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A solar-powered water harvesting device, constructed with a metal organic framework. Unlike other types of water capture devices, this design is able to work under low-humidity conditions. (Hyunho Kim, Evelyn Wang)
A new kind of water-capturing device could be a game-changer for some of the world's driest places. It can pull water vapor out of the air at humidity as low as 20 percent - conditions that may be seen in the Sahara desert during its hottest months - and it can operate entirely off-grid, just using the ambient power of the sun.
This means it could provide water for parts of the world likely to be most vulnerable to water shortages under future climate change, including areas afflicted by recurring drought.
According to a description of the new design, published Thursday in the journal Science, a single tissue box-sized device can harvest up to 2.8 liters, or about three quarts, of water in one day at low humidity - that's a bit more than the half gallon of water experts recommend a person drink over the course of a day.
The design provides a "better way" to capture water under low relative humidity than previous techniques have offered, according to Kyoo-Chul Park, an assistant professor of mechanical engineering at Northwestern University, who was not involved with the new project but has worked on other water capture designs in the past.
"Typically, there have been two technologies developed for extracting water out of air," said Evelyn Wang, an associate professor of mechanical engineering and co-author on the new paper.
The first is fog harvesting, which is just what it sounds like - collecting liquid water straight from clouds of fog. In this case, the fog raises the air's relative humidity to just about 100 percent. The second method is called "dewing" and it involves using water condensers to pull water vapor out of the air and turn it into liquid form. However, this method has typically needed a lot of energy to keep the condenser at the right temperature and still required a fairly high humidity level, Wang noted.
The new design is essentially a form of dewing, Park pointed out - it just works at much lower humidity levels than previous devices have been able to achieve. The key is a special type of compound known as a metal organic framework, or MOF, a crystalline material involving metal ions linked with organic molecules. The MOF has a porous structure that makes it ideal for holding water, similar to a sponge.
MOFs have been used for many different applications in the past, including gas storage, dehumidification and the capture of carbon dioxide emissions. But their ability to serve as water harvesters is a relatively new concept, Wang said.
There are many different types of MOFs, which involve slightly different materials and structures. The MOF used in the new device is optimized specifically to capture water in low humidity conditions. And it requires no external sources of power, other than the sun, to get the water out.
An illustration of the structure of MOF-801, a MOF design that's been optimized specifically to capture water at low relative humidity. (Omar Yaghi)
Over the course of a 24-hour period, this is how it works: During the night, when the air is cool, water vapor enters the device and condenses inside its porous structure, turning to liquid. Then, during the day, the sun strikes the device and warms up its spongy insides. As the water molecules heat up, they begin to vaporize again, slipping away from the porous material and eventually condensing again into reservoirs designed to collect the water.
"That's really where the power of this chemistry comes into play," said Omar Yaghi, a chemistry professor at the University of California Berkeley and another of the paper's co-authors. "Where the composition of organic and inorganic and the balance between these two allows you to craft the interior so that it loves water, but it doesn't hold onto it too tightly."
The entire process relies on phase changes - back and forth between water vapor and liquid water - Park noted, and he added that the technique mimics a natural process that already occurs in desert landscapes.
There, the sand acts as a kind of porous material that collects condensed water during the cool nights, he explained. During the day, it evaporates again when the sun comes up. The new device involves a similar series of steps, with the added component of a reservoir that allows it to collect the water it traps.
The entire device operates off ambient sunlight, Yaghi noted - no solar cells or external power sources are needed to make it work. However, he added that liquid water can only be collected during the day, when the sun heats up the device. To collect water during the night, one would need another power source to warm it up.
The next step is to continue scaling up the design into devices that can collect even more water. The current design, which is no bigger than a toaster, collects about three quarts of water in a day, which is a little more than necessary for one individual.
"If you want to get to a 30-liter quantity, to demonstrate the real viability of this, you would need to incorporate multiple stacks of this MOF layer into a device," Wang said. Such a device would probably be about the size of a suitcase, she noted.
Park pointed out that, when optimizing future devices for practical use, some environments experience dramatic shifts in humidity levels over the course of a single day. In some desert regions, humidity may drop as low as 10 percent during the day and swing up above 50 percent during the night.
"This daily fluctuation of relative humidity is another reason... why we need to think of optimization and improvement of atmospheric water generation systems," he said in a follow-up email to The Washington Post.
But MOFs can also be optimized for different sorts of conditions, Yaghi pointed out. The device described in the new paper is tailored for a 20 percent humidity level, but other MOF designs may work best at 50 percent or higher.
"There would be some other candidates depending on different ambient conditions," he said.
But according to the authors, the real beauty of the new design is specifically that it works in such arid regions - a challenge that previous techniques have struggled to address. And that it requires no electricity is an added bonus.
The success of the prototype described in the new paper has the researchers hopeful that commercial use may be in the not-so-distant future."My vision is really to bring water to households off-grid," Yaghi said. "I don't think we're very far from it because of the fact that this device works."