US researchers have devised a portable, solar-powered water harvester that can deliver around one litre of water per day, even in low humidity.

Developed at the University of California, Berkeley, the microwave-sized device uses fans to draw air over a cartridge containing a metal-organic framework (MOF). The MOF is so porous that a gram has the same surface area as a football pitch. Water molecules from the air are adsorbed by these internal pores, with the increasing humidity causing the H2O to condense. Heating the MOF slightly then releases the collected water.

The MOF cartridge measures about 10 inches square and 5 inches thick. It is intersected by two sets of channels: one set for adsorbing water, the other for expelling it to the condenser, allowing continuous cycling throughout the day. Solar panels connect to batteries so that the harvester can run at night, also powering the small heaters that drive the water out of the MOF.

It’s claimed that the water harvester can produce up to 1.3 litres of water per day for every kilogram of the MOF. During field tests over three days in California’s Mojave Desert, the harvester produced 0.7 litres per kilogram of absorber per day.

“It is well known that in order to condense water from air at a low humidity – less than 40 per cent relative humidity – you need to cool down the air to below freezing, to zero degrees Celsius, which is impractical,” said research lead Omar Yaghi, a UC Berkeley professor of chemistry.

“With our harvester, we are doing this at very low humidity without such cooling; there is no other material that can do that. This is not like a dehumidifier, which operates at high relative humidity. Some people say that 0.7 litres is not a lot of water. But it is a lot of water if you don’t have water.”

Described in ACS Central Science, the water harvester uses an aluminium-based adsorber known as MOF-303, which delivers a 10-fold improvement on the zirconium-based MOF-801 that the Berkeley team had previously developed. Yaghi’s startup, Water Harvester Inc, is now planning on bringing a product to market that can produce 7-10 litres of water per day, which is enough to support two to three adults. There are also plans for a refrigerator-sized device that can harvest 200-250 litres per day, which is enough for a household’s drinking, cooking and washing needs.

“We are making ultra-pure water, which potentially can be made widely available without connection to the water grid,” said Yaghi. “This water mobility is not only critical to those suffering from water stress, but also makes possible the larger objective – that water should be a human right.”

MORE FROM THE ENGINEER