An atmospheric water generator is a device that extracts water from humid ambient air. Water vapor in the air can be extracted by condensation - 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 useful where pure drinking water is difficult or impossible to obtain, because there is almost always a small amount of water in the air that can be extracted. The two primary techniques in use are cooling and desiccants. The extraction of atmospheric water may require a significant input of energy. Some AWG methods are completely passive, relying on natural temperature differences, and requiring no external energy source. Biomimicry studies have shown the beetle Stenocara gracilipes has the natural ability to perform this task.
History
The Incas were able to sustain their culture above the rain line by collecting dew and channeling it to cisterns 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. Several inventors have developed air wells as a way to passively collect moisture from air.
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 in use, but when powered by coal-based electricity it has one of the worst carbon footprints of any water source and it demands more than four times as much water up the supply chain than it delivers to the user. 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. A proposed similar technique combines the use of solid desiccants, such as silica gel and zeolite, with pressure condensation. Direct drinking quality water generating devices using sun light are also under development. It is said to take 310 Wh to make 1 litre of water.
Cooling condensation
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 help keep the water pure and reduce the risk posed by viruses and bacteria which may be collected from the ambient air on the evaporator coil by the condensing water. 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 or the relative humidity drops below 30%. This means they are relatively inefficient when located inside air-conditioned offices. The cost-effectiveness of an AWG depends on the capacity of the machine, local humidity and temperature conditions and the cost to power the unit. Recent efforts have been made attempting to utilize the Peltier effect of semi-conducting materials in which one side of the semi-conducting material heats while the other side cools. In this application, air is forced over the cooling fans on the side that cools which lowers the temperature of the air to its dew point, causing water to condense, the resulting water is then collected. Due to the solid-state nature of the semi-conducting material they are attractive for portable units although the low efficiency of condensing water at commonly experienced humidity is compounded by the high power consumption of Peltier coolers The drinking water generation capacity can be enhanced in low humidity ambient air conditions, first by using the evaporative cooler with a brackish water supply to increase the air humidity near to dew point condition. Thus drinking water is generated using brackish water without depending on ambient air humidity by the water generator.
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 it 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 of water per day, at a ratio of up to 5 gallons of water per gallon of fuel. This technology was contracted for use by the US Army and the US Navy from Terralab and the Federal Emergency Management Agency. 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 is subjected to a partial vacuum and heated. The water vapor is condensed and the liquid water collected, while the renewed 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. Systems combining adsorption, refrigeration and condensation are also being developed.
In greenhouses
A special case is water generation in greenhouses because the air inside a greenhouse is much hotter and more humid than the outside. Particularly in climatic zones with water scarcity, a greenhouse can strongly enhance the conditions necessary for atmospheric water generation. An example is the seawater greenhouse in Oman and the IBTS Greenhouse.
In fuel cell cars
A hydrogen fuel cell car generates one litre of drinking quality water for every 8 miles ride which is of significance in desert conditions.
In air conditioners
In dehumidification type air conditioners, waste water is a by-product, caused by air cooling and condensation, like an atmospheric water generator. The water, in this case, is not purified. Refrigeration air conditioning equipment usually reduces the absolute humidity of the air processed by the system. The relatively cold evaporator coil condenses water vapor from the processed air, much like an ice-cold drink will condense water on the outside of a glass. Therefore, water vapor is removed from the cooled air and the relative humidity in the room is lowered. The water is usually sent to a drain or may simply drip onto the ground outdoors. The heat is rejected by the condenser which is located outside of the room to be cooled.
