Air well (condenser)
An air well or aerial well is a structure or device that collects water by promoting the condensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....

 of moisture
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...

 from air. Designs for air wells are many and varied, but the simplest designs are completely passive, require no external energy source and have few, if any, moving parts.

Three principal designs are used for air wells: high mass, radiative and active. High-mass air wells were used in the early 20th century, but the approach failed. From the late 20th century onwards, low-mass, radiative collectors proved to be much more successful. Active collectors collect water in the same way as a dehumidifier
A dehumidifier is typically a household appliance that reduces the level of humidity in the air, usually for health reasons. Humid air can cause mold and mildew to grow inside homes, which pose various health risks. Very humid climates or air make some people extremely uncomfortable, causing...

; although the designs work well, they require an energy source, making them uneconomical except in special circumstances. New, innovative designs seek to minimise the energy requirements of active condensers or make use of renewable energy
Renewable energy
Renewable energy is energy which comes from natural resources such as sunlight, wind, rain, tides, and geothermal heat, which are renewable . About 16% of global final energy consumption comes from renewables, with 10% coming from traditional biomass, which is mainly used for heating, and 3.4% from...



All air well designs incorporate a substrate with a temperature sufficiently low so that dew
[Image:Dew on a flower.jpg|right|220px|thumb|Some dew on an iris in Sequoia National Park]]Dew is water in the form of droplets that appears on thin, exposed objects in the morning or evening...

 forms. Dew is a form of precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

 that occurs naturally when atmospheric water vapour condenses
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....

 onto a substrate. It is distinct from fog
Fog is a collection of water droplets or ice crystals suspended in the air at or near the Earth's surface. While fog is a type of stratus cloud, the term "fog" is typically distinguished from the more generic term "cloud" in that fog is low-lying, and the moisture in the fog is often generated...

, in that fog is made of droplets of water that condense around particles in the air. Condensation releases latent heat
Latent heat
Latent heat is the heat released or absorbed by a chemical substance or a thermodynamic system during a process that occurs without a change in temperature. A typical example is a change of state of matter, meaning a phase transition such as the melting of ice or the boiling of water. The term was...

 which must be dissipated in order for water collection to continue.

An air well requires moisture from the air. Everywhere on Earth, even in the hottest climates, the surrounding atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 contains at least some water. According to Beysens and Milimouk: "The atmosphere contains of fresh water, composed of 98 percent water vapour and 2 percent condensed water (cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

s): a figure comparable to the renewable liquid water resources of inhabited lands ." The quantity of water vapour contained within the air is commonly reported as a relative humidity
Relative humidity
Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...

, and this depends on temperature—warmer air can contain more water vapour than cooler air. When air is cooled to the dew point
Dew point
The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.The dew point is...

, it becomes saturated, and moisture will condense on a suitable surface. For instance, the dew temperature of air at 20 °C (68 °F) and 80 percent relative humidity is 18 °C (64.4 °F). The dew temperature falls to 10 °C (50 °F) if the relative humidity is only 25 percent.

A related, but quite distinct, technique of obtaining atmospheric moisture is the fog fence
Fog fence
A fog fence or fog collector is an apparatus for collecting liquid water from fog, using a fine mesh or array of parallel wires. Proposed geometries include linear, similar to a fence and cylindrical. It has the advantage of being passive, requiring no external energy source to perform its...


An air well should not be confused with a dew pond
Dew pond
A dew pond is an artificial pond usually sited on the top of a hill, intended for watering livestock. Dew ponds are used in areas where a natural supply of surface water may not be readily available. The name dew pond is first found in the Journal of the Royal Agricultural Society in 1865...

. A dew pond is an artificial pond
A pond is a body of standing water, either natural or man-made, that is usually smaller than a lake. A wide variety of man-made bodies of water are classified as ponds, including water gardens, water features and koi ponds; all designed for aesthetic ornamentation as landscape or architectural...

 intended for watering livestock. The name dew pond (sometimes cloud pond or mist pond) derives from the widely held belief that the pond was filled by moisture from the air. In fact, dew ponds are primarily filled by rainwater.

A stone mulch
In agriculture and gardening, is a protective cover placed over the soil to retain moisture, reduce erosion, provide nutrients, and suppress weed growth and seed germination. Mulching in gardens and landscaping mimics the leaf cover that is found on forest floors....

 can significantly increase crop yields in arid
A region is said to be arid when it is characterized by a severe lack of available water, to the extent of hindering or even preventing the growth and development of plant and animal life...

 areas. This is most notably the case in the Canary Islands
Canary Islands
The Canary Islands , also known as the Canaries , is a Spanish archipelago located just off the northwest coast of mainland Africa, 100 km west of the border between Morocco and the Western Sahara. The Canaries are a Spanish autonomous community and an outermost region of the European Union...

: on the island of Lanzarote
Lanzarote , a Spanish island, is the easternmost of the autonomous Canary Islands, in the Atlantic Ocean, approximately 125 km off the coast of Africa and 1,000 km from the Iberian Peninsula. Covering 845.9 km2, it stands as the fourth largest of the islands...

 there is about 140 millimetres (5.5 in) of rain each year and there are no permanent rivers. Despite this, substantial crops can be grown by using a mulch of volcanic stones, a trick discovered after volcanic eruptions in 1730. Some credit the stone mulch with promoting dew; although the idea has inspired some thinkers, it seems unlikely that the effect is significant. Rather, plants are able to absorb dew directly from their leaves, and the main benefit of a stone mulch is to reduce water loss from the soil and to eliminate competition from weeds.


Beginning in the early 20th century, a number of inventors experimented with high-mass collectors. Notable investigators were the Russian engineer Friedrich Zibold (sometimes given as Friedrich Siebold), the French bioclimatologist Leon Chaptal, the German-Australian researcher Wolf Klaphake
Wolf Klaphake
Wolf Klaphake was a German-born scientist. He emigrated to Australia in 1935, where he was interred as an enemy alien from 1940 to 1944...

 and the Belgian inventor Achille Knapen.

Zibold's collector

In 1900, near the site of the ancient Byzantine
Byzantine usually refers to the Roman Empire during the Middle Ages.Byzantine may also refer to:* A citizen of the Byzantine Empire, or native Greek during the Middle Ages...

 city of Theodosia, thirteen large piles of stones were discovered by Zibold who was a forester
250px|thumb|right|Foresters of [[Southern University of Chile|UACh]] in the [[Valdivian forest]]s of San Pablo de Tregua, ChileA forester is a person who practices forestry, the science, art, and profession of managing forests. Foresters engage in a broad range of activities including timber...

 and engineer in charge of this area. Each stone pile covered just over 900 square metres (1,076.4 sq yd) and was about 10 metres (32.8 ft) tall. The finds were associated with the remains of 75 millimetres (3 in) terracotta pipes that apparently led to wells and fountains in the city. Zibold concluded that the stacks of stone were condensers that supplied Theodosia with water; and calculated that each air well produced more than 55400 litres (117,081.2 US pt) each day.

To verify his hypothesis Zibold constructed a stone-pile condenser at an altitude of 288 metres (944.9 ft) on mount Tepe-Oba near the ancient site of Theodosia. Zibold’s condenser was surrounded by a wall 1 metres (3.3 ft) high, 20 metres (65.6 ft) wide, around a bowl-shaped collection area with drainage. He used sea stones 10–40 cm (3.9–15.7 in) in diameter piled 6 metres (19.7 ft) high in a truncated cone that was 8 metres (26.2 ft) in diameter across the top. The shape of the stone pile allowed a good air flow with only minimal thermal contact between the stones.

Zibold's condenser began to operate in 1912 with a maximum daily production that was later estimated to have been 360 litres (760.8 US pt) – Zibold made no public record of his results at the time. The base developed leaks that forced the experiment to end in 1915 and the site was partially dismantled before being abandoned. (The site was rediscovered in 1993 and cleaned up.) Zibold's condenser was approximately the same size as the ancient stone piles that had been found, and although the yield was very much less than the yield Zibold had calculated for the original structures, the experiment was an inspiration for later developers.

Chaptal's collector

Inspired by Zibold's work, Chaptal built a small air well near Montpellier
-Neighbourhoods:Since 2001, Montpellier has been divided into seven official neighbourhoods, themselves divided into sub-neighbourhoods. Each of them possesses a neighbourhood council....

 in 1929. Chaptal's condenser was a pyramidal
Pyramid (geometry)
In geometry, a pyramid is a polyhedron formed by connecting a polygonal base and a point, called the apex. Each base edge and apex form a triangle. It is a conic solid with polygonal base....

 concrete structure 3 metres (9.8 ft) square and 2.5 metres (8.2 ft) high, it was filled with 8 cubic metres (282.5 cu ft) of limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

 pieces being about 7.5 centimetres (3 in) in diameter. Small vent holes ringed the top and bottom of the pyramid. These holes could be closed or opened as required to control the flow of air. The structure was allowed to cool during the night, and then warm moist air was let in during the day. Dew formed on the limestone pieces and collected in a reservoir below ground level. The amount of water obtained varied from 1 litres (2.1 US pt) to 2.5 litres (5.3 US pt) per day depending on the atmospheric conditions.

Chaptal did not consider his experiment a success. When he retired in 1946, he put the condenser out of order, possibly because he did not want to leave an improper installation to mislead those who might later continue studies on air wells.

Klaphake's collectors

Wolf Klaphake
Wolf Klaphake
Wolf Klaphake was a German-born scientist. He emigrated to Australia in 1935, where he was interred as an enemy alien from 1940 to 1944...

 was a successful chemist working in Berlin during the 1920s and 30s. During that time, he tested several forms of air wells in Yugoslavia
Yugoslavia refers to three political entities that existed successively on the western part of the Balkans during most of the 20th century....

 and on Vis Island
Vis (island)
Vis is the most outerly lying larger Croatian island in the Adriatic Sea, and is part of the Central Dalmatian group of islands, with an area of 90.26 km² and a population of 3,617 . Of all the inhabited Croatian islands, it is the farthest from the coast...

 in the Adriatic Sea
Adriatic Sea
The Adriatic Sea is a body of water separating the Italian Peninsula from the Balkan peninsula, and the system of the Apennine Mountains from that of the Dinaric Alps and adjacent ranges...

. Klaphake's work was inspired by the works of Maimonides
Moses ben-Maimon, called Maimonides and also known as Mūsā ibn Maymūn in Arabic, or Rambam , was a preeminent medieval Jewish philosopher and one of the greatest Torah scholars and physicians of the Middle Ages...

, a known Jewish scholar who wrote in Arabic about 1,000 years ago and who mentioned the use of water condensers in Palestine.

Klaphake experimented with a very simple design: an area of mountain slope was cleared and smoothed with a watertight surface. It was shaded by a simple canopy supported by pillars or ridges. The sides of the structure were closed, but the top and bottom edges were left open. At nights the mountain slope would cool, and in the day moisture would collect on and run down the smoothed surface. Although the system apparently worked, it was expensive, and Klaphake finally adopted a more compact design based on a masonry structure. This design was a sugarloaf
A sugarloaf was the traditional form in which refined sugar was produced and sold until the late 19th century when granulated and cube sugars were introduced. A tall cone with a rounded top was the end product of a process that saw the dark molasses-rich raw sugar, which had been imported from...

-shaped building, about 15 metres (49.2 ft) high, with walls at least 2 metres (6.6 ft) thick, with holes on the top and at the bottom. The outer wall is made of concrete to give a high thermal capacity, and the inner surface was made of a porous material such as sandstone. According to Klaphake:
Traces of Klaphake's condensers have been tentatively identified.

In 1935, Wolf Klaphake and his wife Maria emigrated to Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

. The Klaphakes' decision to emigrate was probably primarily the result of Maria's encounters with Nazi authorities; their decision to settle in Australia (rather than, say, in Britain) was influenced by Wolf's desire to develop a dew condenser. As a dry continent, Australia was likely to need alternative sources of fresh water, and the Premier of South Australia
South Australia
South Australia is a state of Australia in the southern central part of the country. It covers some of the most arid parts of the continent; with a total land area of , it is the fourth largest of Australia's six states and two territories.South Australia shares borders with all of the mainland...

, whom he had met in London, had expressed an interest. Klaphake made a specific proposal for a condenser at the small town of Cook
Cook, South Australia
Cook is a railway station and crossing loop on the standard gauge Trans-Australian Railway from Adelaide to Perth, with no inhabited places around.-History:...

, where there was no supply of potable water. At Cook, the railway company had previously installed a large coal-powered active condenser, but it was prohibitively expensive to run, and it was cheaper to simply transport water. However, the Australian government turned down Klaphake's proposal, and he lost interest in the project.

Knapen's aerial well

Knapen, who had previously worked on systems for removing damp from buildings, was in turn inspired by Chaptal's work and he set about building an ambitiously large puits aerien (aerial well) on a 180 metres (590.6 ft) high hill at Trans-en-Provence
Trans-en-Provence is a commune in the Var department in the Provence-Alpes-Côte d'Azur region in southeastern France.-Air well:-References:*...

 in France. Beginning in 1930, Knapen's dew tower took 18 months to build; it still stands today, albeit in dilapidated condition. At the time of its construction, the condenser excited some public interest.

The tower is 14 metres (45.9 ft) high and has massive masonry walls about 3 metres (9.8 ft) thick with a number of apertures to let in air. Inside there is a massive column made of concrete. At night, the whole structure is allowed to cool, and during the day warm moist air enters the structure via the high apertures, cools, descends, and leaves the building by the lower apertures. Knapen’s intention was that water should condense on the cool inner column. In keeping with Chaptal’s finding that the condensing surface must be rough and the surface tension must be sufficiently low that the condensed water can drip, the central column's outer surface was studded with projecting plates of slate
Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering...

. The slates were placed nearly vertically to encourage dripping down to a collecting basin at the bottom of the structure. Unfortunately, the aerial well never achieved anything like its hoped-for performance and produced no more than a few litres of water each day.

International Organisation for Dew Utilization

By the end of the twentieth century, the details of how dew condenses were much better understood, the key insight being that low-mass collectors that rapidly lose heat by radiation
Thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....

 performed best. A number of researchers worked on this method. In the early 1960s, simple dew condensers made from sheets of polyethylene
Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...

 supported on a simple frame resembling a ridge tent were used in Israel to irrigate plants. Saplings supplied with dew and very slight rainfall from these collectors survived much beter than the control group planted without such aids – they all dried up over the summer. And in 1986 in New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...

 condensers made of a special foil produced sufficient water to supply young saplings.

In 1992 a party of French academics attended a condensed matter
Condensed Matter
Condensed matter may refer to several things*Condensed matter physics, the study of the physical properties of condensed phases of matter*European Physical Journal B: Condensed Matter and Complex Systems, a scientific journal published by EDP sciences...

 conference in Ukraine
Ukraine is a country in Eastern Europe. It has an area of 603,628 km², making it the second largest contiguous country on the European continent, after Russia...

 where physicist
A physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...

 Daniel Beysens introduced them to the story of how ancient Theodosia was supplied with water from dew condensers. They were sufficiently intrigued that in 1993 they went to see for themselves. They concluded that the mounds that Zibold identified as dew condensers were in fact ancient burial mounds
A tumulus is a mound of earth and stones raised over a grave or graves. Tumuli are also known as barrows, burial mounds, Hügelgrab or kurgans, and can be found throughout much of the world. A tumulus composed largely or entirely of stones is usually referred to as a cairn...

 (a part of the necropolis
A necropolis is a large cemetery or burial ground, usually including structural tombs. The word comes from the Greek νεκρόπολις - nekropolis, literally meaning "city of the dead"...

 of antic Theodosia) and that the pipes were medieval in origin and not associated with the mounds. They found the remains of Zibold's condenser, which they tidied up and examined closely. Zibold's condenser had apparently performed reasonably well, but in fact his exact results are not at all clear, and it is possible that the collector was intercepting fog, which added significantly to the yield. If Zibold's condenser worked at all, this was probably due to fact that a few stones near the surface of the mound were able to lose heat at night while being thermally isolated from the ground; however, it could never have produced the yield that Zibold envisaged.

Fired with enthusiasm, the party returned to France and set up the International Organisation for Dew Utilization (OPUR), with the specific objective of making dew available as an alternative source of water.

OPUR began a study of dew condensation under laboratory conditions; they developed a special hydrophilic film and experimented with trial installations, including a 30 square metres (322.9 sq ft) collector in Corsica
Corsica is an island in the Mediterranean Sea. It is located west of Italy, southeast of the French mainland, and north of the island of Sardinia....

. Vital insights included the idea that the mass
Thermal mass
Thermal mass is a concept in building design which describes how the mass of the building provides "inertia" against temperature fluctuations, sometimes known as the thermal flywheel effect...

 of the condensing surface should be as low as possible so that it cannot easily retain heat, that it should be protected from unwanted thermal radiation by a layer of insulation
Thermal insulation
Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...

, and that it should be hydrophilic, so as to shed condensed moisture readily.

By the time they were ready for their first practical installation, they heard that one of their members, Girja Sharan, had obtained a grant to construct a dew condenser in Kothara, India. In April 2001, Sharan had incidentally noticed substantial condensation on the roof of a cottage at Toran
Toran is the name in Hinduism of a sacred or honorific gateway in Buddhist architecture. Its typical form is a projecting cross-piece resting on two uprights or posts...

 Beach Resort in the arid coastal region of Kutch, where he was briefly staying. The following year, he investigated the phenomenon more closely and interviewed local people. Financed by the Gujarat Energy Development Agency and the World Bank
World Bank
The World Bank is an international financial institution that provides loans to developing countries for capital programmes.The World Bank's official goal is the reduction of poverty...

, Sharan and his team went on to develop passive, radiative condensers for use in the arid coastal region of Kutch. Active commercialisation began in 2006.

Sharan tested a wide range of materials and got good results from galvanised iron and aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 sheets, but found that sheets of the special plastic developed by the OPUR just 400 um thick generally worked even better than the metal sheets and were less expensive. The plastic film, known as OPUR foil, is hydrophilic and is made from polyethylene mixed with titanium dioxide
Titanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...

 and barium sulphate.


There are three principal approaches to the design of the heat sinks that collect the moisture in air wells: high mass, radiative and active. Early in the twentieth century, there was interest in high-mass air wells, but despite much experimentation including the construction of massive structures, this approach proved to be a failure.

From the late twentieth century onwards, there has been much investigation of low-mass, radiative
Thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....

 collectors; these have proved to be much more successful.


In a high-mass air well, the design attempts to cool a large mass of masonry with cool night-time air entering the structure due to breezes or natural convection. In the day, the warmth of the sun results in an increased atmospheric humidity, and when moisture-laden daytime air enters the air well, it condenses on the cool masonry. None of the high-mass collectors performed well, and Knapen's aerial well was a particularly conspicuous failure.

The problem with the high-mass collectors was that they could not get rid of sufficient heat during the night – despite design features intended to ensure that this would happen. While some thinkers have occasionally been persuaded that Zibold might have been on the right track after all, the reasoning of an article in Journal of Arid Environments makes it clear that high-mass condenser designs of this type are doomed to failure:
Although ancient air wells are mentioned in some sources, there is scant evidence for them, and persistent belief in their existence has the character of a modern myth
Urban legend
An urban legend, urban myth, urban tale, or contemporary legend, is a form of modern folklore consisting of stories that may or may not have been believed by their tellers to be true...



A radiative air well is designed to cool a substrate by radiating heat
Thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....

 to the night sky. The substrate has a low mass so that it cannot hold onto heat, and it is thermally isolated from any mass, including the ground. A typical radiative collector presents a condensing surface at an angle of 30° from the horizontal. The condensing surface is backed by a thick layer of insulating material such as polystyrene foam and supported 2–3 m (6.6–9.8 ft) above ground level. Such condensers may be conveniently installed on the ridge roofs of low buildings or supported by a simple frame. Although other heights do not typically work quite so well, it may be less expensive or more convenient to mount a collector near to ground level or on a two-storey building.

The 600 square metres (717.6 sq yd) radiative condenser illustrated near the start of this article is built near the ground. In the area of north-west India where it is installed dew occurs for 8 months a year, and the installation collects about 15 millimetre (0.590551181102362 in) of dew water over the season with nearly 100 dew-nights. In a year it provides a total of about 9000 litres (19,020.4 US pt) of potable water
Drinking water
Drinking water or potable water is water pure enough to be consumed or used with low risk of immediate or long term harm. In most developed countries, the water supplied to households, commerce and industry is all of drinking water standard, even though only a very small proportion is actually...

 for the school which owns and operates the site.

Although flat designs have the benefit of simplicity, other designs such as inverted pyramids and cones can be significantly more effective. This is probably because the designs shield the condensing surfaces from unwanted heat radiated by the lower atmosphere, and, being symmetrical, they are not sensitive to wind direction.

New materials may make even better collectors. One such material is inspired by the Namib Desert beetle
Namib Desert beetle
The Namib Desert beetle dwells in one of the most arid areas with only one and half inch of rain per year, and has developed a unique technique to survive by obtaining water from early morning fogs...

, which survives only on the moisture it extracts from the atmosphere. It has been found that its back is coated with microscopic projections: the peaks are hydrophilic and the troughs are hydrophobic. Researchers at the Massachusetts Institute of Technology
Massachusetts Institute of Technology
The Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological education and research.Founded in 1861 in...

 have emulated this capability by creating a textured surface that combines alternating hydrophobic and hydrophilic materials.


Active atmospheric water collectors have been in use since the commercialisation of mechanical refrigeration
Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...

. Essentially, all that is required is to cool a heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

 below the dew point, and water will be produced. Such water production may take place as a by-product
A by-product is a secondary product derived from a manufacturing process or chemical reaction. It is not the primary product or service being produced.A by-product can be useful and marketable or it can be considered waste....

, possibly unwanted, of dehumidification
A dehumidifier is typically a household appliance that reduces the level of humidity in the air, usually for health reasons. Humid air can cause mold and mildew to grow inside homes, which pose various health risks. Very humid climates or air make some people extremely uncomfortable, causing...

. The air conditioning system of the Burj Khalifa in Dubai
Dubai is a city and emirate in the United Arab Emirates . The emirate is located south of the Persian Gulf on the Arabian Peninsula and has the largest population with the second-largest land territory by area of all the emirates, after Abu Dhabi...

, for example, produces an estimated 15 million USgals (56,781.2 m³) of water each year that is used for irrigating the tower's landscape plantings.

Because mechanical refrigeration is energy intensive, active collectors are typically restricted to places where there is no supply of water that can be desalinated
Desalination, desalinization, or desalinisation refers to any of several processes that remove some amount of salt and other minerals from saline water...

 or purified at a lower cost and that are sufficiently far from a supply of fresh water to make transport uneconomical. Such circumstances are uncommon, and even then large installations such as that tried in the 1930s at Cook in South Australia failed because of the cost of running the installation – it was cheaper to transport water over large distances.

In the case of small installations, convenience may outweigh cost. There is a wide range of small machines designed to be used in offices that produce a few litres of drinking water from the atmosphere. However, there are circumstances where there really is no source of water other than the atmosphere. For example, in the 1930s, American designers added condenser systems to airships – in this case the air was that emitted by the exhaust of the engines, and so it contained additional water as a product of combustion. The moisture was collected and used as additional ballast to compensate for the loss of weight as fuel was consumed. By collecting ballast in this way, the airship's buoyancy could be kept relatively constant without having to release helium gas, which was both expensive and in limited supply.

More recently, on the International Space Station
International Space Station
The International Space Station is a habitable, artificial satellite in low Earth orbit. The ISS follows the Salyut, Almaz, Cosmos, Skylab, and Mir space stations, as the 11th space station launched, not including the Genesis I and II prototypes...

, the Zvezda module includes a humidity control system, the water it collects is usually used to supply the Elektron system that electrolyses water into hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 and oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

, but it can be used for drinking in an emergency.

There are a number of designs that minimise the energy requirements of active condensers:
  • One method is to use the ground as a heat sink
    Heat sink
    A heat sink is a term for a component or assembly that transfers heat generated within a solid material to a fluid medium, such as air or a liquid. Examples of heat sinks are the heat exchangers used in refrigeration and air conditioning systems and the radiator in a car...

     by drawing air through underground pipes. This is often done to provide a source of cool air for a building by means of a ground-coupled heat exchanger
    Ground-coupled heat exchanger
    A ground-coupled heat exchanger is an underground heat exchanger loop that can capture or dissipate heat to or from the ground. They use the Earth's near constant subterranean temperature to warm or cool air or other fluids for residential, agricultural or industrial uses...

     (also known as Earth tubes), wherein condensation is typically regarded as a significant problem. A major problem with such designs is that the underground tubes are subject to contamination and difficult to keep clean. Designs of this type require air to be drawn through the pipes by a fan, but the power required may be provided (or supplemented) by a wind turbine
    Wind turbine
    A wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...


  • Cold seawater is used in the Seawater Greenhouse
    Seawater Greenhouse
    The Seawater Greenhouse is a technology that enables the growth of crops in arid regions, using a greenhouse structure, seawater and solar energy...

     to both cool and humidify the interior of greenhouse
    A greenhouse is a building in which plants are grown. These structures range in size from small sheds to very large buildings...

    -like structure. The cooling can be so effective that not only do the plants inside benefit from reduced transpiration
    Transpiration is a process similar to evaporation. It is a part of the water cycle, and it is the loss of water vapor from parts of plants , especially in leaves but also in stems, flowers and roots. Leaf surfaces are dotted with openings which are collectively called stomata, and in most plants...

    , but dew collects on the outside of the structure and can easily be collected by gutters.

  • Another type of atmospheric water collector makes use of desiccant
    A desiccant is a hygroscopic substance that induces or sustains a state of dryness in its local vicinity in a moderately well-sealed container....

    s which adsorb atmospheric water at ambient temperature, this makes it possible to extract moisture even when the relative humidity is as low as 14 percent. Systems of this sort have proved to be very useful as emergency supplies of safe drinking water. For regeneration, the desiccant needs to be heated. In some designs regeneration energy is supplied by the sun; air is ventilated at night over a bed of desiccants that adsorb the water vapour. During the day, the premises are closed, the greenhouse effect increases the temperature, and, as in solar desalination
    Solar Desalination
    Solar desalination is a technique to desalinate water using solar energy. Solar desalination in the modern era extends back to the early 1950s when simple solar stills were studied for remote desert and coastal communities...

    pools, the water vapour is partially desorbed, condenses on a cold part and is collected.

  • A French company has recently designed a small wind turbine that uses an onboard mechanical refrigeration system to condense water and also produce 30kW of electricity.

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