Buildings that Cool like African Elephants

During sunny and sweltering summer days, much relief comes from sweat. As beads of moisture evaporate on the skin, they pull heat away almost invisibly. It feels like a nuisance, but this is one of the most efficient cooling systems in the animal kingdom.

But not all animals have the ability to sweat. The African elephant is one example, enduring intense heat out on the African savanna with dark gray, inches-thick skin. How does it stay cool?

 “Wrinkles,” says Dorit Aviv, an associate professor of architecture at Weitzman. “Elephants have a network of cracks in their skin that trap water. When they spray themselves, that water stays put and evaporates slowly, cooling them over time.”

The concept of cooling without sweat was first pitched to Aviv by her longtime collaborator Shu Yang, a materials scientist at Penn Engineering. The idea stuck with Aviv, whose research focuses on making the built environment more thermodynamically efficient in a warming world, because buildings don’t sweat.

In a paper published in Advanced Materials, Aviv, Yang, and Kun-Hao Yu of Syracuse University turned that insight into a new kind of cement-based tile that captures, holds, and slowly evaporates water, cooling surfaces without fans, compressors, or complex moving parts.

Under infrared heating and periodic watering, the temperature beneath these tiles remained a stable 89.6° Fahrenheit. By comparison, cracked commercial stucco rose to 107.6°, while non-cracked stucco climbed to 125.6°.

“In the United States, we spend roughly 90% of our time indoors, and buildings consume about 40% of primary energy—nearly half of that for heating, cooling, and ventilation,” says Yang. “A passive, bio-inspired cooling façade like this could lower surface temperatures by 10-20° Fahrenheit compared to traditional stucco.”

Unlike mechanical air conditioning, which consumes fossil fuels and contributes to outdoor heat buildup by dumping indoor warmth outside, this is a truly passive system that cools the building’s “skin” itself, she explains.

Trapping water to allow it to evaporate and using that phase change to pull heat away is simple in theory. But the researchers found this particularly challenging, because most conventional building materials are terrible at it since water droplets bead up, bounce off, or run straight down the surface before they can do any useful cooling.

The team needed to devise a way to guide each droplet to maximize its cooling potential, which prompted turning to cracks.

“In conventional construction, cracks signal weakness, the beginning of a material’s decline,” Yang says. “But by engineering where and how they form, we made networks of tiny channels that act like capillaries capable of pulling water across the surface and holding it in place."

Read the full story at Penn Today.