale electronic components (namely diodes and an antenna) rather than a temperature that can be felt with touch.
"If you have two components at the same temperature, obviously you can't extract any work, but if you have two different temperatures you can," says Capasso. "But it's tricky; at the level of the electron behaviors, the explanation is much less intuitive."
Unfortunately, the technology to build these devices doesn't exist — at least not yet. But the groundwork certainly makes it look promising.
"Now that we understand the constraints and specifications, we are in a good position to work on engineering a solution," adds Byrnes. In fact, he envisions a sheet printed with thousands of tiny IR-harvesting rectennas that could be laminated on a solar panel or integrated into a solar water heater.
As an aside, this could be potential bad news as far as the search for extraterrestrial intelligence is concerned; wide-scale energy capture measures like these could make an alien planet appear invisible to us.
Read the entire study at PNAS: "Harvesting renewable energy from Earth's mid-infrared emissions." Additional source: IEEE Spectrum.
Top image: Infrared Earth taken by GOES 9 weather satellite.
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