Access to clean water is one of the world's most pressing problems, but a team of University at Buffalo researchers has come up with a new take on an old technology that uses sunlight to purify water. Led by associate professor of electrical engineering Qiaoqiang Gan, the team has created a device that uses black, carbon-dipped paper to produce fresh water with what is claimed to be near-perfect efficiency.

Solar stills have been around for thousands of years, with references to them going back to Aristotle. They're a common survival kit item, especially for life rafts, and a quick browse of YouTube quickly turns up a surprising number of videos about how to use bits of common rubbish littering a beach to create a sun-powered still that can turn seawater or dirty water into something more potable.

Such stills work on the principle of evaporation. If you leave a dish of water out, the molecules that make it up will eventually gather enough energy from the environment to change from liquid to gas and float away. Heat the water, and the process speeds up.

In a solar still, seawater, dirty water, or even green leaves are set inside a transparent plastic container, which can be as simple as a polythene bag, and set out in the sun. The light heats the air inside the still, encouraging evaporation. The water vapor then rises, comes into contact with the cooler thin plastic, and condenses into liquid. This then trickles down the plastic and is collected into reservoirs to be poured off or sucked out with a straw. The problem is, though the process can be a literal life saver, it isn't very efficient.

"Usually, when solar energy is used to evaporate water, some of the energy is wasted as heat is lost to the surrounding environment," says Gan. "This makes the process less than 100 percent efficient. Our system has a way of drawing heat in from the surrounding environment, allowing us to achieve near-perfect efficiency."

Funded by the National Science Foundation (NSF), the Buffalo still is a surprising advance on previous devices. Before, the theoretical upper limit was 1.68 liters (56.8 oz) per hour per square meter, but the new technology manages 2.2 liters (74.4 oz).

"When you talk to government officials or nonprofits working in disaster zones, they want to know: 'How much water can you generate every day?' We have a strategy to boost daily performance," says team member Haomin Song. "With a solar still the size of a mini fridge, we estimate that we can generate 10 to 20 liters of clean water every single day."

The Buffalo still works by using a strip of black, carbon-dipped paper shaped in an upside-down "V." The bottom edges of the strip are set in water, which it soaks up like a napkin. As sunlight strikes the strip, it heats up, encouraging evaporation. At the same time, the angle of the strip keeps the light from hitting it directly, so it doesn't heat too much. The result is that water evaporates quickly from the strip, causing it to cool below the temperature of the air around it, which draws more energy into the system. This increases the efficiency of the evaporation cycle and generates more water vapor for collection.

The concept isn't new, but where the Buffalo team is ahead is avoiding the expensive materials used in other versions.

"Most groups working on solar evaporation technologies are trying to develop advanced materials, such as metallic plasmonic and carbon-based nanomaterials," says Gan. "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. By developing a technique where the vapor is below ambient temperature, we create new research possibilities for exploring alternatives to high-temperature steam generation."

The team has launched a startup called Sunny Clean Water to commercialize the technology, and are using the support of the NSF Small Business Innovation Research program to create a prototype sun-powered water purifier.

The research was published in the journal Advanced Science.

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