The system allows emission of heat at mid-infrared range to provide cooling of as much as 40 ºC. The proof-of-concept a cooling of 6 ºC.
Other designs have used complex engineered photonic devices that can be expensive to make and not readily available for widespread use. Instead, a simple device built from a combination of inexpensive plastic film, polished aluminum, white paint, and insulation can allow for the necessary emission of heat through mid-infrared radiation if the incoming radiation from the sun is blocked.
“We built the setup and did outdoors experiments on an MIT rooftop using very simple materials,” said researcher Bikram Bhatia. “It’s kind of deceptively simple,” said Evelyn Wang, professor of mechanical engineering. “By having a separate shade and an emitter to the atmosphere — two separate components that can be relatively low-cost — the system doesn’t require a special ability to emit and absorb selectively. We’re using angular selectivity to allow blocking the direct sun, as we continue to emit the heat-carrying wavelengths to the sky.”
This project “inspired us to rethink about the usage of ‘shade,’” says Yichen Shen, a research affiliate and co-author of the paper. “In the past, people have only been thinking about using it to reduce heating. But now, we know if the shade is used smartly together with some supportive light filtering, it can actually be used to cool the object down,” he says.
One limiting factor for the system is humidity in the atmosphere, which can block some of the infrared emission through the air. In a place like Boston, close to the ocean and relatively humid, this constrains the total amount of cooling that can be achieved, limiting it to about 20 ºC. But in drier environments the maximum achievable cooling could actually be much greater, he points out, potentially as much as 40 ºC.
The system might also be useful for some kinds of concentrated photovoltaic systems, where mirrors are used to focus sunlight on a solar cell to increase its efficiency. But such systems can easily overheat and generally require active thermal management with fluids and pumps. Instead, the backside of such concentrating systems could be fitted with the mid-infrared emissive surfaces used in the passive cooling system, and could control the heating without any active intervention.
As they continue to work on improving the system, the biggest challenge is finding ways to improve the insulation of the device, to prevent it from heating up too much from the surrounding air, while not blocking its ability to radiate heat. “The main challenge is finding insulating material that would be infrared-transparent,” Leroy says.
The team has applied for patents on the invention and hope that it can begin to find real-world applications quite rapidly.