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Energy-Harvesting the Earth's Heat

March 10, 2014

As if adding insult to injury, we've needed to scrape a lot of frost from our automobile windshields on the few snow-free mornings we've had this season in Northern New Jersey. When finally finished scraping and on the road, we still needed to be wary of "black ice." Black ice is transparent, not black, but it gets its name from this transparency's allowing the usually black pavement to be seen instead of the ice.

Our ancestors would have blamed all this on Boreas, the god of the
Figure caption north wind. Boreas was sometimes depicted as a old man with shaggy hair, a beard and cloak, holding a conch shell. Substitute a slide rule for the conch shell, and that's the image of some of the old scientists I've known.

We now know that radiative cooling is to blame. The black-body radiation of heat is responsible for cool nights under clear skies and consequent formation of overnight ice.

(Statue of Boreas at the Palace of the Four Winds, Warsaw, Poland, via Wikimedia Commons.)

Such cooling is a consequence of the temperature difference between the Earth's surface and outer space, which is ideally at the temperature of cosmic microwave background radiation of about 3 kelvin. In most cases, clouds act as thermal isolation, so the driving force is the smaller temperature difference between Earth and clouds. Altostratus clouds, which appear several miles above ground, have temperatures as low as about -30°C, while the somewhat higher cirrostratus clouds might be as cold as -50°C.

Thermodynamics tells us that we can extract work from a temperature difference. An object placed between the Earth and the sky would see a difference in thermal radiation between its top and bottom sides. If we're smart enough, we might be able to harvest energy from this effect.

The devil is in the details, and the detail here is that things near room temperature radiate at the longer infrared wavelengths (see figure). Inexpensive silicon will convert radiation to electricity only when the wavelength is less than about 1.1 micrometer (μm). More expensive germanium will work up to 1.7 μm, while the more exotic semiconductor, indium gallium arsenide, will work up to 2.6 μm. Above those wavelengths, things get more difficult.

Blackbody spectral radiance curves near room temperature.Blackbody spectral radiance curves near room temperature.

(Illustration by the author using Inkscape.)

Harvesting the Earth's heat energy in this fashion would be important for at least one reason - It's solar energy harvesting that happens at night. With proper design, it might be possible to create a photovoltaic device that functions both during the day, and at night. Scientists at Harvard University's School of Engineering and Applied Sciences decided to analyze this energy-harvesting idea, and they've published their findings in a current issue of the Proceedings of the National Academy of Sciences.[1-2]

The interesting aspect of their approach is that they're using the emission of thermal radiation, not its absorption, to produce electricity. Says principal investigator, Federico Capasso,
"It's not at all obvious, at first, how you would generate DC power by emitting infrared light in free space toward the cold... 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."[2]

A device for generating electricity from the temperature differential between the Earth and the night sky would be possible without any new technology by using thermoelectric panels. One side of a thermoelectric, coated with a high albedo material, would attain a lower temperature than the other side by radiating heat to the sky. Because of the inefficiency of thermoelectrics, this approach would only generate a few watts per square meter, which is just enough to power a cellphone.[2]

A better option is the emissive energy harvester, which acts like a photovoltaic cell. The Harvard device, as shown in the schematic, uses a resistor as the emissive (or absorptive) device forming one thermal reservoir, and a diode as the other thermal reservoir. The resistor can, in fact, be a nanoscale antenna designed to efficiently radiate the long wavelength light.[2] Capasso says that the essential idea was invented in 1968 by J.B. Gunn, known in microwave research as the inventor of the Gunn diode.[3] An energy-harvesting panel would be composed of a large array of such nanoscale circuits.[2]

Electromagnetic wave noise energy harvester.Mid-infrared energy harvesting circuit. There would be many of these on each collection panel.

(Illustration by the author using Inkscape.)

Such a device is only now enabled by advances in nanofabrication technology and new materials, such as graphene. One problem in this circuit, as in others containing diodes, is that diodes don't function well at low voltages. The Harvard team is looking at alternative diode components, such as tunnel diodes and ballistic diodes. There's the further requirement that the diodes must switch as fast as the optical signals, about 3 x 1013 hertz.[2]

Other members of the research team are lead author of the paper and Harvard postdoc, Steven J. Byrnes, and Romain Blanchard. The research was supported in part by King Abdullah University of Science and Technology.[2]

References:

  1. Steven J. Byrnes, Romain Blanchard and Federico Capasso, "Harvesting renewable energy from Earth's mid-infrared emissions," Proc. Natl. Acad. Sci., Early Edition (published ahead of print, March 3, 2014).
  2. Caroline Perry, "Infrared: A new renewable energy source?" Harvard University School of Engineering and Applied Sciences Press Release, March 3, 2014.
  3. J.B. Gunn, "Microwave oscillations of current in III–V semiconductors," Solid State Communications, vol. 1. no. 4 (September, 1963), pp. 88-91.

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