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How a Sponge, Bubble Wrap and Sunlight Can Lead to Clean Water

With simple materials, MIT researchers have developed a cheap, easy-to-build device to desalinate water and treat wastewater

The prototype solar vapor generator (George Ni)
smithsonian.com

Here’s yet another reason to love bubble wrap, the delightful-to-pop packing material: it can help generate clean water.

Researchers at MIT were looking for a way to clean and desalinate water without using expensive specialty materials or devices. What they came up with is, in layman’s terms, a sponge encased in bubble wrap. This “solar vapor generator” can heat water up enough to make it boil, evaporating the water and leaving behind unwanted products like salt.

The most common way to concentrate sunlight and generate heat is with mirrors, says George Ni, a PhD candidate who led the research. But the problem is that mirrors and other optical heat concentrators are often pricey.

“If you’re going to use this for desalinating water in a developing country, it’s really too expensive for most people to afford,” he says.

The solar vapor generator that Ni and his team developed involves a metallic film that can absorb radiation and trap heat. This spectrally selective absorber is mounted on a piece of special sponge made of graphite and carbon foam, which can boil water to 100 degrees Celsius using ambient sunlight. The whole thing is then wrapped in bubble wrap. The bubble wrap allows the sunlight in, but keeps the heat from escaping when the wind blows across the device, making it much more efficient.

“These are all commercial materials that most people should be able to buy from a home improvement store,” Ni says. The spectrally selective absorber is more specialized, he adds, though not expensive.

The solar vapor generator can float on the water, absorbing it and turning it into steam with the heat from the sun.

The most obvious application for the solar vapor generator is water desalination, Ni says. While the prototype was only about five inches long, there’s no reason why it couldn’t be much bigger—as big as a football field, even. A large solar vapor generator could float on the surface of a pond, lake or ocean. In some areas, where the groundwater is too salty to drink, this could help provide fresh water to communities.

“Access to fresh water is very limited in many places,” Ni says. “It’s not a renewable resource. It’s like oil.”

The other main use would be in wastewater management. The oil and gas industry, in particular, produce tons of wastewater every year, laced with toxic salts, metals and oils. This wastewater is often handled by storing it in ponds, which isn’t a long-term solution, or by dumping it down deep mineshafts, which can cause earthquakes. In theory, a large solar vapor generator could sit on the surface of a wastewater pond. The generator would evaporate most of the water, leaving behind a sludge of waste products that would be much easier to dispose.

The solar vapor generator is still a product in progress, Ni cautions, and it’s certainly not the only product on the market to clean and desalinate water. Still, he thinks it could fill a major niche for cheap, easy-to-build water treatment devices. It won't last as long as other devices, but it's much less expensive to replace, making it ultimately more cost effective. The team’s next steps will involve more testing under real world conditions. Of particular concern is how to deal with the salts that accumulate on the generator after the water is evaporated.

“Our challenge is making sure they don’t build up in the device, muck it up and prevent it from working,” Ni says.

Ni estimates a product may be ready for market in the next several years. No word on whether the MIT researchers found popping the bubble wrap as irresistible as we would. 

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