Extracting water from thin air

Inhabitants of the driest of deserts could one day harvest water from the air around them without using external energy sources. A team at the Center for Nanomaterials for Clean Energy Applications (CENCEA), a collaboration between KACST and University of California, Berkeley, has demonstrated that a new class of highly porous synthetic crystals can condense water vapour by using only ambient sunlight, even in arid conditions.

Current methods of extracting water from air either require high humidity or energy-intensive refrigeration to drive condensation.  

Metal organic frameworks (MOFs), a family of hybrid materials consisting of metal ions linked by organic molecules, are highly porous and their chemical compositions can be fine-tuned, giving them potential for a wide range of applications.

A group led by MOF pioneer, Omar Yaghi, chair of the chemistry department at UC Berkeley and co-director of CENCEA, developed a proof-of-concept water harvesting device consisting of MOF crystals pressed into a sheet of copper placed between a solar absorber and a condenser plate, all in a box. At night, the box is opened, allowing the MOF to absorb vapour from the air into its pores. The box is closed in the morning, when heat from the sun extracts the vapour, which then condenses.  

Last year the group published a study showing that a device containing a compound called MOF-801 could capture water droplets in laboratory conditions. More recently they increased its thermal conductivity by adding graphite, to make MOF-801/G. 

Following tests at a desert site in Arizona, in the United States, they added insulation and altered the angle of the device with respect to the sun. Their optimised device was capable of harvesting the equivalent of 100ml of water per kilogram of MOF-801/G during one day–night cycle. 

MOF-801/G contains the metal zirconium, which, at around 0 per kilogram, would make a machine scaled up for household use prohibitively expensive. Yaghi and colleagues found that a potentially cheaper aluminium-based compound, called MOF-303, could extract more than twice as much water from low-humidity air as MOF-801/G. They also identified some of the main parameters relating to energy, materials, and air for producing water from desert air efficiently. 

“Our prototype can generate drinkable water where there is none,” says Yaghi. This could lead to “potentially profound progress” for regions of the world where water is scarce, he says.

1. Fathieh, F., Kalmutzki, M., Kapustin, E., Waller, P., Yaghi, O. Practical water production from desert air. Science Advances 4(6), eaat3198 (2018).| article