Temperature difference between Earth and space drives Stirling engine

A team of engineers at the University of California, Davis (UC Davis) has succeeded in using the temperature difference between the warm Earth and cold space to generate mechanical energy at night with a low-temperature Stirling engine. In a test setup, the scientists used the Stirling drive to ventilate a greenhouse. However, other applications are also conceivable.

The UC Davis engineers used a technique over 200 years old: the Stirling engine, invented by Robert Stirling and patented in 1816. Stirling's heat engine is relatively simple in construction, and its operation is straightforward. In one cylinder, a gas is heated. The cylinder is sealed by a movable piston. The gas expands and is transferred to a second cylinder, also sealed by a movable piston, where it is cooled and compressed. The gas moves between the two chambers, with the temperature and pressure changing. The two pistons are connected via a linkage to a flywheel, which then performs a circular motion that can be used as a drive. To improve efficiency, a regenerator is often located on the path from the hot to the cold cylinder. It additionally releases absorbed heat as the gas flows from the cold to the hot cylinder.

Temperature difference between Earth and space for propulsion

In practice, heat is artificially supplied to the Stirling engine from the outside by burning fuel to achieve the temperature difference between hot and cold. The UC Davis engineering team, however, pursues a different approach: they use the natural temperature difference between the Earth, warmed by the sun, and cold space, which radiates its coldness to the Earth at night. The coldness is captured by a type of antenna and passed on to the Stirling engine.

Jeremy Munday, Professor of Electrical and Computer Engineering, explains the process: “It doesn't have to physically touch space; it can just interact radiatively with space,” says Munday. “It's like standing outside on a cold, clear night; your head gets cold quickly because heat radiates away.”

In the researchers' setup, the Stirling engine sits on a plate that absorbs heat from the earth. The ground serves to heat one side of the Stirling engine. The antenna, on the other hand, captures the coldness of space and directs it to the cool side of the engine.

According to the engineers, even small temperature differences are sufficient to get the engine running: “These engines are very efficient when only small temperature differences exist, whereas other engine types work better with larger temperature differences and can generate more energy,” explains Munday.

The research results, documented in the study “Mechanical power generation using Earth’s ambient radiation” and published in Science Advances, show that at least 400 mW of mechanical power per square meter can be generated with the Stirling engine used. The scientists used the drive to power a greenhouse fan. In another experimental setup, they coupled it to an electric motor and generated electricity with it as a generator.

US Davis has filed a provisional patent for the process.

(olb)