The infrared light emitted by the Earth could be used as a renewable energy source

Harvesting energy from Earth's heat

A device capturing infrared light the Earth is emitting to outer space could help balance out the lack of solar power at night, American engineers have proposed.

The team from the Harvard School of Engineering and Applied Sciences (SEAS) believes the temperature difference between the Earth, warmed by the Sun during the day, and the surrounding freezing-cold vacuum could be transformed into direct current, making available vast but yet inaccessible energy resources.

In the latest issue of the Proceedings of the National Academy of Sciences journal, the researchers have described a device similar to a photovoltaic solar panel, but designed to generate electricity by emitting infrared instead of capturing visible light.

“It’s not at all obvious, at first, how you would generate DC power by emitting infrared light in free space toward the cold,” said the study's principal investigator Federico Capasso. “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.”

The team has described two types of such an emissive energy harvester – one resembling a solar thermal power generator, the other similar to a conventional photovoltaic cell. Both of these systems, however, would work in reverse.

The thermal power generator would consist of a “hot” plate, as warm as the Earth, with a “cold” plate on top of it. The cold plate, facing upward, would be made of a highly emissive material that cools by very efficiently radiating heat to the sky. During a pilot study in Lamont, Oklahoma, the researchers have calculated that the heat difference between the plates could generate a few watts per square meter, day and night.

“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,” said Steven J Byrnes, a postdoctoral fellow at the SEAS.

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

The second proposed device relies on temperature differences between nanoscale electronic components—diodes and antennas—rather than a temperature noticeable by a human.

“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,” said Capasso. “But it’s tricky; at the level of the electron behaviours, the explanation is much less intuitive.”

Components in an electrical circuit can spontaneously push current in either direction, causing the so-called electrical noise. It has been previously described that if a valve-like electrical component - a diode - is at a higher temperature than a resistor, it will push current in a single direction, producing a positive voltage.

Capasso’s group suggests that the role of the resistor could be played by a microscopic antenna that very efficiently emits the Earth’s infrared radiation toward the sky, cooling the electrons in only one part of the circuit.

“People have been working on infrared diodes for at least 50 years without much progress, but recent advances such as nanofabrication are essential to making them better, more scalable, and more reproducible,” said Byrnes

However, even with the best modern infrared diodes, there is a problem. “The more power that’s flowing through a single circuit, the easier it is to get the components to do what you want. If you’re harvesting energy from infrared emissions, the voltage will be relatively low,” explained Byrnes. “That means it’s very difficult to create an infrared diode that will work well.”

Engineers and physicists, including Byrnes, are already considering new types of diodes that can handle lower voltages, such as tunnel diodes and ballistic diodes. Another approach would be to increase the impedance of the circuit components, thereby raising the voltage to a more practical level. The solution might require a little of both, Byrnes predicts.

Speed presents another challenge.  “Only a select class of diodes can switch on and off 30 trillion times a second, which is what we need for infrared signals,” says Byrnes. “We need to deal with the speed requirements at the same time we deal with the voltage and impedance requirements.”

“Now that we understand the constraints and specifications,” Byrnes adds, “we are in a good position to work on engineering a solution.”

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