A new way to provide cooling without electricity

MIT scientists have developed a new system that provides cooling in extremely sunny weather without using current. This approach to passive radiative cooling exploits the angular confinement of solar radiation in the sky to achieve sub-ambient cooling during the day, regardless of the transmitter's properties in the solar spectrum.

The system can provide cooling down to 20 degrees Celsius (36 degrees Fahrenheit) below room temperature in a place like Boston. During testing, the system proposed to cool down by 6 C (about 11 F) below ambient temperature.

The system works by allowing the emission of heat in a range of average infrared light that can directly cross the atmosphere and radiate in the cold of outer space, passing through gases that act as a greenhouse. To prevent overheating in direct sunlight, a small metal band hanging over the camera blocks direct sunlight.

Scientists have noted: "Other groups have attempted to design passive cooling systems that emit heat in the form of infrared wavelengths of light, but these systems are based on sophisticated complex photonic devices. which can be expensive to manufacture and are not readily available for widespread applications. use."

"The devices are complex because they are designed to reflect almost perfectly all the wavelengths of sunlight and only to emit radiation in the mid-infrared. This combination of selective reflectivity and emissivity requires a multilayer material where the thicknesses of the layers are controlled with nanometric precision. "

The scientists explained: "It turns out that similar selectivity can be achieved by simply blocking the direct sunlight with a narrow band placed at exactly the right angle to cover the path of the sun in the sky, requiring no active tracking by the device. Then a simple device built from a combination of cheap plastic film, polished aluminum, white paint and insulation can allow the necessary emission of heat by means of medium infrared radiation This allows most natural objects to cool while preventing the camera from being heated by direct sunlight. "

"In fact, simple radiative cooling systems have been used since ancient times to achieve nocturnal cooling. The problem was that such systems did not work during the day because the heating effect of sunlight was at least 10 times more powerful than the maximum achievable cooling effect. "

Bikram Bhatia, researcher, said, "But the sun's rays move in straight lines and are easily blocked – as we see, for example, by walking in the shade of a tree on a hot day. . By masking the device by essentially placing an umbrella on it, and by adding insulation around the device to protect it from the ambient air temperature, the researchers made passive cooling more viable. "

"We built the configuration and did outdoor experiments on a MIT roof. This was done with very simple materials "and clearly demonstrated the effectiveness of the system."

Physics professor Marin Soljačić, Evelyn Wang, said: "It's a bit misleading. With a distinct nuance and emitter in the atmosphere, two distinct components that can be relatively inexpensive, the system does not require a special ability to emit and selectively absorb. We use the angular selectivity to block the direct sun while continuing to emit the heat wavelengths in the sky. "

"This would be useful for refrigeration applications, such as food storage or vaccines. The system could also be useful for certain types 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 back of such systems of concentration could be equipped with the emitting surfaces in the middle infrared used in the passive cooling system, and could control the heating without any active intervention. "

Scientists are currently working on improving the system. The main challenge is to find ways to improve the insulation of the device, to prevent it from overheating ambient air, while not blocking its ability to emit heat.

The new system is featured this week in the journal Nature Communications in an article written by researcher Bikram Bhatia, graduate student Arny Leroy, professor of mechanical engineering and department head, Evelyn Wang, professor of physics Marin Soljačić and six other people from MIT.