TIP 1:
Okay, let's look at the physics of water. One quart of water has the cooling capacity of about 2500 BTUs when heating from 50 F to boiling and then from water to steam. I'll be using English (non-metric) units in this description for ease of understanding and comparison.
On a hot day, four conventional sprinklers placed on your ROOF will put out a total of about 16 quarts per minute. When allowed to spray for 3 minutes (the amount of time I have used for cooling without any runoff into the rain gutter), that is the equivalent cooling of 120,000 BTUs!
When one considers that the attic temperature is a combination of outside air temperature and radiative roof heat, the attic temp can be 130 F or higher when the outside air temp is 100 F. Depending on the thickness and placement of attic insulation, the difference between attic and living space ceiling is on the average 30 F.
Thus, if the roof temp can be minimized as a factor of attic space heat, then the only factor left is outside air temperature. In a completely closed house with no access to outside air, it becomes very easy to keep the house cool by removing the added solar heat from the roof.
During the 2006, 110 F heat wave, I set my roof sprinklers up to spray at 10 a.m., 12 noon, 1 p.m., 2 p.m., 3 p.m., and 4 p.m. - for four minutes each time. Although the spray only reached about 50% of the roof, I had approximately 8 gallons of total hot water runoff (water from downspouts averaged 140 F). This meant nearly 88 gallons of cool water was warmed and vaporized on my roof. This was the cooling capacity of 880,000 BTUs!! In comparison, an average window-type air conditioner is rated at about 10,000 BTUs.
Further, my attic temp dropped from 135 F (at 2 p.m.) on a day when the roof was NOT cooled, to 117 F (at 2 p.m.) on a day when it was cooled (two days later). On both days the outside air temp was 108 F (as measured at my house patio).
When I use the roof sprayers to keep my home at 82 F, it required only 40% of the air conditioner use as when I didn't use the sprayers. On the days measured, my A/C use dropped from 7 hours to a fraction more than 3 hours. Based on a measured A/C savings of 0.1 kWh/gal, the minimum A/C savings equated to approx. 9 kWh/day (the energy of 90 screw-in fluorescent light bulbs on for 15 hours a day).
I'll leave it to you, compare the environmental and economic costs of 80 gallons of water turned to steam, to four hours of full house A/C use. There is a drawback however - if hundreds of thousands of homes utilize this method of evaporative roof cooling, the electrical use and carbon dioxide emissions will decrease markedly but the humidity will increase which MAY result in a greater likelihood of regional thunderstorms.
TIP 2:
Air Conditioners rely on outside air temp to effectively remove inner house heat in the vaporization and condensation of a refrigerant chemical within the machine. The efficiency of the A/C is primarily dependent on the outside air temp - the cooler the outside air, the more cooling the inside gets in the least time and with the least energy required.
Swamp coolers operate in a similar fashion but utilize the evaporation of water, blown into the house, to cool off the interior of a house. Since this can make a home in a regionally wet environment unbearably humid, swamp coolers are best used in arid environments.
For this test, I used a thermometer and recorded the air blown out from the circular fan in an outside, all weather, whole house, air conditioner. Setting the thermostat to cool the house at the 82 F mark, I recorded the time it took to cool the house 1 F. Since it was automatically controlled to turn off at 81 F, the same delay time between thermostat and A/C activation removed any guesswork. The outside air temperature was 104 F.
I noted the time it took to cool that 1 F and then used a hose drip line to place water on the heat exchange elements of the air conditioner. The water slowly dripped on the element at the rate of about 1 cup per minute and I retested the A/C using the same degree measurements on the thermostat. The outside temperature during this test was 105 F.
It took the A/C 17 minutes to cool the house 1 F when water was dripped on the heat exchange element and the resulting blown air from the element (measured in the shade 6" above the center of the fan blades) was an amazing 94 F (yes - cooler than the outside air!).
When water was not used to cool the heat exchange element, the blown air from the element was 120 F and the time required to cool the house 1 F increased to 33 minutes!
The water used for the 17 minutes was approx 1 gallon and it saved A/C use by 16 minutes or approximately 48%. By appearances, only about 3/4 of the gallon evaporated, thus it assisted the A/C to the tune of about 8000 BTUs.
The modest increase in my electrical bill is the true tale of the effectiveness of these two simple procedures. During this July, while the area was undergoing record breaking heat for several straight days, when the shade temperature on my patio was over 112 F on two days and over 105 F on five days - my electrical usage was the following:
July 2005: 22.7 kWh/day
June 2006: 23.1 kWh/day
JULY 2006: 27.7 kWh/day (only a 22% increase over last year!)
The entire time, the inside of my house averaged between 79 F and 81 F!
Before investing in the materials to automatically water your roof, test it yourself. If you happen to be home and want to see the effects - use a simple water hose with a narrow sprayer and spray your roof for a few minutes every hour and check the results with your thermostat inside your house. At the same time, very briefly water down your air conditioner once you are sure that it is an all weather type and is safe to operate while raining.
By the way, just today it got up to 98 F (at 3 p.m.) and I didn't have to turn on the A/C at all, with the roof sprinklers operating. With the house closed, the highest the inside temp got was 82 F.
The data for Tip 1 & 2 were taken during the Summer of 2006.
ENVIRONMENTAL ASPECTS OF WATER COOLING
If 18 million gallons of water are utilized in this fashion, they will NOT be going into gutters or sewage treatment but will be returned as rainwater further inland. If used completely the relief on the regional energy usage by this usage will equate to an 180 BILLION BTU saving (54 million kWh!!). 18 million gallons of water is less than that provided by a tiny cloudburst of only 8 square miles dropping less than an eighth inch of rain. If a common single family residential home utilizes an estimated 30,000 kWh per year - this relief is the equivalent of the entire yearly energy for 3000 homes. Even if the VERY conservative test figure of 9 kWh/day is used (the absolute minimal effect), this 18 million gallons will reduce electrical usage by 2 million kWh (the yearly electrical usage for over 70 homes).
This form of evaporative water cooling does not pollute nor contaminate the water used in any fashion. Indeed, in buildings equipped with "industrial water" that is not designed for human consumption, the cooling sprinkler system can utilize non-potable water; non-potable contaminants will be destroyed almost in total by solar UV irradiation..
The uses of evaporative cooling in the United States are best utilized on the west coast and any region that has an air flow that usually moves inland. If used in this manner, the evaporated water will rise to several thousand feet and cool during evening hours to return as rain several hundred miles inland - thus replenishing water storage as well as watering crops and fields and encouraging plant growth. If utilized to a large extent in the Los Angeles basin, the evaporative return to Southern Nevada, Arizona, and New Mexico will likely make these areas more capable of maintaining both produce crops and high carbon dioxide fixating plants (thus, decreasing local carbon dioxide levels and increasing oxygen levels). Even during times of southerly Santa Ana winds, the resulting rainfall will replenish rivers and lakes in the arid regions north of Los Angeles.
Relative to the gain and loss of heat from the evaporation of the water, the heat absorbed by the water in its state change will be transferred to the upper atmosphere and, during the evening, be lost into space as non-radiant heat (and thus not reflected back into the atmosphere (unlike nitrogen, carbon dioxide, and oxygen) - reducing another contribution to global warming).
Although the carbon dioxide and energy benefits will be evident immediately, it will take approximately two full summer seasons to see the inland benefits of the increased rainfall and carbon fixating plant growth. Once manifested, inland areas must be prepared to utilize the increased rainfall. With the loss of the entrapped solar energy to the regions above the atmosphere, there will be consequential cooling of the atmosphere within five years. Hopefully, by this time, there will be a means of utilizing solar energy along with a decreased reliance on fossil and uranium fuels.
SPRINKLERS VS. ATTIC FANS
For those who advocate the use of attic fans for the removal of heat, there are significant disadvantages to the use of electric fans. Those that require a wired connection, are rated at approximately 200 Watts. A timed test of one fan (set for an attic temp of 110 F) during the time period listed above, ran the fan for 18 hours a day for 60 straight days (ave daily high 101 F, ave low 88 F). This was the equivalent of 216 kwh (3.6 kwh per day) which is the cooling capacity of only 76 gallons of water. The roof stored so much heat in the thick masonry tiles that the attic temp didn't drop to below 110 F until around 3-4 a.m.). On one day, the measured ceramic roof temp exposed to the sun was 220 F at 3 p.m.. 200 Watts is the approximate equivalent of 4 standard incandescent lights or 9 standard screw-in fluorescents.
Unless a solar attic fan has a battery backup, it ceases to work when light levels drop below the torque level for the rotor (when shadowed or when the sun is on the wrong face of the silicon panel). Even with the battery, it is often drained within a few hours after dusk.
Furthermore, the attic fan will substantially raise the exterior temperature of the house by 5 F or more. The exhausted heat doesn't just rise, it is also buffered to lower levels and is prevented from rising by the extraordinary temperature of the roof tiles neighboring the fan exhaust. Due to the negative attic pressure, this higher than normal exhaust heat will then be pulled back into the attic through air vents around the attic.
As a side note, any access to the attic through the ceiling light fixtures and other vented openings will find the air conditioned cool air pulled into the attic by the attic fan, thus decreasing the effectiveness of the A/C.
None of these attic fan disadvantages are present with roof sprayers.
EMERGENCIES
Even if homes, businesses, and other facilities rely on air conditioning, swamp coolers, and attic fans, roof sprayers are an inexpensive and reliable source of auxiliary internal cooling. They can function even in a power blackout, brownout, or rolling blackout (even extensive ones) where all other forms of cooling will fail. Furthermore, this type of cooling is effective for buildings without attics, mobile homes, temporary structures, and recreational vehicles. Its only limitations are in sunny areas of high humidity, where the presence of water vapor condensation reduces the effective evaporation and in hot areas that are not directly exposed to the sun. In sunny areas of high humidity, there will be less evaporation although the runoff will be greater and hotter.
COST
Depending on the materials used, the average 2000 square foot home can be provided with manual roof sprayers for as little as $30 (without an automatic watering timer).
If you have a swimming pool - consider the following projects made with mirrored film plastic, made boyant with simple pool floats. This project can keep your pool cool and refreshing and from turning into an alkaline hot spring. During the 2006 heat wave, my neighbor's pool went up to 94 F (being refilled and chlorinated daily due to evaporation and solar burnoff) while mine stayed around 77 F and barely needed any refilling or extra chlorine. The floats are cut sections of a discount and overstock item called a "Water Worm" and the adhesive used was Liquid Nails - Polyurethane. Contact me below for details on construction.
The following pictures of the roof spraying setup can give you an idea of how this is utilized on a single family home. If your concern is the use of water, please consider the available data and statistics about the amount of evaporation from freshwater pools, sprinklers, ponds, lakes, rivers, streams, and especially regional aqueducts.
COMMENTS FROM OTHERS WHO ARE USING THESE METHODS
Regarding the use of water on the cooling fins of an air conditioner:
It was mentioned that water evaporation can build-up mineral deposits on the fins and eventually decrease the utility and eventually the life of the air conditioner. This is true if the water is not allowed to run for a minute or two after the A/C is turned off. For use with air conditioners, the water is NOT supposed to be completely evaporated - sufficient water must be used to prevent mineral build-up. In locations with ample winter rainfall and temperatures that occasionally drop below freezing, I have found that the mineral deposits flake off easily and dissolve in the slightly acidic rainfall.
Regarding the buildup of mineral deposits from hard water evaporation on a roof:
It was mentioned that the continuous use of tap water on a roof would eventually result in staining the roof tiles with the minerals from the tap water. In regions where there is very little winter rainfall, this may be an issue. However in most regions the incredible kinetic energy of raindrops (or snow or hail) along with the heating and cooling (contraction and expansion of the tiles) permits whatever minerals that were on the roof to be flaked and washed off the roof during winter. In the three very distinct regions in which I have lived, I have found that by mid-spring there was NO evidence of the previous summer's mineral accumulation.
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Last Updated January 20, 2008 by Dr. J. S. Reed PhD