Achille Knapen’s Air Well

Billboards and water bottles that produce drinking water out of thin air might look like breakthrough inventions, yet the principles behind them are very basic, and the technology possibly very old. Until very recently, it was believed —based on a discovery made in 1900 by Russian engineer Friedrich Zibold— that the ancient Greeks had successfully recovered water from atmospheric water vapor on a scale large enough to supply water to the city of Theodosia, now known as Feodosia, situated in the Crimean peninsula, in Ukraine.

Zibold was clearing forests in Crimea when he discovered thirteen large piles of stones. Each conical stone pile was 10 meters tall and covered over 900 square meters. He also found fragments of 3-inch diameter terracotta pipes around the piles leading to wells and fountains in the city. Zibold concluded that the stacks of stone were dew condensers that supplied the city with water. He calculated that each “air well” produced more than 55,400 liters of water each day.
To verify his hypothesis, Zibold constructed a stone-pile condenser using large sea stones on top of mount Tepe-Oba near Theodosia. His condenser was 6 meters tall with an 8-meter-diameter top. This was surrounded by a wall, one meter tall and 20 meters wide, creating a bowl-shaped collection area with drainage. By a fortuitous combination of circumstances, Zibold's condenser actually worked. By condensing dews within the pile, the condenser produced up to 360 liters of water every day. Zibold’s experiments with the condenser continued for three years until 1915, when the base developed leaks forcing him to terminate the experiment.

What Zibold identified as air wells near Theodosia were not condersers but actually ancient burial mounds, and the pipes he discovered were medieval in origin and not associated with the mounds. Recent studies have revealed that the yield of dew condensers decrease dramatically as the mass of the structures increase because they are not able to radiate away their heat quickly. Zibold's condenser worked relatively well because, by a good stroke of fortune, the shape of the stones allowed very minimal thermal contact creating thousands a small gaps through which air could pass. This allowed the stones to lose heat rapidly at night. It was also possible that the collector was intercepting fog, which added significantly to the yield.

A model of Zibold’s condenser.
Nevertheless, Zibold’s success with the condenser became an inspiration for many, who began experimenting with different dew-catching mechanisms. Among these was Belgian inventor Achille Knapen, who built a large “air well” on top of a 600-foot high hill in the commune of Trans-en-Provence, in southeastern France, between 1930 and 1931. It still stands, although in dilapidated condition.
Knapen’s air well or “Puit Aerien” is 14 meters high and is made of massive masonry walls about 3 meters thick to keep the inside temperature cool. The walls are punctured by a number of holes that let in warm, moisture-laden air during the day. At night, when the temperature drops, the water vapor in the air condenses against a huge concrete column built inside the structure, and drips down to a collecting basin at the bottom of the structure. Although Knapen’s air well excited some public interest when it was being built, it had disappointingly low yield, generating no more than few liters of water each day, as opposed to 30,000 and 40,000 liters of water that Knapen had hoped. Eventually, the project was abandoned.
It was only in the later part of the 20th century that the mechanics of how dew condenses were better understood. It’s now known that condensers that have low mass perform best because they cool more rapidly by heat radiation, which is why Knapen’s massive air well failed.
In the last few years, several independent developers and organizations in various countries have developed indigenous ways to collect drinking water from dew. In the village of Chungungo, in Chile, where annual precipitation is less than 6 centimeters, fog collectors have been producing 15,000 liters of water a year for past several years. Another fog harvesting project in Lima, Peru, captures fog in huge nets producing more than 2,200 liters of water a day. Many villages in India have been using dew condensers constructed on roofs of houses for years. In coming years, dew and fog harvesting will became even more important as the global climate changes and long spells of drought become more common.