Researchers at the University of California, Davis have successfully demonstrated a new way to generate electricity using a 200-year-old engine design. By harnessing the temperature difference between the Earth and the cold of outer space, their device can produce power continuously, day and night.
This innovative approach repurposes the Stirling engine, an invention from 1816, to create a consistent source of low-level energy. The proof-of-concept, detailed in the journal Science Advances, could open doors for practical applications in off-grid and specialized environments.
Key Takeaways
- UC Davis scientists created a Stirling engine that runs on the temperature gradient between Earth and space.
- The device works 24/7, offering a potential complement to solar power.
- It generates up to 400 milliwatts of power per square meter, suitable for low-energy tasks.
- Initial applications could include ventilating greenhouses or powering small sensors.
A Centuries-Old Engine with a New Purpose
The Stirling engine was first invented by Robert Stirling in 1816 as a safer alternative to the steam engines of the Industrial Revolution, which were prone to dangerous explosions. Unlike internal combustion engines, a Stirling engine operates on an external heat source.
It works by cyclically heating and cooling a sealed gas, such as air or helium. As the gas heats, it expands and pushes a piston. As it cools, it contracts, pulling the piston back. This continuous cycle converts thermal energy into mechanical work.
What is a Stirling Engine?
Invented over two centuries ago, the Stirling engine is known for its high efficiency and ability to run on any external heat source. Its main drawback has always been its lower power output compared to other engines, which limited its widespread adoption for heavy-duty tasks.
While historically overshadowed by more powerful engines, the Stirling engine's unique strength lies in its ability to operate on very small temperature differences. This characteristic is what caught the attention of the UC Davis research team.
Harnessing the Cold of Outer Space
The research team, led by Professor Jeremy Munday and Ph.D. student Tristan Deppe, explored a novel concept: connecting a Stirling engine to a temperature difference that isn't local. Instead of using a conventional heat source, they looked up—to the vast cold of space.
Even during the day, an object on Earth can radiate heat into space, which has an effective temperature of just a few degrees above absolute zero. This process, known as radiative cooling, allows a surface to become significantly colder than the surrounding air.
The team designed a system where the top of the engine is attached to a plate that radiates heat away into the sky, effectively coupling it with the cold of space. The bottom of the engine remains in contact with a plate that absorbs ambient warmth from the ground and 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," explained Jeremy Munday, a co-author of the study.
This setup creates a persistent temperature gradient across the engine, allowing it to run continuously as long as it has a clear view of the sky.
From Theory to a Working Prototype
To test their idea, Munday and Deppe built a small, functional prototype. Their device consisted of a simple Stirling engine—a piston driving a flywheel—mounted on a panel designed to act as a radiative antenna.
The top plate efficiently radiates thermal energy, causing it to cool down. Simultaneously, the bottom plate maintains a warmer temperature by drawing heat from the environment. This small but steady temperature difference is enough to drive the engine's piston.
Power Generation Statistics
The proof-of-concept engine successfully generated upwards of 400 milliwatts of power per square meter. While a modest amount, it demonstrates the viability of the concept for continuous, low-power generation.
A key advantage of this technology is its 24/7 operational capability. Unlike solar panels, which only work when the sun is out, this radiative cooling engine works both day and night. In fact, it often performs better at night under clear skies with low humidity.
Practical Applications for Low-Power Needs
The researchers are quick to point out that this engine is not designed to power homes or cities. Its output is modest, making it unsuitable for energy-intensive applications. However, its practicality lies in niche areas where a small, continuous power source is valuable.
"The first things we’re looking at are possible applications in greenhouses where we need air circulation," Munday stated. "We can use this to create a temperature difference to run a fan... we’ve shown that with these temperature differences we’re able to achieve that we should have enough flow to be useful in that application."
Other potential uses include:
- Powering remote environmental sensors.
- Providing ventilation in off-grid buildings.
- Trickle-charging batteries for small electronic devices.
Its ability to complement solar power is particularly promising. A hybrid system could use solar panels during the day and a radiative engine at night, providing a more reliable off-grid energy solution.
The Path to Improved Efficiency
The current prototype is a proof-of-concept, and the team has already identified several ways to boost its performance. Future iterations could see significant improvements in power output and efficiency.
Potential Enhancements
- Material Upgrades: Using materials with higher thermal conductivity, such as copper, for key components would improve heat transfer and engine performance.
- Thermal Insulation: Incorporating a thermally insulating vacuum enclosure would minimize heat loss to the surrounding air, maximizing the temperature differential.
- Optimized Placement: Locating the engine near a source of waste heat (like an air conditioning unit) could increase the temperature of the bottom plate, thereby increasing the overall power output.
- Surface Area: A larger contacting surface for the radiative panel would allow it to cool more effectively, further enhancing the temperature gradient.
By implementing these improvements, the team believes the engine's output could be scaled up to become a practical and cost-effective solution for a range of low-power energy needs around the globe.