Billions of people lack access to clean water for all or part of the year or must travel far to gather it. Extracting water directly from the air would be an immeasurable boon for them. But existing technologies generally require high moisture and a lot of electricity, which is expensive and often unavailable. The problem is now becoming more tractable. Robust systems are being developed that rely on readily available energy from the sun, are scalable and can work even in arid regions—where a third of the world’s population lives, often in poverty.
Collaborators at the Massachusetts Institute of Technology and the University of California, Berkeley, have tested an approach that requires no electricity at all. The team intends for its technology to overcome a notable problem with most materials capable of absorbing water from the atmosphere (such as the zeolites in humidifiers): aside from needing high humidity, they give up the trapped water only when heated substantially, which takes energy.
The researchers designed their system around a class of porous crystals called metal-organic frameworks (MOFs), developed years ago by chemist Omar M. Yaghi, now in the U.C. Berkeley group. By choosing specific combinations of metals and organics, scientists can select the chemical properties of each MOF and thereby customize its uses. Beyond their versatility, MOFs’ great promise lies with their phenomenally large pores: the surface area inside is almost 10 times that of porous zeolites. For context, one gram of an MOF crystal the size of a sugar cube has an internal surface area approximately equal to the area of a football field.
In April Yaghi’s group, along with that of M.I.T. mechanical engineer Evelyn Wang, reported on a prototype device incorporating MOF-801 (zirconium fumarate), which has a high affinity for water. It pulls moisture from the air into its large pores and readily feeds the water into a collector in response to low-grade heat from natural sunlight. The device can harvest 2.8 liters of water daily per every kilogram of MOF even at relative humidity levels as low as 20 percent, similar to the humidity of deserts, and it requires no additional input of energy. (According to Yaghi, a person needs at least a soda can’s worth, or 355 milliliters, of drinking water a day.) The investigators see room for improvement, however. Further experimentation with MOF composition should make the technology less expensive (zirconium costs $150 per kilogram), increase the amount of water collected per unit of material and allow researchers to tailor MOFs to different microclimates.
Taking a different tack, a start-up called Zero Mass Water in Scottsdale, Ariz., has begun selling a solar-based system that does not have to be hooked up to an electric grid or an existing water system. A solar panel provides energy that both drives air through a proprietary water-absorbing material and powers condensation of the extracted moisture into fluid. A small lithium-ion battery operates the device when the sun is not shining. A unit with one solar panel, the company says, can produce two to five liters of liquid a day, which is stored in a 30-liter reservoir that adds calcium and magnesium for health and taste.
Cody Friesen, founder of Zero Mass Water and a materials scientist at Arizona State University, developed the system with the aim of having it work sustainably and easily anywhere in the world. An installed system with one solar panel sells in the U.S. for about $3,700, including a required 10 percent donation toward reducing costs for installations in parts of the globe lacking a water infrastructure. The same panel that provides luxury, bottle-free water in the U.S., Friesen notes, can also provide clean water to a school that lacks it so that children “are able to get educated and not get sick.” Over the past year, he says, systems have been placed in the southwestern U.S. and several other countries—among them, Mexico, Jordan and Dubai—and the company has recently shipped panels to Lebanon, with funding from the U.S. Agency for International Development, to provide water to Syrian refugees. When most people think about solar, he adds, “they think about electricity. In the future, people will think about water abundance.”