Scientists at the University of California, Davis have found a way to generate mechanical energy after sunset by harnessing the cold of space. The results of their work were published in the journal Science Advances. Led by Professor Jeremy Munday, the team created a prototype Stirling engine that, combined with a radiative cooling panel, is able to operate at night without fuel, using only natural temperature differences between the ground and sky.

Operating principle of the Stirling engine

The Stirling engine is a special heat engine that converts the temperature difference between hot and cold surfaces into mechanical motion. The gas inside the cylinder is heated from the higher temperature side, expands and pushes the piston. The gas then cools on the opposite side, compresses, and the cycle repeats. This principle allows the engine to operate even with small temperature changes, which classic internal combustion engines cannot do. Munday explains:

“These motors are very efficient when temperature gradients are small, while other types perform better at larger gradients and can produce power.”

How the cold of space starts an engine

Typically, to operate the engine, one side is heated and the cold side is kept close to ambient temperature. Munday and his graduate student Tristan Deppe took a different approach: they used the cold of the night sky and space. No physical contact with space is required – the heat is simply radiated from the panel into open space.

“You don’t need to touch space physically, radiation interaction is enough. The effect can be compared to the feeling when a person goes out into the fresh, clear air at night and quickly loses heat,” explains Munday

Night tests and results

For the experiment, the engine was mounted on a radiation cooling panel pointing upward, and the ground was used as the warm side. The temperature difference was sufficient for the piston to begin to move and rotate the flywheel. During a year of overnight testing, the device consistently produced at least 400 milliwatts of mechanical energy per square meter.This was enough to spin a small fan and connect it to an electric motor to generate current.

Scientists note that the method is most effective in regions with low humidity and frequent clear nights. This approach can be useful for ventilating greenhouses, small commercial buildings or residential premises during the warm season, when air needs to circulate after sunset and solar panels are no longer working.

Technology potential and future plans

UC Davis has applied for a patent on the technology and plans to expand the prototype to larger systems.

“Our experiments show that space can provide a practical cold-spot for energy generation at night,” Munday said.

In the future, the team hopes to optimize the device and adapt it for real-life household and industrial applications.Scientists also believe that such systems can become part of hybrid power plants operating around the clock: during the day – on solar panels, at night – using space cooling.