The system is low-tech and easily manufactured, providing small but real gains in efficiency.
Credit: UC Davis
This week, researchers from the University of California published a new study detailing an innovative approach to passive cooling. It can convert the Earth’s passive heat into a small but steady stream of mechanical (or electrical) energy.
It does this by combining passive radiative cooling panel technology with a very old type of mechanical engine, the Stirling engine. Let’s start with the panels, which have been around in principle for a while now.
Basically, these are made of materials that have a passive, unpowered tendency to reflect light in the 8-13μm range, which has only very mild interactions with air. If the reflected light is directed upward, this tendency to pass through the atmosphere ensures that the reflected heat doesn’t simply warm the air around the panel and thereby undo the cooling effect.
This simple figure shows the experimental setup.
Credit: UC Davis
In the past, there have been attempts to capture this radiation and turn it into electricity. However, these have produced limited electrical current, and they’re expensive. Therefore, this study proposes capturing waste power as mechanical energy via a Stirling Engine.
A Stirling Engine is a means of capturing heat energy and converting it to mechanical energy, rather than directly into electricity. It works by using a captured gas that expands and contracts when heated or cooled, and by applying the resulting force to the sealed chamber to drive a rotor.
The Stirling engines used in this case generated 400 milliwatts of mechanical power per square meter of radiative panel. When they instead connected the system to a motor to generate current, it produced less than 1% of the power per square meter of a modern solar panel. Still, the researchers’ approach has clear advantages: These devices are simple and require no exotic materials to produce, and they crucially do not need sunlight to generate electricity.
This yields relatively low overall fan power, but it’s sufficient to circulate air (and CO2) in small- to moderate-sized greenhouses. The researchers believe that they can scale up the electrical output with more efficient Stirling engine designs. In particular, they could more efficiently replace the air in the sealed chamber with other gases, such as hydrogen.
These are the sorts of innovations that can actually move beyond the research world and into the real world. While it doesn’t fully solve a large problem, it is low-cost, low-maintenance, and doesn’t require access to limited resources.
We’re unlikely to see whole skyscrapers with these little things all over them, but modestly sized installations like single homes or even larger greenhouses could certainly benefit. The design will have to be evolved, especially for weather resistance, but in general it’s exactly the sort of innovative renewable technology we need more of.