Super-Efficient Water Purifier Based on Ancient Technology

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In May 2018, a team of University of Buffalo researchers including Haomin Song, Qiaoqiang Gan, and colleagues, unveiled an amazing new water purification device based on — well, stuff we’ve been doing for hundreds of years. Solar stills, as they’re called, purify water using solar energy: you place a container of dirty water beneath some angled surface and leave it out in the sun, which heats the water so that it condenses on the surface and runs down into another container, leaving any minerals, bacteria, and other toxins behind so you’re left with clean, fresh water. Solar stills are so simple that you can make one right now just by watching a few videos on YouTube. Of course, if that’s all the researchers had done, this wouldn’t be news.

Solar stills are great because all they need to work is a little sunlight. The most common method for water purification today is reverse osmosis, and that requires a whole lot of electricity to work — which also means it doesn’t work off the grid. The problem with solar stills, however, is that they’re just not that efficient at vaporizing water. Heat from the sun is hard to control, and a lot of it is simply wasted into the ambient air. Gan’s team fixed this problem by figuring out a way to use just enough solar heat to condense water, but not so much that it’s wasted in the air.

To do that, they used a sheet of carbon-dipped paper folded into an upside-down “V” shape, like the roof of some Silicon Valley birdhouse. The bottom edges of the paper hang in a small pool of water, which the paper soaks up like a sponge. Meanwhile, the carbon coating absorbs energy from the sun and turns it into just enough heat to evaporate the water.

But unlike other solar stills that use a flat surface to collect solar energy, the fact that the carbon-dipped paper is sloped means that the sunlight that hits it is less intense. Think about it: you’re more likely to get a sunburn on your back if you’re lying flat on the ground than you are if you’re standing around playing catch. That helps it stay under room temperature, allowing it to draw in heat from the surrounding area and compensate for the solar energy that’s lost during the vaporization process.

The result? Water purification with near-perfect efficiency. The team was able to evaporate an equivalent of 2.2 liters (0.58 gallons) of water per hour for every square-meter of area illuminated by the sun. The theoretical upper limit, according to the study, is 1.68 liters (0.44 gallons) per hour. Their device surpassed what anyone thought was possible.

“Most groups working on solar evaporation technologies are trying to develop advanced materials, such as metallic plasmonic and carbon-based nanomaterials,” Gan said in a press release. “We focused on using extremely low-cost materials and were still able to realize record-breaking performance. Importantly, this is the only example I know of where the thermal efficiency of the solar evaporation process is 100 percent when you consider solar energy input.”

Of course, evaporation is just one piece of the puzzle. The next step is finding a way to make the actual moisture condensation just as efficient. “We have another manuscript under review talking about how to enhance the moisture condensation,” Gan told Curiosity by email. “It is related to cooling technology to collect water from the air.”

Song, Gan, and the team have launched a startup called Sunny Clean Water to help bring the new technology to the people who need it most. Once they’ve integrated the evaporation system into a full-scale solar still, they could bring inexpensive, efficient water purification to societies all over the world.

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