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Passive Solar Cooling in the Desert - Page 3

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Posted by Toby Anderson on September 13, 2004, 4:50 pm
Greetings Auntie Em,

Here are 4 things you could do:

1. Underground House
2. Solar Chimney - to get airflow
3. Evaporative Cooling (works best with Solar Chimney)
4. water window

I will go into each of these in detail.

1. Underground House

Instead of an Adobe House, you should build and underground house or
an 'almost underground house' (one that is underground except for the
roof). In either case, you should have good ceiling insulation.

The reason? --- the ground is way cooler than the air in the
summertime. Here is some monthly temperature data for various depths
in the ground for Amarillo, Texas.

""feet"""    July    Aug    Sep    Oct    Nov    Dec    Jan    Feb    Mar    Apr    May    Jun
0    79.0    76.1    68.0    57.0    46.0    37.9    35.0    37.9    46.0    57.0    68.0    76.1
0.03    73.9    71.2    64.7    56.1    47.8    41.9    40.1    42.8    49.3    57.9    66.2    72.1
2.6    68.7    63.7    57.0    50.3    45.3    43.5    45.3    50.3    57.0    63.7    68.7    70.5
5.2    61.7    57.0    52.3    48.9    47.6    48.9    52.3    57.0    61.7    65.1    66.4    65.1
10.5    57.0    54.0    51.8    51.0    51.8    54.0    57.0    60.0    62.2    63.0    62.2    60.0
16    55.4    54.2    53.7    54.2    55.4    57.0    58.6    59.8    60.3    59.8    58.6    57.0
20    55.6    55.4    55.6    56.2    57.0    57.8    58.4    58.6    58.4    57.8    57.0    56.2
26    56.2    56.3    56.6    57.0    57.4    57.7    57.8    57.7    57.4    57.0    56.6    56.3

Notice, at 10.5 feet deep, the ground temperature in August is 51.8
degF. Thus, the ground will act as a heat sink (the house heat will
get absorbed into it.

2. Solar Chimney - to get airflow

Build a solar Chimney into the side of your roof (which is above
ground because otherwise, you will need massive supports to hold the
dirt up).

A simple solar chimney is just a chimney (~8ft tall, 1 to 2 ft wide,
and as long as you can make it) with:
1. an opening on top leading to the outside
2. an opening in the bottom leading to the house
3. the outside part of the wall is some type of clear glazing or
4. the inside side of the chimney is painted black to absorb heat.
(the air is flowing on one side of this and there should be insulation
on the other side)

The sun shines through the glazing, heats up the air, hot air rises,
and pulls air out of the house.

3. Evaporative Cooling (works best with Solar Chimney)

The solar chimney pulls air out of the house, and this, you need a
'window' of some sort which let's air get pulled into the house.
Basically, you will need to place some water 'misters' at the entrance
of this 'window', and when the air gets pulled (by the solar chimney)
through this mist, it will evaporate the mist, and cool the air by
about 20 to 30degF from what it was.

4. water window

You will need windows to see, however, most of the summer's heat
enters the house through windows. So, in order to minimize the amount
of heat entering through the window. The windows in an underground
house need be in the roof. So, make your roof window a flat
waterproof, topless, box containing water (distilled is the best). The
water will let the light pass through it, however it will also absorb
the infrared light (which is the light which is the hottest. As the
water heats up, it evaporates, taking the heat out of the water. You
will have to replace the water. Distilled water is best, for it leaves
no minearal traces. You might be able to duct this evaporated water to
a condensing system, and recover it. A condensing system might be as
simple as a water holding tank with 3 to 4 inch diameter flexible rain
water tubing in it ducted to the moist air from the water window. As
the most air comes in contact with the colder surface of the tubing,
the vapor in the air will condense out --- as distilled water.


Posted by Auntie Em on September 14, 2004, 10:34 am

I love those numbers, Toby.  The only problem is the cost.  It looks
like adobe will run about $0 a square foot for just the shell
structure, foundation and wiring (which we are not confident enough to
do).  Judging from what I have read (although I am no expert), an
earth sheltered house is going to run considerably more due to higher
construction costs.  (At least that's the way it looks.  If I am
wrong, please let me know).

Will gravity fed water have enough pressure for misters?  We had a
couple going this summer, at it seemed like you needed an awful lot of
water pressure to get the dang things going.

I really like this.  I had been wanting to put in some sky lights, but
feared the extra solar gain would be too significant.  Now it looks
like I have found a way to have my cake and eat it too.

Thanks for your great input.

Be careful what you wish for....

Posted by Toby Anderson on September 14, 2004, 4:04 pm
 Greetings Auntie Em,

Toby wrote:

Auntie Em  wrote:

Earth Sheltered homes are more expensive ***IF*** one insists on
totally burying them, i.e. putting 2 to 6 feet of dirt on the roof.
Why? Because dirt is heavy, any you need massive trusses and walls to
support it. That is why I don't like those structures in New Mexico
called 'earthships'.

Instead, don't have a dirt roof, but rather have a normal 'fiberglass'
insulated roof.

Put the 'roof' slightly above ground, (and the rest of the house is
below ground). If you do this, then an earth sheltered house is
actually cheaper. Why? Well, you don't have to pay for an exterior
finish -- i.e. stucco or siding. Further, concrete block is still
relatively cheap compared to 'wood' studs. Further, I suspect that
concrete block is cheaper than adobe. THe adobe idea has merit, but
think of the underground dirt replacing the adobe, and you will see
the benefit of going underground. Think of it as a house with a
basement but no first floor. This isn't just theory because the Idaho
potatoe farmers have used earth cooling for years. The typical
'potatoe house' is just a dug out dirt basement (they don't even have
any concrete block wall) covered by an above-ground pole truss roof
system which is covered with horse fence and then straw bales. A thin
(6 inch thick) layer of dirt then covers the straw. This underground
basement keeps the potatoes cool in the summer and prevents them from
freezing in the winter. It is more or less a big root cellar.

We are presently having a contractor build a 2000 sqft house for us. I
had asked what it would have cost to have a 'concrete block' basement
added to it. He said roughly $0,000, i.e. $0,000/2000sqft = $ per

Why is this so cheap?

Well, there are 5 basic costs (totalling $1200):
1. excavating --- I assume it will cost roughly $000.
2. 80 hours of labor (my very rough estimate) x $0/hour = $400
3. the concrete blocks - $300:
a. each block is 8" tall x 16" long (0.8375sqft)
b. a 2000sqft house might be 40' wide by 50' long by 8'tall, so the
wall area would be:  (40+50+40+50)*8 = 1440sqft
c. the number of concrete blocks you need is:
  1440sqft/(0.8375sqft/block) = 1720 blocks
d. each block costs roughly $.75
e. total cost of blocks: 1720*0.75 = $300

4. the 4" (0.33ft) thick concrete floor - $000
(well, your house needs a floor anyway, so it's not really any extra
cost)here  is the cost: 40'x50'x0.33ft * 1 yard of concrete/27cuft *
$5/yd = $800
and about $200 for the footers:
5. gravel for 3ft thick 'pad' and drainage

   3'x 3000sqft (the pad should be wider than the house) * 1 yard of
gravel/27cuft * $00/(10yd) = $500

It depends on how high up your water source is from the misters. One
thing you could do which should greatly improve it, is have your
misters actually underground! You could have a 10ft deep shaft for
airflow from the surface, to where the misters are. The outside air
goes down the shaft makes a 90degree bend, hits the mist, evaporates,
cools and then enters your house.

Let's see if I can calculate the 'water pressure'. If your water tank
is presently 10feet above ground and you are using a 1 inch diameter
pipe of area: pi*(1/2")^2.

Then the wieght of the water will be:
10ft * pi*(1/2" * 1ft/12inch)^2 * 64lb/cuft = 3.49 pounds

This wieght is applied to a cross-sectional area of:
   pi*(1/2")^2 = 0.78sqin

So, the pressure in lb/sqin = 3.49/0.78 = 4.44 psi.

*IF*, you put your misters 10ft below ground, you will double your
pressure get:
20ft * pi*(1/2" * 1ft/12inch)^2 * 64lb/cuft = 7 pounds
7lbs/0.78 = 8.88 psi

Normal pressure is about 30psi.

Another thing you could do, is to buy a water tank with a schraeder
valve (a bicycle air valve) on the top. Basically, you would let all
the air pressure out, then open the inlet water valve, and fill it
like 2/3rds full (make sure the air valve is open the whole time).
Then close the inlet water valve, you could then hook up a tire pump
(or better yet, one of those small air pumps you hook up to your car
or a battery) to the schraeder valve and pressurize the water tank to
an air pressure of say 30psi and close the schraeder valve.

You should plan on the fact that some day this water window will
leak... perhaps make sure there's a floor drain directly below the
window, maybe even have the floor slightly sloped to the drain. Make
sure the sides of this water window (from the top of the roof to the
ceiling) are composed of non-wood waterproof material.

let us know what you all decide.


Posted by Auntie Em on September 19, 2004, 2:48 am
Since water is so precious there, we are wanting to put in a greywater
collection system for watering outdoor plants, etc.  How can you do
this with an underground home when all of the plumbing would be below
grade?  Do you know?

Be careful what you wish for....

Posted by N. Thornton on September 15, 2004, 1:00 pm
Just think of underground building as a necessity in desert. It would
be silly not to do it that way.

If the electrician designs it all, you can do the running of the wires
everywhere, and mounting of the accessories, they just come back to
wire it all in and test. Money saved.

and a lot of water, in the desert. Plus there may be RH issues. Hope
youve done your calculations. An earth pipe may make more sense.

Does that really work? How do you keep it from getting grimey?
Ordinary windows shaded from direct sun would be far easier.

I'd certainly suggest shading the roof, as white paint is not 100%

Regards, NT

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