Solar-powered system can harvest fresh water from air

Scientists have developed a system that can harvest water out of the air using just solar power, an advance ideal for people living in arid areas of the world.

The prototype, developed by researchers at University of California, Berkeley in the US, can extract drinkable water every day/night cycle at very low humidity and at low cost.

"It operates at ambient temperature with ambient sunlight, and with no additional energy input you can collect water in the desert. This laboratory-to-desert journey allowed us to really turn water harvesting from an interesting phenomenon into a science," said Omar Yaghi, who invented the technology underlying the harvester.

The trial in Scottsdale, where the relative humidity drops from a high of 40 per cent at night to as low as 8 per cent during the day, demonstrated that the harvester should be easy to scale up by simply adding more of the water absorber, a highly porous material called a metal-organic framework (MOF).

Metal-organic frameworks are solids with so many internal channels and holes that a sugar-cube-size MOF might have an internal surface area the size of six football fields.

This surface area easily absorbs gases or liquids but, just as important, quickly releases them when heated.

The researchers anticipate that with the current MOF (MOF-801), made from the expensive metal zirconium, they will ultimately be able to harvest about 200 millilitres of water per kilogramme of MOF.

They have also created a new MOF based on aluminum, called MOF-303, that is at least 150 times cheaper and captures twice as much water in lab tests.

This will enable a new generation of harvesters producing more than 400 millilitres of water per day from a kilogramme of MOF.

"There has been tremendous interest in commercialising this, and there are several startups already engaged in developing a commercial water-harvesting device," Yaghi said.

"The aluminum MOF is making this practical for water production, because it is cheap," he said.

For the study published in the journal Science Advances, researchers collected and measured the water and tested the latest generation harvester under varying conditions of humidity, temperature and solar intensity.

The harvester is essentially a box within a box. The inner box holds a two-square-foot bed of MOF grains open to the air to absorb moisture. This is encased in a two-foot plastic cube with transparent top and sides.

The top was left open at night to let air flow in and contact the MOF, but was replaced during the day so the box could heat up like a greenhouse to drive water back out of the MOF.

The released water condensed on the inside of the outer box and fell to the bottom, where the researchers collected it with a pipette.

The extensive field tests lay out a blueprint allowing engineers to configure the harvester for the differing conditions in Arizona, the Mediterranean or anywhere else, given a specific MOF.

"The key development here is that it operates at low humidity, because that is what it is in arid regions of the world," Yaghi said. In these conditions, the harvester collects water even at sub-zero dew points.