As the planet warms and global populations boom, water is becoming a scarce commodity.
Now, two separate projects demonstrate new technology to more efficiently harvest water from air than previous attempts. One shows a low-tech method for squeezing droplets from dry, desert air, while the other uses an electrostatic charge to drastically improve the amount of water collected from humid air. Both aim to create low-cost sources of freshwater that could slack the thirst of humans, crops, and even power plants.
The first water harvester comes from a team at UC Berkeley that has devised a simple box-in-a-box system that works using ambient air temperature without the need for refrigeration. In experiments, the system was able to produce between 2 and 3 1/2 ounces of water per day. That may sound small, but this is bone-dry desert air with a relative humidity that ranges from 40 percent at night to 8 percent in the daytime. The researchers report their findings in journal Science Advances.
A critical component of this system is a kind of powder that consists of metal-organic frameworks, also called MOFs. These tiny, synthetic crystalline grains can be made from a variety of metal ions bound to short organic molecules. MOFs, which were invented decades ago by the research team’s leader, Omar Yaghi, are incredibly porous and have surface structures so intricate that if a gram of them were laid out flat, they would cover several football fields.
For this water harvester, the researchers tested two different MOFs — those made from zirconium and others made from aluminum. For the inner box, a two-foot-tall plastic container, the researchers arranged an inch-deep layer of MOFs near the very top. They placed that box inside a larger, plastic box that has a transparent top and sides.
At night, the lid to the outer box was left open to allow air to permeate the bed of MOFs and their porous structure. During the day, the lid to the outer box was closed, trapping heat inside, which released water vapor trapped inside the MOF. The vapor clung to the inside of the large box, condensed and dripped to the bottom, where it was collected.
“It doesn’t require much energy to get the water out,” graduate student Eugene Kapustin told Seeker.
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Making the current system bigger would offer one solution to collecting more water. But Kapustin said he and his team want to design more efficient MOFs to make the system smaller and portable. This summer, he and colleagues are testing a new design in California’s Death Valley.
“We think MOF based technology will be the only one that will produce actual water in arid regions of the world,” said Kapustin.
In very humid regions, fog collecting has become a popular way to capture water from the air. Typically, these systems are made of giant, passive mesh or nets, where water droplets cling, coalesce, and then stream down into buckets. But this method is quite inefficient, capturing just 1 to 3 percent of the water droplets in the air passing by.
A team at MIT, lead by associate professor of mechanical engineering Kripa Varanasi, has developed a way to increase the efficiency of fog collecting to nearly 99 percent. By emitting a pulse of electrically charged particles, or ions, into the air near the surface of a collector, which creates an electric field and also charges nearby water particles, compelling them toward the mesh of wires. In laboratory experiments the electrostatic charge in combination with a 2 inch by 2 inch mesh collected enough water droplets — created by a mist machine — to fill a 1 ounce beaker in 30 minutes. The team published their research in Science Advances.
Varanasi told Seeker he sees a couple of different applications for this technology and, to commercialize them, has started a company called Infinite Cooling, which recently won MIT's $100K Entrepreneurship Competition.
The first application focuses on power plant cooling towers that billow plumes of water vapor so dense they’re often mistaken for smoke. A 600 MW power plant operating at 55 percent capacity loses roughly 750 million gallons of water every year, said Varanasi. But a lightweight mesh dome fitted over the top of the tower could capture enough vapor to produce 150 million gallons of water.
“That’s a million dollars in savings for the plant,” said Varanasi.
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Currently, the company is building a mesh dome over a geodesic frame for the cooling tower connected to MIT's Central Utility Plant, which generates 20 megawatts of energy for the campus. The dome should be installed by the end of the summer and Varanasi and his team will test its performance over the fall.
Since many power plants are located along coastlines in order to use seawater for cooling, Infinite Cooling’s technology could also be used to desalinate seawater. “The amount of energy we use is 50 times lower than reverse osmosis desalination,” said Varanasi. “We think this is the way to desalinate.”
Water is a crucial resource that everybody needs but nobody wants to pay for, said Varanasi. Instead of developing new membrane technology or filtration, he and his team want to focus on the nexus of energy and water to create an entirely new business.
“We want to disrupt the water market and address global water scarcity,” he said.