Water Harvester Makes It Easy To Quench Your Thirst In The Desert

With water scarcity a growing problem worldwide, University of California, Berkeley, researchers are close to producing a microwave-sized water harvester that will allow you to pull all the water you need directly from the air — even in the hot, dry desert.

In a paper appearing this week in ACS Central Science, a journal of the American Chemical Society, UC Berkeley’s Omar Yaghi and his colleagues describe the latest version of their water harvester , which can pull more than five cups of water (1.3 liters) from low-humidity air per day for each kilogram (2.2 pounds) of water-absorbing material, a very porous substance called a metal-organic framework, or MOF. That is more than the minimum required to stay alive.

During field tests over three days in California’s arid Mojave Desert, the harvester reliably produced 0.7 liters per kilogram of absorber per day — nearly three cups of clean, pure H2O. That’s 10 times better than the previous version of the harvester. The harvester cycles 24/7, powered by solar panels and a battery.

Even on the driest day in the desert, with an extremely low relative humidity of 7% and temperatures over 80 degrees Fahrenheit, the harvester produced six ounces (0.2 liters) of water per kilogram of MOF per day.

“It is well known that in order to condense water from air at a low humidity — less than 40 percent relative humidity — you need to cool down the air to below freezing, to zero degrees Celsius, which is impractical. With our harvester, we are doing this at very low humidity without such cooling; there is no other material that can do that,” said Yaghi, a UC Berkeley professor of chemistry and co-director of the Kavli Energy NanoSciences Institute. “This is not like a dehumidifier, which operates at high relative humidity. Some people say that 0.7 liters is not a lot of water. But it is a lot of water, if you don’t have water.”

Yaghi’s startup, Water Harvester Inc., is now testing and will soon market a device the size of a microwave oven that can supply 7 to 10 liters of water per day: enough drinking and cooking water for two to three adults per day, based on recommendations from the National Academy of Sciences that men should consume 3.7 liters and women 2.7 liters of fluid per day.

An even larger version of the harvester, one the size of a small refrigerator, will provide 200 to 250 liters of water per day, enough for a household to drink, cook and shower. And in a couple of years, the company hopes to have a village-scale harvester that will produce 20,000 liters per day. All would run on power from solar panels and a battery or off the electrical grid.

“We are making ultra-pure water, which potentially can be made widely available without connection to the water grid,” said Yaghi, the James and Neeltje Tretter Chair in the College of Chemistry. “This water mobility is not only critical to those suffering from water stress, but also makes possible the larger objective — that water should be a human right.”

The key: highly porous MOFs

The harvester’s secret ingredient is a type of MOF invented by Yaghi and his UC Berkeley colleagues that easily and quickly takes up water from the air and just as readily disgorges it so the water can be collected. MOFs, which Yaghi has been developing since the mid-1990s, are so porous that a gram has a surface area equivalent a football field. Other types of MOFs capture carbon dioxide from flue gases, catalyze chemical reactions or separate petrochemicals in processing plants.

The researchers came up with their first water-absorbing MOF, called MOF-801, in 2014. Water molecules in ambient air stick to the internal surface — a process called adsorption — and increase the humidity inside the MOF to a point where the water condenses even at room temperature, just as water condenses on cooler surfaces when the humidity is high. When the MOF is heated slightly, the water comes back out and can be condensed and collected.

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