The device is based on a Stirling engine, a type of machine designed to convert heat into mechanical motion. Unlike internal combustion engines, which require a large temperature gap to operate efficiently, Stirling engines can run on much smaller differences in heat. Jeremy Munday, professor of electrical and computer engineering at UC Davis and co author of the study, explained that the temperature contrast can be as modest as that between a hot cup of coffee and the surrounding air.

"These engines are very efficient when only small temperature differences exist, whereas other types of engines work better with larger temperature differences and can produce more power," Munday said.

In a typical setup, one side of a Stirling engine is kept warm while the other side is cooled. The temperature contrast drives a piston, which in turn produces mechanical energy.

"If you just set it on the table, it's not going to produce any power on its own because all sides are the same temperature," Munday said.

Using Deep Space as a Cold Reservoir

Usually, a temperature difference is created by heating one side of the engine with a fuel source. Instead of burning fuel, Munday and graduate student researcher Tristan Deppe explored whether the cold side could be linked to something far colder and much more distant: deep space.

"It doesn't actually have to touch space physically, it can just interact radiatively with space," Munday said. On a clear, cool night, heat naturally radiates away from your body toward the sky, which makes your head feel colder. The team aimed to use that same effect.

Their design places a simple Stirling engine (essentially a piston driving a flywheel) on top of a panel that acts as a heat radiating antenna. The entire system sits outdoors at night. The ground provides warmth to one side of the engine, while the panel releases heat upward, effectively connecting the other side to the cold of space.

Nighttime Experiments Show Promising Results

After a year of testing at night, the researchers found that the compact device could generate at least 400 milliwatts of mechanical power per square meter. In demonstrations, the engine directly powered a small fan. It was also connected to a small electric motor to produce electrical current.

These results show that meaningful amounts of energy can be captured from the night sky. According to Munday, the approach works best in regions with low humidity and consistently clear skies. In the future, the technology could help ventilate greenhouses or residential buildings without conventional energy sources.

UC Davis has filed a provisional patent related to the invention.