The next renewable energy source may be a solar cell-like device that will collect its power at night. If constructed, this new idea in generators would derive its power directly from the heat of the Earth.

Harvard physicists Federico Capasso and Steven Byrnes have proposed this idea that could revolutionize the energy industry. It is based on the simple notion that renewable energy can be generated anytime heat flows from a hotter body to a cooler one. 

Infrared radiation, emitted from the Earth at night, could generate electricity as it is released back into space. Current solar cells collect solar energy in daytime, and could be coupled with an emissive energy harvester (EEH) to provide power at night, as well. 

"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 counter-intuitive," Capasso said in a press release.

The team identified two possible models for such an electrical generator. One idea is to use modified cells to capture infrared radiation. This is called thermal EEH, and would operate with the Earth acting as a hot plate, with a cool plate placed above the ground. This would be made form a material that would quickly radiate thermal energy into the air. 

"[Y]ou get an electric current directly from the radiation process, without the intermediate step of cooling a macroscopic object," Byrnes said

Power delivered by such a device would likely be low, but would draw nearly clean energy from heat that is now just going to waste. Still, the concept is a long way from realization. The team is not sure how much energy can be produced by the process, even if a working model could be constructed. 

"It is possible to harvest energy from Earth's thermal infrared emission into outer space. We calculate the thermodynamic limit for the amount of power available, and as a case study, we plot how this limit varies daily and seasonally in a location in Oklahoma," wrote the authors of the article accompanying the study. 

How to collect energy from the infrared heat of the Earth is profiled in the Proceedings of the National Academy of Sciences

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