Solar-powered water harvester could help in arid climates

As reported by Science magazine, a new spongelike, solar-powered device can collect water vapor from the air even in regions of low humidity, and can produce 2.8 liters of water a day for every kilogram of the spongy absorptive material it contains. Once improved, the device could potentially bring relief to billions of people living in the driest of climates.

An estimated 13 trillion liters of water, or about 10% of all of the planet’s freshwater, is trapped in the atmosphere at any one time, and though researchers have already developed some ways to pull water out, using dehumidifiers or fine nets placed near fog banks, in order to work the approaches need either very humid air or huge amounts of electricity.

To create this newest device, researchers led by UC Berkeley chemist Omar Yaghi used crystalline powders called metal organic frameworks, or MOFs. Yaghi developed the first MOFs over 20 years ago; the structures assemble themselves into unique 3D shapes, and by choosing different metals and organics, chemists can control what gases bind to them, and how strongly they hold on.

In 2014, Yaghi and his colleagues synthesized a MOF that specialized in absorbing water, even in low humidity. Yaghi then partnered with MIT mechanical engineer Evelyn Wang on a water-harvesting device; the duo had previously worked with him to use MOFs in car air conditioning.

Wang and her students created a system that contains a kilogram of tiny MOF crystals pressed into a thin sheet of porous copper. The sheet lays between a solar absorber and a condenser plate within a chamber that opens at night, allowing air to flow through the porous MOFs and gather water molecules in its interior surfaces. Each morning, the chamber closes, and sunlight enters a a window on top of the device, heating the MOFs and forcing the water droplets – now vapor – towards the cooler condenser. The temperature difference and high humidity within the chamber then condense the vapor into liquid water, which drips into a collector.

Still, Yaghi feels the process has much room for improvement. Perhaps most importantly, zirconium costs $150 a kilogram, making the device too expensive to be broadly useful. However, Yaghi’s team has already had success creating water-grabbing MOFs that replace zirconium with aluminum, which is 100 times cheaper. That cost-effectiveness could make future water harvesters accessible not only as a thirst quencher for desert dwellers, but also as a potential water supply for farmers in arid climates.

Source: Service, Robert. “This new solar-powered device can pull water straight from the desert air.” Science. 13 April 2017.

PC: Yaghi Laboratory at UC Berkeley; V. Altounian.

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