Wednesday, 13 June 2018

Solar-powered harvester sucks drinkable water out of thin air


Although there are trillions of litres of water floating around in the air, it's not easily accessible for those who need it. Now engineers at UC Berkeley have developed a device powered only by the Sun that can pluck practical quantities of drinkable water out of thin air, and they've successfully field-tested it in the Arizona desert.
Although it's not the first device to try to wring water out of the atmosphere, UC Berkeley's harvester looks far more practical than systems like the Warka Water, and it seems to be further along in its development than others, such as those on the current shortlist for the Water Abundance XPrize.


The key ingredients in the UC Berkeley water harvester are materials known as metal-organic frameworks (MOF). These synthetic compounds are notable for their extremely high surface areas – a MOF the size of a sugar cube, for example, could theoretically be packing the surface area of six football fields. This makes them great for trapping and storing molecules from the air.
In this case, that molecule is – you guessed it – water, and to trap it the team used the zirconium-based MOF-801. The harvester sees a 2-sq ft (0.19-sq m) bed of this stuff placed inside a box, which is itself placed inside a clear plastic cube. Using the simple process of condensation, over the course of 24 hours the device can adsorb water from the air and then expel it for harvesting.
First, the lid of the box is left open to the air overnight, when the higher humidity lets the MOF grains suck in water molecules from the air. In the morning, the lid is replaced and the Sun heats the box up like a greenhouse, triggering the MOFs to release the trapped water vapour. This condenses on the walls of the outer box and runs down to the bottom, where it can be collected.
The field tests build on lab experiments from early last year, where the team tested a much smaller prototype using just 2 g of MOF with promising results.
The UC Berkeley researchers test the water-harvesting device prototype in Arizona
For the new test run, the UC Berkeley team trialed the larger prototype in the Arizona desert, where humidity ranges from 40 percent at night down to just eight percent during the day. Using up to 1.2 kg (2.6 lb) of MOF-801, they found that the device was able to produce 100 ml of water per kg (1.5 oz per lb) of MOF, but the team says this harvest could be doubled using the same materials.
Using other materials, however, might be better in the long run. The researchers are also experimenting with using an aluminum-based MOF, which is not only 150 times cheaper than zirconium but can capture twice as much water. Future versions of the device could potentially generate over 400 ml (1.5 cups) of water per day per kg of MOF.
The end goal is to allow remote communities to essentially harvest their own clean water from thin air, without requiring any power sources besides the Sun.
"There is nothing like this," says Omar Yaghi, lead researcher on the study. "It operates at ambient temperature with ambient sunlight, and with no additional energy input you can collect water in the desert. This laboratory-to-desert journey allowed us to really turn water harvesting from an interesting phenomenon into a science."
The research was published in the journal Science Advances, and the team demonstrates the device in the video below


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