How to condense water out of air using only sunlight for energy

MIT-UC Berkeley invention may offer hope for the two-thirds of the world’s population experiencing water shortages, including the one third living in desert climates
April 13, 2017

A water harvester designed and built at MIT condenses water from air. The harvester uses sunlight to heat metal-organic framework (MOF) material (inserted just below the glass plate on top), driving off the water vapor and condensing it (in the yellow and red condenser sitting at the bottom) for home use. (photo credit: Hyunho Kim/MIT)

MIT scientists have invented a water harvester that uses only sunlight to pull water out of the air under desert conditions, using a “metal-organic framework” (MOF) powdered material developed at the University of California, Berkeley (UC Berkeley).

Under conditions of 20–30 percent humidity (a level common in arid areas), the prototype device was able to pull 2.8 liters (3 quarts) of water from the air over a 12-hour period, using one kilogram (2.2 pounds) of MOF.

(Left) A schematic of metal-organic framework (MOF) material. The three large yellow, orange, and green balls are porous spaces for capturing and concentrating water molecules. (Right) As ambient air diffuses through the porous MOF, water molecules preferentially attach to the interior surfaces. 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. (credit: UC Berkeley and MIT)

In 2014, a UC Berkeley team team led by chemist Omar Yaghi*, PhD, synthesized a porous MOF — a combination of zirconium metal and adipic acid** — that was able to bind water vapor. He suggested to Evelyn Wang, PhD, a mechanical engineer at MIT, that they join forces to turn the MOF into a water-collecting system.

Today (April 13, 2017), the system was announced in a paper published in the journal Science, with Yaghi and Wang as co-senior authors.***

“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.” — Evelyn Wang

The new solar-powered harvester is a major breakthrough in the long-standing challenge of harvesting water from the air at low humidity, according to Yaghi. “There is no other way to do that right now, except by using extra energy. Your electric dehumidifier at home ‘produces’ very expensive water.”

Regions with desert climates (2011). Red: hot desert climates. Pink: cold desert climates. (credit: Koppen World Map/CC)

“We wanted to demonstrate that if you are cut off somewhere in the desert, you could survive because of this device,” Wang said. “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. … 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.”

Running water and carbon-dioxide capture next

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

The current MOF can absorb only 20 percent of its weight in water, but other MOF materials could possibly absorb 40 percent or more, and the material can be tweaked to be more effective at higher or lower humidity levels, Yaghi believes.

Rooftop tests at MIT confirmed that the water harvester works in real-world conditions. (photo credit: Hyunho Kim/MIT)

“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.”

“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,” Wang added. “Or design the solar collector to allow for this at a much faster rate, where more air is pushed in.”

Some MOFs being developed by Yaghi’s team could hold gases such as hydrogen or methane. The chemical company BASF is testing one of Yaghi’s MOFs in natural gas-fueled trucks; MOF-filled tanks hold three times the methane that can be pumped under pressure into an empty tank.

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

* Yaghi holds the James and Neeltje Tretter chair in chemistry at UC Berkeley and is a faculty scientist at Lawrence Berkeley National Laboratory. He is also the founding director of the Berkeley Global Science Institute, and a co-director of the Kavli Energy NanoSciences Institute and the California Research Alliance by BASF. He 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.

** Metal-organic framework-801 [Zr6O4(OH)4(fumarate)6]

*** 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.

UC Berkeley | Pulling drinkable water out of dry air

Omar Yaghi explains how to make a MOF and their tremendous ability to absorb gases and liquids, including water directly from low-humidity air. A MOF he synthesized was used by MIT engineers to construct a water harvester that sucks water from dry air and condenses it for drinking. Video by Roxanne Makasdjian and Stephen McNally, UC Berkeley. Harvester photos courtesy of MIT.

Abstract of Water harvesting from air with metal-organic frameworks powered by natural sunlight

Atmospheric water is a resource equivalent to ~10% of all fresh water in lakes on Earth. However, an efficient process for capturing and delivering water from air, especially at low humidity levels (down to 20%), has not been developed. We report the design and demonstration of a device based on porous metal-organic framework-801 [Zr6O4(OH)4(fumarate)6] that captures water from the atmosphere at ambient conditions using low-grade heat from natural sunlight below one sun (1 kW per square meter). This device is capable of harvesting 2.8 liters of water per kilogram of MOF daily at relative humidity levels as low as 20%, and requires no additional input of energy.