MIT scientists have developed a new system that offers cooling on extreme sunny days without the use of energy. Such an approach of passive radiative cooling uses angular confinement of solar radiation in the sky to achieve gun cooling during the day, regardless of the properties of the radiator in the solar spectrum.
The system can provide cooling to 20 degrees Celsius (36 degrees Fahrenheit) below ambient temperature in a place such as Boston. When testing, the system suggests cooling to 6 ° C (about 11 F) below ambient temperature.
The system works by allowing it to radiate heat in the mid-infrared region of the light, which can pass directly through the atmosphere and radiate the cold of outer space, punching directly through gases that act as a greenhouse. To prevent heat from direct sunlight, a small strip of metal suspended above the device blocks the direct rays of the sun.
Scientists noted: "Other groups tried to develop passive cooling systems that emit heat in the form of medium infrared wavelengths of light, but these systems were based on sophisticated engineering photon devices that can be expensive and not widely available to use."
“The devices are complex because they are designed to reflect all the wavelengths of sunlight almost perfectly and only to emit radiation in the middle of the infrared range, for the most part. This combination of selective reflectivity and emissivity requires a multilayer material, where the thickness of the layers is controlled to nanometer accuracy. "
Scientists explained: “It turns out that such selectivity can be achieved by simply blocking direct sunlight with a narrow strip at right angles to cover the sun’s path across the sky without requiring active tracking by the device. Then, a simple device built from a combination of inexpensive plastic film, polished aluminum, white paint and insulation can provide the necessary heat emission through medium infrared radiation, as most natural objects cool down without allowing the device to be heated by direct sunlight. "
“In fact, simple radiation cooling systems have been used since ancient times to achieve night 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. ”
Researcher Bikram Bhatia said: “But the sun's rays of the sun move in straight lines and are easily blocked, as we see, for example, on a hot day when you enter the shade of a tree. By painting the device, essentially putting an umbrella on it and supplementing it with insulation around the device to protect it from the ambient temperature, the researchers made passive cooling more viable. ”
“We built the installation and conducted experiments outdoors on the roof of MIT. This was done using very simple materials "and clearly showed the effectiveness of the system."
Evelyn Van, professor of physics Marin Solyachich, said: “This is quite deceptive. Having a separate shade and a radiator to the atmosphere – two separate components that can be relatively inexpensive, the system does not require a special ability to radiate and absorb selectively. We use angular selectivity to block the direct sun, as we continue to radiate heat-carrying wavelengths to the sky. ”
“It would be useful for cold storage such as food storage or vaccines. The system may also be useful for some types of concentrated photovoltaic systems, where mirrors are used to focus sunlight on a solar panel to increase its efficiency. But such systems can easily overheat and, as a rule, require active temperature control using liquids and pumps. Instead, the radiating surfaces of the mid-infrared region used in the passive cooling system can be installed on the back of such concentrating systems and can control the heat without any active intervention. "
Scientists are currently working to improve the system, the most difficult task is to find ways to improve the insulation of the device so that it does not heat up too much from the surrounding air without blocking its ability to radiate heat.
The new system is described this week in Nature Communications magazine in an article by researcher Bikram Bhatia, graduate student Arni Leroy, professor of mechanical engineering and department head Evelyn Wang, professor of physics Marina Solyachich and six others at the Massachusetts Institute of Technology.