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Wringing Water from Dry Air

Simple solar-powered system harvests moisture in arid conditions.
benedikt luft

A new technology could make it possible to obtain clean, fresh water almost anywhere on Earth, by drawing it directly from moisture in the air—even in the driest of locations.

Technologies exist for extracting water from fog or dew, but they require high humidity levels or are very energy-intensive. The new method is the first that has potential for widespread use in virtually any location, regardless of humidity levels, says Evelyn Wang ’00, senior scientist on the project and an associate professor of mechanical engineering at MIT. She and her team developed a solar-powered system based on a foamlike material that draws moisture into its pores.

For air with low humidity, which is common in many parts of the world, no previous technology provided a practical way of getting water. The new system not only fills that pressing need but is “completely passive—all you need is sunlight,” says Wang, who developed the system with graduate student Hyunho Kim, SM ’14, and six others at MIT and the University of California, Berkeley. They reported the results earlier this year in Science.

The key to the new system lies in the porous material it’s based on, which is part of a family of compounds known as metal-organic frameworks (MOFs). These compounds form a spongelike configuration with a large internal surface area. Tuning the exact chemical composition of the MOF can make these surfaces hydrophilic, or water-­attracting. The team found that when the material is placed between a top surface painted black to absorb solar heat and a lower surface kept at the same temperature as the outside air, water is released from the pores as vapor, drips down as liquid, and collects on the cooler lower surface.

Tests showed that one kilogram of the material could collect about three quarts of fresh water per day from air with a humidity level of just 20 percent. That turns out to be just about enough to supply drinking water for one person. Such systems would require attention only a few times a day to collect the water, open the device to let in fresh air, and begin the next cycle.

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