Air well (condenser)
An air well or aerial well is a structure or device that collects water by promoting the condensation of moisture from air.[1] 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.[2]
- Low-mass, radiative collectors: Developed in the late 20th century onwards, proved to be much more successful.[2]
- 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, innovative designs seek to minimise the energy requirements of active condensers or make use of sustainable and renewable energy resources.[3]
Background
All air well design incorporate a substrate with a temperature sufficiently low that
An air well requires moisture from the air. Everywhere on Earth, even in deserts, the surrounding
A related, but quite distinct, technique of obtaining atmospheric moisture is the
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.[7] In fact, dew ponds are primarily filled by rainwater.[8]
A stone
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[10]), the French bioclimatologist Leon Chaptal, the German-Australian researcher Wolf Klaphake, and the Belgian inventor Achille Knapen .
Zibold's collector
In 1900, near the site of the ancient
To verify his hypothesis, Zibold constructed a stone-pile condenser at an altitude of 288 metres (945 ft) on mount Tepe-Oba near the ancient site of Theodosia. Zibold's condenser was surrounded by a wall 1 metre (3 ft 3 in) high, 20 metres (66 ft) wide, around a bowl-shaped collection area with drainage. He used sea stones 10–40 centimetres (3.9–15.7 in) in diameter piled 6 metres (20 ft) high in a truncated cone that was 8 metres (26 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.[3]
Zibold's condenser began to operate in 1912 with a maximum daily production that was later estimated to have been 360 litres (79 imp gal; 95 US gal) – Zibold made no public record of his results at the time.[10] 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.)[3] Zibold's condenser was approximately the same size as the ancient stone piles that had been found,[3] 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 3 metres (9.8 ft) square and 2.5 metres (8 ft 2 in) high, it was filled with 8 cubic metres (280 cu ft) of limestone pieces being about 7.5 centimetres (3.0 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 litre (0.22 imp gal; 0.26 US gal) to 2.5 litres (0.55 imp gal; 0.66 US gal) per day depending on the atmospheric conditions.[15]
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.[2]
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[16] 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.[3]
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 15 metres (49 ft) high, with walls at least 2 metres (6 ft 7 in) 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.[17] According to Klaphake:
The building produces water during the day and cools itself during the night; when the sun rises, the warm air is drawn through the upper holes into the building by the out-flowing cooler air, becomes cooled on the cold surface, deposits its water, which then oozes down and is collected somewhere underneath. It is wrong to think that this process works only on days with dew, as the inner surface becomes much cooler than one should expect. In Dalmatia, that day was a rare exception which failed to produce water.[16]
Traces of Klaphake's condensers have been tentatively identified.[18]
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;[19][20] their decision to settle in Australia (rather than, say, in Britain) was influenced by Wolf's desire to develop a dew condenser.[20] 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,[21] 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.[22][16]
Knapen's aerial well
Knapen, who had previously worked on systems for removing moisture from buildings,[23][24][25] 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 ft) high hill at Trans-en-Provence in France.[1][26] 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.[27]
The tower is 14 metres (46 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.[28] 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.[3] Unfortunately, the aerial well never achieved anything like its hoped-for performance and produced no more than a few litres of water each day.[29]
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.[30] 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.[31] In 1986 in New Mexico condensers made of a special foil produced sufficient water to supply young saplings.[4]
In 1992 a party of French academics attended a
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.[33]
OPUR began a study of dew condensation under laboratory conditions; they developed a special
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
Sharan tested a wide range of materials and got good results from
Types
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.[39]
From the late twentieth century onwards, there has been much investigation of low-mass, radiative collectors; these have proved to be much more successful.[40]
High-mass
The high-mass air well design attempts to cool a large mass of masonry with cool nighttime air entering the structure due to breezes or natural convection. In the day, the warmth of the sun results in increased atmospheric humidity. When moist daytime air enters the air well, it condenses on the presumably cool masonry. None of the high-mass collectors performed well, Knapen's aerial well being a particularly conspicuous example.
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.[3] While some thinkers have believed that Zibold might have been correct after all,[41][42] an article in Journal of Arid Environments discusses why high-mass condenser designs of this type cannot yield useful amounts of water:
We would like to stress the following point. To obtain condensation, the condenser temperature of the stones must be lower than the dew point temperature. When there is no fog, the dew point temperature is always lower than the air temperature. Meteorological data shows that the dew point temperature (an indicator of the water content of the air) does not change appreciably when the weather is stable. Thus wind, which ultimately imposes air temperature to the condenser, cannot cool the condenser to ensure its functioning. Another cooling phenomenon — radiative cooling — must operate. It is therefore at night-time, when the condenser cools by radiation, that liquid water can be extracted from air. It is very rare that the dew point temperature would increase significantly so as to exceed the stone temperature inside the stone heap. Occasionally, when this does happen, dew can be abundant during a short period of time. This is why subsequent attempts by L. Chaptal and A. Knapen to build massive dew condensers only rarely resulted in significant yields. [Emphasis as in original][2]
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.[2]
Radiative
A radiative air well is designed to cool a substrate by
A 550 square metres (5,900 sq ft) radiative condenser illustrated to the left is built near the ground. In the area of northwest India where it is installed dew occurs for 8 months a year, and the installation collects about 15 millimetres (0.59 in) of dew water over the season with nearly 100 dew-nights. In a year it provides a total of about 9,000 litres (2,000 imp gal; 2,400 US gal) of potable water for the school which owns and operates the site.[46]
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.[47]
New materials may make even better collectors.
Active
Active atmospheric water collectors have been in use since the commercialisation of mechanical refrigeration. Essentially, all that is required is to cool a heat exchanger below the dew point, and water will be produced. Such water production may take place as a by-product, possibly unwanted, of dehumidification.[3] The air conditioning system of the Burj Khalifa in Dubai, for example, produces an estimated 15 million US gallons (57,000 m3) of water each year that is used for irrigating the tower's landscape plantings.[54]
Because mechanical refrigeration is energy intensive, active collectors are typically restricted to places where there is no supply of water that can be desalinated 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, South Australia failed because of the cost of running the installation – it was cheaper to transport water over large distances.[22]
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
More recently, on the International Space Station, 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 and oxygen, but it can be used for drinking in an emergency.[56]
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 by drawing air through underground pipes.[57] This is often done to provide a source of cool air for a building by means of a ground-coupled heat exchanger (also known as Earth tubes), wherein condensation is typically regarded as a significant problem.[58] 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.[59]
- Cold seawater is used in the Seawater Greenhouse to both cool and humidify the interior of greenhouse-like structure. The cooling can be so effective that not only do the plants inside benefit from reduced transpiration, but dew collects on the outside of the structure and can easily be collected by gutters.[4]
- Another type of atmospheric water collector makes use of desiccants 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.[60] Systems of this sort have proved to be very useful as emergency supplies of safe drinking water.[61][62] For regeneration, the desiccant needs to be heated.[63] 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 pools, the water vapour is partially desorbed, condenses on a cold part and is collected.[4] Nanotechnology is improving these types of collectors, as well. One such adsorption-based device collected 0.25 L of water per kg of a metal-organic framework in an exceptionally arid climate with sub-zero dew points (Tempe, Arizona, USA).[64]
- A French company has recently designed a small wind turbine that uses a 30 kW electric generator to power an onboard mechanical refrigeration system to condense water.[65]
See also
- Atmospheric water generator
- Condensation trap (Solar still)
- Dew pond
- Fog collection
- Fog drip
- Groasis Waterboxx
- fluorescent cooling, which can cool to below the ambient air temperature
- Rainwater harvesting
- Solar chimney
- Water potential
- Watermaker
References
Notes
- ^ a b Popular Science 1933.
- ^ a b c d e f Beysens et al. 2006.
- ^ a b c d e f g h Nelson 2003.
- ^ a b c d e f Beysens & Milimouk 2000.
- ^ Nikolayev et al. 1996, pp. 23–26.
- ^ "What Exactly Is The Dew Point?". Weather Savvy. Archived from the original on 1 December 2010. Retrieved 10 September 2010.
- ^ Oxford English Dictionary: "dew-pond"
- ^ Pugsley 1939.
- .
- ^ a b c Nikolayev et al. 1996, p. 4.
- ^ Based on diagram by Nikolayev et al., 1996
- ^ Nikolayev et al. 1996, pp. 20–23.
- ^ Nikolayev et al. 1996, p. 2.
- ^ Beysens et al. 2006, p. 4.
- ^ Hills 1966, p. 232.
- ^ a b c Klaphake 1936.
- ^ Sharan 2006, p. 72.
- ^ "In Croatia" (PDF). OPUR Newsletter. OPUR. April 2003. Archived (PDF) from the original on 11 September 2010. Retrieved 10 September 2010.
- ^ Neumann 2002, p. 7.
- ^ a b Klaus Neumann. "Wolf Klaphake – Immigrant or refugee". Uncommon Lives (National Archives of Australia). Archived from the original on 18 February 2011. Retrieved 10 September 2010.
- ^ Klaus Neumann. "Trans-Australian Railway photograph of a condenser cooler at Cook, 10 December 1917". Uncommon Lives (National Archives of Australia). Archived from the original on 18 February 2011. Retrieved 10 September 2010.
- ^ a b Klaus Neumann. "Wolf Klaphake – A rainmaker?". Uncommon Lives (National Archives of Australia). Archived from the original on 18 February 2011. Retrieved 10 September 2010.
- ^ "British Knapen – The Early Years" (PDF). ProTen Services. Archived from the original (PDF) on 9 May 2009. Retrieved 10 September 2010.
- The Manchester Guardian, 27 February 1930 p. 6 column F.
- ^ "ProTen Services Celebrates 80 Years of Service" (PDF). ProTen Services. Archived from the original (PDF) on 24 May 2010. Retrieved 10 September 2010.
- ^ "Well Like Gigantic Ant Hill Gathers Water from Air". Popular Mechanics. 58 (6): 868. December 1932. Retrieved 10 September 2010.
- ^ "Air Well Waters Parched Farms" Popular Science, March 1933
- ^ Achile Knappen. "Improved means for collecting moisture from the atmosphere". European Patent Office. Retrieved 10 September 2010.
- ^ Sharan 2006, p. 70.
- ^ Sharan 2006, p. 22.
- ^ Gindel 1965.
- ^ Nikolayev et al. 1996.
- ^ "OPUR Ou la Conquete de la Rosee – OPUR or The Conquest of Dew" (in French and English). OPUR. Archived from the original on 7 September 2010. Retrieved 10 September 2010.
- ^ Muselli, Beysens & Milimouk 2006.
- ^ Sharan 2006, pp. 20–28.
- ^ Sharan 2006, Acknowledgement section.
- ^ Mukund, Dixit; Sharan, Girha (1 April 2007). "Leveraged Innovation Management: Key Themes from the Journey of Dewrain Harvest Systems" (PDF). Indian Institute of Management Ahmedabad, India. Archived from the original (PDF) on 14 June 2011. Retrieved 10 September 2010.
- ^ Sharan 2006, p. 27.
- ^ Alton Stewart & Howell 2003, p. 1014.
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- ^ Sharan 2006, pp. 20–39.
- ^ Sharan 2006, pp. 40–59.
- ^ a b Sharan 2007.
- ^ Clus et al. 2006.
- ^ Sharan 2006, p. 20.
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- ^ "Yeti Air-Conditioning-12". Everest. Retrieved 15 March 2011.
- ^ "Burj Khalifa: Towering challenge for builders". Gulf News. 4 January 2010. Archived from the original on 25 January 2011. Retrieved 12 January 2011.
- ^ Allen 1931, p. 37.
- ^ "Zvezda". The ISS: Continued Assembly and Performance. NASA. Archived from the original on 25 August 2010. Retrieved 10 September 2010.
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- David Darling. "Earth Cooling Tube". The Encyclopedia of Alternative Energy and Sustainable Living. Retrieved 10 September 2010.
- ^ US patent 4351651, Courneya, Calice, G., "Apparatus for extracting potable water", issued 1980-12-06
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- Clus, Owen; Ouazzani, Jalil; Muselli, Marc; Nikolayev, Vadim; Sharan, Girja; Beysens, Daniel (2006). "Radiation-cooled Dew Water Condensers Studied by Computational Fluid Dynamic (CFD)". arXiv:0707.2514 [physics.flu-dyn].
- Gindel, I. (11 September 1965). "Irrigation of Plants with Atmospheric Water Within the Desert". Nature. 207 (5002): 1173–1175. S2CID 4207774.
- Hills, Edwin Sherbon (1966). Arid Lands: A Geographical Appraisal. Methuen.
- Klaphake, Wolf (1936). "Practical Methods for Condensation of Water from the Atmosphere". Proceedings of the Society of Chemical Industry of Victoria. 36: 1093–1103.
- Muselli, M.; Beysens, D.; Milimouk, I. (January 2006). "Comparative Dew Yields From Two Large Planar Dew Condensers". Journal of Arid Environments. 64 (1): 54–76. .
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This article has been widely reproduced, including extracts in Sharan, 2006.
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- Pugsley, Alfred J (1939). Dewponds in Fable and Fact. Country Life Ltd.
- Sharan, Girja (2006). Dew Harvest. Foundation Books. ISBN 978-81-7596-326-9.
- Sharan, Girja (2007). "Harvesting dew to supplement drinking water supply in arid coastal villages of Gujarat" (PDF). Indian Institute of Management. Archived from the original (PDF) on 14 June 2011. Retrieved 10 September 2010.
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External links
- Rajvanshi, Anil (March 1981). "Large scale dew collection as a source of fresh water supply". Desalination. 36 (3): 299–306. .
- Jaffer, Aubrey (2010). "Optics for Passive Radiative Cooling (Compound Parabolic Concentrator (CPC))".