THREE
Passive Cooling
and Ventilation
This prototype roof pond system in Atascadero, California, was the first passive system to give equal emphasis to cooling and heating (1975).
Passive cooling considerations are important in building design. A building that gets too cool in winter is a problem, but occupants can add a vest or jacket and be comfortable even with temperatures in the 50s. A building that overheats in summer is more problematic, for even with minimal clothing it is hard to stay comfortable as the temperature rises above 80 to 85°F. Passive cooling strategies for new and existing buildings will become increasingly important as global warming causes many more heat waves and increases the need for cooling for comfort in buildings that were historically comfortable in summer—but are no longer.
Building Metabolism and Cooling Demand
Cooling demands vary widely with the orientation of the building windows, the metabolism of the building (see figure 1.13), and occupant behavior. Cooling loads are strongly influenced by unwanted solar heat gain in summer and by internal loads. The opportunities for passive cooling are influenced by building type and size, but good design can bring passive cooling to small-scale skin-dominated buildings and large buildings with low skin-to-volume ratios.
Fig. 3.1. This chart shows the cooling degree days for Yuma, Arizona (base 65°F and 80°F) and Riyadh, Saudi Arabia. In June (30 days) the average temperature is about 95°F in Yuma (900 CDD/30 days = 30°F, 30°F + Base 65°F = 95°F).
Climate-based cooling loads are usually described in cooling degree days, the difference between the average daily temperature and the base temperature. These may be base 65°F or 80°F. In hot tropical areas where people are adapted to higher temperatures (see page 30), base 80°F will often be used. Cooling degree days, base 65°F, range from 4,200 at Yuma, Arizona, to 162 in Seattle.
A new Web site, www.degreedays.net, generates degree days to the base temperature you set almost anywhere in the world.
Cooling degree days alone are not enough for building design development. Humidity and wind speed are very important for passive cooling and comfort. Relative humidity (by hour) and average wind speed and direction are often reported and are helpful to know.
External Loads
In smaller skin-dominated buildings, the external loads commonly dominate the cooling load. Building and window orientation are critical to reduce unwanted heat gain in summer. Fortunately, the best orientation for cooling is also best for passive heating in winter.
In the mid-latitudes, the equator-facing windows have the best winter heat gain and the lowest summer heat gain. By adding an overhang, the equator-window gain in summer can be reduced further. East- and west-facing windows cause overheating in summer, late spring, and fall. They often benefit from lower solar heat gain factor glazing, but equator-facing windows should have high solar heat gain glazing for winter heating.
Fig. 3.2. Louvered shades allow ventilation and control the sun.
Solar control with light-colored walls and roofs, window overhangs, shutters, screens, and shades is inexpensive and very effective. Landscaping can also be used to reduce external loads. Shade trees, green screens, and even living roofs can reduce heat gain. Neighborhoods with extensive landscaping and trees can be 10°F to 15°F cooler than those without.
Internal Loads
Internal heat load must also be managed carefully to reduce cooling demand. As noted on page 40, internal heat gain can be significant. In larger buildings, these internal loads often dominate the cooling demand. An efficient refrigerator provides a double bonus for homeowners by reducing both energy use and cooling cost. In office and commercial buildings, the use of daylighting can reduce unwanted heat gain from lights. More efficient lights also reduce heat gain. Careful placement of large heat producers such as refrigerators, freezers, and computer servers can reduce heat gain in conditioned spaces and help drive ventilation flows.
Passive Cooling Opportunities
Compared with the two passive heating sources of sunlight and internal heat gain, passive cooling is a bit more complex but equally achievable in all climates. The sources for passive cooling are the three thermal sinks of cool night air, night sky radiation, and evaporation from wet surfaces.
Fig. 3.3. Cooling sources.
The use of these sinks is very dependent upon humidity. Humidity in the atmosphere produces a thermal blanket that reduces the temperature differences between day and night, thus reducing the ability to cool with night air. Humidity also substantially reduces the amount of night sky radiation. Passive cooling strategies in hot-dry climates and hot-wet climates are very different. Hot-dry climates have more night sky radiational cooling that can create cooler night air or can be used directly by a building’s elements and configuration. The high humidity of hot-wet climates limits cooling resources, and tropical and subtropical hot-wet climates are considered the most difficult passive cooling situations. However, there are many successful passively cooled buildings in these climates.
The main thermal sink to use in humid climates is wet surfaces. Human skin is admirably designed for cooling if enough airflow is provided. Strategies for passive cooling for these climates include minimizing solar and thermal loads while maximizing ventilation during the day and night.
Natural ventilation is an integral part of the use of two of these sinks, cooling with night air, and cooling by wet surfaces. Ventilation is an important element of almost all passive cooling designs and is covered in more detail on pages 117–129.
These sinks used singly or in combinations can provide sufficient cooling for full comfort in virtually any climate. Passive cooling can eliminate or minimize the use of electricity while providing more comfortable and healthier living conditions.
Fig. 3.4. Terrestrial surfaces radiate energy to anything colder than they are. Therefore, a building at 75°F will radiate heat upward to the cooler sky and space. This radiant energy will travel until it hits a nontransparent barrier that absorbs radiation. In the sky, this barrier is mainly atmospheric water vapor. In dry climates, it takes several thousand feet of atmosphere to contain enough water to be thermally opaque to this upward radiation. The average temperature of the night sky is typically 10°F to 50°F lower than the air temperature near the ground; therefore heat is always radiated from a building out to the sky. The drier the night air, the lower the effective sky temperature and the greater the heat-sink effect. This is why deserts can be cool or even cold at night.
Cooling Considerations
Passive cooling, just like passive heating, works best in an efficient shell. This entails not solar-heating the building during the cooling period by giving careful consideration to solar control and minimizing internal heat gain that could add to the cooling load. The first of these prerequisites is covered here under solar control. The major factor for internal heat reduction is lighting. Replacing electric lighting with natural lighting is in most cases the best way to reduce internal heat gain. Natural lighting is so important that it has its own chapter (chapter 4).
The cooling-load relationships vary depending upon a building’s metabolism, which is related to the size of the building as discussed here. For passive heating, the thermal load decreased with the size of the building because of larger internal loads. For passive cooling, the opposite occurs. Generally speaking, passive cooling and ventilation loads increase as the size and complexity of the building increases, as shown in figure 3.5. Assembly spaces that have peak periods with very large numbers of people add special challenges from the heat of the people, humidity, and carbon dioxide.
Fig. 3.5. (1.) Metabolism: Unlike passive heating, passive cooling and ventilation demands increase as the size and complexity of the building increases, with the exception of assembly spaces. (2.) Scale of cooling and heating needs: Passive cooling capability varies distinctly with humidity. In dry-hot climates, clear night skies allow night radiation and convective cooling with cool night air. In hot-wet climates, the only thermal sinks available are wet surfaces that can cool through evaporation.
Prerequisites for Passive Cooling
Careful orientation of buildings and windows and solar control strategies can block the unwanted heat from the summer sun, yet still allow the sun in for winter heating and daylighting. The critical first step is keeping windows in the shade during the summer. This is easiest for a new building, where summer shading can often be achieved at little or no cost, but it can be done for almost any building even as a retrofit. (Refer to pages 26 and 54 for a reminder of the sun path and radiation characteristics, and here for energy-efficient design.)
Orientation matters most, and a clear understanding of radiation impacts can help you pick the best placement for windows or the best place in an apartment or office complex. Studies in Davis, California, showed that the worst orientation of apartments (east top) in one complex in summer had energy bills that were ten times higher than the best (north bottom) and twice the south-facing apartments (Hammond et al., 1974*).
We want to start with a very efficient building shell, but then we need to remember that the sun’s energy is concentrated on the east and west walls and windows on summer days in the mid-latitudes while the equator-facing and north sides receive considerably less radiation. Direct radiation is most important, but diffuse radiation and reflected radiation must also be considered.
Table 3.1. Solar heat gain for unshaded windows at 40°NL. By comparison the conduction through a light-colored R-40 wall with a temperature of 95°F outside and 75°F inside will be only 12 BTU/ft2/day.
In the summer, unshaded east- and west-facing windows can add more than a hundred times as much energy as a super-insulated wall per square foot and twice as much unwanted solar energy as an equator-facing window. With windows in the right places, a building will be much more comfortable and economical in the summer and winter.
Building Shape and Orientation
A striking example of what can happen if solar control is ignored is demonstrated in the building shown in figure 3.6. This is an exhibition and visitor facility for the ancient Greek city of Selinunte in southern Sicily. It’s a clear design concept with ample parking leading into a sloping plaza and an underground exhibition hall and ticket and sales areas that set the stage for the magnificent ruins. However, it has one fatal flaw—the lack of solar control. The front facade facing southeast below the bermed roof is all glass with no solar control. Direct solar gain is accentuated by reflection from the sloping light-colored plaza. As a result, what is potentially a powerful design is made very uncomfortable by the harsh sun. This glaring sun has also faded out very large expensive drawings of the original city, a huge model of the site, and is slowly destroying the building itself. The very large glass sheets on the front facade are cracking, doors are ripping off their hinges from the constant expansion and contraction, and the concrete surfaces are spalling. The moral of the story is that sunshine can be a wonderful thing, but it can also be a monster if ignored.
Fig. 3.6. Building shape and orientation in Selinunte, Sicily, 37.5°N latitude.
Solar Control
The easiest control device is a horizontal overhang. Overhangs can be very effective for apertures that face the equator because the sun path is high in the summer and low in the winter. Additional challenges to solar control come from east–west orientation, but a variety of shading devices can be used to reduce overheating.
Unfortunately horizontal overhangs do not work well for windows oriented more than 15° east or west of equator facing. Wing walls, louvers, or fins can be used to protect these orientations but must be carefully designed for appearance and durability. Movable fins are best but can be costly and complex to build and maintain. These may be easier to build and maintain if they are included within a double facade.
Shading Devices
There are many approaches to providing shade, privacy, security, and flexibility to windows and doors, as shown in figure 3.8. These fall into two general categories, exterior and interior shading devices.
Exterior shading devices are usually better because once the sun enters a window, heat has effectively entered the house, and although its impact can be reduced with an interior shade that is well sealed, some heat trapped between the shade and the window will still pass into the room.
In traditional management of courtyard buildings in the Mediterranean region, a toldo or fabric shade may be closed over a courtyard when it is hot. These shades are often made of fabric that is open weave or translucent to allow light in but reduce heat gain. John Reynolds found that these could reduce temperature more than 30°F. A large courtyard in San Diego’s Balboa Park was transformed from a hot-in-summer, cold-in-winter space to a very pleasant environment by adding a permanent translucent fabric cover.
Fig. 3.7. Toldo system of sun control. (a) Retracted. (b) Extended.
Fig. 3.8. (a) Exterior shading devices. (b) Interior shading devices.
An exterior porch that gets too hot in summer, particularly east- or west-facing ones, can be shaded with homemade vertical sun screens hung at the end of the porch. These can be louvers, shade screen, shade cloth, or diagonally crisscrossed wood strips, such as plaster lath, set into a frame; or woven willows, bamboo, or cane. These will still admit enough light so the porch won’t be dull and dim, but will keep the porch and the building much cooler. Motorized or mechanical awnings can be used to provide sun when wanted and shade when needed. Those developed for the recreational vehicle market are very robust.
The ultimate design of a total exterior shading retrofit may include a mix of the options discussed here, depending on the many variables of orientation, window placement, available space (for wings and fences), and certainly on how the retrofits will affect your building’s appearance.
Additional Approaches to Thermal Control
Cool Roofs
Although insulated ceilings can reduce heat gain from a hot roof, it is desirable to further reduce gains with light-colored roofing. Light colors can reduce roof temperature dramatically, keeping the building cooler and extending the life of the roof. Certain roofing materials also naturally “run cooler” than others. Tarred roofs, dark metal roofs, and asphalt shingles are among the hottest roofs, while living roofs are the coolest.
Measurements of roof temperatures in Davis, California, found that black roofing reached 162°F, two coats of aluminum paint reached only 143°F, and smooth white paint topped out at 117°F, a cooling benefit of 45°F essentially for free.
There is, however, an aesthetic nuance here that should be discussed. Light-colored buildings are much more visually intrusive in most landscapes than dark-colored. Most landscapes, at least in temperate biomes, are dominated by green, so darker buildings and roofs fit in much better. If the roof can be seen from public spaces and you still want it to be as cool as possible, you should consider “cool roof” technology. By using tricks with emissivity, you can look dark and yet act light and stay cool.
Fig. 3.9. Be discreet: Fit the building into the environment.
Ten Approaches to Shading
A double roof may make sense in the hot desert or the tropics. Double roofs were included on early Land Rovers for tropical use for this reason. The upper roof should be light in color and well vented so that the heat buildup between the two roofs sets up a convection airflow to exhaust hot air.
Landscaping
Landscape elements can help a great deal with solar control. Plants on an arbor can provide different solar control between spring and fall, but arbors are often made with such large elements they block too much of the winter sun. A tension arbor made of wire works very well. A wire arbor can provide good support and almost full sun in the winter after the vines or plants are pruned. If wood or metal elements are used for an arbor, keep the structural members as small as possible so they won’t cast shadows on equator-facing windows in the winter.
Trees, vines, and shrubs are attractive and can provide very effective control for roofs, walls, and windows. Trees can have a shading coefficient of 0.2, which is the same as an overhang. However, care must be taken to minimize unwanted shading in the winter. A deciduous tree to the south can shade in the winter as well as the summer because limbs, bare branches, and twigs can block much of the desired sun. Placing deciduous trees to the southeast and southwest instead of directly toward the equator can allow summer shading and unimpeded winter sun.
Landscaping is also perfect for providing needed shade for windows or walls that face east or west. An arbor or netting should be held back from the window or wall 2 to 3 feet, if possible, to allow for better ventilation and to avoid damage to the siding. A small misting system that uses a high-pressure nozzle to create a mist of water on the outside of the vines can add additional cooling.
A solar-control landscape plan for a house or commercial building might include tall trees to the southeast and southwest for wall and roof shade, deciduous vines for shade on a horizontal arbor above the equator-facing windows, a low-growing darker green ground cover on the equator-facing side to reduce ground reflection and heat buildup, hedges to the east and west to shield those walls and windows, and shrubs or small trees on the polar side to the east or west to block late-evening and early-morning sun in the hottest period of the summer. Shade plants also contribute to a cooler building environment, especially compared with artificial grass, concrete, asphalt, or gravel surfaces. These high-mass surfaces get very hot and retain heat late into the evening or night.
Not only does landscaping cool the house and yard, it also makes a quieter, cleaner, and more satisfactory environment. In Sacramento, California, cutting down trees in one neighborhood resulted in a 10°F jump in ambient summer air temperature and considerably increased cooling loads in adjacent homes.
Improved Glazing
Attempts to use tinted glass in the past were often not very effective. Much of the glass used cut down on the light transmission in the visual spectrum but not the UV or infrared, so the occupants got less light and still much of the heat. It would have been better just to use less glass. However, the advent of heat-reducing low-e glass has corrected this situation, allowing in much more of the visual spectrum and much less heat. But do not use this glass on south windows where passive heating is wanted in the winter.
Space Planning
For more extreme cooling conditions like the tropics and semitropics it can pay to zone the heat-producing parts of the building so they don’t contribute cooling load to the rest of the building; kitchens, laundry, and utility rooms are the most common offenders in residences. You can see an example of zoning to move heat producing activities out of the living space in figure 3.15 on page 97.
Fig. 3.10. Solar control options include, solar control glass screens, most expensive, exterior operable blinds, roll-down awnings, and awnings.
Classic Approaches to Passive Cooling
Passive cooling has a long-established history dating back thousands of years with a legacy of very sophisticated techniques. Recent improvements have come with new materials, more sophisticated analysis, and the development of performance-prediction techniques for the four systems shown in figure 3.11.
All of these cooling approaches are affected by local climate and microclimate. The choice among different systems is based on the humidity levels and temperatures throughout the year, as noted in the introduction. Places with low humidity have much more potential for night sky radiation and accompanying cool night air. This allows cooling with system 1 or 3.
Quiet and Cool
Fan noise can be an issue with cooling fans. If the fans are noisy, they won’t be used as often or feel as cool. A quiet ceiling fan should be less than 60 dB. An average 18-inch hall fan emits 76 dB on high, 66 dB medium, and 53 dB low, while a 7-inch fan puts out 56 dB on high and 48 dB on low. Sound ratings may also be given in sones (a subjective measure of sound intensity as heard by humans)—get the lowest you can find. Panasonic makes an 80 cfm bathroom fan that is only 0.3 sone.
High humidity acts as an obscuring blanket for night radiation and keeps night temperatures relatively high. This reduces the capability of approach 1 and 3. For this reason, tropical and subtropical hot-wet climates are considered the most difficult passive cooling situations. Still, there are many successful passively cooled buildings in these climates utilizing systems 2 (evaporative cooling by air flowing over the skin) and/or 4 (architectural-based systems utilizing cool towers or landscape elements such as fountains).
Combinations of many of these systems are possible.
System 1: Night Ventilation Cooling
This approach is most applicable to dry climates with cool night air. It is essentially the same as the direct-gain/distributed-mass system for passive heating described on page 58. These components work equally well for heating and cooling. Therefore, the details and sizing information on page 60 apply here as well—with the addition that for cooling, we need to provide good airflow over the distributed mass at night. Information on these techniques is covered under the ventilation section of this chapter.
System 2: Ventilation Cooling
This approach is most applicable to hot humid climates such as the Gulf Coast of the United States, much of South America, and South Asia. Owing to high humidity, cool night air and night sky radiation are less available for cooling, and we need to rely on airflow over moist skin both day and night for cooling. This approach has been used successfully for thousands of years. The key to success, other than total protection from the sun, is getting sufficient airflow. Ideally, you want on the order of thirty to sixty air changes per hour. Thermal mass is of less help, although thermal lag through roof elements can help reduce peak afternoon temperatures. Walls need to be highly porous to improve airflow.
The traditional approaches shown in figure 3.12 have large overhangs for protection from the hot sun and either no walls at all or walls that are highly porous to air. Screened porches offer large free-flowing surfaces. Traditional materials such as thatch roofs and woven-fabric walls perform well because they shed water but also breathe, which helps with ventilation and minimizes the development of mold problems in wet and humid environments. Traditional Mayan homes with thatched roofs remained comfortable in summer, but when the switch was made to black corrugated roofing they became very uncomfortable.
Fig. 3.11. Classic passive cooling systems.
Ventilation Cooling for Hot Humid Climates
1200
Fig. 3.12. Traditional ventilation cooling responses in the vernacular architecture of the tropics. Tall ceilings, porous but insulated roofs, and large openings for ventilation all facilitate cooling. Shown are examples in the Amazon Basin and the South Pacific that use highly developed organic fabric construction techniques.
1500
Fig. 3.13. The medieval architecture of Japan evolved a responsive hot-humid-climate architecture of highly modular construction, sliding walls, sophisticated use of wood and thatch, and maximum airflow day and night, with large openings to the outside and large openings for internal circulation and cross-ventilation.
1600
Fig. 3.14. The formal architecture of Mogul India evolved wonderful methods of ventilation cooling while also accomplishing a high degree of privacy. (a) At Fatehpur Sikri, near Agra, cross-ventilation was achieved by eliminating walls; (b) in the harem at Fatehpur Sikri, privacy was obtained by screens similar to those shown in (c) at the Red Fort.
With our over-reliance on mechanical air-conditioning, cooling for hot, humid conditions has almost become a lost art. It is instructive to look at successful historical examples. One of the best is the staff housing built in Gamboa, Panama, built by the United States for the families of the engineers creating the Panama Canal. These houses were built in 1906 before the availability of mechanical air-conditioning, so passive cooling was the basis of the designs using the following architectural techniques:
- Three stories high with living facilities on the higher levels, where breezes are stronger and more accessible.
- A ground level used mostly for services and largely open.
- Long, thin buildings carefully designed for maximum cross-ventilation. Interior walls, where they exist, have open grilles above privacy walls.
- Large, well-ventilated attic spaces and ample overhangs at each floor for shading from the always high tropical sun.
- Ample banks of screened casement windows on all sides.
- Strict zoning separating heat-producing areas such as bathrooms, kitchens, and laundry rooms.
- Provision of ceiling fans as an integral part of each room.
Proven Performance in the Tropics
1906
Fig. 3.15. Staff Housing Gamboa, Panama. With the return of the Panama Canal facilities to Panama, Gamboa is now a resort run by the Panamanians. If you visit, rent a suite in these facilities, shut off the noisy wheezing window air conditioner that’s been recently added, open the casement windows wide, smell the fragrant tropical breeze, listen to the sounds of exotic birds, turn on the ceiling fan, sit back in the wicker furniture, and pour yourself a cool gin and tonic. (a, b, c) The small structure on the ground floor is the laundry room. (d) Typical interior. (e) Kitchens, bathrooms, and laundry are thermally isolated.
Ventilation Cooling, Recent Examples
Taiwan is an example of a hot, humid climate where the dogma is that you need mechanical air-conditioning to survive. Shown are three examples that help demonstrate that modern versions of passive cooling by ventilation are applicable in the hot humid climates of the world.
Classrooms for Dharma Drum University
Dharma Drum is a new Buddhist university being built on the northern coast of Taiwan. Shown here are studies done to illustrate the passive cooling capability for high-density classroom buildings. These long, thin buildings utilize enhanced cross-ventilation, natural lighting, and ceiling fans powered directly by photovoltaic panels.
Fig. 3.16a. Ventilated classroom energy analysis using Energy10 modeling software (continuous vent @ 2 cfm/sq. ft. work and nonwork days).
Fig. 3.16b. Section drawing.
Chi Family Compound in Dragon Well
The Chi family wanted a traditional house without air-conditioning, which they found uncomfortable. The plan shows some of the ventilation studies used to create a passively cooled building. It was constructed in 2003 and has successfully operated as a passively cooled building.
Treasure Hill
As one of the last post–World War II squatter settlements, Treasure Hill in Taipei is now considered a cultural resource. This retrofit of two abandoned hovels into the Community Planning Center involved making it passively cooled by utilizing the breezes from the adjacent riverfront, as well as adding openings, ventilation cowls, and insulation.
System 3: Radiant Cooling
Radiant Cooling Design
High mass roofs with operable insulation, either of concrete or with roof ponds, provide the functions of cold collection and storage with one element and, therefore, these types of radiant cooling are effecting in providing daylight cooling, practically at any region with low cloudiness at night, regardless of the air humidity.
—Baruch Givoni, 1991
The potential for natural cooling using cold night sky and cool north sky in the day has received much less attention than it deserves. Although these microclimate/site resources are not as powerful as evaporative cooling, they can be used as important elements in natural cooling design, particularly in areas with clear skies and lower humidity. The rapid chilling in the desert after a hot day results from the powerful effect of night sky radiant cooling.
The cooling potential of the night sky was first utilized by societies many centuries ago. In Iran, night sky radiant cooling was used in the yakh-chal to produce ice, even when ambient air temperatures fail to drop below freezing (see figure 3.19). A tall U-shaped mud wall facing north was used to fully shade a small shallow trough of water. The cooling effect of the north sky in the day and the colder sky at night allowed ice to form. The ice was then collected and stored for use during the hot days that follow. Similar practices have been used in the desert of Chile and with ice pits in the West Indies.
Radiant cooling works because sky temperatures may be 10 to 40°F below air temperature. Frederick A. Brooks reported a night radiant-cooling-driven air temperature observation of 28°F over straw-covered ground near Sacramento after a daytime high of 98°F. The heat loss may be on the order of 20 BTU per square foot per hour (sf/hr) with a clear sky and low humidity, although peak losses in the desert have been measured at 30+ BTU/sf/hr; the nighttime loss under clear dry skies can be even higher.
Radiant cooling results when the incoming direct and indirect radiation is less than the energy radiated to the sky vault. During the daytime, the short-wave radiation from the sun dominates, but at night long-wave radiation from earth exceeds the counter-radiation from molecules and particles in the atmosphere. This loss is referred to as the net outgoing radiation and is primarily at wavelengths between 7 and 14 microns. Most of the net outgoing radiation occurs to the cold night sky, but radiation to space also occurs during the day. If this outgoing long-wave radiation was not taking place, the earth would get warmer and warmer; and if the losses were not reduced by water vapor and carbon dioxide in the atmosphere the surface of the earth would get very cold at night.
The rate of outgoing radiation is not uniform across the sky vault. Research has been done on the variation in radiation across the cold night sky. At night the greatest radiation loss occurs directly overhead.
Table 3.2 Net outgoing radiation. FA Brooks, 1959.
The radiation losses also vary with the nature of the sky view. Trees, walls, clouds, or structures can decrease night sky cooling, but more than 80 percent of the outgoing radiation flows to area of the sky vault above 30° from the horizon, so relatively low obstructions can be ignored. Trees and taller nearby structures should be considered, however. You can see this with the pattern of frost or dew formation on car windows. The windows protected by being near a building or under a tree will be free of frost or dew because the radiant loss has been reduced.
Observations on radiant cooling to the sky by Tod Neubauer,* Richard Cramer, and N. R. Ittner added more insight on the cool day sky. In a comprehensive test in Davis, California, the nighttime and daytime radiant exchanges across the sky dome were measured. The average nighttime temperature for a horizontal black panel was 10°F below air temperature, and even vertical walls with sky exposure were 5°F below air temperature. White panels were considerably cooler during the day, but similar at night—as would be expected with paints with comparable emissivity. White panels sloped facing north at 60° and 70° stayed below air temperature throughout the day. Further studies identified 65° as the coolest spot, as shown in figure 3.20.
Tests in the very hot Imperial Valley identified the cool spot in the north sky at 60°N in August. This cool spot had previously been noted as a minimum at a compass point opposite the sun and approximately at a right angle. The cool spot was 40°F cooler than air temperature at 3:30 pm, 23°F cooler at 6 pm, and 13°F cooler at 2 pm. Observations of animals suggest they are able to detect the cool spot in how they locate themselves in relation to shelter.
Fig. 3.20. Day sky cool spot: Relation of roof plane to temperature rise on a typical summer day in August. Neubauer and Cramer, 1965.
Increasing moisture in the atmosphere 30 percent can reduce outgoing radiation as much as 15 percent, while cutting it in half can increase cooling by 25 percent. Increasing moisture content can reduce the radiant losses as the angle away from the zenith increases. The strongest radiant cooling will be achieved with horizontal surfaces exposed to unobstructed, dry, and cloudless skies. Cooling may also be enhanced by convective cooling if cool breezes are blowing; but these may reduce radiant-cooling benefits if the cool air from a radiating surface is carried away instead of being captured by the building.
Even in more humid areas, cooling rates up to 50 percent of those in low humidity areas are possible, which means that up to 100 to 150 BTU per square foot per day of cooling effect may still occur on a clear night. Although radiant sky cooling is more effective in the drier areas, it can be worthwhile elsewhere when combined with evaporation, convective cooling, or ventilation.
Design Techniques
Many traditional buildings with high-mass roofs benefit from night sky cooling. Without it they would become ovens. The thermal lag provided by the slow energy transfer through a thick roof can make a classroom or office more livable during the late afternoon, but will add unwanted heat later at night. If the high-mass roof could be insulated in the day, it will work better. But faster energy transfer is desirable.
Fig. 3.21. The Winters House, Jon Hammond, Living Systems.
Harold Hay resolved this problem in his development of the Skytherm houses. These use bags of water on the roof covered with movable insulation to take advantage of radiant night sky cooling. These systems have been very effective. 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 room. In summer, the roof insulation panels are drawn back at night to initiate cooling by night sky radiation and then closed during the day to retain the coolth. Open water, sprinkling, or misting can be added to provide the added benefit of evaporative cooling. The roof water bags can also be used for heating by reversing the operation, opening the covers during the day in winter to collect solar heat and closing them at night.
The radiant transfer with a high-mass ceiling makes for very uniform and comfortable temperatures. A test facility in Phoenix, Arizona, and then a house in Atascadero, California, demonstrated the year-round performance of this design. The problem remained the management of the horizontal insulation panels. (See the Atascadero prototype on the section title page.)
These problems were resolved with the Winters house designed by Jon Hammond. This used hydraulic rams to tilt up a reflective, insulated roof panel over the water bags. This is more robust and develops a much better seal. Performance fully met the expectations of the clients, who were from Alaska and wanted a cool home in summer. This building is shown on figure 3.21.
Water roof ponds have been used because they have good thermal capacity and by circulation move the warmest water to the radiant surface. But concrete, phase-change materials, or other high-mass materials can also be used.
Fig. 3.22. Radiant cooling with high-mass roofs.
Radiant Cooling Details
The widespread use of high-mass roofs in developing countries offers a great opportunity in developing further options for low-cost movable insulation retrofits to improve radiant cooling. An insulated louver system may be a good solution for a reliable and robust system that would work for both heating and cooling as shown below.
Steve Baer, always the most practical idealist, was also influenced by Harold Hay (see page 103) and developed what he calls the Double Play System essentially doing what Hay’s roof ponds did but using off-the-shelf components that have been time-tested—components like 12-inch-diameter PVC pipe as water containers inside the building and polypropylene swimming pool solar collectors as the radiator/absorber. The result is a system of passive heating and radiant cooling that is simple, inexpensive, easy to understand, and easy to maintain.
Baruch Givoni, working in the Negev Desert, has developed a range of approaches for using radiant cooling using air systems rather than water systems.
Other more exotic cooling systems that use the combined effects of evaporation and night sky radiation are the wetted roof pond, the cool pool, and heat exchangers coupled to a shaded pool or swimming pool, shown in Cool Pools.
“Night sky” cool roof systems that pump water to a white roof for evaporation and night sky radiant cooling can also be effective. Several options were developed by the Davis Energy Group. A manufacturing facility in Vacaville found that this system provided a 67 percent cost saving with a payback of 2.5 years. In Australia, this type of system is being used to provide cooling through floor slabs and ceiling beams.
Fig. 3.23. Baer high-mass ceiling with water pipes.
Fig. 3.25. White Cap F System for nighttime cooling.
Calculations
The rate of heat loss can be calculated using any one of the formulas developed for calculating outgoing radiation at night. These generally provide reasonable agreement with observed data. Geiger* provides a graphic solution that accounts for humidity, relative temperatures, and water vapor. Radiant losses are increased when the radiator is warmer than air temperature. They can be reduced up to 30 percent by high humidity, but there are two frequency windows in the atmosphere where moisture content doesn’t affect radiation loss so they can work even in more humid areas.
The relative humidity equation developed by McDonald* develops estimates for night sky cooling. It uses readily available information and it may be the easiest formula to use. ε is the emissivity of the surface, which would typically be around 0.95 to 0.99.
Io = ε (0.165 – 0.000769 RH) langley/minute
With ε assumed to be 0.95, and relative humidity of 20 percent, this estimates net radiant loss at 0.16 ly/min or 34.7 BTU/sf/hr.
The Passive Solar Research Group in Omaha explored different methods for calculating radiant losses. One approach was possible when an upward-facing infrared thermometer (165° aperture) was used to determine sky temperature. The equation for radiant loss then becomes
R = εr (σTpond 4 – σTsky4) w/m2
εr = roof pond emissivity (perhaps 0.95)
σ = Stefan-Boltzman constant (5.7 × 10-8 watts/meter2/°K)
If the sky temperature is 15°C and the roof pond is 20°C, then the radiant heat loss will be about 7 watts/m2/hr or 20 BTU/sf/hr.
Clouds can reduce net night sky radiation by blocking radiant loss and increasing the counter-radiation. We feel this effect when thick cloud cover at night reduces heat loss to space, moderating a winter night, or when clouds reduce cooling effects on a hot summer night. Even if there are no clouds, increasing the moisture reduces heat loss to space, but when very dry atmosphere conditions develop the heat loss can increase, helping us keep a house cool in summer.
Most of the long-wave-radiation blocking occurs very close to the earth. Dense fog can reduce outgoing radiation to zero; high thin cirrus might reduce radiation loss only 10 percent. Relatively little work has been done to provide tables for calculating impact of different cloud types on outgoing radiation. The formula for calculating the effect of clouds on outgoing radiation is: I = Io(1 – knm), where k, n, and m are constants with different values calculated by different investigators. For general estimates, n and m can be around to equal 1 and the formula becomes: I = Io (1 – k). The table below presents experimentally determined values for 1 – k.
Table 3.3. The impact of clouds on outgoing radiation.
The radiant cooling rates observed are typically 10 to 50 BTU/sf/hr, but in the desert with very high temperatures and dry, clear skies the rate may be higher. Even with more commonly observed rates, a 1,200-square-foot roof would provide 12,000 to 60,000 BTUs of cooling per hour and perhaps 100,000 to 500,000 BTUs each night, more than enough for an energy-efficient building shell to perform well. Still higher radiant-cooling rates may be possible with selective surfaces and better isolation of cooling surfaces from conductive and convective heat gain.
Cooling a high-mass roof surface that is exposed to the room below works well, and collecting cool- or cold-air drainage has shown some promise. Radiant cooling has great potential and has been demonstrated in many buildings, but commercialization of this approach is still lagging. This is a result of limited development of movable insulation systems and the ease of installing low-cost air-conditioning made possible by heavily subsidized nonrenewable-fuel-based generation of electricity and no payment for external costs of air pollution and global climate change. As prices for electricity climb and fees for climate change impact are added, or in places where electricity is not available, these radiant systems will be more widely used. They can improve security because they can work well even when power supplies are interrupted.
System 4: Evaporation
Evaporating water has a very powerful cooling effect. The phase change from water to water vapor, the latent heat of vaporization, takes considerable energy—about 1,050 BTUs per pound of water at 75°F. Evaporating a cubic foot of water will provide about 65,000 BTUs of cooling for a passive building. Many traditional building systems used evaporation-based cooling to great advantage, perhaps with a pool or fountain in the courtyard or building, sprinkling the tile or floor with water, or utilizing the evapotranspiration from landscaping to provide cooling. With most evaporative systems airflow is important, so refining the ventilation design is important, as is protecting cooling breezes. These remain among the most important cooling options for the world’s hot climates because they can often minimize or eliminate air-conditioning systems.
Lessons from these historic cooling options can be used to improve comfort for the five hundred million people who live in warm climates but earn less than a dollar a day. In ancient Rome, for example, villas would often have a shallow landscaped pond for summer cooling. The maziara of the Middle East uses terra-cotta water jars in the wind flow to add evaporative cooling to a building. Frescoes from Egypt show slaves fanning terra-cotta jars full of water to cool the room. In traditional Iranian designs, many buildings have running water in areas with ventilating towers, called badgirs, above to improve airflow. Wetted fabric or fiber can also be used for cooling. In parts of India, the solid doors were removed in summer and tatties or a thermantidote made of dried khusskhuss grass was installed. These fiber nets over a framework are wetted to create a cool airflow by sprinkling or with an automatic drip or tipping trough that rolls over when full to wet the pad. Similar practices can be found in Iran and other parts of the Middle East. In Rajasthan, a grille was wetted with rosewater and used in place of a door. The blades of the large ceiling fans in India were sometimes wetted as well. Tents were also wetted to keep occupants more comfortable.
The evaporative cooler using excelsior pads (aspen fiber) developed from these simple systems. Indirect evaporative coolers, which offer cooling without humidity, are once again reaching the market. These are very energy-efficient and with improved heat exchangers can reach temperatures below the wet bulb temperature.
Key Terms
- Dry bulb temperature: the air temperature measured with a bare thermometer.
- Wet bulb temperature: the air temperature measured with a thermometer bulb with a wetted fabric sleeve exposed to rapidly moving air (often done on a sling).
- Relative humidity: the percent of moisture in the air compared with the maximum possible.
- Dew point temperature: the temperature to which a surface must be cooled to condense water. This can be determined with a metal cup full of ice water and a thermometer. Pour cool water into the cup and add ice. Measure the temperature periodically until drops form on the outside of the cup.
Landscaping with larger trees can provide very powerful evaporative cooling and solar control. Tests have showed temperature reductions of 22°F with full tree shade, dropping temperatures from 108° to 86°F in a test trailer. The cooling effect of landscaping can be enhanced by spraying or sprinkling. Peter the Great reportedly used a similar system to add water from pipes to the outer foliage of a tree to increase summer cooling at his residence.
Evaporative cooling doesn’t leak ozone-layer-killing or global warming gases like air conditioners do. The most economical systems involve simple evaporation from fountains, landscaping, or mist systems. More complex evaporation-based cooling systems with tanks or pools that add night sky radiation cooling as well can provide comfort even in the most extreme deserts.
How Much Cooling Can We Expect?
The evaporative cooling that can be expected for a building site can be estimated from the nearest evaporation pan data. Standard daily pan evaporation is measured using a 4-foot-diameter Class A evaporation pan. Class A pans offer actual measured evaporation rates and integrate wind, humidity, and temperature. Although the pans are smaller than cooling system tanks or ponds would typically be, they suggest the potential for evaporative cooling pretty clearly, as shown in figure 3.26.
Fig. 3.26. A rough idea of cooling potential can be estimated by calculating the evaporation per square foot per month. This is shown for Yuma, Arizona, in figure 3.27. If the evaporation is 12 inches, then 1 cubic foot per square foot of water surface is evaporating every month. This provides 65,000 BTUs per square foot of cooling per month, or about 2,000 BTUs per square foot of pond per day. More typically, cooling rates will be in the range of 300 to 700 BTU/sf/night. In an energy-efficient building with good solar control, this will be enough to provide cooling for comfort with many evaporative cooling systems.
Fig. 3.27. Evaporation cooling potential for Yuma, Arizona.
The potential for evaporative cooling is related to the site microclimate. Humidity and wind affect evaporation rate. Evaporation is most powerful in arid environments but can be helpful in more humid environments if it is indirect and does not increase the humidity levels where the people are living. Evaporation can be used to cool the air in a building, around the building, or the thermal mass. Common options include cooling towers, misters, sprinkling the tile or concrete, fountains, pools, or landscaping. Indirect cooling options include roof ponds and roof sprays.
Cooling Towers
The Environmental Research Lab at the University of Arizona–Tucson developed a very effective cooling-tower design for buildings using evaporation. This modern version of a Persian wind tower was a better investment than a solar chimney for cooling. More recently, evaporative cooling has been added to traditional wind towers in the Middle East to improve cooling. These experiments have included pads, wetted curtains, and clay pipes. Airflow has been slightly reduced but cooling has been increased. The cooling towers developed by ERL use a pad or set of evaporative pads at the top of the tower. The intake air is cooled and the denser air falls down the tower and into the building. Systems have been built for both homes and greenhouses and performed very well.
The tower is 6 feet by 6 feet in cross section and 25 feet tall. A 4-inch-thick vertically placed evaporation pad of cellulose sits at the top and is kept wet with a 1/25-horsepower pump. A high-quality mister might provide the same benefits, but would probably require more careful control and maintenance. A plywood X-baffle in the tower helps collects wind from any direction. Adding the wind effect improves tower performance significantly, reducing the size needed. With a 4-meter-per-second wind, the tower downdraft doubles. A test system installed on a home provided good comfort, never exceeding 78°F inside, despite air temperatures reaching 108°F. The inside air temperature was held to within a couple of degrees of the outdoor wet bulb temperature.
Cooling towers can be easily incorporated in new buildings (see figure 3.30), but are not hard to retrofit to some existing buildings. The cool draft should flow into the building through a large inlet and perhaps into a hallway or central room with good airflow paths to other rooms. A solar chimney or stack effect may be used to help draw cool air through the building. Greenhouses with a solar chimney at one end and cooling tower at the other allow for passively generated cross-ventilation with cool air.
Fig. 3.28a. Evaporative cooling tower with solar chimney for exhaust.
Fig. 3.28b. Cooling-tower house.
Fig. 3.29. Cooling-tower thermal performance.
Fig. 3.30. Two passive downdraft cooling towers at Zion Visitors Center, Zion National Park, Springdale, Utah.
Additional Cooling Techniques
Misters
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 is often a medium-priced system with a booster pump and metal fittings, running at 100 to 200 psi.
Misting systems can provide cooling comfort for a courtyard or exterior space as well as cooling for the building.
Fig. 3.31. Misters cooling the Audubon Center in Los Angeles.
Sprinkling Interiors
Many traditional buildings used evaporative cooling from sprinkling the tiles or floor. This can be effective in a building or in a courtyard. Temperatures can be dropped 20 to 60°F in a matter of minutes. The lower radiant temperatures and cooler air both improve comfort.
Fountains
Traditional buildings often incorporated a fountain to cool spaces. The effect comes from evaporation primarily, but the psychological benefits of the sound of running water are also important. For maximum cooling, the fountain should create small water particles. A fountain in a pool can help keep pool temperatures down. This cooled water could be used in a radiant cooling system or fan coils. Waterfall features can also be used inside or outside to increase cooling.
Landscaping
Plants evaporate water as part of transpiration. A courtyard tree can provide delightful shade and considerable evaporative cooling for a building. A green wall or screen can provide evaporative cooling and solar control. Arbors, vines, and other landscaping can also provide evaporative cooling. The evapotranspiration of water from plants provides very significant cooling. This can be augmented by sprinkling the vegetation with water. A weeping hose or sprinkling hose can be threaded through a hedge or tree to improve cooling.
Green Roofs
A green roof is a roof that is partially or completely covered with vegetation and soil, or with a growing medium, planted over a waterproof membrane. Also known as living roofs, eco-roofs, oikosteges, and vegetated roofs, green roofs can also reduce ceiling or attic temperatures and stormwater runoff. Effective design and choice of plant material is critical. The challenge is developing a lightweight soil mixture and secure waterproofing system. The soil layer can improve roof material lifetimes by shielding them from UV radiation and extreme temperatures. Maintenance is also required for most living roofs. Some cities are supporting these living roofs with financial incentives to help reduce stormwater runoff and water-quality problems.
Green roofs can be categorized as intensive or extensive, depending on the depth of planting medium and the amount of maintenance required. Intensive roofs typically have deeper soil and require more maintenance and irrigation.
Fig. 3.33. (a) Green roof diagram. (b) Comparison of dark roof and green roof surface temperature at 40°N latitude.
Hybrid Applications
Cool Roofs
A light-colored roof is cooler than a dark roof, but roof temperatures can be dropped even more by sprinkling or misting, which lowers the temperature of the roofing material, which in turn helps to reduce attic temperatures in poorly insulated roof systems.
Evaporation on the roof can also be used to cool water in a reservoir for daytime cooling. A pump can be used to move water at night to a white roof, where evaporation and night sky radiation chill the water for use in cooling during the day. These systems are discussed in more detail in the section on radiant cooling.
A traditional sleeping room in the desert was sometimes made using a lightweight metal shell with a sprinkler running on the outside, creating a desert submarine. Not only did it sound cool, but it was cool. The monsoon or rain palaces of Asia could be updated to create a cool room. A lightweight metal roof would be chilled with a flow of water dropping off the eaves into a trough or pond and recirculated to the roof.
A roof pond that is fully shaded all the time and has an open surface for evaporation is called a cool pool. These systems are well suited for areas where temperatures are very high in summer. They are noteworthy because they can provide indirect evaporative cooling and can work even in more humid climates. Cool-pool performance tests in California’s low hot desert showed that they can provide comfort under extreme conditions. One of the test cells included a heat load simulating internal heat gains, and it showed very little change in temperature, for as water temperature increases so do evaporation and cooling. The importance of shading is clearly shown, with the sun-exposed pond getting very hot.
A cool pool with a water pond over the ceiling coupled to a vertical waterwall was extremely effective and maintained comfort even in the middle of a very hot parking lot at the state fairgrounds in Sacramento. This configuration provides low radiant temperatures on two surfaces, improving comfort. The demonstration unit by Living Systems maintained cool temperatures even though it only used plastic flap doors and experienced considerable human traffic (letting in hot outside air) with daytime temperatures exceeding 100°F.
Fig. 3.34. Cool-pool performance, Indio test.
Fig. 3.35. Section of cool-pool test cell (left). Cool-pool test cell performance chart, Sacramento, California, State Fair (right).
Roof Pond and Cool-Pool Performance
Not many studies of cool pools, roofs wetted at night, and flooded roof ponds have been done, so the relation between climate and cooling potential is not well developed. If average wind speeds from the nearest station are used to calculate the cooling rate for a cool-pool house, the cooling potential may be underestimated because the wind speed is generally highest when the relative humidity is lower. Conversely, if average wind speed is used to estimate cooling for a misted water bags in a Skytherm house with insulated lids closed in the day, it will tend to overestimate evaporation because the pond is being exposed at night when wind speed and relative humidity are higher.
A ground-mounted cool pool, a swimming pool, lake, pond, or the ocean, can also be used as a source of cool water for radiant cooling within a building if the transfer distance is short. The cool water is pumped through radiant tubing in the ceiling, walls, or floor to cool the space. But adding pumps and controls adds to the cost, complexity, and unreliability. An existing swimming pool could be converted to a cool pool by adding a screened shade. This large mass could be kept cool and used in a radiant cooling system or fan-coil system.
Supplemental Cooling
There are three basic types of mechanical systems for adding cooling: direct and indirect evaporative coolers and air conditioners. Air conditioners are commonly used and well understood, but direct and indirect evaporative coolers are more efficient, delivering comparable cooling capacity for one-quarter to one-half the energy cost of an air conditioner.
The direct evaporative cooler or “swamp cooler” draws outside air through wetted pads and blows the cooled air through the house or building. These were first used extensively in New England and the southeastern seaboard to cool large mill buildings. They worked there, but they are more effective in drier climates where the added humidity won’t be a problem. Models with the fan driven by a photovoltaic panel are available.
The indirect evaporative cooler couples evaporative cooling with a heat exchanger. It may draw air from the building through evaporative pads and exhaust it through a heat exchanger, dumping the now cooler but humid air to the atmosphere. Or it may use fresh air for both the cooling and the makeup air. A second fan draws drier exterior air through a heat exchanger, where it is cooled off and blown into the building. These were first used in Arizona and California in the 1930s on both homes and larger buildings. These are less efficient than a direct evaporative cooler, but they don’t add humidity to the living space, and they improve comfort. Indirect evaporative coolers were once sold by several manufacturers and were also widely used on commercial buildings. Neal A. Pennington developed a commercially sold home system in the early 1940s that was bulky but effective (see figure 3.36).
Cooling Efficiency
The energy-efficiency ratio of an air conditioner is its British thermal units rating over its wattage. For example, if a 10,000 BTU air conditioner consumes 1,200 watts, its EER is 8.3 (10,000 BTU/1,200 watts). Air conditioners with EERs ranging from 9 to 16 for central systems are available; and more efficient models, EER 20, use a groundwater source heat pump. In contrast, the EER may reach 40-plus for indirect evaporative coolers. These units offer double the efficiency, and are typically quieter than air conditioners.
Fig. 3.36. Indirect evaporative cooler, 1940s.
Cool and Comfortable
Large Buildings
Direct and indirect evaporative systems can be included in large-building design. Fountains, waterfalls, and pools can be included in lobbies and large rooms. Large-scale roof ponds and cool pools have not been built, but as noted before indirect evaporative systems were once widely used in large commercial buildings. Both approaches will be considered and used more frequently in the years to come as air-conditioning costs rise.
The experience with evaporative cooling in larger buildings was quite good, but is not appreciated by most architects or engineers today. The Walt Disney building and a medical complex used the successful Stockly plate system in the 1930s, providing up to 1.8 million BTUs of cooling at very low energy cost. A cooling shaft for direct or indirect evaporative cooling can extend the full height of a building and could be open or regenerative, with a system that recycles water. The administration building at the University of Arizona–Tucson was cooled with a very efficient system that ran from 1936 to 1952 and provided 500,000 BTUs of cooling with only 17.5 hp. The cooling shaft was 4 by 5 feet and extended from the basement to the attic. About 135 gpm of water were sprayed down the shaft as 16,000 cfm flowed up it. The cooled water was used in counterflow radiators providing 10,800 cfm to the conditioned space. The moist cool air was emptied into the attic, further reducing heat gain to the occupied space. The system was simple: two fans, a pump, and a 10 hp refrigeration system used during the infrequent hot moist monsoon weather in Arizona. It was totally concealed and had no exterior features that could weather or freeze. It maintained temperatures 15°F below exterior dry bulb.
Indirect Evaporative Coolers Return
Arizona’s hard (mineral laden) and corrosive water limited the use of indirect evaporative coolers there, but they remained in use in California until air-conditioning systems and energy became cheap. Fortunately they are coming back, with several new models on the market. Temperatures at or below wet bulb are possible with these new systems. Improved models use a more complex heat exchanger that involves multiple steps and are more powerful and efficient. The Climate Wizard can deliver significantly colder air than traditional evaporative cooling with air temperatures near, and at times below, those produced by refrigerated air-conditioning. The Coolerado Cooler uses a patented heat and mass exchanger (HMX) and cools the supply air in twenty stages. At each stage, the humidified air is exhausted and enhances the cooling effect of the supply air. The MasterCool 2-Stage Indirect Cooling Module can be added to any MasterCool evaporative cooler to provide even greater cooling power and energy/cost efficiency. Another version is also offered by OASys. These indirect evaporative coolers and their larger cousins are also well suited for commercial and industrial uses and may be integrated with a conventional AC system.
- Coolerado (residential). Coolerado Corporation, 4430 Glencoe Street, Denver, CO 80216. Phone: 303-375-0878. Email: CustomerService@Coolerado.com. Web site: www.coolerado.com.
- Climate Wizard (residential and commercial). Seeley International Americas Corporation, 1202 North 54th Avenue, Building 2, Suite 117, Phoenix, AZ 85043. Phone: 602-353-8066. Web site: www.climatewizard.com.au.
- OASys (residential). Speakman, PO Box 191, Wilmington, DE 19899. Web site: www.oasysairconditioner.com.
- MasterCool (residential & commercial). Champion/Essick EPX (commercial). Des Champs Technologies, 225 South Magnolia Avenue, Buena Vista, VA 24416. Phone: 540-291-1111. Web site: www.deschamps.com.
- Stage II (commercial). Spec-Air, 6850 McNutt Road, Anthony, NM 88021 Phone: 505-589-6200. Web site: www.specair.net.
A home system can be built by a skilled do-it-yourselfer. J. R. Watts tested a system built using a truck radiator for the heat exchanger with a ¼-horsepower 18-inch fan to move air into the house. The water was chilled in a redwood packed cooling tower of nominal 90,000 BTU capacity that was capable of cooling water to within 1°F of wet bulb temperature and circulated with a ½-horsepower pump. None of these was optimized, but the system was very effective. A plate-type system built by a grad student at the University of Texas provided 57,400 BTUs of sensible cooling per hour using less than 1 horsepower.
Evaporation cooling systems using fabric tubes might prove very economical. These would use a counterflow system with an updraft of moist air and/or sprinklers in the center tube and a downdraft of cooling air in the outer tube. The same approach could also be taken with a pair of nested culverts. The center culvert might be wrapped with a weeping hose. The inner culvert would be used for downdraft makeup air.
Radiant Cooling for Comfort
Water chilled by evaporation can also be used in a radiant cooling mode. L. W. Neubauer at UC Davis demonstrated this use in a test trailer in the 1950s. Metal pans full of water in the ceiling were cooled by airflow through the attic, providing 15°F degrees of cooling in the room. Comfort increased from the reduced temperature and the impact of radiant cooling surfaces above the head.
Indirect radiant systems have also been developed using panels or coils chilled by water cooled by a cooling tower or other form of evaporation. R. R. Irwin at Oklahoma State University–Stillwater used water from a cooling tower to chill radiant serpentine copper coils in the ceilings and walls of student apartments. Temperatures never exceeded 82.5°F and varied only a degree over the day, despite outdoor air temperatures of 106°F. A similar system in an energy-efficient building shell would be much more effective.
Fig. 3.37. How much cooling? July wet bulb is a good indicator of evaporative cooling potential. The Southeast has less cooling potential, but the comfort temperatures are higher and indirect evaporative cooling can still be very useful. In the arid Southwest, evaporative cooling is very effective, and indirect evaporate cooling is even better.
Evaporative cooling is effective!
Ventilation is the key element in passive cooling systems 1 and 2, and is needed to meet the health requirements mentioned on page 51. It is also a large subject with many facets, so it deserves its own section. Air flow plays a key role in passive cooling and a major role in comfort and health.
Ventilation based on natural forces is quiet, requires no electricity, does not create global warming gases, and works even when the power is off. Passive ventilation can improve comfort, offer occupants more control over their comfort, and reduce operating costs. Building costs can be reduced even further when the HVAC system can be eliminated or reduced in size.
While a cooling breeze can be delightful, a hurricane is not. Most people find airspeeds up to 100 feet per minute (fpm) comfortable, while airspeeds between 100 and 200 fpm are often acceptable. Air movement on legs can be at higher speeds, but beyond 150 fpm, hair, loose paper, and light objects on desktops may start to blow around. Outside dust, pollen, and smog may also require special treatment, although most studies have shown outside air is often less polluted than inside air due to stagnation, molds, and concentrations of toxic materials, finishes, and household products in buildings. Noise can also be an issue. Building configurations and landscaping elements need to be designed to limit noise problems so that vents or windows can be open.
While ventilation can provide needed cooling in summer, ventilation in winter is needed to provide fresh air. This can be provided by trickle vents, makeup air vents for fireplaces and furnaces, and ventilation for bathrooms, kitchens, and other areas where moisture or pollution are concentrated. In a very efficient passive solar home, the performance is often good enough that windows can be opened as needed to let in more fresh air.
There are three general approaches for natural ventilation. Although they are often combined in various ways, we will cover each separately for clarification. The three approaches are:
1. Cross-ventilation.
2. Stack ventilation.
3. Various augmentation techniques.
Cross-Ventilation
Cross-ventilation techniques were highly developed in many traditional architectures and buildings. A visit to a plantation home in the Deep South offers a living textbook for natural ventilation. The large window openings, vents, tall ceilings, large doors, hallways, stairwells, cupolas, and orientation of these buildings were all refined to produce the greatest possible comfort without air-conditioning.
Planning for Cross-Ventilation
Where natural ventilation is sufficient to provide comfort, urban planning and subdivision design should protect or enhance natural wind flows. Streets and landscaping should be laid out to protect natural breezes for cooling. This is rarely done, but the wind flow around windmills in Holland is protected by “wind rights,” and a similar concept can be considered in the layout of subdivisions and cities. Wind flow can be protected by running the streets at only a small angle to the wind and using trees and landscaping to channel the wind into the houses. Where strong winds occur regularly, landscaping and design can be used to block the unwanted gusts, yet still protect the cooling breezes.
Cross-ventilation will vary from hour to hour as the wind speed and direction change. It is best to develop a very flexible ventilation system that can adapt to changes in wind direction, changes in surrounding landscaping and structures, and changes in use within a building. The goal is always to provide occupants with the ability to adjust ventilation to meet their comfort requirements.
Cross-Ventilation Details
If there is only one opening into a room, breezes generally will not flow into it very far. These cave-type rooms may experience much less than half the ventilation of a comparable room with window openings on two sides. Cave-type rooms are prone to air stagnation and the buildup of indoor air pollution from furnishings, finishes, appliances, and mold. Although these nonventing rooms are most common in apartment buildings, motels, and office complexes, they can also be created by poor design choices in homes and commercial buildings.
Cross-ventilation should be a guiding element in design starting with the schematics. It is relatively easy to include by rethinking design, and reconsidering older design strategies that provided cross-ventilation. David has asthma and allergies that make modern hotels and motels with cave rooms a problem, but older motels almost always have cross-ventilation that allows clearing scents and reducing mold density to tolerable levels. So it can be done, often for free with good window placement.
Fig. 3.38a. Room and window orientation. If windows are on opposite sides, the room should be oriented askew to the wind direction. If the windows are on adjacent walls, the room should be oriented to face directly into the wind. Increasing height difference between inlet and outlet helps induce natural ventilation during still times.
Fig. 3.38b. Inlet treatment. Air patterns in a room are largely determined by the inlet location and its relationship to the exterior surfaces of the building. It is important for night-vent cooling to wash thermal masses with cool night air via the technique shown. The same principles apply to the vertical dimension. Overhangs can have the same effect as wing walls and other barriers in the horizontal dimension.
Fig. 3.38c. Outlet treatment. Outlet size in relation to inlet size largely determines the speed of interior airflow. Change in direction causes greater spread but less speed.
Fig. 3.38d. Landscape elements. Trees, shrubs, walls, et cetera, can often be used to improve natural ventilation even if the building cannot be optimally oriented to the wind.
Fig. 3.38e. Reducing unwanted ventilation in the heating season. For a typical house, heat loss by air infiltration can be two and a half times as great in a5 mph wind versus no wind. Since infiltration may account for up to half of the building’s heating load, protection from wind with a windbreak can reduce heating requirements.
Predicting Cross-Ventilation Benefits
The pressure difference between the intake and exit points on the building, vent placement, screening materials, and interior arrangements all affect ventilation, but estimates can be made using simple formulas and rules of thumb. For more detailed predictions, we can use computational fluid dynamics, but this is costly and generally restricted to larger buildings.
To calculate the potential rate for cross-ventilation cooling when inlets and outlets are about the same size and the flow path is straightforward, the basic formula (adapted from the American Society of Heating, Refrigerating and Air-Conditioning Engineers, ASHRAE) is:
Qw = EAfVf Sf where
Qw = volume flow rate, cubic feet minute
Af = free area of inlet openings, square feet
Vf = wind velocity, feet per minute (miles per hour × 88)
E = effectiveness of openings (E = 0.5 for perpendicular winds and 0.3 for diagonal winds)
Sf = screen factor (see table 3.5)
For example, if the perpendicular wind speed is 5 mph and the room has one 2-by-4 foot casement window on each side with dirty fiberglass screens, the flow rate would be:
Qw = 0.5(E) × 8 ft2 × (5 × 88) × 0.5(Sf) or 880 cubic feet minute or 52,800 cubic feet hour
A slightly more generous estimate is developed using the equation:
Qh = IORcAfVm Sf
IORc = inlet outlet ratio constant
Qh = volume flow rate per hour
Af = free area inlet, square feet
Vm = wind velocity, mph
Sf = screen factor
Qh =3150 × 8 ft2 × 5 mph × 0.5(Sf) or 63,000 cubic feet per hour
In the more conservative calculation, a 10-by-14 foot room with 10-foot ceilings would still experience about forty air changes per hour. Other adjustment factors for the second equation include the effect of different inlet and outlet sizes.
Table 3.4. Cross ventilation I-O ratio
For a cave-type room:
Qw = 53(Af/10.8) × (Vmph/2.2) × (Sf)
Using the same example, a 140-square-foot room with a 2-by-4 foot casement window on one side only, the flow rate would be:
Qw = 53(8/10.8) × (5/2.2) × (0.5) = 43 cubic feet per minute
43 cfm × 60 minutes = 2,580 cfh 2,580 cfh/1,400 cf = 1.8 ACH
so it would provide only about two air changes per hour, and many pockets of stale air would be left.
The energy transfer at a given flow rate will depend on flow paths, vent configuration, interaction with wind flows, air moisture content, and temperature differences and will be influenced by thermal mass. At its simplest:
Energy transfer = airflow × specific heat of air × density of air × temperature difference °F.
See page 122 for an example.
The climate information needed for ventilation design is available for many locations around the world, from NOAA to climate-data centers, Department of Defense engineering design guides, and other local, regional, and state climate resources. The Internet has made this much more accessible, but some areas still lack easily accessible information. Even if you are able to find a nearby weather station, consider the possible microclimate differences if your site is on a hill, in a canyon or forest, or shaded or wind-blocked by trees or a building next door. It can be helpful to take notes on the wind flow and temperature on site and compare them with the nearest weather station data.
Table 3.5. Screen effects: Insect screens are a necessity in most locations. They can greatly reduce airflow, particularly when they are dirty. Airflow calculations should take screen type and condition into consideration. And clean screens regularly. Aynsley, 2007.
Stack Ventilation
This second type of natural ventilation utilizes the fact that hot air is less dense than cool air to generate air movement in a building with a low intake and high outlet. The intakes should be as low as possible and from an area with cooler air, perhaps a landscaped area or shaded courtyard with a fountain. Louvers can be used on intakes to help direct cooling air toward people. The outlet should be placed as high as possible.
Stack ventilation was used in many traditional building designs, and usually included very high ceilings, multiple floors, thin buildings, open stairwells, and roof monitors, dormers, or cupolas with operable windows. Usually the stack effect will supplement cross-ventilation. Stack-effect ventilation can also help remove pollutants from interior spaces as well as provide cooling. Performance can be estimated with simple formulas, calculated with software or computational fluid dynamics, or tested with models if well-developed design and climate data exist. For larger buildings, computational fluid dynamics programs allow for more accurate modeling.
The stack effect becomes more useful for larger and taller buildings and can augment or replace cross-ventilation. Taller ceilings help air stratification and airflow and also improve occupant satisfaction. Open plans are generally best. Cubicles and partitions can interrupt the flow of window-driven cross-ventilation and may limit natural ventilation benefits unless floor vents are used.
The stack effect works best when the exit air is warmer than the outside air. The interior air will be warmed by the people and equipment inside the building or by gains from artificial lights and sunlight.
Design Choices for Stack Ventilation
Taller buildings with atria, open stairwells, ventilation shafts, or courtyards are well suited for stack-effect cooling. Windows and vents are needed to provide flow paths for warmer, lighter air to escape and for cooler makeup air. The inlets and outlets should be as far apart as possible in height, with 5 to 6 feet as a minimum and 15 or 20 feet preferable. Multistory building offer even greater potential for stack-effect ventilation. The airflow induced by thermal force is proportional to the effective area of the apertures and the square root of the vent height difference and the inside–outside temperature differential. Bigger vents are better, although a combination of smaller automatic vents and user-controlled windows and doors can make a good combination.
Over-door transom windows or vents can help maintain ventilation flow between rooms while providing privacy and security.
To work effectively, the stack effect needs knowledgeable operators. Building users or owners should be trained and provided with a simple guide to proper operation. This should be printed, placed in a plastic sleeve, and mounted in a secure location where users can see it, perhaps in a hallway or on the inside of a closet door. Clear temperature guidelines can be provided with paired thermometers showing exit and intake temperatures to guide operators. The mechanisms for operable inlets and outlets should be well designed and maintained. Although a long pole with a hook can be used to open and close high windows and vents, a permanently mounted crank mechanism is better and cannot be misplaced.
Predicting Stack-Ventilation Benefits
The stack effect of simple, open buildings is relatively easy to estimate. More complex buildings are better studied with computational fluid dynamics to estimate the distribution of airflow in the many flow cells in a building and to integrate the effects of stack and cross-ventilation. Most passive buildings are designed to integrate stack and cross-ventilation effects, and the high outlets are often placed where wind will increase the exhaust rate.
The value of the stack effect with no wind (for example, on a clerestory house with operable windows) can be estimated from the following formula if inlet and outlets about the same size and the outdoor temperature is around 80°F (ASHRAE).
In English measurements
Q = 9.4 √H(Ti–To)
cubic feet/min
Q = airflow cubic feet per minute
A = free area of inlets or outlets (assumed equal) in square feet
H = height from inlet center to outlet center
Ti = average temperature of indoor air in height H °F
To = temperature of outdoor air °F
9.4 = constant of proportionality for 65 percent effective openings, use constant of 7.2 if 50 percent effective (to take into account screens, grilles, et cetera, see table 3.5)
For a sample home with a free area of inlets (equal to outlets) of 16 square feet, a height difference of 15 feet to a clerestory window, indoor air at 78°F, and outdoor air in the early evening at 74°F, the flow would be: = 1164 ft3min
Heat transfer = 1164 ft3min × 0.24 BTU/lb/°F × 0.075 lb/ft3 × 4°F = 84 BTU min, or 5,000 BTU/hr
This expression requires adjustment in cases when the area of outlets is appreciably different from the area of inlets according to the following ratios:
Table 3.6. Outlet-inlet relations affect ventilation
Combining Cross-Ventilation and Stack Ventilation
Cross- and stack ventilation are commonly combined and can often provide full comfort. Special opportunities are offered in larger commercial buildings. And when these simple methods are not enough, ventilation cooling can be augmented with solar chimneys, cowls and ventilators, wind catchers, and mechanical systems. Thermal mass can be added to improve cooling performance.
Fig. 3.39. Davis experiences a cooling breeze most summer nights. This can provide very effective cooling with night ventilation.
Fig. 3.40. David leans on the first rectangular metal waterwall in his home in the Village Homes solar subdivision. We now know that this tank was taller and thicker than needed, but it worked well.
Ventilation cooling can be particularly effective if the night air temperature drops significantly; simply open the windows and doors to let breezes cool the inside at night and store the coolness or coolth using the thermal mass of the house (sheetrock, tile floors, waterwalls, and so on). The potential for night ventilation can be estimated from the high and low temperatures, dry and wet bulb temperatures, prevailing wind patterns, and diurnal temperature patterns.
Ventilation for Large Buildings
Hybrid ventilation for larger buildings may include cross- and stack ventilation, a wind catcher or wind tower, roof monitors or cowls, a solar chimney, and a mechanical system. Costs may be comparable to those for a pure mechanical system, but energy savings, improved comfort, and reduced energy use have long-term payoffs. Current codes and standards often discourage use of natural ventilation, and a reworking of ASHRAE standard 62.1 is in order. Human comfort, health, and sustainability should be given greater attention.
Buildings work best if occupants understand how they work. In commercial buildings, passive ventilation may be automated to some extent, but occupants often play an important role in opening and closing windows and vents. Vents and openings can be adjusted with control logic based on temperature, moisture, airflow, and concentration of carbon dioxide.
It can be helpful to isolate major heat-generation sources from occupied space. Servers, laser printers, copiers, and other heat-generating equipment can be screened from occupied areas. This will also reduce noise problems. Computers and office equipment should be chosen for energy efficiency and low noise generation.
One of the most flexible approaches for ventilating larger buildings is under-floor ventilation, which can also be very effective for heating. This ventilation is often done with a fairly large separation so that the utilities can be run in the same space and be easily reached for repair and revision. Running, connecting, and revising IT systems is much easier. In some lab buildings, the space may grow to become a separate utility floor, but in a home it might be kept to air passages under a slab or sand-suspended stone, paver, or urbanite flooring.
Under-floor ventilation in larger buildings works best when it is an open system and vents can be easily placed or moved. This makes it possible to set the vents where they are needed to keep people comfortable and supplied with fresh air. When desks, tables, and cubicles are moved, the vents can be moved as well.
Computational fluid-dynamic modeling becomes more important for larger buildings where stack and cross-ventilation effects are more complex and where more accurate predictions may be needed for code approval.
The actual ventilation rate can also be determined for larger buildings after the building is built to provide more accurate estimates of ventilation performance and control issues. This is usually done with a study of the time decay for an introduced marker gas such as nitrous oxide, helium, or hydrogen. These tracer gases should be stable, not react with materials, and be easy to detect. Venting characteristics and leakage can also be evaluated with a blower door.
Building design should incorporate full solar control and good ventilation features to make use of ventilation cooling. Buildings should also be designed, commissioned, and operated with the users in mind. Educational materials or indicators may be needed to help occupants open and close windows, shutters, and vents in the appropriate pattern. Retrofitting buildings is also possible, and many successful projects have been undertaken. However, what is free or almost free in a new building can be more costly and challenging to apply to a poorly oriented and built older home or office.
Techniques for Enhancing Ventilation
Augmenting Airflow with a Wind Catcher
In areas where the wind almost always flows from the same direction, a fixed wind catcher or wind tower may be incorporated in the design of the building. These fixed wind catchers are used in many traditional designs. They usually extend above the buildings to reach into the stronger and more laminar flow winds and can be very effective. However, studies suggest they need not be as tall as traditional designs might suggest; a 12-foot tower might suffice. There is, however, less dust and debris farther off the ground. These towers can become effective rain catchers in storms, so design details must allow for rain entry or exclusion—perhaps as part of the rainwater harvesting system.
Wind catchers and wind towers were traditionally used to ventilate and cool buildings where winds are common. There are sophisticated designs throughout the Middle East, from Egypt to Iran. They may be big wind scoops in areas where the winds reliably come from the same direction, wind towers (which will work with winds from any direction), or wind sails. The wind sails used along the Persian Gulf and Gulf of Oman use four sailcloth vanes to deflect wind into the house. Although wind towers and fixed wind scoops like those in Isfahan and Hyderabad Sind, shown in figure 3.41a, might be most applicable in commercial buildings, they can also be included in residential structures when the cooling breezes blow consistently from one direction. Wind sails can also provide a delightful and effective ventilation boost.
Augmenting Airflow with a Cowl
Where wind direction is more variable, a roof-mounted cowl is effective, allowing winds from any direction to be used. If the roof vent is placed in the low-pressure area, ventilation will be improved.
Exhaust venting might also be done with an exhaust cowl or attic turbine ventilator on a roof monitor or near the peak of a cathedral ceiling. An exhaust cowl can be fixed or rotate with wind direction. Buckminster Fuller had a large cowl on his Dymaxion dwelling, figure 3.41b. Much larger rotating exhaust cowls have been used on some commercial buildings. Commercial exhaust cowls are also being developed and used in Europe, figure 3.41c. The Wind Jetter uses a wing profile to enhance exhaust.
Turbine ventilators were first commercialized in Australia and are now used around the world. These include a set of curved or straight vanes, with taller vents performing better than lower vents. Research suggests the straight vanes are more effective than the more commonly seen curved vanes. These are lightweight and with high-quality bearings will spin in even very light breezes. In an 18 mph wind, a 9-inch-diameter chimney cowl vent provided 3 cfs (87.5 l/s) of ventilation. These vents can be augmented with a PV-driven fan. These rotary vents can provide a reasonably weatherproof vent.
While seemingly simple, the airflows of cross wind, stack effect, vents, windows, doors, and wind catchers add up to a fairly complex problem of understanding and modeling ventilation flow.
The wind direction and wind speed information will also help determine the best way of venting a house, building, or simply an attic. Venting the attic of a building can help reduce unwanted heat gain, particularly in a building with poor ceiling insulation. A wide variety of vents are available, including gable vents, ridge vents, skylight vents, different types of roof vents, and turbine vents. Ridge vents are often desirable. Soffit vents should be kept unobstructed when insulation is installed. In wildfire-prone areas, venting should be carefully designed to prevent embers from entering the attic.
One of the greatest challenges in cross-ventilation is finding ways to retrofit existing sealed buildings. This is a problem at all levels, from single-level commercial buildings to apartment towers and high-rises. It is likely some clever solutions can be developed, even for double-facade buildings.
Fig. 3.41a. Hyderabad Sind, Pakistan (1928).
Fig. 3.41b. Buckminster Fuller, Dymaxion house (1950).
Augmenting Airflow with a Solar Chimney
A solar chimney can be used to enhance the stack effect and drive ventilation. At its simplest, this can be just a pipe painted black, but more commonly is a more sophisticated solar air heater with glass face and black interior. A typical solar chimney has glazing on the sun side, an air channel, and thermal mass painted black to capture and store solar energy to enhance airflow. Insulation helps keep temperatures high, and the thermal mass enables the solar chimney to work for a while after the sun sets.
For maximum flow, the chimney should be very smooth with generous dimensions. The most basic solar chimney consists of a section of large-diameter black pipe (8-inch minimum) extending up the wall and a few feet up from the roof. These pipes are used on composting toilets to enhance ventilation. For better results, a rectangular chimney with east, west, and south glazing might be used, with an insulated high-thermal-mass back wall with selective surface or flat black paint. The chimney can be set at the roof pitch to reduce visual impact and reduce the angle of incidence of the solar radiation on the collector in midsummer when cooling is needed most.
A wind-powered turbine vent may be a suitable cap for the chimney top if the area is windy. You’ll also want an insulated inside shutter to close it off in winter or a return vent to capture the warmed air. Again, remember to supply a cool air intake (basement, crawl space) along with a solar chimney. There will be little benefit in using the stack effect only to pull in hot outside air.
To be effective, the solar chimney has to have a clear flow path, good solar absorption, and good insulation and heat storage. The solar chimney may be incorporated in the wall of a building, typically facing the equator, or placed on the roof or in a separate stack that reaches above the roof. It can be vertical or inclined. Modeling and tests suggest an optimum airflow-rate value was achieved when the chimney inclination was between 45° and 70° for a latitude of 28.4°. This is a typical solar collector orientation in relation to latitude for good performance. Time-of-day ventilation needs may suggest a westerly or southwesterly orientation for optimum gain in the afternoon on the hottest summer days.
Considerable research has been done on solar chimneys, but no clear consensus has developed on the best shape or characteristics for them. More empirical studies are needed to support extensive modeling and simulation. The width of the chimney, the depth of the channel, the thermal-mass type and distribution, and the inlet and exit configurations all can be adjusted to fit the particular goals of the building design or engineering considerations. The width has been found to be important (the wider the better), and the channel depth should probably be in the range of 10 to 20 inches, although some have been only 4 inches and some air collectors were only 1 inch. Taller chimneys are much more powerful, but even models as small as a 12-foot-tall simple wall chimney in Athens, Greece, were able to generate a temperature difference of 9°F and airflow of up to 82 cfm on a summer day.
Architectural Configurations and Concerns
An aerodynamically efficient reversible ceiling fan may be incorporated in a tower or roof monitor to increase airflow. In summer, the fan would help exhaust hot air; in the winter it could return hot air from the thermal chimney down to the floor at its lowest setting. These fans use very little energy yet provide very good ventilation and can be PV-powered.
Air-conditioning systems should also be designed to work with natural ventilation. A displacement design can be used to help drive stack ventilation. Displacement ventilation is essentially a buoyancy-driven “displacement” process where “fresh” cool ventilation air is introduced at low velocity and at low level into the occupied space. The supply air spreads out across the floor, forming a reservoir of cool fresh air. Any heat source in the room, such as a person at a desk, generates a positively buoyant thermal plume rising upward. This plume draws air from the reservoir of cool fresh air at low level in the room. Heat and pollutants are transported up. The incoming air and thermal plumes help drive the stack ventilation.
Replacement air could also be passed through a cooling chamber that is cooled by evaporation, a solar-absorption chiller (using solar heat to power the cooler instead of mechanical energy), or seasonal storage. In traditional designs in very hot areas, the makeup air would come from yards, courtyards, or streets shaded with arbors or shades. In some cases these designs contain multiple layers, and may include thermal-stack ventilation to cool the lower-shade levels so the people on the street below are more comfortable. Shades, such as the courtyard toldo of Spain, may also be designed to enhance convective cooling.
The best configuration for a building with very high summer temperatures and consistent sun might include two vertical solar chimneys, one on the east wall for morning ventilation and one on the west for afternoon ventilation. Solar chimney performance might also be enhanced with a solar air collector below the chimney to preheat air. A quick early pop in temperature might also be provided with an internal black mesh or screens often used in solar air collectors.
Selective surfaces on the absorber plates or mass can be helpful as higher temperatures are desired. Double-wall or high-performance glazing may also be of value. Phase-change materials are very promising as a solar chimney thermal mass, offering higher temperature storage and good energy return.
Performance will depend on solar exposure, air temperature, wind, and the design and operation of the chimney. Solar-chimney performance can be further enhanced if the makeup air is drawn into the building from a cooling cavity with thermal mass, chilled by night ventilation, evaporation, or solar adsorption refrigeration. Modeling performance in an 8-foot-tall chimney and a solar adsorption unit of 53 square feet, 1.8 inches thick, using methanol and activated carbon suggested the solar chimney was more important, but the two worked well together. Ventilation rates at night were higher than during the day.
Courtyards and Atria
Courtyards and atria (a courtyard with glazing) can be very helpful in facilitating cross-ventilation and are used in many traditional designs. The courtyard makes it easier to provide windows or vents on both sides of rooms. It can also eliminate hallways, substituting covered walkways around the courtyard. Airflow across the courtyard may provide extract ventilation, pulling air through all the rooms around the courtyard. Atria should have large operable vents and may need to be shaded in the summer.
Acoustics
Acoustical aspects are a big part of natural ventilation. Ideally there should be cross-ventilation without losing acoustical privacy of major rooms. There are many clever design devices for doing this, such as large individual ceiling plenums, use of secondary spaces, and so forth. Vent placement can be used to minimize sound transmission. Landscaping, berms, and walls can also be used to reduce traffic noise.
[a.] Wing walls, besides providing solar control as shown in figure 2.25b, can also direct ventilation. For example, they can double the air penetration in a cave room as show here.
[b.] Casement windows can act as small wing walls if placed correctly.
[c.] Screened vents with insulated covers can usually be installed for less cost than a window; they will gain less heat in summer and lose less heat in winter.
[d.] A sill vent like this one in a bay window allows ventilation while providing protection from driving rain and allowing an unscreened view. Sill vents can also limit noise.
[e.] Interior walls and doors can impede or support ventilation depending on their placement and design. Louvered doors, vents, or transoms over doors can provide cross-ventilation while still maintaining privacy.
[f.] Venturi effect. Providing a larger air exit than entrance will increase the airspeed in the room for ventilation cooling.
[g.] Insect screening can reduce ventilation, as described on page 121. Therefore, large areas like screened porches can often be more effective than smaller screened windows for ventilation cooling.
[h.] Ceiling fans. Newer, more optimum ceiling fans such as the Windward II ($120) are quiet, reversible, and can be set to start at any temperature. They should be placed in every room in a hot humid climate. Ceiling fans can also be helpful outside on porches and patios (if weatherproofed).
For smaller residential applications, stairwells can be extended to become a tower for stack ventilation (see fig. 3.45). Or a clerestory window can be placed in an open, full-height room (see fig. 3.42h).
The goal is to optimize the interaction of the many parts of a building—to get more and spend less.
Fig. 3.43. Security can be maintained by installing stop locks and fixed or locked grilles, screens, and vents or by providing other security for open windows and vents. Grilles or louvered vents can be use to provide ventilation without a loss in privacy or security. The magnificent grilles in the Middle East, Spain, and Mexico are prime examples.
Fig. 3.44. Plan A (top). Plan B (bottom). Statistics for related areas (below).
A common impression is that providing solar access and natural ventilation is not applicable to buildings other than low-density custom homes. Fig. 3.44, Plan B illustrates that this is not true.
Plan A, Taylor Towers, is a fifteen-story apartment complex in San Jose, California. Several have been built. We suggested that it would be possible to provide solar access and natural ventilation equally for all units and still fit the site, building envelope, and budget. In San Jose’s climate, this could save 100 percent of the cooling cost and 80 percent of the heating costs of the units. The developer was very skeptical of this, so he commissioned this design study.
The result is Plan B “Green Towers” and a comparison figure showing that limitations of passive applications are only limitations of thinking, not limitations of the passive approach.
Ventilation comfort is also affected by furniture choices. Wicker will be cooler than solid furniture.
Allergies might be created with the introduction of pollen, fungi spores, dust, or smog with ventilation. These can best be dealt with using large free-flowing filters. These need to be large, since they will reduce airflow. Allergies can also be minimized by careful landscaping choices using the Ogren Plant Allergy Scale (www.allergyfreegardening.com/opals.php).
Where stack ventilation is important, increasing the height between incoming air and outgoing air increases its efficiency. Wonderful examples of doing this exist in historical architecture in the Middle East, especially Iran.
Fig. 3.46 Wind catchers, called badgirs, are common in Yazd, Iran. These may draw cool makeup air from over reservoirs, fountains, or the underground stream channels called qanats.
Fig. 3.47. Modern use of ventilation cooling and traditional materials in Bali.
Fig. 3.48. BedZED (zero energy development), Beddington, London (2006), incorporates wind scoops for ventilation, translucent photovoltaics, gardens, rainwater harvesting, mixed-use occupancies of commercial and residential space, and an integrated wastewater treatment system.
Example of Integrated Design for Passive Cooling
Many excellent examples of integrated design for passive cooling can be found in historic buildings. It is less common in modern buildings, but it can be done. It is often felt that assembly buildings are the most difficult to passively cool because of high internal loads and ventilation needs. To show how cooling such a space can be done, let’s look at a passively conditioned assembly building that also integrates regional, environmental, and social considerations.
Planning Considerations
The 92-acre site for the synagogue at the edge of the city of San Luis Obispo is a beautiful locale with striking views of the entire sweep of volcanic peaks known as the Seven Sisters, stretching from San Luis Obispo to the coast. The proximity to the city and natural beauty were advantageous, but the site came with some challenging environmental considerations. It contains an ecologically important wetland, and the ocean winds that regularly blow down the valley are unusually cold. In addition, heavy traffic noise flows onto the site from the adjacent highway, accentuated by the wind direction. Thus the site is thermally uncomfortable and acoustically difficult almost all year. Bioregional planning was needed to respond to the challenging conditions. The construction area for the build-out of this 20,000-square-foot facility was minimized so that site disturbance involved only 9 of the 92 acres. Most of the remaining land was put into an open-space easement, protecting the extensive wetlands and visually adding to the city’s developing greenbelt at one of its most publicly visible locations. In addition, a major component of the site-planning process was the development of a 10-foot-high earth berm in an arc from the southwest to the northwest of the building and parking area, creating a first layer of noise and wind protection as well as a planting area for native vegetation above the high water table. Public view of the building from the highway is greatly diminished by this berm, helping to protect the public viewshed.
Architectural Considerations
Assembly buildings can be a challenge to design for passive conditioning, but in this case this was accomplished without providing any centralized HVAC system. This saved $160,000, some of which was spent on passive solar components such as high-performance windows, light shelves, thermal mass, and automation of natural ventilation capability. Natural lighting is a critical part of any high-performance building. This synagogue is designed so that artificial lighting is not needed during daylight hours—the design utilizes daylight as an integral part of the whole passive solar strategy.
The final design concept that was chosen from four alternatives was given the nickname of “the onion bagel”—onion because both the site plan and floor plan of the building consist of a series of layers. The outer layers protect from the cold winds and the noise that dominate the site—the layers represent a sequential ordering from the busy outer world to a quieter, inner spiritual world. A “bagel” came to represent the final scheme because of the hole in the middle of the building, the courtyard.
Actually, upon phase-two construction, there will be two courtyards. The main one will allow south light into the social hall and aid in natural ventilation. A smaller one will serve the expanded school area—a kind of fractal self-similar-scale progression of older to newer, bigger to smaller, wisdom to regeneration, and so forth.
Thermal Analysis
The sanctuary area for the synagogue utilizes solar gains to replace heating by conventional means. South-facing glazing is used in the occupied space and in the overhead lantern area. The two spaces are separated by acoustical panels, or “clouds,” that were included for acoustical, lighting, aesthetic, as well as thermal purposes. Initial thermal modeling of the space assumed solar heat gain from the lantern area could be effectively distributed to the lower occupied space when needed, and therefore the two areas were treated as a single zone. In order to ensure that thermal comfort was possible, a three-dimensional CFD model was used to test various designs for distributing the solar gains from the lantern in January. The final configuration uses a high-efficiency but inexpensive ceiling fan mounted just above the Bema (pulpit) to provide the destratification required. This fan can also be reversed to help with ventilation when necessary.
[a.]
[b.]
[c.]
[d.]
[e.]
[f.]
Fig. 3.49a–f. The Beth David synagogue, San Luis Obispo, California. (a) Site analysis and plan. (b) Floor plan. (c) Performance chart. (d) Ventilation cross-section and computational fluid-dynamics model. (e) Chart of passive strategies and modeling. (f) Ventilation plan.
Social Response
Paul Wolff, a member of the design committee, made the following comments regarding the social dimension of the building.
This facility has dramatically influenced our congregants and community in unexpected ways. The building provides an outstanding illustration of a major concept of environmental psychology—that architecture directly influences human behavior. Green architecture can have a very positive effect on the behavior of its users. Inspired by our new building, Congregation Beth David initiated the Green Shalom program, intended to bring into the daily behavior of our congregants the same principles that governed the building’s sustainable design.
We introduced monthly programs to promote activities, education and life styles that will increase awareness (with subsequent actions) of the need to protect our environment and conserve energy.
These programs have included seminars on fair trade coffees, teas, chocolates, etc., recycling, green gardening, shopping with a conscience (anti sweatshops), green cleaning supplies, socially responsible investing, and compact fluorescent light bulbs. Through the local ministerial association, we have now widened the Green Shalom program to reach out to our whole community. Our relatively small congregation has long sought to make a more significant contribution to the local social fabric; the functionality, sustainability, and aesthetics of this building has allowed us to do just that in positive and unforeseen ways.
The key to backup cooling is to acknowledge that we can provide the majority of the cooling needs by passive systems so that mechanical cooling is eliminated or reduced to a much more economical and manageable backup system. It is important to avoid the redundancy of having two complete cooling systems just because the passive system may need an occasional boost. This is even true in the hot humid climates considered so difficult by today’s mechanical cooling mind-set (see Figure 3.15).
Once the majority of the cooling load is handled by passive design, several traditional cooling-augmentation approaches can be considered. Recent technological improvements in three of these allow them to fulfill this function efficiently and economically.
Ceiling Fans
Backup cooling can be provided by the traditional ceiling fan, which has been vastly improved by Paul McCready’s group of aeronautical engineers (figure 3.50). If powered by photovoltaics, this mechanical device essentially becomes part of the building’s passive system.
Fig. 3.50. Aerodynamic design improved efficiency dramatically.
Whole-House Fans
There has been recent improvement in large fans that can upgrade the traditional whole-building night-ventilation capability.
Fig. 3.51. Whole-house fans enhance night ventilation cooling.
Evaporative Cooling
Another old standby has been improved and is capable of providing cooling backup to passive systems (see Indirect Evaporative Coolers Return).
Radiant Cooling
Eventually radiant cooling systems may become well developed and commercialized for both new buildings and retrofits.
Ventilation for Health
In the 1970s, rising energy prices and demand for conservation led to a disastrous forty-year emphasis on sealed buildings with mechanical ventilation and air-conditioning. The code development process and regulations illustrate the problem of narrow focus and dominance by special-interest groups. The most active participants in the process were quite naturally those with a vested interest in selling mechanical equipment—and they succeeded. Sadly, the occupants and environment lost. Occupants suffered sick building syndrome from moldy and chemically contaminated air, with costs estimated at $58 billion a year in health-related expenditures and as much as $200 billion in lost productivity (Fisk, 2000). The focus on mechanical systems also helped increase building impact on global warming gas production to 40 percent of the US total. Many of these buildings will be hard to retrofit, but others will be easy to upgrade. In addition, many once naturally ventilated and cooled historic buildings were sealed up, and thereby ruined, in order to meet “modern” codes.
As a result, there is now a great deal of emphasis in green buildings on indoor air quality. Avoiding unhealthy materials, finishes, cleaning products, and behavior (like smoking indoors) is a first step. Careful fresh-air-supply management and venting of combustion appliances, including woodstoves, is also important. Material choices and detailing to avoid mold and rot are also important, and moisture from kitchens and bathrooms must be well managed. When groups of people are involved, the management of CO2 and moisture is also critical. Finally, the ventilation system should be understandable and transparent to the user so that deferred maintenance or mechanical failure does not diminish effectiveness.
Fig. 3.52. Radiant cooling study, Cortez, Colorado. We need to encourage more research on passive cooling techniques.
Many passive systems are equally capable of providing heating, cooling, and ventilation as an integral part of architectural design. Too often, passive cooling is neglected or treated as the lesser twin of passive heating. In many parts of the United States and the world, however, more energy is used for cooling than heating. Passive cooling can improve comfort and save money.
We must also overcome the neglect of ventilation that remains an unfortunate legacy of the “energy conservation” approach of the 1970s. Sealed buildings should be opened, and windows should operate.
Knowledge of passive cooling principles and improved modeling techniques, including the availability of computational fluid dynamics for airflow performance predictions, can allow a majority of cooling and ventilation to be provided by passive systems in any climate.