MIT scientists have developed a new system that offers cooling in extreme, sunny, energy-free days. This passive radiation cooling approach uses angular sunlight in the sky to achieve underwater cooling throughout the day regardless of the properties of the emitter in the solar spectrum.
The system can provide cooling up to 20 degrees Celsius (36 degrees Fahrenheit) below the ambient temperature in a place like Boston. During testing, the system offered to cool at 6 ° C (about 11 ° F) below ambient temperature.
The system works by allowing the emission of heat in the mid infrared range of light that can pass directly out of the atmosphere and radiate into the cold space in space and flow directly through the gases that act as a greenhouse. To prevent heating in direct sunlight, a small metal strip, suspended above the device, blocks direct sunlight.
Scientists noted: "Other groups have tried to design passive cooling systems that emit heat in the form of medium infrared wavelengths of light, but these systems were based on complex photonic structures that can be costly and not available to the general public."
"The devices are complex because they are designed to almost reflect all sunlight wavelengths and mostly only emit radiation in the mid range of infrared radiation." This combination of selective reflectivity and emissivity requires multilayer material where layer thicknesses are controlled by nanometer accuracy.
Scientists have explained, "It has been shown that such selectivity can be achieved by simply blocking direct sunlight with a narrow stripe at right angles to cover the sun's path through the sky, which does not require any active device tracking." A simple device built from a combination of cheap plastic films, polished aluminum, white colors and insulation can allow for the necessary heat emission through infrared radiation, which is how most natural objects cool down and prevents the device from being heated by direct sunlight. "
"In fact, simple radiation cooling systems have been used to achieve night-time cooling, and the problem was that these systems did not work during the day because the sunlight heating effect was at least ten times the maximum cooling effect."
Research scientist Bikram Bhatia said: "But the sun's rays are moving in a straight line and are easily blocked-for example, we have experienced a climb in the shade of a tree on a hot day, by shielding it by basically putting an umbrella on it and adding it to the insulation around the device to protect it from the ambient air temperature, scientists have made passive cooling more viable. "
"We've put together a set of outdoor experiments on MIT's roof, made using very simple materials," and clearly demonstrated the effectiveness of the system. "
Evelyn Wang, professor of physics, Marin Soljačić, said, "It is deceptively simple: having a separate shade and emitter into the atmosphere – two separate components that can be relatively low, the system does not require a special ability to selectively radiate and absorb. to allow direct sunlight to be blocked because we continue to emit wavelengths with warmth to the sky. "
"This would be useful for cooling applications such as storing food or vaccines and can also be useful for some types of concentrated photovoltaic systems where mirrors are used to focus solar radiation on a solar cell to increase its efficiency. they can easily overheat and generally require active heat control by means of liquids and pumps instead of the backsheet of such concentrating systems can be equipped with a medium infrared emission surface used in the passive cooling system and can control heating without any active intervention.
Scientists are currently working on improving the system, the biggest challenge being to find ways to improve device isolation to avoid overheating of the surrounding air without blocking its ability to radiate heat.
The new system is described this week in Nature Communications in the contribution of Bikram Bhatia, a research fellow, Arny Leroy, a professor of engineering and head of Evelyn Wang, physics professor Marina Soljacic, and six others at MIT.