Without the Sun, the Earth would slowly cool down. This fact is particularly evident on clear nights, when the heat of our planet, unblocked by clouds, spreads into the darkness of space. Now researchers from Stanford University have developed a device that can take advantage of this heat flow and create electricity.

The experimental device is a bit like an anti-solar panel, given that it has a similar optoelectronic system. However, instead of using incoming radiation from the Sun, it uses outgoing radiation from the Earth. As reported in Applied Physics Letters, the system uses the negative illumination effect. The infrared semiconductor generates electricity as the heat flows away from the surface of the device.

"The vastness of the universe is a thermodynamic resource," co-author of the study, Shanhui Fan, said in a statement. "In terms of optoelectronic physics, there is really this very beautiful symmetry between harvesting incoming radiation and harvesting outgoing radiation."

There is a large temperature difference between the Earth and outer space. Even at our planet's coldest locations, there is still a difference of at least 200°C (360°F). The limitations for the device are not in the external conditions but in the technology. The current device produces only a small current, 64 nanowatts per square meter.

"The amount of power that we can generate with this experiment, at the moment, is far below what the theoretical limit is," lead author Masashi Ono explained.

The theoretical limit for the device is expected to be 4 watts per square meter, roughly 1 million times more than what was obtained in the first experiments. While that would be a huge improvement, it would still be far below what a solar panel can do (up to 200 watts per square meter) but enough to be of interest. At this stage, it is just a proof of concept. No other optoelectronic device has been shown to be able to create electricity from the coldness of space.

The team is now interested in ways to improve the device to make sure the materials used are the best for the job at hand. The applications for this technology are not just about using the coldness of the universe, it could in principle be used to recover some of the wasted heat from machines. That said, the researchers caution that it is still very early days.