Solar powered device harvests water from air

(Credit: MIT/Hyunho Kim)

The harvester, described in the journal Science, uses a material known as a metal organic framework (MOF) to first extract the water from the air. MOFs combine metals with organic molecules to form porous structures that are excellent for storing gases and liquids. Different compositions of MOFs can be used to capture and store various substances, such as hydrogen, carbon dioxide or natural gas.

Using an MOF derived from zirconium metal and adipic acid, the new device is able to harvest water vapour from the atmosphere. Sunlight then heats the MOF and drives the water towards a condenser, which is at the same temperature as the outside air. The vapour condenses into liquid form and drips into a collector. On the roof of an MIT building, a prototype containing 1kg of the MOF material was able to collect 2.8 litres of water over a 12-hour period, under conditions of 20-30 per cent humidity.

A schematic of a metal-organic framework (Credit: UC Berkeley, Berkeley Lab)

“This is a major breakthrough in the long-standing challenge of harvesting water from the air at low humidity,” said Berkeley chemist Omar Yaghi, one of the paper’s senior authors, who first discovered MOFs more than 20 years ago. “There is no other way to do that right now, except by using extra energy. Your electric dehumidifier at home ‘produces’ very expensive water.”

Atmospheric water is equivalent to about 10 per cent of the Earth’s fresh water that exists in rivers and lakes. The new device promises a cheap and efficient way to provide access to this resource, and should even work in arid conditions such as the desert. While the current MOF can absorb about 20 per cent of its weight in water, Yaghi believes other MOFs could potentially double that capacity to 40 per cent. Combined with better harvesting technology, the device’s efficiency could improve dramatically.

“There is a lot of potential for scaling up the amount of water that is being harvested,” said Yaghi. “It is just a matter of further engineering now.”

“To have water running all the time, you could design a system that absorbs the humidity during the night and evolves it during the day. Or design the solar collector to allow for this at a much faster rate, where more air is pushed in. 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."