Atmospheric water generator

An atmospheric water generator (AWG), is a device that extracts water from humid ambient air, producing potable water. Water vapor in the air can be extracted either by condensation - cooling the air below its dew point, exposing the air to desiccants, using membranes that only pass water vapor, collecting fog,[1] or pressurizing the air. AWGs are useful where potable water is difficult to obtain, because water is always present in ambient air.

AWG may require significant energy inputs, or operate passively, relying on natural temperature differences. Biomimicry studies have shown the Stenocara gracilipes beetle has the natural ability to perform this task.

History
"Atrapanieblas" or fog collection in Alto Patache, Atacama Desert, Chile.

The Incas were able to sustain their culture above the rain line by collecting dew and channeling it to cisterns for later distribution.[2] Historical records indicate the use of water-collecting fog fences. These traditional methods have usually been completely passive, employing no external energy source and relying on naturally occurring temperature variations.

Air wells are one way to passively collect moisture from air.

Brine extraction technology was contracted by the US Army and the US Navy from Terralab and the Federal Emergency Management Agency (FEMA).[3]

DARPA's Atmospheric Water Extraction program that aims to develop a device which can provide water for 150 soldiers and be carried by four people. In February 2021 General Electric was awarded 14 million dollars to continue development of their device.[4]

In 2022, a cellulose/konjac gum-based desiccant was demonstrated that produced 13 L/kg/day (1.56 US gal/lb/day) of water at 30% humidity, and 6 L/kg/day (0.72 US gal/lb/day) at 15% humidity.[5]

Technologies

Cooling-based systems are the most common, while hygroscopic systems are showing promise. Hybrid systems combine adsorption, refrigeration and condensation.[6][7]

Cooling condensation
Example of cooling-condensation process.

Condensing systems are the most common technology in use.

A cooling condensation type AWG uses a compressor to circulate refrigerant through a condenser and then an evaporator coil that cools the surrounding air. Once the air temperature reaches its dew point, water condenses into the collector. A fan pushes filtered air over the coil. A purification/filtration system keeps the water pure and reduces the risk posed by ambient microorganisms.[8]

The rate of water production depends on the ambient temperature, humidity, the volume of air passing over the coil, and the machine's capacity to cool the coil. AWGs become more effective as relative humidity and air temperature increase. As a rule of thumb, cooling condensation AWGs do not work efficiently when the ambient temperature falls below 18.3 °C (65 °F) or the relative humidity drops below 30%. The cost-effectiveness of an AWG depends on the capacity of the machine, local humidity and temperature conditions, and power costs.

The Peltier effect of semiconducting materials offer an alternative condensation system in which one side of the semi-conducting material heats while the other side cools. In this application, the air is forced over the cooling fans on the side that cools which lowers the air temperature. The solid-state semiconductors are convenient for portable units, but this is offset by low efficiency and high power consumption.[9]

Potable water generation can be enhanced in low humidity conditions by using an evaporative cooler with a brackish water supply to increase the humidity. A special case is water generation in greenhouses because the inside air is much hotter and more humid. Examples include the seawater greenhouse in Oman and the IBTS Greenhouse.

In dehumidifying air conditioners, non-potable water is a by-product. The relatively cold (below the dewpoint) evaporator coil condenses water vapor from the processed air.

When powered by coal-based electricity it has one of the worst carbon footprints of any water source (exceeding reverse osmosis seawater desalination by three orders of magnitude) and it demands more than four times as much water up the supply chain than it delivers to the user.[10]

The most efficient and sustainable method is to use an adsorption refrigerator powered by solar thermal, particularly useful in desert climates due to the abundance of sunlight. The captured waste heat can be used to further efficiency by pumping the collected water to a storage site.

Hygroscopy

Hygroscopic techniques pull water from the air via absorption or adsorption. These materials desiccate the air. Desiccants may be liquid ("wet") or solid.

Wet desiccants

Liquid desiccants include lithium chloride or lithium bromide.[11]

Another wet desiccant is concentrated brine. The brine absorbs water, which is then extracted and purified. One portable device runs on a generator. Large versions, mounted on trailers, produce up to 1,200 US gallons (4,500 L) of water per day, at a ratio of up to 5 gallons of water per gallon of fuel.[12]

Another variation claims to be more environmentally friendly, by relying on passive solar energy and gravity. Concentrated brine is streamed down the outside of towers, absorbing water vapor. The brine then enters a chamber, subjected to a partial vacuum and heated, releasing water vapor that is condensed and collected. As the condensed water is removed from the system using gravity, it creates a vacuum which lowers the brine's boiling point.[13]

Solid desiccants

Silica gel and zeolite desiccate pressurized air. Direct potable water generating devices using sunlight are under development.[14] One device takes 310 watt-hours (1,100 kJ) to make 1 liter of water. It uses a zirconium/organic metal-organic framework on a porous copper base, attached to a graphite substrate. The sun heats the graphite, releasing the water, which then cools the graphite.[15]

Fuel cells

A hydrogen fuel cell car generates one liter of drinking quality water for every 8 miles (12.87 kilometers) traveled by combining hydrogen with ambient oxygen.[16]

Power
The minimum energy for atmospheric water harvesting [1]

Unless the air is super-saturated with vapor, an energy input is required to harvest water from the atmosphere. The energy required is a strong function of the humdity and temperature. It can be calculated using Gibbs free energy.

Potable water can be generated by rooftop solar hydropanels using solar power and solar heat.[17][18][19]

Hydrogels can be used to capture moisture (e.g. at night in a desert) to cool solar panels[20] or to produce fresh water[21][22] – including for irrigating crops as demonstrated in solar panel integrated systems where these have been enclosed next to[23][24] or beneath the panels within the system.[25][26][27][28][29][30]

One study reported that such devices could help provide potable water to one billion people, although off-the-grid generation could "undermine efforts to develop permanent piped infrastructure".[31][32][33]

See also

References
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  22. Shi, Ye; Ilic, Ognjen; Atwater, Harry A.; Greer, Julia R. (14 May 2021). "All-day fresh water harvesting by microstructured hydrogel membranes". Nature Communications. 12 (1): 2797. doi:10.1038/s41467-021-23174-0. ISSN 2041-1723. PMC 8121874. PMID 33990601. S2CID 234596800.
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