Tuesday, August 5, 2014

passive cooling works like a charm

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.
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!
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

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