Current atmospheric water harvesting (AWH) technologies are hindered by issues related to size, cost, and efficiency. However, recent research from University of Utah engineering researchers has introduced innovations that could significantly enhance efficiency, bringing the possibility of drawing drinking water from the air in arid regions closer to reality.
The study reveals a novel compact, rapid cycling, fuel-fired AWH device. This two-step prototype uses adsorbent materials to capture water molecules from non-humid air and then employs heat to release these molecules into liquid form, explained Sameer Rao, senior author of the study published Monday and an assistant professor of mechanical engineering.
"Hygroscopic materials intrinsically have affinity to water. They soak up water wherever you go. One of the best examples is the stuff inside diapers," said Rao, who happens to be the father of an infant son. "We work with a specific type of hygroscopic material called a metal organic framework."
Rao compared metal organic frameworks to Lego blocks that can be rearranged to create various structures. In this case, they are designed to form a molecule ideal for gas separation. "They can make it specific to adsorb water vapor from the air and nothing else. They're really selective," he added. Developed with graduate student Nathan Ortiz, the study's lead author, the prototype uses aluminum fumarate fashioned into panels that collect water as air passes through.
"The water molecules themselves get trapped on the surfaces of our material, and that's a reversible process. And so instead of becoming ingrained into the material itself, it sits on the walls," Ortiz said. "What's special about these absorbent materials is they have just an immense amount of internal surface area. There's so many sites for water molecules to get stuck."
According to Rao, just a gram of this material has as much surface area as two football fields, allowing a small amount of material to capture a significant amount of water. "All of this surface area is at the molecular scale," Rao noted. "And that's awesome for us because we want to trap water vapor onto that surface area within the pores of this material."
The research was funded by the DEVCOM Soldier Center, a Department of Defense program supporting technology transfer for Army modernization. The Army's interest lies in keeping soldiers hydrated in remote areas with scarce water sources.
"We specifically looked at this for defense applications so that soldiers have a small compact water generation unit and don't need to lug around a large canteen filled with water," Rao said. "This would literally produce water on demand."
Rao and Ortiz have filed for a preliminary patent for the technology, which also addresses civilian needs. "As we were designing the system, I think we also had perspective of the broader water problem. It's not just a defense issue, it's very much a civilian issue," Rao said. "We think in terms water consumption of a household for drinking water per day. That's about 15 to 20 liters per day."
In this proof of concept, the prototype produced 5 liters of water per day per kilogram of adsorbent material. In the field, this device would surpass the efficiency of carrying water within three days, Ortiz noted.
In the device's second step, water is condensed into liquid by applying heat from a standard-issue Army camping stove, taking advantage of the exothermic nature of the water collection process. "As it collects water, it's releasing little bits of heat. And then to reverse that, we add heat," Ortiz said. "We just put a flame right under here, anything to get this temperature up. And then as we increase the temperature, we rapidly release the water molecules. Once we have a really humid airstream, that makes condensation at ambient temperature much easier."
Atmospheric water harvesting technologies typically perform better in humid conditions, but this device stands out for arid environments due to its use of energy-dense fuel like white gasoline, rather than relying on solar power. "If you're reliant on solar panels, you're limited to daytime operation or you need batteries, which is just more weight. You keep stacking challenges. It just takes up so much space," Ortiz said. "This technology is superior in arid conditions, while refrigeration is best in high humidity."
Research Report:Compact rapid cycling fuel-fired atmospheric water harvesting device for all-day water production
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