Scientists create new engine that runs on the cold of space

Solar panels do a remarkable job during the day; however, the hours when people need the most electricity rarely coincide with the hours when sunlight is strongest. That mismatch has always created a real hurdle for a clean-energy future. Batteries help, but remain costly and complex.

A group of engineers in California took a different path and showed that the night sky itself can power a machine. Their small engine turns the natural chill of space into steady motion long after the sun has set. The work, led by researchers at the University of California, Davis, offers an unexpected tool for a world searching for dependable and sustainable energy.

The idea came from a simple question. Could the cold of space act like the "cool side" of a heat engine, like the sun heats the "hot side" of a solar panel? Jeremy Munday, a professor of electrical and computer engineering at UC Davis, and graduate researcher Tristan Deppe decided to give it a try.

They constructed a lightweight Stirling engine, a device known to work well with small temperature differences. Many engines have to start with a lot of heat. A Stirling engine can hum along with a small thermal contrast and only have to develop power with a small temperature differential.

The researchers pressed the base of the engine against the ground, which, after dark, stays relatively warm. They then coated the top plate with special infrared paint and left it in the open night sky. On clear nights, the top plate cooled rapidly, with field tests showing temperature drops of up to 10 °C.

This cooling caused heat to radiate into space through a natural atmospheric window just above Earth’s surface, occurring between 8 and 13 micrometers in wavelength. When subsequent field nights were conducted, the researcher measured the top plate cold in Davis, Utah, particularly on nights with low humidity and relatively warm nighttime air temperatures.

The research found empirically that nighttime clouds, humidity, and warm air would decrease the cooling performance of the thermal cooler on the top plate. Even on the warmest nights, the thermal cooler was capable of reducing the plate temperature by about 10 °C before the next morning. On average, the device was successful year-round. During the summer, there were revealed to be even more efficient ways to expel heat from the top plate of flywheel movement in a reasonable period of time, and it could successfully create usable electricity output.

In order to discover the limitations of the device, the engineering team brought the engine into the laboratory and replicated the heated-cooled plates in a controlled environment. Then they tested how quickly the flywheel sped up or slowed down and measured the mechanical power from these plate differences. The testing showed for this particular design that the output increased proportionally to the temperature difference tested.

The researchers estimated that delivering the needed 0.1 milliwatt of power from a reported radiative cooling surface with sides of 1.58 cm. When scaled, the Engine was capable of producing 400 milliwatts of mechanical power per square meter of sky-facing area. Although that might not appear very substantial when compared to larger energy systems, it was sufficient to perform targeting operations without using any fuel.

When the team placed a miniature DC motor, the Engine transferred the produced power into electrical current. It was only a very low efficiency, a few percent, but the small device still maintained approximately half of the mechanical energy output while producing some electricity. In a realistic setting, this could be used to power sensors, charge small batteries, and provide an increase in power to users in circumstances when clouds cause the top plate to warm.

Possibly the most applicable demonstration came from substituting the flywheel with a fan blade prudently 3D-printed. In greenhouse-like conditions, the rotating fan produced air speeds close to 0.3 meters per second, which helped sweep carbon dioxide and thus convey it to plants growing in the greenhouse.

Lower temperature differences produced milder airflows in the range of 0.15 to 0.2 meters per second were preferable to the recommended indoor comfort levels proposed by the American Society of Heating, Refrigerating, and Air Conditioning Engineers. With air temperature differences of over 30 degrees Celsius, the air speed increased to nearly 5 cubic feet per minute. This is sufficient to achieve baseline ventilation rates appropriate for large public spaces.

The pairing of a rooftop radiative cooler with internal warm air would be preferred as a means of pulling fresh air into and through a building overnight with no electricity. In heat-load or resource-limited areas, these fans could provide relief.

The team looked at global satellite data provided by NASA to estimate locations where this technology might provide the most effective power. Dry regions, as well as high altitudes, provided the best performance due to clear air, which allows for less obstruction of heat escaping into the environment.

Parts of the Sahara, parts of the Eurasian Steppe, and even summer conditions in Antarctica scored solidly in consideration. Moist, forested regions performed the least, due to water vapor trapping infrared thermal energy and reducing effective cooling capacity.

The concept carries implications for climate as well; Earth is gaining approximately one watt per square meter, more energy than it emits, which equates to global warming. Radiative coolers such as this technology, can help shift that imbalance by increasing heat loss to space. A radiative cooling engine creates not only heat loss, but it also produces a very small percentage of that thermal energy as a useful mechanical work output.

The prototype indicates potential for higher output. Better materials from an emissivity perspective would help shed more heat. A vacuum chamber around the top plate would help reduce unwanted warming of the heat loss by air. A better thermal link to the ground would aid in developing a stronger temperature contrast.

Gases like Helium or Hydrogen could be used to increase the efficiency of the engine by minimizing friction inside the Engine. The pairing of the bottom plate with a hot source, a waste heat from buildings or industry, could also achieve stronger performance without adding a separate energy user; the waste heat provides heat transfer into the engine and reduces the thermal load bottom plate.

The basic premise is what stands out as a major consideration in the design and application of the Engine. This engine has no fuel source, nor batteries, nor other electronics, and very few moving parts. All the system does is sit outside and draws power from the continual cooling that permeates our environment every night.

Research findings are available online in the journal Science Advances.

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