New Device Pulls Water from Dry Air, Powered Only by the Sun

The water harvester, built at MIT, uses MOFs synthesized at Berkeley to suck water from dry air. The harvester uses sunlight to heat the MOF, driving off the water vapor and condensing it for use.

 MIT photo by Hyunho Kim.

“One vision for the future is to have water off-grid, where you have a device at home running on ambient solar for delivering water that satisfies the needs of a household,” said Yaghi, who is the founding director of the Berkeley Global Science Institute, a co-director of the Kavli Energy NanoSciences Institute and the California Research Alliance by BASF. “To me, that will be made possible because of this experiment. I call it personalized water.”

Tinker toys

Yaghi invented metal-organic frameworks more than 20 years ago, combining metals like magnesium or aluminum with organic molecules in a tinker-toy arrangement to create rigid, porous structures ideal for storing gases and liquids. Since then, more than 20,000 different MOFs have been created by researchers worldwide. Some hold chemicals such as hydrogen or methane: the chemical company BASF is testing one of Yaghi’s MOFs in natural gas-fueled trucks, since MOF-filled tanks hold three times the methane that can be pumped under pressure into an empty tank.

 

A schematic of a metal-organic framework. The lines in the models are organic linkers, and the intersections are metal ions. These are the building blocks that Yaghi stitches together into crystalline sponges using what he calls reticular chemistry. The yellow balls represent the porous spaces that can be filled with gas or liquid. The background image shows individual MOF crystals, which are packed into the water harvester.

UC Berkeley, Berkeley Lab image.

Other MOFs are able to capture carbon dioxide from flue gases, catalyze the reaction of adsorbed chemicals or separate petrochemicals in processing plants.

In 2014, Yaghi and his UC Berkeley team synthesized a MOF – a combination of zirconium metal and adipic acid – that binds water vapor, and he suggested to Evelyn Wang, a mechanical engineer at MIT, that they join forces to turn the MOF into a water-collecting system.

The system Wang and her students designed consisted of more than two pounds of dust-sized MOF crystals compressed between a solar absorber and a condenser plate, placed inside a chamber open to the air. As ambient air diffuses through the porous MOF, water molecules preferentially attach to the interior surfaces. X-ray diffraction studies have shown that the water vapor molecules often gather in groups of eight to form cubes.

Sunlight entering through a window heats up the MOF and drives the bound water toward the condenser, which is at the temperature of the outside air. The vapor condenses as liquid water and drips into a collector.

“This work offers a new way to harvest water from air that does not require high relative humidity conditions and is much more energy efficient than other existing technologies,” Wang said.

The harvester sitting atop a roof at MIT. The MOF is just below the glass plate on top, which lets sunlight in to heat the MOF and drive off the absorbed water. The yellow and red condenser sitting at the bottom is covered with water droplets.

MIT photo by Hyunho Kim.

This proof of concept harvester leaves much room for improvement, Yaghi said. The current MOF can absorb only 20 percent of its weight in water, but other MOF materials could possibly absorb 40 percent or more. The material can also be tweaked to be more effective at higher or lower humidity levels.

“It’s not just that we made a passive device that sits there collecting water; we have now laid both the experimental and theoretical foundations so that we can screen other MOFs, thousands of which could be made, to find even better materials,” he said. “There is a lot of potential for scaling up the amount of water that is being harvested. It is just a matter of further engineering now.”

Yaghi and his team are at work improving their MOFs, while Wang continues to improve the harvesting system to produce more water.

“To have water running all the time, you could design a system that absorbs the humidity during the night and evolves it during the day,” he said. “Or design the solar collector to allow for this at a much faster rate, where more air is pushed in. We wanted to demonstrate that if you are cut off somewhere in the desert, you could survive because of this device. A person needs about a Coke can of water per day. That is something one could collect in less than an hour with this system.”

Co-authors of the paper with Yaghi and Wang are Eugene Kapustin and Hiroyasu Furukawa of UC Berkeley and Hyunho Kim, Sungwoo Yang, Sameer Rao, Shankar Narayanan and Ari Umans of MIT. The work was supported in part by ARPA-E, a program of the U.S. Department of Energy. The work on MOFs in Yaghi’s laboratory is supported by BASF and the King Abdulaziz City for Science and Technology in Riyadh, Saudi Arabia.

Contacts and sources”

Robert Sanders

University of California, Berkeley

Water harvesting from air with metal-organic frameworks powered by natural sunlight (Science)
Omar Yaghi website
Evelyn Wang’s website