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A bubble-wrapped sponge may revolutionize clean water

Bubble wrap, combined with a selective absorber, keeps heat from escaping the surface of the sponge. (Credit: George Ni/MIT)

Providing people with clean water has long been touted as the one basic necessity that can change everything for poor communities, by preventing diseases and reducing the time-and-labor burden on the women and children, mostly, who walk long distances to fetch it.

Now, a floating, bubble-wrapped sponge may change how easily people can access clean water.

Simply by absorbing ambient sunlight, the unassuming device, developed by a team at the Massachusetts Institute of Technology, can actually boil water.

Requiring no fuel, mirrors or lenses, the device is intentionally low-tech and cost-effective with a wide range of applications – including heating residential water, treating wastewater, desalinating small bodies of water, sterilizing medical tools when electricity isn’t available and purifying water in areas of the world without access to clean water.

“It’s kind of a different approach where before, people were doing high-tech and long-term,” says George Ni, MIT graduate student and lead author of the study published Monday. “We’re doing low-tech and short-term.”

The device – called a solar vapor generator – would need to be replaced every one to two years, but the low price would still outweigh the higher initial and maintenance costs of other systems.

Most importantly, it works – even on cloudy days.

The device is actually based on the team’s 2014 design of a similar solar sponge made of graphite and carbon foam that can “convert 85 percent of incoming sunlight to steam,” the university said in a release. However, in order to heat the water to a boil, the researchers had to expose the sponge to sunlight 10 times the intensity of natural sunlight.

“But I kept asking myself, ‘Can we basically boil water on a rooftop, in normal conditions, without optically concentrating the sunlight?’ That was the basic premise,” senior author Gang Chen, head of MIT’s Department of Mechanical Engineering, told the university.

The black graphite, while effective at absorbing ambient sunlight, was not great at trapping it, radiating heat back into the environment. To combat that problem, the researchers covered the sponge with a thin copper sheet coated with a “spectrally-selective absorber,” and for the most part, it worked.

But heat was still escaping through convection, or air motion like the wind. That’s when Chen’s 16-year-old daughter’s science fair project – a homemade greenhouse – inspired him with bubble wrap.

“She was able to heat it to 160 degrees Fahrenheit, in winter!” Chen said. “It was very effective.”

Chen proposed the idea to Ni, who says he was skeptical at first: “I thought it was not a high-performance material. But we tried the clearer bubble wrap with bigger bubbles for more air trapping effect, and it turns out, it works. Now because of this bubble wrap, we don’t need mirrors to concentrate the sun.”

While the current device seems to be about the size of a person’s palm, the researchers have visions for a larger system. But useful applications exist large or small, and at the very least, it’s a reminder that revolutionary solutions can be as simple and cheap as bubble wrap.


About Author

Joanne Lu

Joanne Lu is a South Carolina-based writer and editor dedicated to global development, poverty alleviation and social justice. After a year in Rwanda, she now covers the Asia-Pacific and economics. Find her on Twitter @joannelu or email