On Tatooine, the desert planet from "Star Wars" that Luke Skywalker calls home, some people work as moisture farmers, collecting water from the desert air using moisture vaporators. But that's all science fiction, right? We couldn't possibly have anything like that in the real world, could we? Well, thanks to a joint effort between University of California, Berkeley, and MIT, we may be one step closer to "Star Wars" technology.
Previous attempts to collect water have either been not particularly effective since they require very humid air or they've been energy hogs and thus aren't viable for long-term use. The new device, detailed in Science, can produce almost three liters of water over a 12-hour period, even if the humidity level hits 20 percent, which is typical of arid regions. What powers this water harvester? The sun, of course.
Pulling water out of thin air
To collect the water, scientists turned to metal organic frameworks, or MOFs. These Tinkertoy-like networks of atoms were first developed by UC Berkeley's Omar Yaghi more than 20 years ago. The basic principle of MOFs is to join different metals together with organic molecules in a rigid but porous structure that can contain gases or liquids. By selecting the right combinations of metals and organics, scientists can hone the MOF's ability to bind to other molecules and determine how strongly they hold onto them. Since Yaghi developed MOFs, over 20,000 different types of MOFs have been created around the world that can hold hydrogen and methane or that can contain petrochemicals in processing plants.
In 2014, Yaghi and a team of researchers created a MOF out of zirconium metal and adipic acid, and it binds to water vapor. Realizing the potential, Yaghi reached out to MIT's Evelyn Wang to team up to develop a water harvester using this new MOF.
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. (Photo: Hyunho Kim/MIT)
With the baton handed off, Wang and her students developed the device. They took about two pounds of dust-sized MOF crystals and pressed it into a thin sheet of porous copper. This sheet was then placed between a solar absorber and a condenser inside a chamber. At night, the chamber was opened to allow air to move through the MOF sheet and for water molecules to stick to the surface. During the day, it was closed, and sunlight would heat up the MOF from above, freeing droplets of water and driving them, as vapor, toward the cooler condenser below.
This combination of night and day settings resulted in the vapor collecting as liquid water on the condenser. This drips into a collector and, presto, usable water from air. Wang and her students managed to collect 2.8 liters of water when it was used continuously.
"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 told Berkeley News.
More work to do
The device is billed as a proof of concept by Yaghi, so it's just a starting point.
For instance, the MOF used in the prototype can only absorb about 20 percent of its weight in water. Other MOFs might be able to bump that number up to 40 percent, Yaghi explained to Berkeley News. Along those lines, the high cost per kilogram of zirconium (about $150) is a limiting factor. Aluminium is 100 times cheaper, and Yaghi's team has already had some success with water-grabbing MOFs that use it.
"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 told Berkeley News. "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."
So not only could people in desert regions have drinking water, but farmers in deserts could have much-needed water for crops and livestock.
Luke's moisture farmer uncle on Tatooine would be thrilled.