Just add heat: Innovative device harvests water from thin air

Posted 20 April 2017

A solar-powered household appliance could harvest litres of water from the air each day – even in very arid regions. 

US scientists have demonstrated a system for obtaining fresh water using a foam-like material that draws moisture into its pores and is powered entirely by solar heat.

Tests have showed that 1kg of the material can collect about 3L of fresh water per day – enough to supply drinking water for one person – from very dry air with a humidity as low as 20%. 

“This is a major breakthrough in the longstanding challenge of harvesting water from the air at low humidity,” said faculty scientist at Lawrence Berkeley National Laboratory, and senior author of a paper on the research, Omar Yaghi.

“There is no other way to do that right now except by using extra energy. Your electric dehumidifier at home produces very expensive water.”

The key to the new system is the porous material, which is part of a family of compounds known as metal-organic frameworks (MOFs). 

These compounds form a sponge-like configuration with large internal surface areas that can be made hydrophilic – or water-attracting – by tuning the exact chemical composition of the MOF. 

When the material is placed between a top surface that is painted black to absorb solar heat, and a lower surface that is kept at the same temperature as the outside air, water is released from the pores as vapour. 

The temperature and concentration difference then naturally cause the vapour to drip down as liquid and collect on the cooler lower surface.

“One vision for the future is to have water off-grid, where you have a device at home running on ambient solar for delivering water that satisfies the needs of a household,” Yaghi said.

“To me, that will be made possible because of this experiment. I call it personalised water.”

Research team member and MIT postdoc Sameer Rao added that the system does not even require sunlight — all it needs is some source of heat.

“There are a lot of places where there is biomass available to burn and where water is scarce,” Rao said.

While initial experiments have proven the concept can work, there is more work to be done to refine the design and search for more effective varieties of MOFs. 

The present version can collect water up to about 25% of its own weight, but with further tuning researchers think that could double.