Sustainable Civilization

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Water Technology

Water Collection from "Dry" Air. Before modern dehumidifiers, there were methods shown usefull in precipitating water from the air.

Dew ponds appear to predate history. They are large but shallow artificial pools, smooth rock to protect the water tight layer, with the entire pond insulated from the ground below and around.


A pond described in Popular Science (September 1922 is a concrete cistern about 5 feet deep, with sloping concrete roof, above which is a protective fence of corrugated iron said to aid in collecting and condensing vapor on the roof and prevent 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 roof at a lower temperature 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.


In 1932, Achille Knapen built an "air well" in France . The structure was described in Popular Mechanics Magazine to be about 45 feet tall with walls 8 to 10 feet thick. The claim is the aerial well will yield 7,500 gallons of water per 900 square feet of condensation surface.

At night, cold air pours down the central pipe and circulates through the core... By morning the whole inner mass is so thoroughly chilled that it will maintain its reduced temperature for a good part of the day. The well is now ready to function. Warm, moist outdoor air enters the central chamber, as the daytime temperature rises, through the upper ducts in the outer wall. It immediately strikes the chilled core, which is studded with rows of slates to increase the cooling surface. The air, chilled by the contact, gives up its moisture upon the slates. As it cools, it gets heavier and descends, finally leaving the chamber by way of the lower ducts. Meanwhile the moisture trickles from the slates and falls into a collecting basin at the bottom of the
well.

The French inventor L. Chaptal built a small air well near Montpellier in 1929. The pyramidal concrete structure was 3 meters square and 2.5 meter in height (10 x 10 x 8 ft), with rings of small vent holes at the top and bottom. Its 8 cubic meters of volume was filled with pieces of limestone (5-10 cm) that condensed the atmospheric vapor and collected it in a reservoir. The yield ranged from 1-2.5 liters/day from March to September. The total weight of water was 190 lb; the maximum yield was 5.5 lb/day. Chaptal found that the condensing surface must be rough, and the surface tension sufficiently low that the condensed water can drip. The incoming air must be moist and damp. The low interior temperature is established by
reradiation at night and by the lower temperature of the soil. Air flow was controlled by plugging or opening the vent holes as necessary.

Calice Courneya patented an air well in 1982 (USP #4,351,651): A heat exchanger at or near subsurface temperature. .. is in air communication with the atmosphere for allowing atmospheric moisture-laden air to enter, pass through, cool, arrive at its dew point, allow the moisture to precipitate out, and allow the air to pass outward to the atmosphere again. Suitable apparatus may be provided to restrict air flow and allow sufficient residence time of the air in the heat exchanger to allow sufficient precipitation. Furthermore, filtration may be provided on the air input, and a means for creating a [negative] movement pressure, in the preferred form of a turbine, may be provided on the output... The air well is buried about 9 feet deep. The entrance pipe is 3-inch diameter PVC pipe (10 ft
long), terminating just near the ground... This is an advantage because the greatest humidity in the atmosphere is near the surface." (7, 8) (Figure 4)


Air flows through the pipes at 2,000 cubic feet per hour at 45oF with a 5 mph wind. This translates to about 48,000 ft3/day (over 3,000 lb of air daily). Courneya’s first air well used a turbine fan to pull air through the pipes. Later designs employed an electric fan for greater airflow. At 90oF and 80% Relative Humidity (RH), the air well yields about 60 lb water daily. At 20% RH, the yield is only about 3 lb/day. The yield is even lower at lower temperatures. The yield depends on the amount of air and its relative and specific humidity, and the soil temperature, thermal conductivity, and moisture.

Acoustic resonance within the pipes might enhance condensation. The more recent invention of acoustic refrigeration could be used to advantage, as well as the Hilsch-Ranque vortex tube.

It is necessary to cool the air to the "Dewpoint". All of the preceding devices appear to rely on night cooled mass to provide the needed temperature difference, yet leave the device open to daytime heating by the sun. I find indications that even in the daytime in certain conditions it might be possible to radiate to the sky 100 to 200 BTU per hour, which strictly in math could represent 1 pint or so of operation for every 10 square foot or radiation area.

Once the water has condensed the "dry" air, now cool, needs to be exhausted. This points out the flaw in all of the above. None of the above low-tech devices provide for heat exchange directly between the incoming and outgoing air[i], therefore
the "coolness", essential to precipitation, imparted to the incoming air is directly exhausted, and rapidly eroded.

Ideally, there should be sufficient heat exchange between intake and exhaust air that at the pipe open ends, they are virtually at the same temperature, despite being cycled thru a chilled spot. The transition between liquid and vapor water is, absent unknown science or magic, a matter of the transfer of 970 BTU per each pint condensed. (7760 BTU per gallon)

Using a sky radiation approach[ii] to cooling your condenser core, if the latent heat of vaporization of water is 2.26 × 106 J/kg a 1 m2 radiator can provide 50 W/m2 of cooling, enough to condense 1 g of water in 45 s; 1 kg in 12.6 hr; or 1.9 kg per day. Reportedly production rates in the Southwest U.S. can average about 2 liters per day in the winter to over 6 liters per day during the summer, per square meter.” At the low end 10 m2 (1/4 acre) of radiators cooling humid air could produce 19 L of water per day. The humid air must of course be moved thru the cooling unit, and the “coolness” used to change air temperature recovered during expulsion of the “dried” air.

A commercial, powered water condenser is sold under the name Aqua-Cycle, invented by William Madison, introduced in 1992. It resembles a drinking fountain and functions as such, but it is not connected to any plumbing. It contains a refridgerated dehumidifier and a triple-purification system (carbon, deionization, and UV light) that produces water as pure as triple-distilled. Under optimal operating conditions (80o/60% humidity) the unit claims to produce up to 5 gallons daily.

ATMOSPHERIC CONDENSER DISCUSSION

At any given pressure and temperature, in a fixed volume only a limited amount of water will evaporate into vapor, the limit is referred to as the dew/saturation point. A cubic meter of normal sea level air has a mass of 1.292 kilo. A cubic meter of water vapor at sea level pressure has a mass of .804 kilo, this would occur at a temperature of 100C (212F).

At 0 C (32 F), saturation point is about 1.1 gm of water per cubic meter. A rule of thumb is that raising the air temperature 18°F (10°C) doubles its moisture capacity. This means that air at 86°F (30°C) can hold eight times as much water as air at 32°F.

Degrees C Degrees F Gram H2O
0 32 1.1
10 50 2.2
20 68 4.4
30 86 8.8
40 104 17.6
50 122 35.2
60 140 70.4
70 158 140.8
80 176 281.6
90 194 563.2
100 212 1126.4

Relative humidity is the percentage of water vapor present as compared to how much there could be at the present temperature.

Take a typical Tucson fall day of 84 F and relative humidity of 7%. There is roughly 7% of 8.8 grams of water in each cubic meter of air (.616 gram). Lower the temperature to 66 F, and relative humidity doubles to 14%. Lower the temperature to 48 F, and relative humidity again doubles, now to 28%.
If we cool air without changing its moisture content, eventually we'll reach a temperature at which the air can no longer hold the moisture it contains. Then water will have to condense out of the air, forming dew or fog. The dewpoint is this critical temperature at which condensation occurs.

But, water does not immediately change state as the temperature reaches the "right" point. The "Latent heat of condensation" (Lc) refers to the heat that must be removed from water vapor for it to change into a liquid. Lc=2500 Joules per gram (J/g) of water or about 600 calories per gram (cal/g) of water.
Specific heat is defined as the amount of heat energy required to raise 1 g of a substance by 1° Celsius. If the specific heat of air is .25 calories per gram of air per degree C change, then each degree C change in a cubic meter represents 323 calories. The specific heat of water is 1 calorie per gram per degree C. In our Tucson fall day above there was .616 grams of water in a cubic meter of air. Air and water vapor together take a change of about 324 calories per degree C. We need to lower the temperature by around 40 C, or get rid of 12,960 calories of heat to reach the dew point. An additional 379 calories of heat needs to be removed to compensate for the latent heat of condensation, for a total of 13,339 calories.

Assume a daily water need of 174 gallons (658.6 liters) - 658,660 grams of water. In a Tucson fall day, each of us would need to "wring" all of the water out of more than a million cubic meters of air (1,069,252) - a cube 100 meters on a side.
The heat to be moved is about 14 billion calories. The water portion of this number is about 450 million calories. Depending on device efficiency, SOME part of the other 1 billion calories should be able to be conserved in a heat exchanger.
Increasing the pressure also changes the dew point. Double the pressure and relative humidity doubles. Assume normal atmospheric pressure of 14 PSI. Pump the fall Tucson air into a tank at 28 PSI and the relative humidity inside is now 14%. Make it 56 PSI - 28%. 102 PSI - 56%. 204 PSI - 102%, and you've got water accumulating in the bottom of the tank.

[i] See chapter 7 of the book "Passive Annual Heat Storage", by John Hait

[ii] Radiative Cooling in Hot Humid Climates
Aubrey Jaffer February 2006


Ronald Frederick Greek
Moderator (Electronic Janitor)
Sustainable Tucson

"Stabilization of human numbers is no solution... To speak of an actual reduction of human population - exactly what is needed if the world is to avoid unprecedented human dieoff through famine, pestilence, and war - is unthinkable and unspeakable, at least in polite company. Not just Catholics and conservatives, but liberals as awll become positively apoplectic if the subject is broached. And so the discussion necessary to understanding our econlogical dilemma, and dealing effectively with it, never occurs."
- Richard Heinberg, Power Down