More than 2 billion people worldwide lack access to clean drinking water, with global warming and competing demands from farms and industry expected to worsen shortages. But the skies may soon provide relief, not in the form of rain but humidity, sucked out of the air by atmospheric water harvesters. The devices have existed for decades but typically are too expensive, energy-hungry, or unproductive to be practical.
Now, however, two classes of materials called hydrogels and metal-organic frameworks have touched off what Evelyn Wang, a mechanical engineer at the Massachusetts Institute of Technology (MIT), calls an explosion of efforts related to atmospheric water harvesting.
So far, none of the devices can compete with established approaches to augment water supplies, such as desalinating seawater. But some applicationscooling data centers and slaking the thirst of soldiers on the movecould support higher costs until the technology scales up, says Samer Taha, CEO of Atoco, a California-based startup. There are many applications where atmospheric water harvesting can help.
Water capture technology may date back to the Inca who, living on the desert coast of South America, are thought to have collected morning dew on mesh nets, feeding it into cisterns. More recently, companies have deployed devices that use air-conditioners to cool air below the dew point, causing water vapor to condense, or water-absorbing desiccant materials such as salts, which are then heated to release the liquid. But both approaches require lots of energy, raising costs and limiting their reach.
The trick is to find a material that captures lots of water but readily frees it, too. Hydrogelssoft, porous networks of polymer fibers often impregnated with saltsseem to fit the bill. In a June report in Nature Water, Xuanhe Zhao, a mechanical engineer at MIT, and his colleagues describe a water harvester that, thanks to a novel hydrogel, requires no external energy input at all.
The team sandwiched the hydrogel, which contains lithium-chloride salt, between two glass sheets. At night, water vapor enters the gel and is trapped by the salts. During the day, sunlight heats the gel, evaporating the water. The vapor condenses on the glass panels, forming droplets that trickle down and are captured. So far, the results are modest: Prototypes can produce up to 1.2 liters of water per kilogram of hydrogel per day in the dry desert air of Death Valley, California.
Other researchers are using modest amounts of energy and dirt-cheap materials to harvest much more water. For example, in a February report in Advanced Materials, Guihua Yu, a chemist at the University of Texas at Austin, and his colleagues describe a promising hydrogel made by altering cellulose, chitosan, and starchcomplex carbohydrates common in agricultural and food waste. The biomaterials have a dense structure that limits the amount of water they can store, and they tend to hold onto much of what they snag, even when heated.
So Yus team modified its hydrogel with chemical compounds known as zwitterionic groups that repel one another, stretching open the carbohydrates and making them more porous. Then the researchers added other compounds that cause the hydrogel to shrink when heat is applied, helping to squeeze trapped water out. Together, these changes enabled a prototype device to harvest up to 14 liters of water per kilogram of hydrogel daily. For now, getting the water out still requires raising temperatures to 60�C with an electric heater. However, Yaxuan Zhao, a graduate student in Yus lab, says the low cost of the biomaterials mean the device could be deployed along with solar panels in off-grid communities and emergency relief efforts.
Jeremy Cho, a mechanical engineer at the University of Nevada, Las Vegas, and his colleagues believe dividing a water capture device into two layers can help keep energy costs down. They use a hydrogel membrane containing salts that attract water vapor. Once concentrated, the water is pulled farther into a salty liquid desiccant layer for storage. The process empties the pores in the hydrogel, freeing it up to capture more water. Releasing the water from the desiccant takes only a modest amount of heat. Its a lot easier to heat a liquid than a solid, Cho says, which raises efficiency. According to a October 2024 report in the Proceedings of the National Academy of Sciences, the setup could collect nearly 17 liters of water per day for each kilogram of absorbing material in humid environments, and a still respectable 5.5 liters in a Las Vegas-type arid environment.
Some of the most productive devices rely on a different kind of material: metal organic frameworks (MOFs). These porous atomic scaffolds have channels and pockets that can be designed to attract and store specific moleculesin this case, water. Although MOFs tend to hold less water than hydrogels, they can capture and release it more quickly, allowing them to go through dozens of such cycles in the time it takes hydrogels to go through one.
The key is to tailor them with alternating chemical groups that attract and repel water, says University of California, Berkeley chemist Omar Yaghi. In 2023 he and his colleagues reported an aluminum-based MOF that was cheap to make in bulk and that could wring water from desert air. In preliminary, unpublished tests, Yaghi says, prototype devices using a tweaked version of his teams MOF can produce 200 liters of water per kilogram per day with only small amounts of added heat.
Yaghi has licensed the technology to Atoco, which is exploring using it to generate water to cool data centers, harnessing their waste heat to speed the cycling. Atoco plans to open pilot scale facilities in Texas and Arizona next year to test scaled-up versions, Taha says.
Despite all of these emerging solutions, there is still room for improvement, says Cody Friesen, an atmospheric water harvesting pioneer at Arizona State University. Today, he notes, desalination plants can convert large amounts of seawater to drinking water at a cost of less than 1 cent per liter. Water harvesting devices are orders of magnitude more expensive and nowhere near as prolific. With the community still sorting through various materials options and device designs, We are not all singing out of the same hymn book, he says.
But Friesens own company, Source Global, is an encouraging example. It has installed water-producing hydropanels, which use a proprietary desiccant, at more than 450 sites worldwide, mostly in remote, off-grid locations. And Friesen believes costs will drop as manufacturing is scaled up, much as has happened with solar panels and batteries.
Atmospheric water harvesting will eventually be the lowest cost delivered potable water on the planet, he predicts.
