How to Stay Cool in the Hot Desert
(with less power)
When the thermometer starts to hit 90ºF nearly every day, even though "it is a dry heat" as we say here in the desert , we start thinking seriously about ways to stay cool. More than 14 years ago when we were planning to build a renewable energy powered home, cooling our home was the big question.
We had no doubt our new home, to be constructed on a 20 acre hilltop
near Vail, Arizona, would be powered with wind and solar. We chose
the site with wind power in mind. The domestic hot water system
would be a passive solar system. We would use solar for space
heating the structure, but how do we cool the home using alternative
energy?
No Information on Low Energy Cooling
Air conditioning is not practical for a renewable energy (RE) powered
home because the compressor and blowers consume a lot of energy.
Evaporative coolers work well and use considerably less energy, but
the blower still requires lots of energy. Plenty of books and
information discuss all types of solar heating, but little to none
describe passive or low energy use cooling.
I first thought about building most of the house underground. After
choosing a site on the property to construct the house, I realized that
excavating and removing the rock at the site would be difficult.
Secondly, an underground house would deny us the outstanding views
at the house site. We decided to build at a different site on the
property. The house would be a two story structure. The downstairs
would be mostly (80%) earth-sheltered, and the upstairs completely
above ground with many windows.
Underground Cooling Tubes
The downstairs would not require much cooling because it is
thermally connected to the earth, but the upper portion of the house
would require considerably more cooling. I had researched
underground cooling tubes and thought this could be part of the
answer. I would feed air through a tube about 150 feet long and two
feet in diameter. The air would pass through an evaporative cooler pad
as the air entered the house. This cooler would be located
underground. To move the air I would use an upwind air scoop at the
cooling tube's intake. A solar chimney at the top of the house would
help move the air through the house. No blowers would be required to
move the air. So I started digging the ditch for the cooling tubes. I
soon found the rocks that I had abandoned at the other higher site had
deep roots. In addition I still had to come up with a material for the
tubes: it had to be rust proof, a good heat conductor, the proper size,
workable, and affordable.
Finding A Better Way
The ditch and the search for the tube material became an ongoing project. Then one day, about three years into the search, I stopped by the Environmental Research Lab where a friend, Bill Cunningham, worked as an engineer. He told me about a low energy use passive cooling system — cool towers. A cool tower requires no blowers or fans to move the cool air. The only power required is for a small DC pump to circulate water over the pads. A cool tower seemed the perfect answer for cooling an RE powered dwelling. From that day on, some major design changes took place in the already half completed structure. The solar chimney planned for the west end of the house changed to a cool tower. We filled in the mini Grand Canyon (the ditch) and avoided many hours of digging.
Normal Evaporative Cooling
Folks that live in places other than the desert may not be familiar
with an evaporative cooling system. Blowers are used to move air
through wet pads. As the air flows through the wet pad, water
evaporates and cools the air. You cannot recirculate this air because
the humidity increases and evaporation stops. At that point your
evaporative cooler becomes a humidifier only. With evaporative
coolers you must leave an exit for the air to escape from your house.
Many newcomers to the desert don't realize you must open a window
to make an evaporative cooler work properly.
How Cool Towers Work
Cool towers operate on the same principle as a standard evaporative
cooler. The magic starts with the way the air is moved. Special pads
made of CEL-dek sit at the top of a tower with a pump recirculating
water over these pads. Air passes through the special pads with little
resistance and is cooled by evaporation of the water. This cool moist
air is heavier than the hot dry outside air and drops down the tower
and into the structure to be cooled.
In order for the cool air to flow in, hot air must be exhausted from
the structure. Open windows exhaust this air with conventional
evaporative coolers. If the wind blows hard against the side of the
house with the open windows, the cool tower air flow will be
reversed: no cooling. A large solar chimney can be used to exhaust air
from the structure, which eliminates constantly watching the wind
and opening the appropriate windows on the lee side. Downwind
scoops are another alternative.
The Normal Cool Tower
Most cool towers have the pads around the very top of the tower. They
use baffles inside the pads to keep the wind from blowing through the
pads and out the other side.
My Cool Tower
I never do anything the way most people do a similar task. Maybe my
situations are always different. I wanted to reduce the cost of the
system as much as possible. The pads are expensive, so the fewer
pads used that still accomplished the job, the better. I also used some
cooling tube ideas in the design of the cool tower. Since the wind
blows at a good steady pace here most of the time, I wanted to use
wind power directly to help move the cool air through the house.
To create the additional flow down the cool tower I installed one
large upwind scoop above the pads in the cool tower. This is an air
scoop with a tail to keep the scoop oriented into the wind, thus
creating a positive pressure. Instead of one large outlet for the hot
air, like a solar chimney, I installed smaller openings in the roof with
down wind scoops to help remove heat. With these scoops the wind
can blow from any direction and the cool tower continues to work
properly.
On my design the pads are just below the scoop. This reduces the size
and area of the pad, thus reducing cost. I have 18 square feet of four
inch thick pads in my tower. Placing pads at the top of the tower
would have required 72 square feet of pads. Pads down below the
scoop are protected from direct sun, so they last longer. The tower
itself is six feet square and 27 feet tall. The air scoop occupies the
top three feet. Two pads three feet square by four inches thick are
located just below the air scoop. Just below the pads is a tank
containing 20 gallons of water with a float valve keeping this tank
topped up. Located outside the tank is a small 12 Volt Teel bilge
pump. This is a submergible pump, but I found the hard way not to
submerge this pump. The first pump only lasted two months. The
replacement pump mounted outside the tank lasted six seasons.
Some General Design Rules
I am not an engineer. I build things by what many refer to as "back
yard engineering". I suspect some of you have completed projects
engineered in a like fashion. Most of the time things work out pretty
well. I did get some suggestions from my friend Bill Cunningham, an
engineer and co-inventer of the cool tower.
A good way to visualize the air flow is to compare air flow to water.
Water is of course a much denser fluid than air, but the principle is
the same. Tower height, or the distance from the bottom of the pads
to the air outlet, will determine the velocity or pressure of the air.
The greater this distance the more air pressure created, similar to a
water column. We are using a column of cool moist air (compared to
the hot dry outside air) to create this pressure.
To determine tower width, or cross section, use the water analogy
here, too. The larger the size of a pipe, the greater the volume passes
through the pipe at a given pressure.
Enhancements will increase the air flow; upwind and downwind
scoops are my choice. Other methods include rigid and movable cloth
baffles. Barometric operated louvers also work to direct the air
through the pads and create increased pressures.
Pad material choice for me is CEL-dek. At first I installed the
expanded paper pads that are much less expensive. Even the old
standby for coolers, aspen pads, will work. Water must flow down the
pads and air must pass through the chosen medium. The CEL-dek pad
works best because it has low resistance to air passing through it.
Duct work must be as large as possible. Having the air move through
hallways and doors of the structure is best. An open floor plan works
well. Cooling a large open area is much easier than cooling many
rooms. If you use duct work with the cooling tower, the ducts must
have a larger cross sectional area than ducts in a forced air system.
Vents must have a larger opening than those used with a forced air
system such as conventional air conditioning or evaporative coolers.
We are moving the air naturally with small pressure differences. Use
large openings that don't restrict air movement.
What Kind of Water?
Evaporating water is what creates the cooling and makes evaporative
coolers and cool towers work. Rain water is the perfect source for
the water used in cool towers because it does not have dissolved
salts or minerals. Well water can contain dissolved minerals. As the
water evaporates from the pads, whatever minerals it contains are
left behind. This buildup will eventually clog the pads and block air
flow.
We chose to get water for all our needs from the water harvesting
systems we installed. Yes, we live in a desert, with an average annual
rainfall of only 12 inches and we have plenty of water for all uses.
The CEL-dek pads in our cool tower have only had rain water on them
since 1986. They have a very small amount of mineral buildup on the
surfaces.
Normally you can expect to replace cooler pads every year, or at best
every other year. I have seen cooler pads fed with ground water that
have more buildup after less than one season than my eight year old
pads fed with rain water.
How Much Water
Approximately 1000 BTUs of cooling is created per one pound of
water evaporated. On a hot summer day with low humidity you can
expect to use 50-100 gallons of water. The most we have used in one
day is about 60 gallons to cool the entire house. When we only cool
parts of the house ("zone cooling"), we reduce this by 50-75%.
Other Benefits to a Cool Tower
Would you believe the cool tower helps heat our home in the winter?
Our greenhouse has excess solar gain. We open a small door in the cool
tower leading to the greenhouse. The upwind scoop on the cool tower
forces cool outside air into the greenhouse and excess heat is pushed
downstairs. Cool air escapes through a vent located low in the
downstairs room and is replaced by more warm fresh air from the
greenhouse . We call this our fresh air heating system.
When we go away for an extended period of time in the summer, we
open all the vents from the cool tower but leave the water pump off.
With a slight breeze fresh air flows through the house. This keeps the
house from building excess heat.
Bill Cunningham built a cool tower on his office and shop/garage with
south and east facing windows in the cool tower. They provide light
and heat to both areas in the winter. In the summer they provide soft
indirect light.
Conclusion
We started construction on the cool tower in the spring of 1985 and used it that summer. The system has undergone several changes. The first upwind scoop was metal, and not a good choice unless you use aluminum. Our scoop now has a framework of steel covered with heavy canvas. The cool tower has been in operation nine years. On a hot dry day (100ºF with 10% humidity) the air coming from the tower is 65-70ºF. We are very pleased with the performance. I am saving the finishing touches for a 110ºF day — that's when working inside the cool tower is quite enjoyable!
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Author: Charles Van Meter, Alternative Research Center Inc., PO Box 383 Vail, AZ 85641-0383; 602-647-7220
Custom Cool Tower & Solar Design, Bill Cunningham, 5085 S Melpomene Way, Tucson, AZ 85747; 602-885-7925
Suppliers of CEL-dek: Munters Corporation, Mrs. Pat Thomas, Box 6428, Fort Myers, FL 33911; 1-800-446-6868
12 Volt Teel bilge pump: Stock # 1P811, W.W. Grainger Inc., local phone book
by Charles Van Meter
copyright 1994 Charles Van Meter