Passive Cooling Techniques

[COMPILED DATA ON PASSIVE COOLING TECHNIQUES]

Architectural Design 7 | A Passively-Cooled Commercial Hub 1

CLIMATE RESPONSE AND BUILDING DESIGN

STRATEGIES OF CLIMATE CONTROL FOR HOT – HUMID CONDITIONS

Humid overheated conditions are most severe along the Gulf Coast, but occur across the entiresoutheastern U.S. Atmospheric moisture limits radiation exchange, resulting in daily temperature rangesless than 20 degrees F. High insolation gives first priority to shading. Much of the overheated period isonly a few degrees above comfort limits, so air movement can cool the body.

Ground temperatures are generally too high for the earth to be useful as a heat sink, although slab-on-grade floor mass is useful. The strategies are to resist solar and conductive heat gains and to take bestadvantage of ventilation.

Minimize Solar Gains

1. Plant trees to shade roof and east and west walls.2. Shade building to minimize solar load on envelope.3. Shade all glazing during overheated period.4. Shade north elevation in subtropical latitudes.5. Use light-colored surfacing on walls and roof.

Minimize Conductive Gain

1. Insulate envelope components in proportion to sol-air-indoor temperature difference.2. Use radiant barrier in attic space.3. Consider thermally massive envelope materials to reduce peak air-conditioning loads.4. Use slab-on-grade instead of crawl space and insulate only at perimeter.

Promote Ventilation Losses

1. Orient building to benefit from breezes.2. Use plantings to funnel breezes into building, but be careful not to obstruct vent openings.3. Use wing walls and overhangs to direct breezes into building.4. Locate openings and arrange floor plan to promote cross ventilation.5. Plan interior for effective use of whole-house fan.6. Ventilate building envelope (attic or roof, walls).



SPACE VENTILATION

"Air-change ventilation" brings outdoor temperatures indoors by breezes or whole-house exhaust fans.Whole-house fans yield about 20 air changes per hour (ACH) and are useful only as long as outdoorconditions are within comfort limits (72 degrees -82 degrees F). They may offer 30-50% savings inelectricity costs over air conditioning. Whole-house fans do not provide high enough airflow rates forbody ventilation. Ceiling (paddle) fans are recommended for air movement and can maintain comfortwith indoor temperatures up to 85 degrees F ET*. Air conditioning is necessary above 85 degrees F ET*.The issue of when to ventilate and when to air condition is a function of building type, occupancy hours,heat and moisture capacity of the structure, and climatic subregion. Humidity is a factor, as night airmay be cool but excessively humid.

ROOFS AND ATTICS

The attic should be designed to ventilate naturally. Most of the heat gain to the attic floor is by radiationfrom the underside of the roof. While ventilation is unable to interrupt this transfer, most of it can bestopped by an aluminum foil radiant barrier. Foil facings on rigid insulation and sheathing can be used asradiant barriers when installed facing airspace.



Roof spray systems can dissipate most of the solar load, leaving the roof temperature near the ambientdry-bulb instead of the sol-air temperature. The theoretical lowest temperature that the roof can becooled to by evaporation is the wet-bulb, but is not attainable under real daytime conditions. The cost-effectiveness of spray systems depends on the roof section, R value, building type, climatic region, andother factors. Spray systems are most advantageous for poorly insulated flat roofs.

WALLS

Radiant barriers enhance the performance of walls by reducing solar gain. They are most effective oneast and west walls and are recommended for predominantly overheated regions [<2000 heatingdegree days (HDD), >2500 cooling degree days (CDD)]. They are not recommended on south wallsexcept where CDD exceed 3500. Radiant barriers must face an airspace and can be located on eitherside of the wall structure. Outside placement allows the cavity to be vented. This enhances summer wallperformance, but admitting cold air degrades it during winter. Venting is recommended for regionshaving more than 3500 CDD. Discharging the cavity into the attic ensures best vent action. Thermal massin walls reduces peak air-conditioning loads and delays peak heat gain. By damping off some of the peakload, massive walls help keep indoor temperatures in the range where ceiling (paddle) fans and airflowfrom cross ventilation provide comfort.

DAYLIGHTING

Windows and skylights should be shaded to prevent undesired heat gain. North- and south-facingglazing is shaded most easily for predictable daylighting. Light-colored reflective sunshades and groundsurfaces will bounce the light and minimize direct gain. Cloudy or hazy sky conditions are a source ofbrightness and glare.

References

1. S. Chandra et al. Cooling with Ventilation, Solar Energy Research Institute, Golden, CO, 1982.

2. K. E. Wilkes, Radiant Barrier Fact Sheet, CAREIRS, Silver Spring, MD.

3. P. Fairey, S. Chandra, A. Kerestecioglu, "Ventilative Cooling in Southern Residences: A Parametric Analysis," PF-108-86, Florida Solar Energy Center, Cape Canaveral, 1986.



ALTERNATIVE ENERGY SYSTEMS

The term "alternate energy systems" describes uses of climatic resources--sun, wind,precipitation/humidity, and temperature--to provide all or part of the energy requirements of a building.Their development has paralleled the uncertain cost and availability of conventional energy supplies.New design concepts--passive solar and cooling and daylighting designs--have become part ofrecommended building practice. More advanced technologies have been developed, but theirwidespread use awaits either more experience with them or more penalizing energy prices. Some canbe easily incorporated into a building design, requiring only careful design integration of architecturaland heating, cooling, and lighting systems. A number of factors can change the economic constraintsupon what is and is not cost-justified: the need for emergency preparedness, the prospect ofinterruptible or increasingly costly conventional fuel supplies, environmental pollution from fossil fuelcombustion, and limited capacity of existing power plants. These concerns suggest that they be givenfull consideration together with energy conservation/load reduction techniques so that our long-termreliance upon conventional and nonrenewable energy sources can be minimized.

The practical approach to alternate energy system design begins with analysis of the energyrequirement of the building "end use": the temperature, humidity, air flow, and lighting levels requiredfor human comfort, and the related power demands for productive activity. The various sources forsupplying heating, cooling, and lighting, and electric power can then be matched to the end use in termsof "thermodynamic" efficiency, comfort, operational costs, and reliability. High levels of energyconservation and renewable energy use can make life-cycle economic gains possible, such as bydownsizing mechanical system sizes or through "off-peak" loading of the building's energy requirementto reduce or eliminate "demand charges," as is possible when a building has a large energy storagesystem.

The figure, "Site and Building as Energy Collection, Storage, and Distribution System," diagrams thevarious alternate energy system components. The building itself is shown as an energy collection,



storage, and distribution system. Choices include system components that are separate from thebuilding (though presumably nearby) and those that must be integrated with it.

REMOTE ENERGY COLLECTORS (ELECTRICAL)

Three contenders for alternate electric power are windmills, microhydro dams, and photovoltaic panels.Photovoltaic systems use the photons of sunlight to generate electricity across a grid of cells in a solarcollector. These can be mounted on the roof of a building or can be "remote," since electricity is easilydistributed from its point of collection. Site engineering concerns are major, but building design criteriaare minor, limited only to storage battery location and the electric distribution system within thebuilding. The economic viability of these choices is greatly improved by reduction of the electric loadrequirement achievable by energy-efficient lighting and equipment.

REMOTE ENERGY STORAGE (THERMAL)

Energy storage near a building site has proved to be viable when the site is large enough, made part ofseasonal (6-month) storage, and serves groups of buildings (district heating/cooling). These include:

Underground thermal storage: Heat generated by solar collectors (either air type or liquid type) can bestored within a large mass of earth, in existing caverns, or in newly dug clay or soil beds. In Kerava SolarVillage near Helsinki, Finland, solar collectors mounted on the south-facing roofs of 44 apartmentssupply solar-heated water to a 400,000 gal water tank which in turn heats 338,500 cu ft of rocksurrounding the tank embedded 66 ft in the earth.

Acquifier systems: A variation of thermal storage that is "charged" by solar collectors are systems usingnatural or man-made acquifiers for seasonal storage, thus utilizing groundwater temperature forheating and cooling, generally relying upon a water-to-water heat pump to change the groundwatertemperature to the end-use requirement for heating and cooling.

Ice storage systems: Ice storage systems use ice-making, either "seasonal" for 6-month storage or"diurnal," at night for next-day use, to provide building cooling. The advantage of making ice in winter isobvious, imposing only the cost of large storage area logically located within the subgrade basement ofa building, but which can also be separate. Diurnal systems are cost-effective when there areadvantages of "off-peak" utility rates and/or significantly cooler nighttime temperatures.

Solar ponds: Solar ponds are salt ponds that exploit the temperature gradient effect of salt water. Firstdocumented by Russian scientist von Kaleczinky in 1902, water a few feet below a confined body of saltwater reaches temperatures up to 185 degrees F due to the varying salinity of the water: The bottom ofthe pond is a bed of salt in which heat is efficiently stored because heatedsalt-rich water does not rise,while the surface of relatively fresh water above is clear, allowing solar heat to be transmitted through itand at the same time insulating the denser layers below. In Israel and Australia, such solar ponds havebeen used as a source of thermal energy and to drive engines for electric generation. While only half asefficient as solar collector, the relatively low cost of solar ponds (reportedly ten times less costly per unitof collector surface) indicates their potential.



INTEGRATED BUILDING SYSTEMS

A building designed to efficiently use climatic resources for heating, cooling, lighting, and electric powergeneration is properly considered an alternate energy system. Means for doing so are tabulated in thetable "Energy-Efficient Architectural Elements" and summarized as a checklist for designers.

South wall: The south-facing wall of a building (in the Northern hemisphere) is an efficient energyresource. The low-angled winter sun can bring into a building interior the benefits of winter heat andlight. Shading the south facade in summer can be efficiently accomplished with relatively shoroverhangs. Because of this, passive solar heating, summer shading, and year-round daylighting can andought to be made part of the south-wall design. Solar heat can be stored in thermal storage placed inthe sun behind glass or ducted/piped to the building interior.

Roof: The roof of a building can be used for mounting "active" solar collectors for heating, photovoltaiccollectors for generating electricity, or skylights for daylighting. In hot climates, the roof is also analternate energy resource if used for evaporative or radiant cooling.

Atrium: Atria design can be integrated into a "whole building" daylighting system and combined withthe mechanical air movement system wherein it can economically replace ducting in ventilative coolingand heat recovery systems. Skylights, enhanced with light and heat reflectors, can be designed to reflectsunlight deep within a building.

Below-ground/basement: The below-ground construction of a building can be used for thermal storage,as described above. In single-storied or low-rise buildings, "ground coupling" utilizes the relatively stabletemperatures of the surrounding earth to provide an economical heating/cooling flywheel effect.



ENERGY-CONSERVING DESIGN FOR COMMERCIAL BUILDINGS

Energy-conserving design for commercial buildings is justified by savings in operating costs which resultin a lower "life-cycle" investment. For large buildings of all types, the best opportunities are most likelyto be found in electricity costs; depending upon the demand charges of the local utility, "peakload"reduction and/or "shifting" (diurnal or seasonal) measures may prove to be cost effective. Concurrently,lower electric use by effective daylighting and by cooling load reduction (window orientation and solarcontrols) will be cost-effective, since these loads are typically interrelated and use expensive forms ofenergy. When these loads and costs are reduced, heating cost reduction by solar and energy-conservingtechniques also applies to larger buildings. Energy-conserving opportunities are best addressed by awhole-systems team approach of architecture, HVAC, lighting, and controls engineering. For example,high levels of insulation or of thermal mass may be cost justified when these also result in substantiallyreduced mechanical system sizes and power requirements.

The architect should consider "Site Planning and Orientation," "Daylighting," "Energy-Efficient Lighting,""Thermal Construction," "Energy-Efficient Mechanical Systems," and "Smart Building Controls" indesigning an energy-efficient nonresidential building, regardless of size and building type.

SITE PLANNING AND ORIENTATION

1. Orient The Longer Walls Of A Building To Face North-South

Walls that face the equator (e.g., the noonday sun) are ideal for windows oriented to admit daylightingwith minimum cost for shading or sun control (i.e., relatively small horizontal overhangs create effectiveshading). Walls and windows facing east andwest, on the other hand, are sources ofundesirable overheating and are difficult toshade effectively. In a cool climate, windowsfacing the equator can gain useful wintertimeheating from the sun.

2. Provide Sun Shading To Suit Climate AndUse Variations

Buildings can be located in group s to shadeone another. Landscaping and sun shading canbe used to shade building surfaces, especiallywindows, during overheated hours. Functionscan be located within a building to coincidewith solar gain benefit or liability. Forexample, cafeterias are ideally exposed tonoontime winter sun in cool and temperate



climates or placed in the midday shade in warm climates; low-use areas (storage areas) can be used asclimatic buffers placed on the east or west in hot climates or on the north in cool climates.

3. Create Courtyards And Enclosed Atriums

Semi enclosed courtyards (in warm climates) and enclosed atriums (in temperate and cool climates) canbe formed by groups of buildings to provide areas for planting, shading, water fountains, and othermicroclimatic benefits. Atriums can also be used as light courts and ventilating shafts. Indoor or outdoor

planted areas provide evaporative cooling for local breezes when located near buildings.

4. Use Earth Berms For Climatic Buffering

Earth berms (sloped or terraced, formed simply be grading earth against the wall of a building) help tobuffer the building against temperature extremes of both heat and cold. The planting on earth bermsalso provides evaporative cooling near the building. Earth berms can be construction cost saversbecause the foundation does not have to be as deep (in single-storied construction); the earth andground cover is often less costly than other wall finishing materials. Its long-term maintenance can alsobe lower than conventional materials.

DAYLIGHTING

5. Place Windows High In The Wall Of Each Floor

Windows placed high in the wall near the ceiling provide the most daylight for any given window area,permitting daylight to penetrate more deeply into the interior.

6. Use Light Shelves

Light shelves are horizontal projections placed on the outside and below a window to reflect sunlightinto the interior. Typically placed just above eye level, the light shelf reflects daylight onto the interiorceiling, making it a light-reflecting surface (instead of a dark, shaded surface typical of a conventionalinterior ceiling). At the same time, the light shelf shades the lower portion of the window, reducing theamount of light near the window, which is typically overlit. The result is more balanced daylighting withless glare and contrast between light levels in the interior.

7. Size Windows According To Use And Orientation

Because window glass has little or no resistance to heat flow, it is one of the primary sources of energywaste and discomfort. Window areas should be shaded against direct solar gain during overheatedhours. Even when shaded, windows gain undesired heat when the outdoor temperature exceeds thehuman comfort limit. Window areas should therefore be kept to a reasonable minimum, justified byclearly defined needs for view, visual relief, ventilation, and/or daylighting. Double glazing should beconsidered for all windows for energy efficiency and comfort in cool and temperate climates. In warmclimates, double, tinted, or reflective glass should be considered, depending upon building size and use.



8. Use Skylighting For Daylighting, With Proper Solar Controls

Skylighting that is properly sized and oriented is an efficient and cost-effective source of lighting.Consider that for most office buildings, sunlight is available for nearly the entire period of occupancyand that the lighting requirement for interior lighting is only about 1% of the amount of light availableoutside. Electric lighting costs, peak demand charges, and work interruptions during power brownoutscan be greatly reduced by using daylighting. Cost-effective, energy-efficient skylights can be small,spaced widely, with "splayed" interior light wells that help reflect and diffuse the light. White-paintedceiling and walls further improve the efficiency of daylighting (by as much as 300% if compared withdark interior finishes). Skylights should includesome means to control undesired solar gain by oneor more of the following means: (a) Face theskylight to the polar orientation; (b) provideexterior light-reflecting shading; (c) providemovable sunshades on the inside, with a means tovent the heat above the shade.

ENERGY-EFFICIENT LIGHTING

9. Use Task Lighting, With Individual Controls

Lamps for task lighting are ideally located near thework surface and are adjustable to eliminatereflective glare. The energy-efficient advantagesare that less light output is required (reducedgeometrically as a function of its closer distance tothe task) and the lamp can be switched off whennot needed.

Note: General light levels should be reduced below conventional standards and sources of reflective glarefrom ceiling lights and windows eliminated in areas where cathode ray tubes (CRTs) are used.

10. Use The Ceiling As A Light-Reflective Surface

By using "uplights," either ceiling pendants or lamps mounted on partitions and/or cabinets, the ceilingsurface can be used as a light reflector. This has several advantages: (a) fewer fixtures are required forgeneral area ("ambient") lighting; (b) the light is indirect, eliminating the sources of visual discomfortdue to glare and reflection, (c) if light shelves are used, the ceiling is the light reflector for both naturaland artificial light, an advantage for the occupant's sense of visual order.

11. Employ A Variety Of Light Levels

In any given interior, a variety of light levels improves visual comfort. Light levels can be reduced in low-use areas, storage, circulation, and lounge areas. Daylighting can also be used to provide variety oflighting, thereby reducing monotone interiors.



12. Provide Switching Choices, To Accommodate Schedule And Daylight Availability

Areas near windows that can be naturally lit should have continuous dimming controls to dim lights thatare not needed. Other areas should have separate switching to coincide with different schedules anduses. Consider occupant-sensing light switches in areas of occasional use, such as washrooms, storage,

and warehouse areas.

13. Use Energy-Efficient Lights AndLuminaires

Use the most efficient light source for therequirement: these might be fluorescentbulbs, high-intensity discharge lamps, orhigh-voltage/high-frequency lights.Compact fluorescent lights with high-efficiency ballasts have advantages of lowwattage, low waste heat, long life, and goodcolor rendering. Incandescent lights use lessenergy when switched on, so these areappropriate for occasional use and short-term lighting. Luminaires should also beevaluated for how efficiently they diffuse,direct, or reflect the available light.

THERMAL CONSTRUCTION

14. Place Insulation On The Outside Of The Structure

Insulation is one of the most cost-effective means of energy conservation. Insulation placed on the outerface of a wall or roof protects the structure from the extremes of the outside temperature (with theadded benefit of lengthening the life of the roof waterproofing membrane) and adds the massiveness ofthe structure to the thermal response of the interior. In localities where "resistance insulation" is notavailable, the combination of airspaces and high capacitance materials (such as masonry and/or earthberms) should be designed for effective thermal dampening or time lag (the delay and diffusion ofoutside temperature extremes that are transmitted to the interior). As a n alternative to insulating roofstructures in hot climates, a "radiant barrier" consisting of a continuous sheet of reflective foil with alow emissivity coating and an airspace around it serves as an effective shield against undesired heatgain.

15.Utilize Thermal Mass On Building Interior

In office buildings, thermally massive construction (such as masonry and concrete which have good heatstorage capacity) benefits the energy-efficient operation of heating and cooling equipment as follows:



(a) Cooling benefits: Thermal mass absorbs the "overheating" that is inevitable in an office space due tothe buildup of heat from people, equipment, lighting, rising afternoon temperature, and solar gain. Themore thermal mass that is effectively exposed to an interior space (ceiling and walls), the greater is thesaving on air conditioning in the afternoon, with the potential to delay the overheating until earlyevening when electric rates may be lower and/or outdoor air may be low enough to cool the mass bynight ventilation. (The "night cooling" option is especially favorable in warm, dry climates due topredictably cooler nighttime temperatures.)

(b) Heating benefits: In temperate and cool climates, thermal mass helps absorb and store wintertimepassive solar heat. This is especially effective ifthe thermal mass is on the building interior anddirectly heated by the sun (made possible bydesign of various corridor, stairway, and half-height partition arrangements).

16. Use Light-Constructed Ventilated Roofs InHot Climates

In hot climates, the roof is the primary sourceof undesired heat gain. Energy-efficient roofdesigns should be considered. One of the bestfor hot climates is a ventilated double roofwherein the outside layer is a light-colored andlightweight material which shades the solarheat from the inner roof, which should be wellinsulated. As described in Strategy 14, a"radiant barrier" can be considered as analternative to resistance insulation to serve asa shield against thermal transfer through theceiling portion of the roof structure.

ENERGY-EFFICIENT MECHANICAL SYSTEMS

17. Use Decentralized And Modular Systems

Heating and cooling equipment is most efficient when sized to the average load condition, not the"peak" or extreme condition. Use modular unit boilers, chillers, pumps, and fans in series so that theaverage operating load can be met by a few modules operating at peak efficiency rather than a singleunit that is oversized for normal conditions. Zone the distribution systems to meet different loads due toorientation, use, and schedule. Use variable-air-volume (VAV) systems to reduce fan energyrequirements and to lower duct sizes and costs (the system can be designed for the predominant load,not the sum of the peak loads). Decentralized air-handling systems have small trunk lines and ductlosses. Dispersed air handlers, located close to their end uses, can be reduced in size from conventional



system sizes if hot and chilled water is piped to them (a decentralized air-handling system with acentralized plant).

18. Use Economizer/Enthalpy Cycle Cooling

Economizer/enthalpy cycle cooling uses outdoor air when it is cool enough for direct ventilation and/orwhen the outdoor air has a lower heat content than indoor air (so that it can be cooled evaporativelywithout raising indoor humidity). Although useful in all climates, direct or indirect evaporative coolingsystems are especially effective in hot, dry climates.

19. Use Energy-Efficient Equipment

The energy efficiency of mechanical equipment varies greatly. Consider heat pumps for cooling and forheating to replace separate chiller and boiler units. Heat pumps can also use local water sources orwater storage. Newly developed mechanical heating equipment, such as gas-fired pulse combustionboilers, is achieving very high (up to 85%) annual operating efficiencies.

20. Use Energy Storage For Cooling

Chilled water storage has several advantages: it permits water chilling or ice-making at night under morefavorable ambient conditions and possible lower electric rates; perhaps more important, it reduces oreliminates peak-hour energy consumption, thereby reducing demand charges.

21. Use Heat Recovery For Heating

In cool and temperate climates, heat can berecovered from warm zones of a building andrecirculated to underheated areas. Recoverableheat sources include equipment, process heat,and passive solar gain. Heat recovery wheels orcoils can be used where indoor air needs to beventilated, transferring heat into the incomingfresh airstream. In all climates, process heat oractive solar heat (e.g., from solar collectors) canbe used for domestic hot water or fortempering incoming fresh air.



"SMART BUILDING" CONTROLS

22. Use Smart Thermostats

"Duty-cycling" temperature controls can be programmed for different time schedules and thermalconditions, the simplest being the day-night setback. Newer controls are "predictive," sensing outdoortemperature trends and then selecting the system operation most appropriate to the condition.

23. Use Occupancy And Daylight-Sensing LightingControls

Automatic switching of lights according to thebuilding occupant schedule and the daylightcondition is recommended, with manual override fornighttime occupancy. Photosensors should be placedin areas that can be predictably lit by natural light.

24. Be Prepared For Rapid Innovation In BuildingControl Systems

Newly developing "smart" building systems includemicroprocessing for thermal and light control, fireand air-quality precautions, equipment failure, andoperations/maintenance requirements (along withnew communication and office managementsystems). These innovations require that electricwiring be easily changed, such as through "double-floor" construction.

References

Burt Hill Kosar Rittelmann Associates: Small Office Building Handbook, New York: Van Nostrand Reinhold, 1985.

Burt Hill Kosar Rittelmann Associates: Commercial Building Design, New York: Van Nostrand Reinhold, 1987.

McGuiness, Stein, and Reynolds: Mechanical and Electric Equipment for Buildings, New York: John Wiley & Sons,7th Edition, 1986.

Solar Energy Research Institute: Design of Energy-Responsive Commercial Buildings, New York: John WileyInterscience, 1985.

Watson, Donald, editor: Energy Conservation through Building Design, New York:McGraw-Hill Book Company,1979.



NATURAL VENTILATION

BASIC PRINCIPLES

The "Natural Ventilation" diagrams presented in this discussion are based on an isolated building.Neighboring buildings and landscaping can substantially affect airflow and should be taken into accountwhen evaluating ventilation strategies.

As wind approaches the face of a building the airflow is slowed, creating positive pressure and a cushionof air on the building's windward face. This cushion of air, in turn, diverts the wind toward the buildingsides. Airflow as it passes along the sidewalls separates from building wall surfaces and, coupled withhigh-speed airflow, creates suction (negative pressure) along these wall surfaces. On the buildingleeward side a big slow-moving eddy is created. Suction on the leeward side of the building is less thanon the sidewalls (see "Natural Ventilation: Basic Principles," Figure 1).

If windows are placed in both windward and leeward faces, the building would be cross ventilated andeddies will develop against the main airflow direction (see "Natural Ventilation: Basic Principles," Figure2). Ventilation can be enhanced by placing windows in sidewalls due to the increased suction at thislocation; also, greater air recirculation within the building will occur due to air inertia (see "NaturalVentilation: Basic Principles," Figure 3). Winds often shift direction, and for oblique winds, ventilation isbest for rooms with windows on three adjacent walls (see "Natural Ventilation: Basic Principles,"



Figure 4) than on two opposite walls (see "NaturalVentilation: Basic Principles," Figure 5). However, ifwind is from the one windowless side, thenventilation is poor, since all openings are in suction(see "Natural Ventilation: Basic Principles," Figure 6).

If the building configuration only allows for windowsin one wall, then negligible ventilation will occur withthe use of a single window, because there is not adistinct inlet and outlet. Ventilation can be improvedslightly with two widely spaced windows. Airflow canbe enhanced in these situations by creating positiveand negative pressure zones by use of architecturalfeatures such as wing walls (see "Natural Ventilation:Basic Principles," Figure 7). Care must be exercised indeveloping these features to avoid counteracting thenatural airflow, thereby weakening ventilation (see"Natural Ventilation: Basic Principles," Figure 8).

AIR JETS

As airflow passes through a well-ventilated room, it forms an "air jet." If the windows are centered in aroom, it forms a free jet (see "Air Jets," Figure 9). If, however, the openings are near the room walls,ceiling, or floor, the airstream attaches itself to the surface, forming a wall jet (see "Air Jets," Figure 10).Since heat removal from building surfaces is enhanced with increased airflow, the formation of wall jetsis important in effecting rapid structure cooling. To improve the overall airflow within a room, offsettingthe inlet and outlet will promote greater mixing of room air (see "Air Jets," Figure 11).

WINDOW SIZE

Airflow within a given room increases as window size increases, and to maximize airflow, the inlet andoutlet opening should be the same size. Reducing the inlet size relative to the outlet increases inletvelocities. Making the outlet smaller than the inlet creates low but more uniform airspeed.

VENTILATION AIR CHANGE RATE

The natural air change rate within a building depends on several factors: speed and direction of winds atbuilding site; the external geometry of building and adjacent surroundings; window type, size, location,and geometry; and the building's internal partition layout. Each of these factors may have an overridinginfluence on the air change rate of a given building.

Natural ventilation can be accomplished by wind-driven methods or by solar chimneys (stack effect).However, the stack effect is weak and works best during hours when air temperatures are highest andventilation may not be desirable. In many areas ventilation is best accomplished during the night hours



when temperatures are lowest. The night average wind speed is generally about 75% of the 24-hraverage wind speed reported by weather bureaus. Often wind speeds are insufficient to accomplisheffective people cooling; therefore, ventilating for structure cooling rather than people cooling shouldbe the first design goal. As a rule of thumb, an average of 30 air changes per hour should provideadequate structure cooling, maintaining air temperatures most of the time within 1.5 degrees F ofoutdoor temperatures.

EXTERNAL EFFECTS

The leeward wake of typical residential buildings extends roughly four and one-half times the ground-to-eave height. For buildings spaced greater than this distance, the general wind direction will remainunchanged. For design purposes, vegetation should be considered for its effect on wind speed, whichcan be as great as 30-40% in the vegetation's immediate vicinity. Its effect on wind direction is not wellestablished and should not be relied upon in establishing ventilation strategies.

PASSIVE SOLAR DESIGN

Passive solar heating and cooling systems which rely on natural energy flow through and around abuilding, are divided into three generic categories, including:

1. DIRECT SYSTEMS: Heat is collected directly within the space or, for cooling, lost or dissipateddirectly from the space.

2. INDIRECT SYSTEMS: Heat gain or loss occurs at the weatherskin.3. ISOLATED SYSTEMS: Heat gain or loss occurs away from the weatherskin. Cooling, for example,

can include induced air precooled from the earth's mass using air to earth heat exchangers("coolth" tubes) or cooling ponds.

Systems can be combined depending on thermal needs.

SPACE HEATING CONCEPTS

As part of any passive system's development, energy conservation elements should be considered. Withpassive solar heating, minimizing and preventing heat loss is fundamental to ensure that the heatingsystem is most effective. These elements include adequate insulation, building orientation, surface-to-volume ratios, and appropriate materials, texture, and finish choices. The space heating successdepends on adequate solar energy collection, storage, distribution, and control, all of which occur bynatural, nonchemical means using the three basic heat transfer processes: conduction, convection, andradiation. Efficient passive system operation often involves some user control to alter or override energyflows within a building or at its weatherskin.



1. Solar collection surfaces generally are transparent or translucent plastics, fiberglass, or glassoriented in a southerly direction. Material degradation can be caused by solar exposure andother weather elements. Insulating these collection areas to control nighttime loss is especiallyimportant in extreme climates.

2. Thermal storage materials include concrete, brick, sand, tile, stone, and water or other liquids.Phase change materials such as eutectic salts and paraffins also are feasible. Storage should beplaced to receive maximum solar exposure, either directly or indirectly. Adequate thermalstorage capacity allows the sun's heat to be absorbed and retained until it is needed, and ithelps to reduce internal temperature fluctuations.

3. Heat distribution occurs naturally by conduction, convection, and radiation. Generally, fans andother mechanical energy distribution equipment are avoided; however, sometimes they arerequired for fine-tuned operations.

4. Control mechanisms such as vents, dampers, movable insulation, and shading devices can assistin balanced heat distribution.

SPACE COOLING CONCEPTS

Passive solar cooling, like passive heating, tempers interior space temperatures using natural thermalphenomena. A structure designed for natural cooling should incorporate features that reduce externalheat gains and dissipate internal heat gains, including adequate insulation, overhangs, shading,orientation, surface color and texture, proper ventilation, and similar factors. When possible, externalheat gain should be controlled before it reaches or penetrates the weatherskin.

When cooling is necessary, heat dissipation is accomplished by cooling interior thermal mass, air, orboth with conduction, convection, and radiation. Evaporation in hot arid regions and dehumidification inhot humid regions are primary cooling design concerns. Many passive cooling concepts and methodsexist:

1. Site cooling: through vegetative control, water bodies, and adjacent land forms and materials.2. Earth cooling: by using groundwater or the earth's mass with earth sheltering or "coolth" tubes.3. Radiative cooling: heat loss to the sky or cooler objects.4. Ventilative cooling: cross ventilation through spaces, double roofs, attics, or walls, induced or

forced ventilation by pressure or temperature differences.5. Vapor cooling: evaporative cooling to remove sensible heat, dehumidification to remove latent

heat.6. Flywheel cooling: cooling by internal thermal mass or rockbeds.

Passive Cooling Techniques

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