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A new way to provide powerless cooling



Two versions of the device designed by MIT researchers, using metal strips to block direct sunlight, are built and tested on the roof of an MIT building to ensure that they can provide cooling below the ambient air temperature. Photo by Bikram Bhatia
Two versions of the device designed by MIT researchers, using metal strips to block direct sunlight, are built and tested on the roof of an MIT building to ensure that they can provide cooling below the ambient air temperature.

Photo by Bikram Bhatia

MIT scientists have developed a new system that offers cooling on very sunny days without using power. This passive radiation cooling approach exploits the sun's angle of irradiation in the sky to achieve daytime sub-ambient cooling regardless of the emitter properties in the solar spectrum.

This system can provide cooling of up to 20 degrees Celsius (36 degrees Fahrenheit) below ambient temperature in locations such as Boston. When testing, the system offers cooling 6 C (around 11 F) below room temperature.

This system works by allowing heat emissions in the infrared light range that can penetrate directly through the atmosphere and emit into cold outer space, punching through gases that act like greenhouses. To prevent heating in direct sunlight, a small strip of metal hanging over the device blocks direct sunlight.

Scientists note, "Other groups have tried to design passive cooling systems that emit heat in the form of mid-light infrared wavelengths, but this system has been based on complex designed photonic devices that can be expensive to make and not available for use. widely use. "

"This device is complicated because it is designed to reflect all the wavelengths of sunlight almost perfectly, and only emits radiation in the mid-infrared range, for the most part. The combination of selective and emissivity reflectivity requires a multilayer material where the thickness of the layer is controlled to nanometer precision. "

The scientists explained, "It turns out that similar selectivity can be achieved only by blocking direct sunlight with a narrow strip placed at the right angle to cover the sun's path in the sky, which does not require active tracking by the device. Then, simple devices built from a combination of cheap plastic films, polished aluminum, white paint, and insulation can allow the required heat emissions through mid-infrared radiation, which is the most natural way of cooling objects while preventing the device from being heated by direct sunlight. "

"In fact, simple radiation cooling systems have been used since ancient times to achieve nighttime cooling; the problem is that such a system does not function during the day because the effect of warming the sun is at least 10 times stronger than the maximum cooling effect that can be achieved. "

Scientist researcher Bikram Bhatia said, "But sunlight that travels in a straight line and is easily blocked – as we experience, for example, by stepping into the shadow of a tree on a hot day. By shading the device by placing an umbrella on it, and adding it to the insulation around the device to protect it from air temperature, the researchers made passive cooling more feasible. "

"We built a configuration and conducted an outdoor experiment on the MIT roof. It is done using very simple material "and clearly shows the effectiveness of the system."

Evelyn Wang, physics professor Marin Soljačić said, "This is rather simple. By having separate shadows and emitters into the atmosphere – two separate components that can be relatively low cost – the system does not require a special ability to selectively transmit and absorb. We use angular selectivity to allow direct sun blocking, because we continue to emit long waves that bring heat to the sky. "

"This will be useful for cooling applications, such as storing food or vaccines. This system may also be useful for several types of concentrated photovoltaic systems, where mirrors are used to focus sunlight on solar cells to improve efficiency. But such a system can easily heat up and generally requires active thermal management with liquids and pumps. In contrast, the rear of such a centering system can be fitted with a mid-infrared emitting surface used in a passive cooling system, and can control heating without active intervention. "

Scientists are currently working to improve the system, the biggest challenge is finding ways to improve the isolation of the device, to prevent it from overheating from the surrounding air, while not blocking its ability to emit heat.

The new system was explained this week in the journal Nature Communications in a paper by research scientist Bikram Bhatia, graduate student Arny Leroy, professor of mechanical engineering and department head Evelyn Wang, physics professor Marin Soljačić, and six others at MIT.


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