Atmospheric water generator

An atmospheric water generator (AWG), is a device that extracts water from humid ambient air. Water vapor in the air is condensed by cooling the air below its dew point, exposing the air to desiccants, or pressurizing the air. Unlike a dehumidifier, an AWG is designed to render the water potable. AWGs are very useful in locations where pure drinking water is difficult or impossible to obtain, as there is almost always a small amount of water in the air. The two primary techniques in use are cooling and desiccants.

The extraction of atmospheric water may not be completely free of cost, because significant input of energy is required to drive some AWG processes, sometimes called "trading oil for water". Certain traditional AWG methods are completely passive, relying on natural temperature changes, and requiring no external energy source. Research has also developed AWG technologies to produce useful yields of water at a reduced (but non-zero) energy cost.

History

Water has been collected from the air for at least 2,000 years using air wells in Middle Eastern deserts, and later in Europe. The Incas were able to sustain their culture above the rain line by collecting dew and channeling it to cisters for later distribution. Historical records indicate the use of water-collecting fog fences. These traditional methods have usually been completely passive, requiring no external energy source other than naturally-occurring temperature variations.

Modern technologies

Many atmospheric water generators operate in a manner very similar to that of a dehumidifier: air is passed over a cooled coil, causing water to condense. 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. These systems reduce air temperature, which in turn reduces the air's capacity to carry water vapor. This is the most common technology.

An alternative available technology uses liquid, or "wet" desiccants such as lithium chloride or lithium bromide to pull water from the air via hygroscopic processes.^[1] A proposed similar technique combines the use of solid desiccants, such as silica gel and zeolite, with pressure condensation.

Cooling condensation

Example of cooling-condensation process.

In a cooling condensation type atmospheric water generator, a compressor circulates refrigerant through a condenser and then an evaporator coil which cools the air surrounding it. This lowers the air temperature to its dew point, causing water to condense. A controlled-speed fan pushes filtered air over the coil. The resulting water is then passed into a holding tank with purification and filtration system to keep the water pure.^[2]

The rate at which water can be produced depends on relative humidity and ambient air temperature and size of the compressor. Atmospheric water generators become more effective as relative humidity and air temperature increase. As a rule of thumb, cooling condensation atmospheric water generators do not work efficiently when the temperature falls below 18.3°C (65°F) or the relative humidity drops below 30%. The cost-effectiveness of an atmospheric water generator depends on the capacity of the machine, local humidity and temperature conditions and the cost to power the unit.

Wet desiccation

One form of wet desiccant water generation involves the use of salt in a concentrated brine solution to absorb the ambient humidity. These systems then extract the water from the solution and purify them for consumption. A version of this technology was developed as portable devices which run on generators. Large versions, mounted on trailers, are said to produce up to 1200 gallons of water per day, at a rate of up to 5 gallons of water per gallon of fuel.^[3] This technology was contracted for use by the US Army and the US Navy from Terralab^{[citation needed]} and the Federal Emergency Management Agency (FEMA).^[4]

A variation of this technology has been developed to be more environmentally friendly, primarily through the use of passive solar energy and gravity. Brine is streamed down the outside of towers, where it absorbs water from the air. The brine then enters a chamber and subjected to a vacuum and heated. The vacuum significantly lowers the boiling point of the brine, so the boiling process requires only a small amount of energy. The steam is collected and condensed, while the remaining brine is recirculated through the system. As the condensed water is removed from the system using gravity, it creates the vacuum which lowers the boiling point of the brine.^[5]

In greenhouses

A special case is the water-generation in greenhouses because the air inside a greenhouse is much hotter and more humid than the outside. Particularly in climatical zones with water scarcity a greenhouse can offer strongly enhanced ambience for atmospheric water generation. Examples are the seawater greenhouse in Oman and the proposed IBTS.^[6]

References

Water portal

1. ^ Patents; Draw water from air, measure how much water you drink and be kind to the fish you catch. New York Times. July 2, 2001
2. ^ Latest Willie Nelson venture: Water from Air. Atlanta Journal Constitution.
3. ^ Water Extracted from the Air for Disaster Relief. National Public Radio; by Nell Greenfieldboyce; October 19, 2006
4. ^ Innovation Awards: Ahead of the Pack. Wall Street Journal. October 30, 2007.
5. ^ Drinking Water From Air Humidity. ScienceDaily (June 8, 2009)
6. ^ Out of the Box. H2O magazine (March 12, 2011)