Perspiration is Inspiration for New Cooling Rooftop
Researchers used small, inches-high model houses to test a new cooling roof that 'sweats' in hot weather.
CREDIT: Aline Rotzetter, from Advanced Materials, published by Wiley
Plunk this new mat on the roof, and it could keep the house cool in hot weather while saving the homeowner 60 percent on air conditioning costs, the mat's creators say. The mat's cooling mechanism is designed to kick in automatically once temperatures reach 90 degrees Fahrenheit (32 degrees Celsius), it works for up to three hours, and it doesn't require any electricity.
So how does it work? The mat sweats, the way humans and other mammals do, exuding drops of water that wick away heat when they evaporate.
"The urgent need to reduce our energy consumption led to the idea," said Aline Rotzetter, one of the mat's creators, who is a doctoral student in materials science at the Swiss Federal Institute of Technology in Zurich.
The inspiration for the idea was "the sweating action of humans," Rotzetter told TechNewsDaily in an email.
At this point, the mat is still in its earliest stage of development. Rotzetter and her colleagues have tested it on model houses a few inches tall, but they plan to leave further work to other scientists.
The mat probably will need much more work to ensure it's affordable and reliable over long periods of time, said Hashem Akbari, a building engineer and cool roofs expert at Concordia University in Quebec. That's true of all early-stage lab ideas – and all working tech started as lab ideas, added Akbari, who was not involved in the Swiss research. [SEE ALSO: 5 Easy Ways to Go Green — and Save Money]
"This is a very good idea," he said. "It is an encouragement to see that new materials are being developed and knowing that the cooling is significant in buildings that have advanced roofing materials is welcome research."
Rotzetter's mat is made with a material, called PNIPAM, that soaks up water whenever it's at a temperature below 90. Rotzetter and her colleagues imagine that a real roof with a PNIPAM mat would store water from rain.
Once temperatures climb above 90, things would change. "It changes its chemical property from hydrophilic to hydrophobic," Rotzetter said. As the PNIPAM material switches from wanting to absorb water to wanting to repel it, the mat squeezes out its stored water. The water is then able to evaporate, cooling the roof.
In laboratory tests using the small model houses and a sun lamp, a fully soaked PNIPAM mat kept a roof at 77 degrees F (25 C) or cooler for an hour. Meanwhile, the ground in front of the house reached a scorching 140 F (60 C).
After the hour, the PNIPAM-covered roof slowly warmed for the next two hours, until all of the mat's stored water evaporated and the roof temperature reached 95 F (35 C).
At its peak performance, the PNIPAM mat kept roofs cooler than reflective paint or insulated roof tiles did, Rotzetter and her colleagues found. It worked about as well as a rooftop garden.
"A big advantage compared to a rooftop garden is the possibility to install it on every kind of house, since the mat is only a few millimeters thick," Rotzetter said. "Perhaps a combination of a reflective and a sweating surface would be interesting, too." [SEE ALSO: Redesigned Roofs Withstand Tornadoes and Hurricanes]
The Swiss researchers think the mat would work best for homeowners in tropical regions, where rain is likely every few days. A dry period of 16 days doesn't affect the mat's cooling ability once it's been soaked, the researchers found. During the dry time, however, the mat can't sweat.
The team has no plans to keep developing the PNIPAM roof, Rotzetter said. They only wanted to show that, in principle, sweating roofs work. The next steps would be up to other scientists: finding ways to make and install mats onto real roofs. "This is a challenge, because the mat has to survive over a long period," Rotzetter said.
Rotzetter and her labmates wrote about their work Oct. 2 in the journal Advanced Materials.