Billions of people lack access to clean water for all or part of the year or must travel far to collect it. Extracting water directly from the air would be an immeasurable boon for them. But existing technologies generally require a high-moisture climate and a lot of electricity, which is costly and often unavailable. This problem is now becoming more tractable, thanks to robust systems in development that rely on readily available energy from the sun. They are scalable and 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 2017 Yaghi’s group, along with that of M.I.T. mechanical engineer Evelyn Wang, reported on a prototype device incorporating MOF-801, or 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 for every kilogram of MOF even at relative humidity levels as low as 20 percent, similar to those of deserts. (According to Yaghi, a person needs at least a soda can’s worth, or 355 milliliters, of drinking water a day.) Plus, it requires no additional input of energy.
The investigators see more room for improvement. Further experimentation with MOF composition should make the technology less expensive (zirconium currently 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. Called a SOURCE Hydropanel, the system uses both solar photovoltaic and solar thermal to drive air through a proprietary water-absorbing material and condenses the extracted moisture into fluid. A small lithium-ion battery operates the device when the sun is not shining. Each Hydropanel, the company says, can produce an average of 60 to 150 liters of liquid water a month, 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 Hydropanels with the aim of having them work sustainably and easily anywhere in the world. An installed residential array sells in the U.S. for about $6,500. The company partners with nonprofits to build arrays of the technology in parts of the globe lacking water infrastructure. The same Hydropanels reducing the need for bottled water in the U.S., Friesen notes, can also provide clean water to a school that lacks it so kids “are able to get educated and not get sick.”
Over the past few years, he says, Hydropanels have been installed across the U.S. and 22 other countries—among them Mexico, Zimbabwe and Australia—and the company has recently completed a project funded by the U.S. Agency for International Development to provide water to Syrian refugees in Jordan and Lebanon. When most people think about solar, Friesen adds, “they think about electricity. In the future, they will think about water abundance.”