Paul van Gerven
13 May

The vast cold space of the universe is a neglected energy source that now can be tapped.

Whenever heat flows from a hotter to a colder body, there’s an opportunity to siphon off energy. In fact, most energy sources on earth depend on a temperature difference. As the extremely hot sun beams energy to the lukewarm earth, a solar panel converts some of it to electricity. The energy contained in oil similarly stems from the sun. Even wind power and hydroelectricity rely on the energy flow from hot to cold areas in the earth’s atmosphere.

As it turns out, though, we’ve been neglecting a temperature difference as a potential source of energy: that between the earth’s surface and cold outer space. While not as big a difference as between the sun and the earth, about 1017 J of infrared thermal radiation is beamed into space every second. And all of that goes to waste.

Over the past couple of years, several research groups have been trying to come up with ideas on how to capture the earth’s outgoing thermal radiation. Only recently, however, a measurable amount of electricity has been generated from it. Researchers from Stanford University and Fujifilm’s Frontier Core-Technology Laboratories succeeded in building a device that converts heat into electricity before it dissipates into space.

Room for improvement

The new energy harvesting device basically operates as a solar cell in reverse. In case of a solar cell, the sun is the hot object and a photodiode is the cold one, resulting in a net photon flux from the sun to the diode. The energy is extracted from this flux.

Now if the diode is the hot object and outer space the cold one, the net photon flux is going in the opposite direction. Energy can be extracted from that as well. “In terms of optoelectronic physics, there is really this very beautiful symmetry between harvesting incoming radiation and harvesting outgoing radiation,” says Stanford researcher Shanhui Fan, co-author of the study published in Applied Physics Letters.

The device, then, is little more than a photodiode pointed towards the sky. One important thing to consider, however, is that not every infrared photon is able to reach space because of atmospheric absorption. Only IR light with a wavelength ranging 8 to 13 μm can, for the most part, escape — if the sky is clear, that is. The bandgap of the HgCdTe photodiode is tuned to that range.

The infrared photodiode generates electricity by harvesting the outflow of energy towards space. The researchers included an optical chopper in their setup, which periodically interrupts the path to the sky. The on-off-on-off-pattern of electricity they observed proves a ‘connection’ with space is required. Credit: Masashi Ono

The group of researchers found that their ‘negative illumination diode’ generated about 64 nW/m2 of power. That’s a tiny amount and nowhere near what could be considered usable, but the team believes there’s a lot of room for improvement. Theoretically, the device can generate close to 4 W/m2 under typical conditions. By increasing the temperature difference, for example by warming the device using industrial waste heat, even more power may become available.

Considering the diode-space photon flux is massively lower than the sun-diode flux, however, the energy harvester will never produce as much power as a solar cell, which generates up to 200 W/m2. Unlike a solar panel, however, the device works just fine at night.