Fresh water is scarce in many parts of the world and must be obtained at great expense.
Communities near the ocean can desalinate sea water for this purpose, but doing so requires
a large amount of energy.
Further away from the coast, practically often the only remaining option is to condense atmospheric
humidity through cooling, either through processes that similarly require high energy input or
by using "passive" technologies that exploit the temperature swing between day and night.
However, with current passive technologies, such as dew-collecting foils, water can be
extracted only at night.
This is because the sun heats the foils during the day, which makes condensation impossible.
Researchers at ETH Zurich have now developed a technology that, for the first time, allows
them to harvest water 24 hours around the clock, with no energy input, even under the
blazing sun.
The new device essentially consists of a specially coated glass pane, which both reflects solar
radiation and also radiates away its own heat through the atmosphere to the outer space.
It thus cools itself down to as much as 15 degrees Celsius below the ambient temperature.
On the underside of this pane, water vapour from the air condenses into water.
The process is the same as can be observed on poorly insulated windows in winter.
The scientists coated the glass with specifically designed polymer and silver layers.
This special coating approach causes the pane to emit infrared radiation at a specific wavelength
window to the outer space, with no absorption by the atmosphere nor reflection back onto
the pane.
Another key element of the device is a novel cone-shaped radiation shield.
It largely deflects heat radiation from the atmosphere and shields the pane from incoming
solar radiation, while allowing the device to radiate the aforementioned heat outward
and thus to self-cool, fully passively.
As tests of the new device under real-world conditions on the roof of an ETH building
in Zurich showed, the new technology can produce at least twice as much water per area per
day as the best current passive technologies based on foils: the small pilot system with
a pane diameter of 10 centimetres delivered 4.6 millilitres of water per day under real-world
conditions.
Larger devices with larger panes would produce more water accordingly.
The scientists were able to show that, under ideal conditions, they could harvest up to
0.53 decilitres of water per square metre of pane surface per hour.
This is close to the theoretical maximum value of 0.6 decilitres per hour, which is physically
impossible to exceed.
Other technologies usually require the condensed water to be wiped from a surface, which requires
energy.
Without this step, a significant portion of the condensed water would cling to the surface
and remain unusable while hindering further condensation.
The ETH Zurich researchers applied a novel superhydrophobic (extremely water-repellent)
coating to the underside of the pane in their water condenser.
This causes the condensed water to bead up and run or jump off on its own accord.
In contrast to other technologies, this technology can really function without any additional
energy, which is a key advantage.
The researchers' goal was to develop a technology for countries with water scarcity and, in
particular, for developing and emerging countries.
Now, they say, other scientists have the opportunity to further develop this technology or combine
it with other methods, such as water desalination, to increase their yield.
The production of the coated panes is relatively simple and building water condensers that
are larger than the current pilot system ought to be possible.
Similar to the way solar cells feature several modules set up next to each other, several
water condensers could also be positioned side by side to piece together a large-scale
system.