You can’t squeeze blood from a stone, but wringing water from the desert sky is now possible, thanks to a new spongelike device that uses sunlight to suck water vapor from air, even in low humidity. The device can produce nearly 3 liters of water per day for every kilogram of spongelike absorber it contains, and researchers say future versions will be even better. That means homes in the driest parts of the world could soon have a solar-powered appliance capable of delivering all the water they need, offering relief to billions of people.
There are an estimated 13 trillion liters of water floating in the atmosphere at any one time, equivalent to 10% of all of the freshwater in our planet’s lakes and rivers. Over the years, researchers have developed ways to grab a few trickles, such as using fine nets to wick water from fog banks, or power-hungry dehumidifiers to condense it out of the air. But both approaches require either very humid air or far too much electricity to be broadly useful.
To find an all-purpose solution, researchers led by Omar Yaghi, a chemist at the University of California, Berkeley, turned to a family of crystalline powders called metal organic frameworks, or MOFs. Yaghi developed the first MOFs—porous crystals that form continuous 3D networks—more than 20 years ago. The networks assemble in a Tinkertoy-like fashion from metal atoms that act as the hubs and sticklike organic compounds that link the hubs together. By choosing different metals and organics, chemists can dial in the properties of each MOF, controlling what gases bind to them, and how strongly they hold on.
The new water harvester is made of metal organic framework crystals pressed into a thin sheet of copper metal and placed between a solar absorber (above) and a condenser plate (below).
Device pulls water from the air.
At night setup soaks up water vapor from air, and uses heat from the sun to release it as liquid water during the day.
The system Wang and her students designed consists of a kilogram of dust-sized MOF crystals pressed into a thin sheet of porous copper metal. That sheet is placed between a solar absorber and a condenser plate and positioned inside a chamber. At night the chamber is opened, allowing ambient air to diffuse through the porous MOF and water molecules to stick to its interior surfaces, gathering in groups of eight to form tiny cubic droplets. In the morning, the chamber is closed, and sunlight entering through a window on top of the device then heats up the MOF, which liberates the water droplets and drives them—as vapor—toward the cooler condenser. The temperature difference, as well as the high humidity inside the chamber, causes the vapor to condense as liquid water, which drips into a collector. The setup works so well that it pulls 2.8 liters of water out of the air per day for every kilogram of MOF it contained.
Yaghi says his group has already had early success in designing water-grabbing MOFs that replace zirconium with aluminum, a metal that is 100 times cheaper. That could make future water harvesters cheap enough not only to slake the thirst of people in arid regions, but perhaps even supply water to farmers in the desert.
From Science Magazine .