Solar still in water

Solar still in water.

Qiaoqiang Gan, SUNY Buffalo

Sunlight-powered purifier could clean water for the impoverished

One-tenth of the world’s population lacks clean water. Now, researchers report they have developed a cheap solar still, which uses sunlight to purify dirty water up to four times faster than a current commercial version. The raw materials cost less than $2 per square meter. The technology will “allow people to generate their own drinking water much like they generate their own power via solar panels on their house roof,” says Zhejun Liu, a visiting scholar at the State University of New York (SUNY) in Buffalo and one of the study’s co-authors.

Solar stills have been around for millennia. Most are simple black-bottomed vessels filled with water, and topped with clear glass or plastic. Sunlight absorbed by the black material speeds evaporation, which is trapped by the clear topping, and funneled away for drinking water. Most pollutants don’t evaporate, and so are left behind. But much of the sun’s energy is wasted in the slow heating of a full vessel of water. Even the best stills need to be about 6 square meters in size to produce enough water for a single person for a day.

In recent years, researchers have improved stills using two approaches. First, they design their stills so that only the very top layer of water in the vessel is heated and evaporated, which means less energy is lost. Second, they’ve turned to nanomaterials to absorb more of the sun’s rays. But efficient light-absorbing nanomaterials can cost hundreds of dollars per gram, making them unrealistic for widespread use in developing countries where the technology is needed most.

Qiaoqiang Gan, an electrical engineer at SUNY Buffalo, saw that problem firsthand. His lab was already developing new nanomaterials as absorbers for solar power cells, and wanted to also use them in a solar still. But it quickly became apparent that the material’s cost would never allow the technology to be viable. So Gan began looking for cheap alternatives.

V. Altounian/Science

His team’s new device has three main components. Gan and his colleagues start with a fiber-rich paper—sort of like the paper used to make currency. They coat this with carbon black, a cheap powder left over after the incomplete combustion of oil or tar. Next, they take a block of polystyrene foam—the stuff used to make coffee cups—and cut slices through it making 25 connected sections. The foam floats on the untreated water and acts as an insulating barrier to prevent sunlight from heating up too much of the water below. The researchers then layer pieces of their paper over each section, folding the ends down so that they dangle into the water. The paper wicks water upward, wetting the entire top surface of each of the 25 sections. Finally, a clear acrylic housing sits on top.

During operation, evaporated water from the carbon paper is trapped by the acrylic and funneled to a collection vessel, and the paper wicks up additional water to replace it. Gan and his colleagues report this week in Global Challenges that the setup not only works, but that it’s 88% efficient at channeling the energy in sunlight into evaporating water. This allows a 1-square-meter-sized device to purify 1 liter of water per hour, which is about four times faster than commercially available versions, Gan says.

Equally important Gan adds, is that the still is cheap. He estimates the materials needed to build it cost roughly $1.60 per square meter, compared with $200 per square meter for commercially available systems that rely on expensive lenses to concentrate the sun’s rays to speed evaporation. At that price, providing the minimal water needed for a family of four might cost as little as $5 for the raw materials per device. That cheap cost may not only help people in impoverished regions, but also help aid workers deploy cheap water purifiers to people affected by natural disasters that wipe out safe drinking water sources. “We think this is an immediate application,” Gan says.

The new work is “good progress,” says Gang Chen, a mechanical engineer at the Massachusetts Institute of Technology in Cambridge, who has developed his own version of the technology in recent years, which uses slightly different materials. The new setup not only uses cheaper starting materials than anything on the market, but makes freshwater much more quickly, he notes. “This is really important in solving many water challenges.”

The authors of the report have formed a company—Suny Clean Water—to commercialize the work and are already in discussions with other companies around the world to make the new technology available.