In his 1977 film Star Wars, George Lucas depicted Luke Skywalker, his earnest young rebel without a cause, making his living as a moisture farmer, pulling water out of thin air on his arid home planet of Tatooine. Luke's family barely got by on the rickety technology, but hey – that was a long time ago, in a galaxy far, far away.
Now comes word that engineers have created a solar-powered moisture farming device that can pull significant amounts of water out of low-humidity or desert environments. According to the research team, the harvester is so efficient that it could power an entire new category of off-grid water delivery systems.
Think of it as another example of science fiction identifying technologies that real-word scientists raced to develop.
As reported in the journal Science, the water harvester is a collaboration between chemists and mechanical engineers at MIT and the University of California, Berkeley. The prototype unit was able to pull nearly three quarts of water from the air over a 12-hour period, using only the power of ambient sunlight.
It's a major breakthrough in the centuries-old challenge of harvesting water from the air at low humidity, according to Omar Yaghi, one of two senior authors of the paper and chemist at UC Berkeley. "There is no other way to do that right now, except by using extra energy,” Yaghi said in a press statement.
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Moisture harvesting technologies have actually been around for a very long time. The Incas were pretty good with dew-collection devices, for instance, and you can find innovative contemporary examples all over the world.
This new water harvester could prove to be a significant advance because it delivers substantial amounts of water with minimal energy requirements.
Its novelty centers around the use of metal-organic frameworks (MOFs), cage-like structures of porous materials and organic elements typically used in industrial facilities to store liquids and gases. More than 20,000 different types of MOFs have been developed by industry over the years.
For the water-harvesting device, scientists at Berkeley created an MOF specifically designed to bind water vapor. (If you're keeping notes at home, the MOF uses zirconium metal and adipic acid as the organic element.) From there, the Berkeley team collaborated with mechanical engineers at MIT to turn the MOF into an operational water-collecting system.
It works like this: Around two pounds of MOF crystals are pressed between a solar panel and a condenser plate. When water vapor passes through the MOF, the H20 molecules are snagged and held at the molecular level. One interesting detail: X-ray diffraction studies reveal that water molecules tend to bind to the interior surface in groups of eight, forming microscopic water cubes.
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As the solar panel heats up, water is shuttled to the condenser where the vapor condenses as liquid water and drips into the collector.
The prototype MOF units can absorb about 20 percent of their weight in water, but the researchers suspect they can get that number up to 40 percent or more. Further refinements in the water collection unit should further improve efficiency.
“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," said researcher Evelyn Wang, a mechanical engineer at MIT. "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."
MIT's Yaghi thinks the device can probably produce water at an even faster rate, if engineers can figure out an energy-efficient way to actually push air through the MOF unit.
“We wanted to demonstrate that if you are cut off somewhere in the desert, you could survive because of this device,” he 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."
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