Device that extracts drinkable water from humid air
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 Onymacris unguicularis beetle has the natural ability to perform this task.[2]
History
The
Incas were able to sustain their culture above the rain line by collecting dew and channeling it to
cisterns for later distribution.[3] 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.[citation needed]
Air wells are one way to passively collect moisture from air.
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.[5]
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.[6]
Technologies
Cooling-based systems are the most common, while hygroscopic systems are showing promise. Hybrid systems combine
adsorption, refrigeration and condensation.[7][8]
Cooling condensation
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.[9]
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.[10]
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.[11]
Perhaps the most efficient and sustainable method is to use an adsorption refrigerator powered by solar thermal, which outperforms photovoltaic-powered systems.[12] Such systems also may have beneficial uses for waste heat, e.g. for pumping or for operation overnight, where humidity tends to rise.
Hygroscopy
Hygroscopic techniques pull water from the air via
absorption or
adsorption. These materials desiccate the air. Desiccants may be liquid ("wet") or solid. They need to be regenerated (typically thermally) to recover the water.
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.[15]
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.[16]
Solid desiccants
Silica gel and
zeolite desiccate pressurized air. Direct potable water generating devices using sunlight are under development.[17] 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.[18]
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.[19]
Power
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 humidity 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.[20][21][22]
Solar still – Water distillation and purification system using solar energy
Watergen – Israel-based atmospheric water generator company
Watermaker – Device used to obtain potable water by reverse osmosis of seawater
Water scarcity – Lack of fresh water resources to meet water demand
References
^
abRao, Akshay K.; Fix, Andrew J.; Yang, Yun Chi; Warsinger, David M. (2022). "Thermodynamic limits of atmospheric water harvesting". Energy & Environmental Science. 15 (10). Royal Society of Chemistry (RSC): 4025–4037.
doi:
10.1039/d2ee01071b.
ISSN1754-5692.
S2CID252252878.