MIT researchers have developed a new way to cool hot, sunny days by using inexpensive materials that do not require fossil fuel energy. The passive 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 high-tech version of an umbrella.
The system allows the emission of heat in the range of average infrared light that can directly cross the atmosphere and radiate in the cold of the 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.
The new system is described this week in the newspaper Nature Communications in an article written by researcher Bikram Bhatia, graduate student Arny Leroy, mechanical engineering professor and department head, Evelyn Wang, physics professor Marin Soljacic and six other people at MIT.
In theory, the system they designed could provide cooling up to 20 degrees Celsius (36 degrees Fahrenheit) below room temperature in a place like Boston, according to the researchers. Until now, during the first validation tests, they reached 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 passive heat-emitting cooling systems in the form of average infrared light wavelengths, but these systems are based on sophisticated complex photonic devices that can be expensive to manufacture. and are not readily available for widespread use. say. 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 where the thicknesses of the layers are controlled with nanometric precision.
But it turns out that a similar selection 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 l & # 39; device. Then a simple device built from a combination of inexpensive 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 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 stronger than the maximum achievable cooling effect.
But the sun's rays move in straight lines and are easily blocked, as seen for example by walking in the shade of a tree on a hot day. By shading the device with an umbrella on top, and 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 carried out outdoor experiments on a MIT roof," says Bhatia. "This was done with very simple materials" and clearly showed the effectiveness of the system.
"It's a bit misleading," Wang says. "By having a separate nuance and transmitter in the atmosphere, two separate components that can cost relatively little, the system does not require a special ability to transmit and transmit. We use an angular selectivity to allow direct sun blocking, while we continue to transmit the wavelengths that carry heat to the sky. "
This project "has prompted us to rethink the use of" shadow "," says Yichen Shen, associate 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 explains.
According to Leroy, one of the limiting factors of the system is the humidity in the atmosphere, which can block some of the infrared emission in the air. 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 degrees Celsius. But in drier environments, such as the southwestern United States or many desert or arid environments around the world, the maximum achievable cooling could actually be much greater, he says, potentially up to $ 25,000. at 40 ° C (72 ° F).
Although most research on radiative cooling has focused on larger systems that can be used to cool buildings or entire buildings, this approach is more localized, explains Wang: "This would be useful for refrigeration applications, such as than 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 could be well placed to solve.
Even if the system was not enough to lower the temperature to the required level, "it could at least reduce the loads" of the electric refrigeration systems, to provide only the final cooling, Wang explains.
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 concentration systems 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.
While they continue to improve the system, the biggest challenge is to find ways to improve the insulation of the device, so that it does not overheating the air, while not blocking its ability to emit heat. "The main challenge is to find an infrared-transparent insulating material," says Leroy.
The team has filed a patent application and hopes that it can begin to find real applications fairly quickly.
Harvesting renewable energy from the sun and space at the same time
Bikram Bhatia et al, passive directional sub-ambient daytime radiative cooling, Nature Communications (2018). DOI: 10.1038 / s41467-018-07293-9
Massachusetts Institute of Technology
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A new way to provide cooling without electricity (November 28, 2018)
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