Crystalline materials similar to these can now harvest water vapor from the air.
YAGHI LABORATORY AT UC BERKELEY
You can’t squeeze blood from a stone, but wringing water from the desert sky is now possible, thanks to a new spongelike device that uses sunlight to suck water vapor from air, even in low humidity. The device can produce nearly 3 liters of water per day for every kilogram of spongelike absorber it contains, and researchers say future versions will be even better. That means homes in the driest parts of the world could soon have a solar-powered appliance capable of delivering all the water they need, offering relief to billions of people.
You can’t squeeze blood from a stone, but wringing water from the desert sky is now possible, thanks to a new spongelike device that uses sunlight to suck water vapor from air, even in low humidity. The device can produce nearly 3 liters of water per day for every kilogram of spongelike absorber it contains, and researchers say future versions will be even better. That means homes in the driest parts of the world could soon have a solar-powered appliance capable of delivering all the water they need, offering relief to billions of people.
The new water harvester is made of metal organic framework crystals pressed into a thin sheet of copper metal and placed between a solar absorber (above) and a condenser plate (below).
WANG LABORATORY AT MIT
Over the past 2 decades chemists have synthesized more than 20,000 MOFs, each with unique molecule-grabbing properties. For example, Yaghi and others recently designed MOFs that absorb—and later release—methane, making them a type of high-capacity gas tank for natural gas–powered vehicles.
In 2014, Yaghi and his colleagues synthesized a MOF that excelled at absorbing water, even under low-humidity conditions. That led him to reach out to Evelyn Wang, a mechanical engineer at the Massachusetts Institute of Technology (MIT) in Cambridge, with whom he had previously worked on a project to use MOFs in automobile air conditioning. After synthesizing the new zirconium-based MOF, dubbed MOF-801, Yaghi met Wang at MIT and said, “Evelyn we have to come up with a water-harvesting device.” She agreed to give it a shot.
Device pulls water from the air
At night setup soaks up water vapor from air, and uses heat from the sun to release it as liquid water during the day.
The system Wang and her students designed consists of a kilogram of dust-sized MOF crystals pressed into a thin sheet of porous copper metal. That sheet is placed between a solar absorber and a condenser plate and positioned inside a chamber. At night the chamber is opened, allowing ambient air to diffuse through the porous MOF and water molecules to stick to its interior surfaces, gathering in groups of eight to form tiny cubic droplets. In the morning, the chamber is closed, and sunlight entering through a window on top of the device then heats up the MOF, which liberates the water droplets and drives them—as vapor—toward the cooler condenser. The temperature difference, as well as the high humidity inside the chamber, causes the vapor to condense as liquid water, which drips into a collector. The setup works so well that it pulls 2.8 liters of water out of the air per day for every kilogram of MOF it contained, the Berkeley and MIT team reports today in Science.
“It has been a longstanding dream” to harvest water from desert air, says Mercouri Kanatzidis, a chemist at Northwestern University in Evanston, Illinois, who wasn’t involved with the work. “This demonstration … is a significant proof of concept.” It’s also one that Yaghi says has plenty of room for improvement. For starters, zirconium costs $150 a kilogram, making water-harvesting devices too expensive to be broadly useful. However, Yaghi says his group has already had early success in designing water-grabbing MOFs that replace zirconium with aluminum, a metal that is 100 times cheaper. That could make future water harvesters cheap enough not only to slake the thirst of people in arid regions, but perhaps even supply water to farmers in the desert.
*Update, 14 March, 12:28 p.m.: This item has been updated to reflect the fact that the device pulls nearly 3 liters of water out of the air for every kilogram of the water-absorbing material that is used.
