Infrared energy emitted from the Earth into space may be potentially harnessed as a renewable energy source in the future, US researchers said Monday.

Physicists at the Harvard School of Engineering and Applied Sciences (SEAS) proposed a device that would resemble a photovoltaic solar panel. But instead of capturing incoming visible light, it would generate electric power by harvesting energy from Earth's infrared emissions into outer space.

"The energy flow from earth to outer space provides a hitherto neglected opportunity to generate a potentially enormous amount of renewable energy," the researchers reported in the US journal Proceedings of the National Academy of Sciences.

"The technology does not yet exist to siphon renewable energy out of this flow, but we argue that it is possible to make a device that does exactly that."

The researchers proposed two possible designs for an emissive energy harvester (EEH) to harness such infrared emissions.

The first design, a thermal EEH, would generate electricity by drawing the heat of surface ambient air through a cold plate that could radiate the energy into the atmosphere, with the flow of heat generating work.

Keeping the cold plate cooler than the ambient temperature would be difficult, but this device illustrates the general principle: differences in temperature generate work, the researchers said.

The challenges of efficiently cooling the cold plate led the researchers to propose another EEH design, inspired by photovoltaics.

The alternative design uses rectifying antennas, or rectennas, warmed by ambient air as part of a circuit that generates direct current using temperature differences between electrical components.

Current rectenna technology can generate "only negligible power ", but technological advances could enhance rectenna EEH efficiency, the researchers said.

"Today's technology is not sufficient to make an efficient, cost-effective, optoelectronic EEH, but we have described a number of paths that could plausibly lead there over time," the researchers wrote.

"We hope that a vigorous research effort will open up this frontier and create a role for EEHs in the renewable energy landscape."