Scientists at MIT have come up with a new method of providing cooling on a hot sunny day, using low-cost materials and without the need for fossil fuel-generated power. The passive system is basically a high-tech version of a parasol and could be used to supplement other cooling systems to preserve medications and food in hot, off-grid locations.
The system is designed to allow emission of heat at mid-infrared range of light that can pass straight out through the atmosphere and radiate into the cold of outer space, piercing right through the gases that act like a greenhouse. To avoid heating in the direct sunlight, a small strip of metal suspended above the device obstructs the sun’s direct rays.
The new system is 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, professor of physics Marin Soljačić, and six others at MIT.
Theoretically, the system built by them could offer cooling of as much as 20 °C (36 °F) below the ambient temperature in a place like Boston, the scientists say. Thus far, in their preliminary proof-of-concept testing, they have realized a cooling of 6 °C (about 11 °F). For applications that require a lot more cooling, the remainder could be accomplished through traditional refrigeration systems or thermoelectric cooling.
Other research teams have tried to design passive cooling systems that radiate heat in the form of mid-infrared wavelengths of light, but these systems have been based on complex engineered photonic devices that may not be cost-effective to make and not readily available for extensive use, the scientists say. The devices are complex as they are engineered to reflect all wavelengths of sunlight almost flawlessly, and only to discharge radiation in the mid-infrared range, for the most part. That combination of selective reflectivity and emissivity necessitates a multilayer material where the thicknesses of the layers are manipulated to nanometer precision.
Nonetheless, it turns out that similar selectivity can be attained by just blocking the direct sunlight with a thin strip positioned at just the right angle to cover the sun’s path across the sky, needing no active tracking by the device. Then, a simple device developed from a combination of cheap plastic film, white paint, polished aluminum, and insulation can allow for the required emission of heat through mid-infrared radiation, which is the way most natural objects cool off, while stopping the device from being heated by the direct sunlight. Actually, simple radiative cooling systems have been used since olden times to attain nighttime cooling; the problem was that such systems did not function in the daytime as the heating effect of the sunlight was no less than 10 times stronger than the maximum attainable cooling effect.
But the heating rays of the sun travel in straight lines and are easily obstructed — as you experience, for instance, by stepping into the shadow of a tree on a hot day. By shading the device by basically placing an umbrella over it, and supplementing that with insulation around the device to protect it from the ambient air temperature, the scientists made passive cooling more feasible.
“We built the setup and did outdoors experiments on an MIT rooftop,” Bhatia says. “It was done using very simple materials” and vividly showed the system’s effectiveness.
It’s kind of deceptively simple. By having a separate shade and an emitter to the atmosphere—two separate components that can be relatively low-cost—the system doesn’t require a special ability to emit and absorb selectively. We’re using angular selectivity to allow blocking the direct sun, as we continue to emit the heat-carrying wavelengths to the sky.
Evelyn Wang, Professor of Mechanical Engineering and Department Head, MIT.
This project “inspired us to rethink about the usage of ‘shade,’” says Yichen Shen, a research affiliate and co-author of the paper. “In the past, people have only been thinking about using it to reduce heating. But now, we know if the shade is used smartly together with some supportive light filtering, it can actually be used to cool the object down,” he says.
One factor that limits the system is humidity in the atmosphere, Leroy says, which can obstruct some of the infrared emission through the air. In a city like Boston, near the ocean and comparatively humid, this limits the total amount of cooling that can be attained, restricting it to about 20 °C. However, in drier environments, such as the southwestern U.S. or many desert or arid environments globally, the maximum attainable cooling could actually be a lot greater, he remarked, potentially as much as 40 °C (72 °F).
While a majority of research on radiative cooling has concentrated on larger systems that might be applied to cooling whole rooms or buildings, this method is more localized, Wang says: “This would be useful for refrigeration applications, such as food storage or vaccines.” Certainly, protecting vaccines and other medicines from decay in hot, tropical environments has been a huge ongoing challenge that this technology could be well-positioned to solve.
Even if the system was not adequate to lower the temperature all the way to desirable levels, “it could at least reduce the loads” on the electrical refrigeration systems, to deliver just the final bit of cooling, Wang says.
The system might also be beneficial for certain kinds of concentrated photovoltaic systems, where mirrors are used to direct sunlight on a solar cell to boost its efficiency. But such systems can easily overheat and mostly require active thermal management using fluids and pumps. Instead, the backside of such concentrating systems could be equipped with the mid-infrared emissive surfaces used in the passive cooling system, and could regulate the heating without any active intervention.
As they carry on working to enhance the system, the biggest challenge is discovering ways to enhance the insulation of the device, to stop it from heating up excessively from the surrounding air, while not obstructing its ability to radiate heat.
The main challenge is finding insulating material that would be infrared-transparent,” Leroy says.
Arny Leroy, Graduate Student, MIT.
The team has applied for patents on the invention and is looking forward to finding real-world applications quite fast.
The research team included Lin Zhao, Melissa Gianello, Duanhui Li, Tian Gu, and Juejun Hu, all at MIT. The research was supported as part of the Solid-State Solar Thermal Energy Conversion (S3TEC) Center, an Energy Frontier Research Center of the U.S. Department of Energy.