A desert harvest

Water, a precious resource, is in perilously short supply. But researchers, led by Evelyn Wang at the Massachusetts Institute of Technology (MIT), and KACST-affiliated Omar Yaghi, recently developed a promising solution for efficiently harvesting water in even the driest places.

Their system is based on a class of porous polymers known as metal organic frameworks (MOFs). Under the right conditions, water vapour readily accumulates on the surface of MOFs, as Yaghi and Wang have previously demonstrated in the temperate environment of Cambridge, Massachusetts.  However, this approach was ill-suited to desert conditions, where days are hot and extremely arid, while the nights bring higher humidity, but frigid temperatures.

The researchers have now demonstrated a MOF-based system that can effectively collect vapor in the Arizona desert, where daytime humidity can be as low as 10 percent.  Water vapor is collected after dark, when humidity reaches around 20–40 percent. The MOF layer is cooled in the night air, lowering its temperature to improve the efficiency of water vapor binding. When the sun is up, the system is covered and quickly heats up, and the MOF layer releases the captured water on to a condenser, from which it is collected for use.

This demonstration harvest was on a small scale, hardly sufficient to quench the thirst of one person, but enough to prove the system’s potential. From a single overnight collection, Yaghi and Wang’s team calculated that they were able to collect 250 mililitres of water for every kilogram of MOF material. They achieved consistent performance over multiple cycles of collection and condensation, and quality testing confirmed that the harvested water was safe, and free of chemical contaminants. Critically, this approach is far more cost-effective than existing vapor condensation approaches, which generally rely on refrigeration and are too energy-intensive for use in very hot environments.

With a variety of MOF materials available, there are opportunities to improve the system’s performance. The authors describe alternatives that could potentially deliver three times as much water from the same MOF surface area, or which operate more effectively at even lower night-time humidity levels. The authors conclude that “our demonstration in an exceptionally arid climate indicates that adsorption-based water harvesting strategy is a promising solution to alleviate water scarcity in such regions.”

1. Kim, H., Rao, S.R., Kapustin, E.A., Zhao, L., Yang, S., Yaghi, O.M., & Wang, E.N. Adsorption-based atmospheric water harvesting device for arid climates. Nat. Commun. 22, 1191 (2018). | article