Droughts have recently made headlines worldwide, including the California drought and Cape Town’s extreme water crisis. Research suggests that 25 percent of the world may end up in perpetual lack because of climate change. But what if we could pull water out of the air?
That’s the idea behind the latest technology developed by University of California, Berkeley researchers. It’s a system for water harvesting that can extract water from atmospheric air even in arid climates, with only ambient sunlight.
The key ingredient of the water harvester’s success is a new type of material called metallic-organic frameworks (MOFs). They are solid but porous materials that have huge surfaces. A MOF, the size of a sugar cube, can have an internal surface area as large as a football field. They can absorb liquids and gases and release them rapidly after adding heat.
“Certain MOFs as we showed here have an extraordinary ability to suck in water vapor from the atmosphere, but then at the same time do not hold on to the water molecules inside their pores too tightly so that it is easy to get the water out,” says Omar Yaghi, a professor of chemical chemistry at Berkeley who led the study.
The researchers tested the harvester at Scottsdale, Arizona, a desert town with the highest humidity of 40% humidity at night and 8 percent throughout the day. Based on the results of tests that used a zirconium-based mof, the harvester can get about 3 ounces of moisture per mound each day.
The harvester is a box within an outer box. Inside the box is a MOF bed. The box on the outside is a two-foot clear plastic cube. The researchers at night removed the top of the box’s exterior so that air could flow past the MOFs. In the daytime, they put the top back in place so that sunlight could heat the package. The sun’s heat would draw the water from the MOFs, and it would expand on the inside of the cube’s plastic before it dripping down to the bottom where it could be gathered.
“The most important aspect of this technology is that it is completely energy-passive,” says Eugene Kapustin, a Berkeley graduate student who participated in the study.
It does not require any energy other than the sun, making it eco-friendly and available for those living in areas with limited power. The findings of the experiments were released earlier this month in the science journal Science Advances.
The team must run more tests on the current models to determine which variables, like the device’s size and the MOF’s location, have the most significant impact on the amount of water harvested. They also want to learn more about how environmental conditions impact water yield. The next test is scheduled for the end of summer at Death Valley, where the nighttime humidity can drop as low as 25 percent.
Yaghi also has developed an aluminum-based MOF that is 150 times more affordable and will take more water than the current MOFs. The team he is working with is creating a new water harvester that pulls air into MOFs at high speed, thereby delivering more water.
The team is currently collaborating with the industry to study harvesters at an industrial scale. They continue to search for better, more efficient, and less expensive MOFs that are less expensive.
“I am very happy to see that more and more researchers around the world are joining our efforts in this regard,” Yaghi states.
The concept of sucking water from the atmosphere isn’t a new idea, according to Eric Hoek, an engineering professor at the University of California, Los Angeles, and editor of the journal NPJ Clean Water. It’s been observed for years that when you use an air conditioning system, you see water drip out. This happens because the device cools the air until the dew point. This is the temperature at which air is saturated by water vapor. This is when condensation occurs.
However, the process of creating water harvesters that are based on cooling technology is highly energy-intensive. In arid climates, the dew point is less than zero. The air cooling to this temperature on a large scale is not feasible.
“The real innovation [of Yaghi’s research] is a materials innovation,” Hoek states. “These materials [the MOFs] pull water out and more easily give it up.”
However, the idea is challenging to implement, Hoek cautions, as the amount of water produced by a harvester per square centimeter is low. A giant harvester could cover a vast area of land.
“But maybe for a household or village scale it could be a very interesting way for someone off the grid to get fresh water,” Hoek adds.
Yaghi envisions a world in which everyone who can’t access water has a harvester in their backyard.
“My vision is to achieve ‘personalized water,’ where people in water stressed regions have a device at home running on ambient solar, delivering the water that satisfies the basic needs of the individuals,” the researcher declares. “More than a third of the population around the world lives in water stressed areas or suffers due to a lack of clean water. The possibilities of this technology in changing people’s lives and improving world’s public health are enormous.”