MOF device harvests water from dry air

A NEW water-harvesting device based on a metal-organic framework (MOF), developed in the US, can pull water out of the air – even in arid conditions.

University of California, Berkeley chemistry professor Omar Yaghi and MIT associate professor of mechanical engineering Evelyn Wang have made and tested a prototype device which pulled almost 3 l of water in 12 hours from air at just 20–30% humidity. This humidity level is similar to that found in desert conditions. The researchers envisage the new device being used as a domestic appliance, collecting water for an individual home using the power of sunlight. Yaghi says the device could lead to the possibility of off-grid water.

The device is based on an MOF developed by Yaghi and his team in 2014, made from zirconium and adipic acid. The MOF binds to water vapour. The system designed by Wang and her team uses around 1 kg of dust-sized MOF crystals, which are compressed between a solar heat absorber and a condenser plate. Ambient air flows through the MOF, and any water vapour present binds to it. As the solar absorber heats up, it heats the MOF, driving the water vapour bound to it towards the condenser plate. The liquid water then drips into a collector.

Wang says that the proof-of-concept device is a new way to harvest water from air, which is much more energy efficient than other existing technologies.

“It's not just that we made a passive device that sits there collecting water; we have now laid both the experimental and theoretical foundations so that we can screen other MOFs, thousands of which could be made, to find even better materials. There is a lot of potential for scaling up the amount of water that is being harvested. It is just a matter of further engineering now,” said Yaghi.

The team is now working on that engineering. Wang is focussed on improving the collector, while Yaghi is concentrating on the MOF. Yaghi says developments could include a system that absorbs the humidity at night to give out water during the day, or a solar collector which takes in more air and absorbs water at a much faster rate. At present, the MOF they use can only absorb 20% of its own weight in water, but Yaghi believes that other MOF materials could absorb 40%, while tweaking the material could make it more effective at different humidity levels.

“We wanted to demonstrate that if you are cut off somewhere in the desert, you could survive because of this device. A person needs about a Coke-can of water per day. That is something one could collect in less than an hour with this system,” said Yaghi.

Science doi.org/b5t3