Nowadays, architects, engineers and designers are ignoring the potential value of the most basic principles of natural cooling:
- Solar control (shade)
- Convective cooling
- Conductive cooling
- Evaporative cooling
- Radiant cooling
In the first part of the article, solar control, convective cooling and conductive cooling were discussed in detail. When they cannot provide enough cooling then we may have to make use of evaporation.
Evaporative Cooling
The hottest areas tend to be dry and are ideally suited for evaporative cooling if water is available. When water is limited it can be used sparingly for just personal cooling. In general, direct evaporative cooling is most helpful if the humidity at noon in July is 40 per cent or less.
Many traditional building systems used evaporation-based cooling to great advantage, perhaps with a pool or fountain in the courtyard or in the building, sprinkling the tile or floor with water, or utilizing the evapo-transpiration from landscaping to provide cooling.
In traditional Iranian designs many buildings pull cool air from underground water tunnels, called qanats, with wind towers. This can provide air that is 15°C (27°F) cooler than outside. One of my students said this often worked too well for her and she was often cold even in summer where she worked in Yazd.
Ceramics have a well-established history for use in evaporative cooling in different parts of the Middle East. Maziara jars are porous and capillary flow keeps the outer side moist providing cooling by evaporation of water in the wind flow.
Early Anglo settlers in the American Southwest would hang water filled ollas in a doorway or window to help improve the cooling. The Ecooler system from Egypt is a modern adaptation with a ceramic water filled lattice made with modular units that interconnect with a special connector.
Each tile is 33 x 33 cm and 2.7 cm deep. The modular structure enables great flexibility, and users can create a cooling lattice at whatever size and shape they want by fitting together more tiles. Each tile might evaporate 0.25 liters (a half pint) a day. The jars and Ecooler would be filled every evening during peak cooling season.
Patios, sidewalks and other surfaces can be wetted for evaporative cooling. Peter the Great reportedly wetted the foliage of trees to increase summer cooling in St. Petersburg. Wetted fabric or fiber can be used for cooling. In India the solid doors in some areas were removed in summer and panels made of dried khus (vetiver) grass were installed. These were wetted by sprinkling or with an automatic drip or tipping trough that rolled over when full to wet the pad and create a cool air flow.
Similar practices can be found in Iran and other parts of the Middle East. In Rajasthan the grill might be used in place of a door and wetted with rose water. The blades of the large ceiling fans in India were sometimes wetted as well. Tents were wetted in some areas to keep occupants more comfortable. Wetted sheets were hung in windows to provide cooler air. Desert dwellers sometimes wrapped themselves in a wet sheet for a cooler night.
In the hot Southwestern desert in the 1930s many people fitted burlap covered boxes over windows. The burlap was wetted and cooled air was blown into the house with a fan. By 1933, thousands of homemade drip type coolers were in use. In 1939 two University of Arizona professors, Martin and Paul Thornburg, put out an excellent extension bulletin on making an evaporative cooler. A fan draws outside air through pads soaked with water. The evaporation of water lowers the temperature of the air. This cooled air is blown through an opening into the building with ceiling or wall vents. Unlike refrigeration systems which recycle air within the home, coolers blow large volumes of fresh cooled outside air into the living areas. This air needs to be vented from the home.
These more powerful modern systems provided more cooling but use more water. The Arizona Department of Water Resources estimated that a typical Tucson household cooler uses a year-round average of 61 liters (16 gallons) a day for evaporative cooling. Research by the Office of Arid Lands Studies at the University of Arizona and the Water Services Department monitored evaporative coolers in 46 homes. They found water use could be as high as 29 liters (7.6 gallons) an hour at a time when the typical home was using 1,600 liters (427 gpd) for all uses.
The use of evaporative cooling declined in the US even though it can save energy in dry climates. Evaporative systems are very competitive in terms of first cost and provide significant reductions in operating energy use, as well as peak-load reduction benefits. Innovative designs can provide even greater savings but have not been pursued.
The Davis Energy Group developed a two-stage system that could reduce energy use 90%. The Speakman Company developed a commercial model of this system. The OASys™ indirect/direct evaporative cooler contains an Indirect Cooling Module, that first cools incoming fresh air without adding moisture. The air then passes through the Direct Cooling Module and enters the space and then exits by roof vents or other openings. The small amount of water used in this process is renewed periodically by a self-purging reservoir. The key to OASys™ is a unique heat exchanger combined with a single pump/blower. The unit produces up to 3.5 tons of cooling while using less than 600 watts for an energy efficiency equivalent to better than 40 SEER, almost four times better than the US Department of Energy standard.
Architects can utilize evaporation for more powerful cooling without electricity. The Environmental Research Lab in Arizona developed a very effective cooling system with a downdraft evaporative cooling shaft and updraft exhaust shaft at the other end of the building.
I visited this house and it worked very well. In India evaporative cooling is still important and even office complexes may be designed for natural cooling. Industrial systems often use cooling towers, where evaporation of water is used to cool process water for fan coils or direct contact. Home size cooling towers have rarely been used, but can be effective and low energy use if a PV powered pump and fans are used.
Evaporative cooling can also be provided with an indirect evaporative cooler. Several commercial models are sold, including the EvaPolar Coolator 8 and Seeley Internationals Coolerado. Large units are available for commercial buildings. These all provide the advantage of powerful evaporative cooling without raising indoor air humidity. Two airstreams are involved-cool dry air is supplied to the house and a secondary air stream is cooled by evaporation that is exhausted outside after passing through a heat exchanger. Improved models using can chill air to near or below the wet bulb temperature using half the energy of an air conditioner! In Jaipur for example the wet bulb is 27°C (81°F). Installation is easy and they are quiet, efficient and durable. In areas with high mineral content water rainwater may be better to avoid mineral buildup. Size, performance and installation details and availability vary
Direct evaporative coolers (sometimes called swamp coolers) use evaporation of water from a pad to reduce outside air temperatures to a comfortable level for indoor comfort. These can be very effective in areas with low humidity, and may be run with solar panels. One commercial unit for livestock uses a pad mounted on a drum which rotates through a pan of water. On very humid days, like the monsoons of Arizona, direct evaporative coolers provide less relief.
Misters can provide evaporative cooling for outside spaces and intake air. They come in a range of prices and effectiveness. The best systems use very high pressure and metal fittings to produce a very fine fog. The cheapest systems are all plastic and use just standard water pressure, they make big drops and are not as reliable. The best compromise may be a medium price system with a booster pump and metal fittings, running at 700-1400kp (100-200 psi). Misting systems can provide cooling comfort for a courtyard or exterior space as well as cooling for the building. They do require regular attention. A personal mist can be made with a spray bottle, This is how I survived work in the hot desert without a truck AC system.
Landscaping with larger trees can provide very powerful evaporative cooling and solar control. Tests showed temperature reductions of 12°C (22°F) with full tree shade, dropping temperatures from 42°C (108°F) to 30°C (86°F) in a test trailer. The cooling effect of landscaping can be enhanced by spraying or sprinkling. Evaporative cooling doesn’t leak ozone layer killing or global warming gases like air conditioners do.
Radiant cooling
Radiant cooling results when the incoming direct and indirect radiation is less than the energy being reradiated. Our thermal comfort includes radiant exchange with our surroundings. If there are cool surfaces we can be comfortable with higher air temperatures. The planet is also exchanging energy all the time. During the day incoming shortwave radiation from the sun dominates. At night the long wave radiation from earth to deep space exceeds the counter-radiation from molecules and particles in the atmosphere. Global warming is in part a result of reductions in the radiant sky cooling effect from increasing concentrations of CO2 and other radiation blocking gases and moisture.
Radiant cooling works because the night sky temperatures may be 5.5-22°C (10-40°F) or more below air temperature. F. A. Brooks measured a night sky radiant cooling driven air temperature of -2°C (28°F) with frost on straw covered ground near Sacramento after a daytime high of 37°C (98°F). The Persian yakh chal uses night sky radiant cooling to make ice. In my tests the temperature drop was lare enough to make ice with air temperatures almost 10°C. Most of the net outgoing radiation occurs to the cold night sky, but radiation to space also occurs during the day. A cool spot was found at a compass point opposite the sun and approximately at a right angle. This cool spot in the sky was 22°C (40°F) cooler than air temperature at 3:30 PM in a test in the low desert of California. These observations led to innovations in shade structures for animals. Studies suggested animals would take advantage of the cool spot in how they locate themselves in relation to shelter.
Many traditional buildings with high mass roofs benefit from night sky cooling. Without it they could become even hotter during the day and at night. The thermal lag in high mass roofs slows energy transfer through a thick roof and helps makes it livable inside during the day, but hotter at night. High mass roofs of concrete, adobe, or with roof ponds, with operable insulation can provide the functions of coolth collection and storage with one element. This might be done with bags of straw spread in the morning and rolled back at night.
Innovative designs improved performance. Harold Hay’s Skytherm™ houses using bags of water on the roof covered with movable insulation during the day and opened at night.
The water for the roof pond is typically placed in bags on a metal roof support system that doubles as a ceiling for the room below. This ensures excellent radiant transfer from the water bags to the people below. The water will circulate so the hottest water is on the radiating surface and the coolest water is in contact with the ceiling. Open water can be added to provide the added benefit of evaporative cooling. The roof water bags can also be used for heating in the winter by reversing the operation, opening the covers during the day and closing them at night. The radiant transfer with a high mass ceiling makes for very uniform and comfortable temperatures.
The roof pond concept developed by Jon Hammond with engineering help by Loren W. (Tod) Neubauer at Living Systems used insulated panels that could be opened at night. This design used hydraulic rams to tilt up reflective, insulated roof panels. On this well insulated and passive solar oriented building they also added more thermal mass to store more coolth and to provide more cool surfaces for radiant exchange cooling for comfort. Performance fully met the expectations of the clients, who were from Alaska and wanted it cool even in summer. A backup bicycle powered pump could operate the panels if the power went out.
We conducted tests of the Cool Pool concept in both Sacramento and Indio, California. These used fixed shades with cool north sky exposure. The Sacramento test building was set up at the State Fair in an asphalt parking lot. It was hot! The cool pool room stayed cool even with just a drape instead of a door.
Sacramento State Fair Room in asphalt parking lot with plastic film strip door
Average Cool Pool Temperature 24°
Average Air Temperature 33°
Average Low Air Temperature 25°
Average High Air Temperature 39°
Our Indio test in the low desert reached an outside air temperature of 38°C (100°F), but just 24°C (75°F) in the cool pool.
The “Night sky” cool roof system pumps water to a white roof for evaporation and night sky radiant cooling. A manufacturing facility in Vacaville found this system provided a 67% energy cost saving with a payback of 2.5 years. Recent research has begun to evaluate more of these systems and to develop design parameters. The Davis Energy Group installed a large system in Los Angeles on a 2,554 m2 (27,500 sf) building with projected energy savings of 50%.
In Australia, a similar system is being used to provide radiant cooling through floor slabs and ceiling beams. This type of cooling would also integrate very well with the increasing use of large thermal masses in commercial buildings for cool thermal energy storage. The cool water in tanks or panels can be used for direct radiant cooling for occupants.
Integrated Design for Natural Cooling
These five methods of natural cooling can provide full comfort throughout the summer in many areas. The cost of natural cooling will be below the cost for mechanical air conditioning systems particularly when the natural cooling system is an integral part of the house design and solar heating system. Solar control is the most cost effective option. Shade can be provided by roof overhangs, awnings, and many kinds of shades. Direct and indirect evaporative coolers can provide comfort without high cost for installation and operation. The most economical system for convection cooling is to optimize ventilation. Windows and doors are a first start.
Down draft evaporative cooling can be done for homes or large buildings. Evaporation from fountains is cooling and soothing. Mist systems require some maintenance but help cool homes, courtyards and patios. Costs can range from $30 to more than $1,000 for a mist system. Hard water can clog mister jets. More complex evaporation based cooling systems with down draft cooling, and or tanks or pools that add night sky radiation cooling can provide comfort in deserts. In areas with limited water supplies night sky cooling may be key.
One of the great benefits of most natural cooling options is that they will work even if the power is off after a natural or social disaster. Natural cooling can provide significant reductions in peak electrical demand at very low cost. In the Central Valley of California every fifty square feet of west facing glass can increase the heat load at the peak energy demand point on a summer afternoon enough to require 1 ton of air conditioning. This in turn requires the addition of approximately two kw of capacity to the system electrical generating capacity. The approximate cost of the increased electrical generation capacity and air conditioner is around $2,000.
Alternatively, a bamboo shade could be purchased and installed for about $50 dollars, essentially eliminating the need for the addition cooling capacity. Evaporation can also be used in a dual stage system with supplemental air conditioning.
Further reading
Bainbridge, D. A., Haggard, K. 2011. Passive Solar Architecture. Chelsea Green. 294 p.
Bainbridge, D. A. 1978. Natural cooling: practical use of climate resources for space conditioning in California. pp. 138-153. In Clark, E. F.and de Winter, F. eds. Proceedings of the 3rd Workshop on the Use of Solar Energy for the Cooling of Buildings, San Francisco, California, US Department of Energy/University of Colorado, Boulder, CO.
Bainbridge, D. A. 1978. The Indio cool pool experiment. Alternative Sources of Energy.32:6-10. https://www.academia.edu/127442568/The_Indio_cool_pool_experiment.
Bourne, R. 2015. PV indirect-direct evaporative cooling. Davis Energy Group. RETD California Energy Commission. 500-2013-071.
Goss, J. R. 1956. Cooling of Water by Nocturnal Radiation and Evaporation. Master of Science Thesis in Agricultural Engineering. University of California Davis.
Haggard, K., Cooper, P. and Rennick, J. 2005. Chapter 3. Natural Conditioning of Buildings in Elizabeth, L. and Adams, C. eds. Alternative Construction: Contemporary Natural Buildings.Wiley.
Kelly, C. F., Bond, T. E. and Ittner, N. R. 1975. Cold spots in the sky may help cool livestock. Agricultural Engineering, October.
Niles, P. W. B. 1976. Thermal evaluation of a house using a movable-insulation heating and cooling system. Solar Energy. 18(5):413-419.
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