Squeezing water from the air

In arid regions, fresh water is a scarce and precious resource that must be delivered from a distance—maybe by truck or pipe, or via a desalination plant.

An emerging alternative is to harvest fresh water from the atmosphere. Even the driest environments harbour an abundance of water vapour. The problem remains to convert it into liquid form so it can be drunk—or used to irrigate crops.

Traditionally, research in this area has concentrated on creating cold surfaces where water vapour can condense. However, the high energy requirement means that a variety of other methods have come to the fore. And, while largely unproven commercially, they hold promise for the future.

Efficient in dry air

Researchers at the University of Berkeley, USA have developed chemical compounds that efficiently absorb water from dry air – and release it again very easily. The compounds, called metal oxide frameworks (MOFs), have been designed to absorb as much moisture as possible.

“They have internal surface areas up to 7,000 cubic metres per gramme of material, to trap molecules like water,” says Omar Yaghi, Professor of Chemistry at Berkeley, who led the research.

Yaghi’s team has designed and tested a type of MOF based on aluminium. Each kilogram of material harvests around 230g of water. It is twice as effective as an earlier version— based on zirconium – and much cheaper.

Its structure is such that, once the first water molecules are absorbed, they act as ‘seeds’ that attract more water. And, in sharp contrast to a desiccant like silica gel—which only releases its absorbed water at around 300°C—the MOFs release their water at much lower temperatures.

A water-harvesting device incorporates the MOF into a thin, porous copper sheet. Exposing the copper sheet to sunlight—within a sealed plastic container—is enough to release trapped moisture, which then condenses as fresh water. The device has been tested in the Mojave Desert, which provided “the most extreme test conditions”.

Under the sun

Dutch company Sunglacier’s solar-powered Peltier element cools an aluminium block below the dew point, causing moisture to condense. The device is cheap to run, but must still be energy-efficient: it takes 5W to cool a cubic metre of air, so cooling 50 cubic metres requires 250W (the capacity of a typical solar panel).

Sunglacier also has a second technique that sprays cold water onto the inner surface of a tube, which sucks in ambient air. The air vapour then condenses and adds to the volume of water in the tube.

“We get ‘extra’ water because the inner surface is colder than the air that is sucked in,” says Peter Van Geloven, who leads technology development at Sunglacier.

Devices based on each design have been tested in the Sahara Desert. Sunglacier has built a device combining both designs and will test it in a chamber that mimics desert conditions for the Dutch Ministry of Defence.

“They want a machine that is easy to carry and can supply 20 litres of water per day—enough for a military unit to stay alive,” he says.

The device could be used to irrigate crops by placing it in a field and “letting it drip water where needed”. Several companies are interested in licensing the technology but are waiting until the latest tests have been completed.

Water from biomass

Scarcity of fresh water is not restricted to arid regions, because natural disasters – such as storms – can knock out local water infrastructure. In ‘mild’ climates, a water-from-air system is unlikely to work.

The Wedew system uses biomass gasification to convert green feedstocks into electricity and humid air. The electricity powers a water-harvesting machine from US-based Skysource, which extracts fresh water from the humid air. Wedew is housed in a portable shipping container – where it generates an artificial ‘high humidity’ environment that works in different regions and climate types.

“We make our own climate,” says David Hertz, who led the project to develop Wedew and also runs Skysource.

Wedew provided disaster relief in the unlikely setting of Malibu, California during serious forest fires there last year. Disaster areas are a perfect application for Wedew, says Hertz: they lack power and fresh water, yet have plenty of biomass – such as waste vegetation – that can be used to run the system.

Last year, Wedew won a $1.5 million environmental award called the Xprize, where 97 teams competed to produce 2,000 litres/day of water—costing no more than 2 cents/litre— using only renewable energy.

“We were the only team that met the challenge,” says Hertz.

Harvesting water from air is still some distance from everyday reality: many devices have minimal outputs or are still in development. However, the fact that the atmosphere contains more freshwater than all the world’s rivers means that more researchers are looking to tap this abundant, though elusive, source.


About the author: Lou Reade writes as a correspondent for the International WaterCentre, charged with exploring water challenges and the ways these challenges are managed around the world.

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