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November 29, 2018
MIT researchers have developed a new way to cool hot, sunny days by using inexpensive materials and requiring no energy from fossil fuels. The pbadive system, which could be used in addition to other cooling systems to store food and medicine in hot and off-grid locations, is essentially a state of the art version of an umbrella.
The system allows the emission of mid-infrared heat. Range of light that can cross the atmosphere and radiate in the cold of outer space, crossing gases that act as a greenhouse. To prevent overheating in direct sunlight, a small metal band hanging over the camera blocks direct sunlight.
The new system is described this week in the journal by Nature Communications in an article written by a scientist Bikram Bhatia, graduate student Arny Leroy, professor of mechanical engineering and department head, Evelyn Wang, professor in physics, Marin Soljacic and six other people at MIT.
In theory, the system they designed could provide cooling up to 20 degrees Celsius. (36 ° F) below room temperature in a place like Boston, say the researchers. Until now, during the initial concept validation tests, they achieved a cooling of 6 ° C (about 11 ° F). For applications requiring even more cooling, the rest could be achieved through conventional refrigeration systems or thermoelectric cooling.
Other groups have attempted to design pbadive cooling systems that emit heat in the form of average infrared light wavelengths. are based on complex sophisticated photonic devices that can be expensive to manufacture and are not readily available for widespread use, say the researchers. The devices are complex because they are designed to reflect almost perfectly all the wavelengths of sunlight and to emit radiation in the mid-infrared range, for the most part. This combination of selective reflectivity and emissivity requires a multilayer material in which the thicknesses of the layers are controlled to a nanometric precision.
However, similar selectivity can be achieved by simply blocking direct sunlight with a narrow band at the same distance. right angle to cover the race of the sun in the sky, requiring no active tracking by the camera.
Then, a simple apparatus built from a combination of cheap plastic film, polished aluminum, white paint and insulation can allow the necessary emission of heat by Medium infrared radiation, which allows most natural objects to cool while preventing the device 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 effect of heating the sunlight was at least 10 times more powerful than the maximum possible cooling effect.
But the heating sun's rays move in straight lines and are easily blocked – as we experience, for example, entering the shade of a tree by a day hot. By masking the device by essentially placing an umbrella and adding insulation to protect it from the ambient air temperature, the researchers made pbadive cooling more viable.
"We built the configuration and performed outdoor experiments on a MIT roof," says Bhatia. "This was done using very simple materials" and clearly showed the effectiveness of the system.
"It's a bit misleading of simplicity," Wang said. "By having a distinct hue and a transmitter in the atmosphere, two separate components that can cost relatively inexpensive, the system does not require a special ability to emit and absorb We use an angular selectivity to block Sun, while we continue to transmit heat wavelengths to the sky. "
This project" prompted us to rethink the use of "l". shadow, "says Yichen Shen, badociate researcher and co-author of the paper. . "In the past, people were only thinking of using it to reduce heating, but now we know that if shading is used intelligently with light filtering, it can actually be used to cool the object, "he says.
One of the limiting factors of the system is the humidity in the atmosphere, which can block part of the infrared emission in the atmosphere. In a place like Boston, close to the ocean and relatively humid, this limits the total amount of cooling that can be achieved, limiting it to around 20 ° C. But in drier environments, such as the Southwest of the United States or many desert or arid environments in the world, the maximum achievable cooling could actually be much greater, he pointed out, reaching up to 40 ° C.
Although most of the research on radiative cooling has focused on larger systems that can be used to cool rooms or entire buildings, this approach is more localized, he says. This would be useful for refrigeration applications, such as food storage or vaccines.
Indeed, the protection of vaccines and other drugs against deterioration in hot and tropical weather is a major and permanent challenge that this technology may well be able to solve.
Even if the system was not enough to lower the temperature According to Mr. Wang, "the system could at least reduce the loads" on electric refrigeration systems, to provide only the final cooling.
This system could also be useful for some 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 pbadive cooling system, and could control the heating without any active intervention.
While they continue to improve the system, the most important The challenge is to find ways to improve the insulation of the device, to prevent it from overheating ambient air, while by not blocking its ability to emit heat. "The main challenge is to find an insulating material that is transparent to infrared," says Leroy.
The team has filed a patent application for the invention and hopes to begin to find real applications fairly quickly.
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