Instead of absorbing energy from the sun to produce electricity, a new class of devices generates power by absorbing heat from its surroundings instead and beaming it at outer space. Such devices, which do not require exotic materials as their predecessors did, could help ventilate greenhouses and homes, researchers say. ****

In 2014, scientists invented superthin materials that can cool buildings without using electricity by beaming heat into outer space. When these materials absorb warmth, their compositions and structures ensure they emit heat outward as very specific wavelengths of infrared radiation, ones that air does not absorb. Instead, the radiation is free to leave the atmosphere, carrying energy with it, and cooling the area around the material in a process called radiative cooling. The materials could help reduce demand for electricity. Air conditioning accounts for nearly 15 percent of the electricity consumed by buildings in the United States alone.

Researchers then began exploring whether they could harness radiative cooling to generate power. Whereas solar cells produce electricity from the flow of energy into them from the sun, thermoradiative devices could generate power from energy flowing out from them into space.

“Thermoradiative devices operate like solar cells in reverse,” says Jeremy Munday, professor of electrical and computer engineering at the University of California, Davis. “Rather than pointing them at a hot object like the sun, you point them at a cool object, like the sky.”

However, these devices were typically semiconductor electronics that needed rare or expensive materials to operate efficiently. In a new study, Munday and his colleagues investigated using Stirling engines, which “are mechanically simple and do not rely on exotic materials,” he says. “They also directly produce mechanical power—which is valuable for applications like air movement or water pumping—without needing intermediate electrical conversion.”

A Stirling engine meets a heat-emitting antenna

At the heart of a Stirling engine is a gas sealed in an airtight chamber. When the gas is heated, it expands and pressure increases within the chamber; when it is cooled, it contracts, reducing pressure. This creates a cycle of expansion and contraction that drives a piston, generating power.

Whereas internal combustion engines rely on large differences in temperature to generate power, a Stirling engine is very efficient when it comes to small differences in temperature.

“Stirling engines have been around since the early 1800s, but they always operated by touching some warm object and rejecting waste heat into the local, ambient environment,” Munday says. Instead, the new device is heated by its surroundings and cooled when it radiates energy into space.

The new device combines a Stirling engine with a panel that acts as a heat-radiating antenna. The researchers placed it on the ground outdoors at night.

A year of nighttime experiments revealed that the device could generate more than 10 degrees C of cooling most months, which the researchers could convert to produce more than 400 milliwatts of mechanical power per square meter. The scientists used their invention to directly power a fan and also coupled it to a small electrical motor to generate current.

Close-up of Jeremy Munday’s experimental engine, which resembles a mechanical pinwheel and is mounted on a metal sheet.Jeremy Munday

Since the source of the new device’s energy is Earth’s ambient heat instead of the sun, its power output “is much lower than solar photovoltaics—roughly two orders of magnitude lower,” Munday says. “However, the goal is not to replace solar. Instead, this enables useful work when solar power is unavailable, such as at night and without requiring batteries, wiring, or fuel.”

The researchers calculated the device could generate more than 5 cubic feet per minute of air flow, the minimum air rate the American Society of Heating, Refrigerating and Air-Conditioning Engineers requires to minimize detrimental effects on health inside public buildings. Potential applications may include circulating carbon dioxide within greenhouses and improving comfort inside residential buildings, they say.

Munday and his colleagues note there are many ways in which they could further improve the device’s performance. For instance, they could replace the air sealed in the device with hydrogen or helium gas, which would reduce internal engine friction. “With more efficient engine designs, we think this approach could enable a new class of passive, around-the-clock power systems that complement solar energy and help support resilient, off-grid infrastructure,” Munday says.

In the future, “we would like to set up these devices in a real greenhouse as a first proof-of-concept application,” Munday says. They would also like to engineer the device to work during the day, he notes.****

The scientists detailed their findings** **in the journal Science Advances.