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Device makes clean water with paper and sunlight

Researchers have created a method for using sunlight to generate clean water with nearly perfect efficiency.

The idea of using energy from the sun to evaporate and purify water is ancient. The Greek philosopher Aristotle reportedly described such a process more than 2,000 years ago.

The researchers’ method involves draping black, carbon-dipped paper in a triangular shape and using it to both absorb and vaporize water.

“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.”

“Our technique is able to produce drinking water at a faster pace than is theoretically calculated under natural sunlight,” says lead researcher Qiaoqiang Gan, associate professor of electrical engineering in the University at Buffalo School of Engineering and Applied Sciences.

“Usually, when solar energy is used to evaporate water, some of the energy is wasted as heat is lost to the surrounding environment. 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.”

The low-cost technology could provide drinking water in regions where resources are scarce or where natural disasters have struck.

solar still (water purification concept)
The new water purification technique involves draping a sheet of carbon-dipped paper in an upside-down “V.” The paper’s bottom edges soak up water, while the carbon coating absorbs solar energy and transforms it into heat for evaporation. (Credit: Huaxiu Chen/U. Buffalo)

“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 Haomin Song, an electrical engineering PhD graduate. “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.”

Solar stills have been around for a long time. These devices use the sun’s heat to evaporate water, leaving salt, bacteria, and dirt behind. Then, the water vapor cools and returns to a liquid state, at which point it’s collected in a clean container.

The technique has many advantages. It’s simple, and the power source—the sun—is available just about everywhere. But unfortunately, even the latest solar still models are somewhat inefficient at vaporizing water.

Gan’s team addressed this challenge through a neat, counterintuitive trick: They increased the efficiency of their evaporation system by cooling it down.

A central component of their technology is a sheet of carbon-dipped paper that they folded into an upside-down “V” shape, like the roof of a birdhouse. The bottom edges of the paper hang in a pool of water, soaking up the fluid like a napkin. At the same time, the carbon coating absorbs solar energy and transforms it into heat for evaporation.

As Gan explains, the paper’s sloped geometry keeps it cool by weakening the intensity of the sunlight illuminating it. (A flat surface would be hit directly by the sun’s rays.) Because most of the carbon-coated paper stays under room temperature, it can draw in heat from the surrounding area, compensating for the regular loss of solar energy that occurs during the vaporization process.

Using this set-up, researchers evaporated the equivalent of 2.2 liters of water per hour for every square meter of area illuminated by the regular sun, higher than the theoretical upper limit of 1.68 liters, according to the new study. The team conducted its tests in the lab, using a solar simulator to generate light at the intensity of one regular sun.

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“Most groups working on solar evaporation technologies are trying to develop advanced materials, such as metallic plasmonic and carbon-based nanomaterials,” Gan says. “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 researchers have launched a startup, Sunny Clean Water, to bring the invention to people who need it. With support from the NSF Small Business Innovation Research program, the company is integrating the new evaporation system into a prototype of a solar still, a sun-powered water purifier.

A study describing the research appears in the journal Advanced Science.

The National Science Foundation funded the project. Additional researchers contributing to the work are from UB, Fudan University in China, and the University of Wisconsin-Madison.

Source: University at Buffalo

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