But that’s not the case in our system, which is much closer to reality,” says Gan. “The cost and availability of materials has been the barrier for this type of application. “You can’t set out to solve the water problem without addressing energy.” Courtesy of UW–Madison College of Engineering “Water sustainability is a global issue,” says Zongfu Yu, an associate professor of electrical and computer engineering at UW–Madison. “The cooling power obtained via this thermal contact enables daytime water condensation with no need for an external power source.”Īnother benefit is that polydimethylsiloxane is widely available and relatively cheap, and the silver backing is not necessary for the condenser to work. “Fundamentally, our radiative condenser is engineered to be in ‘thermal contact’ with the vast cold reservoir in the upper atmosphere and in outer space,” says co-author Mikhail Kats, a UW–Madison electrical and computer engineering associate professor and expert on managing infrared radiation. The polydimethylsiloxane was the only material that condensed water vapor while in direct sunlight. The team pumped humidified air into the three chambers, which they positioned on top of a UW–Madison building and, during another test, a parking garage. Zhou tested the device by placing it inside a condensation chamber alongside chambers containing a commercially available dew-collecting material as well as a simple “black body,” an object used for experimental comparisons because of the consistent way it absorbs and emits radiation. The combination of the two is able to cool the condenser below the dew point, leading to condensation. They layered that over silver, which reflects sunlight. In this project, the team, led by UW–Madison postdoctoral researcher Ming Zhou and supported by the National Science Foundation, constructed a small vapor condenser using a thin film of material called polydimethylsiloxane, which is very efficient at releasing thermal radiation in the atmospheric-transparency window. ![]() The problem is that those collectors only work at night since sunlight produces more heat than the materials can give off. Over the last few decades, researchers have designed dew collectors based on the same principle, using special materials that efficiently shed heat like the beetle shell does. ![]() Photo courtesy of the Photonics Lab at UW–Madison Water condensed from air beads on the surface of a new water vapor condenser designed by UW–Madison engineers. The beetle is then able to harvest that water, using special grooves and structures to direct the moisture toward its mouth. This heat loss lowers the beetle’s temperature below the dew point, or the temperature at which water vapor in the air condenses into droplets on cooler surfaces (think of a glass of iced tea on a hot day). That heat naturally radiates toward the cool upper atmosphere of Earth and the chilly void of space. During clear nights when ambient temperatures are cool, darkling beetle shells shed extra heat in the mid-infrared range, also known as the atmospheric-transparency window. In fact, it’s used in nature by insects like the darkling beetle found in the Namib Desert in southwest Africa. The idea of radiative cooling is not new. Yu, with University at Buffalo Professor Qiaoqiang Gan and their students, described the new radiative vapor condenser this week in the journal Proceedings of the National Academy of Sciences. ![]() Researchers tested their water condenser in direct sun atop UW–Madison buildings.
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