Solar panels and solar energy harvesting obviously require sunlight, but an international team of scientists devised an innovative energy harvester that can get energy from the night sky. It doesn’t get energy from moonlight. It gets energy from the temperature difference between the Earth and deep space (the night sky).
Space is known to have a frigid temperature. So a device facing outward towards space could harvest the cooling outflow of energy using the same optoelectronics physics as photovoltaic cells.
An international team of scientists from Stanford University and Fujifilm Corporation was able to do just that. They published their work in an issue of Applied Physics Letters. (See schematic of the experimental infrared photodiode that generated electricity directly from the coldness of space — courtesy of Masashi Ono).
For the first time, the scientists demonstrated that the coldness of the universe could directly generate a measurable amount of electricity in a diode. They used an infrared semiconductor device facing the sky to produce the electricity.
“The vastness of the universe is a thermodynamic resource,” said Shanhui Fan, an author on the paper and Director, Edward L. Ginzton Laboratory, and professor of electrical engineering at Stanford University. “In terms of optoelectronic physics, there is really this very beautiful symmetry between harvesting incoming radiation and harvesting outgoing radiation.”
Unlike using incoming energy as a regular solar cell would, the negative illumination effect allows the harvesting of electrical energy as heat leaves a surface.
However, this new technology does not capture energy over these negative temperature differences as efficiently.
By pointing their device toward space, whose temperature approaches mere degrees from absolute zero, the group was able to find a great enough temperature difference to generate power through an early design.
“The amount of power that we can generate with this experiment, at the moment, is far below what the theoretical limit is,” said Masashi Ono, from Fujifilm corporation and another author of the paper.
The group observed that their negative illumination diode produced about 64 nanowatts per square meter, a tiny amount of electricity, but a significant proof of concept, that the authors can improve by enhancing the quantum optoelectronic properties of the materials.
Calculations made after the diode created electricity revealed that, when atmospheric effects are taken into consideration, the current device could theoretically generate nearly 4W/m2. This would be roughly one million times what the group’s device generated, and could be enough to help power machinery that needs to run at night.
By comparison, today’s solar panels can generate between 100 to 200W/m2.
While the results show promise for having ground-based devices directed towards the sky, Fan said the same principle could also be used to recover waste heat from machines.
For now, he and his group are concentrating on improving their device’s performance.
Ono, M. Santhanam, P., Li, W., Zhao, B., Fan, S. “Experimental demonstration of energy harvesting from sky using the negative illumination effect of a semiconductor photodiode,” Applied Physics Letters, April 23, 2019. (DOI: 10.1063/1.5089783).