An air well or aerial well is a structure or device that collects water by promoting the condensation of moisture 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, designated as high mass, radiative, and active: High-mass air wells: used in the early 20th century, but the approach failed. Low-mass, radiative collectors: Developed in the late 20th century onwards, proved to be much more successful. Active collectors: these collect water in the same way as a dehumidifier; although the designs work well, they require an energy source, making them uneconomical except in special circumstances. New designs seek to minimise the energy requirements of active condensers or make use of sustainable and renewable energy resources.

Background

All air well designs incorporate a substrate with a temperature sufficiently low that dew forms. Dew is a form of precipitation that occurs naturally when atmospheric water vapour condenses onto a substrate. It is distinct from fog, in that fog is made of droplets of water that condense around particles in the air. Condensation releases latent heat which must be dissipated in order for water collection to continue.

An air well requires moisture from the air. Everywhere on Earth, even in deserts, the surrounding atmosphere 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 (clouds): 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, this depends on temperature, warmer air contains more water vapour than cooler air. When air is cooled to the dew point, it becomes saturated, and moisture will condense on a suitable surface. For instance, the dew point temperature of air at and 80 percent relative humidity is . The dew point temperature falls to if the relative humidity is 50 percent.

A related, but quite distinct, technique of obtaining atmospheric moisture is the fog fence.

An air well should not be confused with a dew pond. A dew pond is an artificial pond 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.Oxford English Dictionary: "dew-pond" In fact, dew ponds are primarily filled by rainwater.

A stone mulch can significantly increase crop yields in arid areas. This is most notably the case in the Canary Islands: on the island of Lanzarote there is about of rain each year and 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.

History

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, and the Belgian inventor .

Zibold's collector

In 1900, near the site of the ancient Byzantine city of Theodosia, thirteen large piles of stones were discovered by Zibold, who was a forester and engineer in charge of the area. Each stone pile covered just over , and was about tall. The finds were associated with the remains of 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 he calculated that each air well produced more than each day.

To verify his hypothesis, Zibold constructed a stone-pile condenser at an altitude of on mount Tepe-Oba near the ancient site of Theodosia. Zibold's condenser was surrounded by a wall high, wide, around a bowl-shaped collection area with drainage. He used sea stones in diameter piled high in a truncated cone that was 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 – 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 in 1929. Chaptal's condenser was a pyramidal concrete structure square and high, it was filled with of limestone pieces being about 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 to 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 was a successful chemist working in Berlin during the 1920s and 1930s. During that time, he tested several forms of air wells in Yugoslavia and on Vis Island in the Adriatic Sea. Klaphake's work was inspired by Zibold and by the works of Maimonides, 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 night 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-shaped building, about high, with walls at least thick, with holes on the top and at the bottom. The outer wall was 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. 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, whom he had met in London, had expressed an interest. Klaphake made a specific proposal for a condenser at the small town of Cook, 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 moisture from buildings,Prevention of Damp in Buildings. The Manchester Guardian, 27 February 1930 p. 6 column F. was in turn inspired by Chaptal's work and he set about building an ambitiously large puits aerien (aerial well) on a high hill at Trans-en-Provence 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."Air Well Waters Parched Farms" Popular Science, March 1933

The tower is high and has massive masonry walls about 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. 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 mechanics of how dew condenses were much better understood. The key insight was that low-mass collectors which rapidly lose heat by radiation perform best. A number of researchers worked on this method. In the early 1960s, dew condensers made from sheets of polyethylene 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 better than the control group planted without such aids – they all dried up over the summer. In 1986 in New Mexico condensers made of a special foil produced sufficient water to supply young saplings.

In 1992 a party of French academics attended a condensed matter conference in Ukraine where physicist 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 part of the necropolis of ancient Theodosia) and that the pipes were medieval in origin and not associated with the construction of 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.