Air wells and dew ponds. Answer to my water problem?

[Brent Geery]

OK, it looks like most methods of collecting rain water will be very
expensive in an arid location, not to mention the need to store lots
of water. So, even a $15,000 deep-well, and treatment of the brackish
water, will probably be more practical.

Gigawatt mentioned looking into air wells and dew ponds as a possible
alternative. I didn't find many useful links, but here they are
anyway.

http://www.rexresearch.com/airwells/airwells.htm
http://www.bagelhole.org/article.php/Water/350/
http://www.opur.u-bordeaux.fr/index.htm

The theory for both is to cool down the collector surface below
dew-point of the air, and condense the moisture out. Most of the
devices use the chilling effect cold space during a clear night to do
the cooling. Dew ponds insulate themselves from the earth via
insulation, and most air wells depend on mass to hold the cool.
However, at least Courneya's air well uses the earth itself as the
heat sink, instead of nighttime radiation into space.

Courneya's design seems the simplest and cheapest to construct, but
I'm confused if the device will work at all, and how much water can be
expected. My main source of concern is that the ground, even many
meters down, can still be as hot as 75 degrees F. I don't think Hot
(100-120 degree F) low humidity air (10-20%) will give up any moisture
at such a high ground temp.

Dew ponds also look relatively simple to construct, and using
radiation into space, they can get much colder than Courneya's air
well design that depends on earth heat-sink. But, I still have a
question on how cold the dew pond cold get, how long it would stay
cool, and how much dew would be collected for a particular size pond.


Anyone have a clue about the possibilities of air wells and dew ponds
under these conditions. I'm way out of my league.

 

[William P.N. Smith]

Courneya's design seems the simplest and cheapest to construct, but
I'm confused if the device will work at all, and how much water can be
expected. My main source of concern is that the ground, even many
meters down, can still be as hot as 75 degrees F. I don't think Hot
(100-120 degree F) low humidity air (10-20%) will give up any moisture
at such a high ground temp.

I don't have it on the tip of my fingers, but I bet Google can find a
dew point calculator for you that'll take temperature and RH and give
you the dewpoint. If you can't get below the dewpoint, you aren't
going to condense any water...

 

[Ecnerwal]

Anyone have a clue about the possibilities of air wells and dew ponds
under these conditions. I'm way out of my league.

Are you at your site, or near enough to your site that experiments where
you are would be valid at your site (or you could set them up at your
actual site and monitor them)?

Make a "dew puddle" and see what you get - such as I understand the
concept, your testbed could be a hunk of black pondliner or plastic on
some foamboard insulation (with and/or without additional things sitting
on top of it - ping pong balls or small stones - experiment, see if such
things help), sloped to collect/drain in one corner. A typical hunk of
foam is 2x8 feet or 4x8 feet - so you have an area, and a volume of
water collected, and that will give a much better idea of what water a
larger area will collect than all the hand-waving websites in the
world...try a couple of different orientations, perhaps.

One question that comes to mind is how much dew you might be able to get
off of your roof, if you built your roof to maximize dew formation and
runoff.

Not much point in me experimenting with this here in the wet NorthEast.

 

[[email protected]]

I don't have it on the tip of my fingers...

Td (R) = T/(1+Tln(RH)/9621), approx., with RH as a fraction. At 110 F, ie
460+110 = 570 R and RH = 0.15, Td (R) = 570(1+570ln(0.15)/9621) = 512 R,
ie 512-460 = 52 F.

The condensing/filtering greenhouse seems more interesting,
especially with something edible inside. Mangos? Bananas?

Nick

 

[Bob]

Answer to your question:

If you are in an "arid region", air wells and dew or fog collectors won't
produce enough output to make it worth your effort. Arid zones normally
have less than 10% humidity on a day to day basis, and air wells and dew
collectors usually need better than 65% to be effective.

If you simply have good deep water, or shallow brackish water, its all a
matter of equipment to provide good potable water. Usually either deep
water or brackish filtered system will run around $15,000.00 to
$20,000.00(including digging the well). For that kind of money, you are
best to setup a co-op with neighbors with one good well serving 3 or 4
homes. $4,000 to $5,000 for a good well per house is a good average dollar
ratio.

Bob

 

[Brent Geery]

Answer to your question:

If you are in an "arid region", air wells and dew or fog collectors won't
produce enough output to make it worth your effort. Arid zones normally
have less than 10% humidity on a day to day basis, and air wells and dew
collectors usually need better than 65% to be effective.

If you simply have good deep water, or shallow brackish water,

Actually, it's deep, brackish water.

its all a
matter of equipment to provide good potable water. Usually either deep
water or brackish filtered system will run around $15,000.00 to
$20,000.00(including digging the well). For that kind of money, you are
best to setup a co-op with neighbors with one good well serving 3 or 4
homes. $4,000 to $5,000 for a good well per house is a good average dollar
ratio.

There is a couple of site on the net that show how to build a RO water
treatment system for $1K-$2K, that would cost $5K-8K to buy pre-made.
So that makes the treatment costs a lot more reasonable.