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NEWSCIENCENEWSCIENCENEWSCIENCE roof at a lower tem- perature than the atmosphere, thus assuring continuous condensation. "At one side of the reservoir is a concrete basin set in the ground. By means of a ball valve, this basin is automatically kept full of water drawn from the reservoir." Reservoir Figure 2: Russell's Dew Pond . 9 Air Wells roof at a lower tem- central pipe with its upper opening above Hedge & Fence perature than the _ the top of the outer dome. “ig. + 4's] atmosphere, thus "At night, cold air pours down the central i ‘|| assuring continuous _ pipe and circulates through the core... By condensation. morning the whole inner mass is so thor- "At one side of — oughly chilled that it will maintain its jit | the reservoir isa reduced temperature for a good part of the ______— ~~~ | concrete basin set day. The well is now ready to function. Roof» in the ground. By "Warm, moist, outdoor air enters the cen- . means of a ball tral chamber, as the daytime temperature P . valve, this basin is rises, through the upper ducts in the outer Reservoir automatically kept wall. It immediately strikes the chilled full of water drawn core, which is studded with rows of slates = ————| from the to increase the cooling surface. The air, reservoir." chilled by the contact, gives up its moisture ond upon the slates. As it cools, it gets heavier Air Wells and descends, finally leaving the chamber «In 1930, the Belgian inventor Achille by way of the lower ducts. Meanwhile, the Knapen built an "air well" atop a 600-foot- moisture trickles from the slates and falls high hill at Trans-en-Provence in France. into a collecting basin at the bottom of the Its construction took him 18 months to _ well."** complete. The unique structure was Unfortunately, however, the structure did described in Popular Mechanics magazine: not perform as hoped; at best, it collected "The tower...is about 45 feet tall. The — only about five gallons per night. walls are from 8 to 10 feet thick to prevent Knapen was inspired by the work of bio- the heat radiation from the ground from _ climatologist Leon Chaptal, who built a influencing the inside temperature. Itis small air well near Montpellier in 1929. estimated that the aerial well will yield The pyramidal concrete structure was 3 7,500 gallons of water per 900 square feet metres square by 2.5 metres in height (10 x of condensation surface."* 10 x 8 feet), with rings of small vent holes An article in Popular Science magazine _ at the top and bottom. Its 8 cubic metres of (March 1933) also featured Knapen's air volume was filled with pieces of limestone well and included these details of its con- (5-10 cm) that condensed the atmospheric struction: vapour and collected it in a reservoir. The "[The air well has] a mushroom-like — yield ranged from 1-2.5 litres/day from inner core of concrete, pierced with numer- March to September. In 1930, the structure ous ducts for the circulation of air; anda collected about 100 litres from April to Fi Sa e Knapen Air Well (Photo: International Organization For Dew Utilization) Reservoir Another form of dew pond was invented by S. B. Russell in the 1920s. It was described in Popular Science (September 1922): "A dew reservoir 30 feet square will col- lect 24,000 gallons of water in a year, or an average of 120 gallons daily during the hot summer months and 50 gallons daily for the remainder of the year... "The Russell reservoir consists of a con- crete cistern about 5 feet deep, with sloping concrete roof, above which is a protective fence of corrugated iron which aids in col- lecting and condensing vapor on the roof and prevents evaporation by the wind. The floor of the cistern is flush with the ground, while sloping banks of earth around the sides lead up to the roof. "Moisture draining into the reservoir from the low side of the roof maintains the Cold air enters through pipe at night Concrete core is chilled during night Day ‘ air enters Air gives up ona its moisture. F here = Outlet for f Circular air after ——| iL i ¥ i reservoir condensation _aaaaanaal i : _— ey ©" condensed water ; ce! : Figure 3: Knapen's Air Well 44 = NEXUS www.nexusmagazine.com DECEMBER 2002 — JANUARY 2003