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                              TECHNICAL PAPER #48
                             UNDERSTANDING PASSIVE
                                COOLING SYSTEMS
                                 Daniel Halacy
                                Illustrated By
                                George R. Clark
                              Technical Reviewers
                                Thomas Beckman
                                 Daniel Dunham
                                 Daniel Ingold
                                 Published By
                       1600 Wilson Boulevard, Suite 500
                         Arlington, Virginia 22209 USA
                    Tel:  703/276-1800 . Fax:   703/243-1865
                    Understanding Passive Cooling Systems
                             ISBN: 0-86619-265-4
                 [C]1986, Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in
Technical Assistance to provide an introduction to specific
state-of-the-art technologies of interest to people in developing
countries.  The papers are intended to be used as
guidelines to help people choose technologies that are suitable
to their situations.  They are not intended to provide
construction or implementation details.   People are urged to
contact VITA or a similar organization for further information
and technical assistance if they find that a particular
technology seems to meet their needs.
The papers in the series were written, reviewed, and illustrated
almost entirely by VITA Volunteer technical experts on
a purely voluntary basis.  Some 500 volunteers were involved
in the production of the first 100 titles issued, contributing
approximately 5,000 hours of their time.   VITA staff included
Bill Jackson as editor, Suzanne Brooks handling typesetting
and layout, and Margaret Crouch as project manager.
The author, reviewers, and illustrator of this paper are all
VITA Volunteers.  The author, VITA Volunteer Dan Halacy, is
past Vice Chariman and Director of the American Solar Energy
Society and presently on the Editorial Board of the International
Solar Energy Society.  He has served with the Arizona
Solar Energy Commission and the Solar Energy Research Institute,
holds three solar patents, and has published eight
books and papers on solar energy.   Reviewer Thomas Beckman is
currently studying artificial intelligence at the Massachusetts
Institute of Technology, and has studied solar energy
applications at George Washington University in Washington,
D.C. Reviewer Dan Dunham is a professor at Columbia University
in New York City.  He has worked in Asia, Africa, and the
Caribbean on building design, rural housing, and settlement
planning projects.  Reviewer Dan Ingold is a test engineer for
the Hayward Tyler Pump Company in Burlington, Vermont.   Illustrator
George Clark teaches drafting, design, and technical
illustration at Kellogg Community College in Battle Creek,
VITA is a private, nonprofit organization that supports people
working on technical problems in developing countries.
VITA offers information and assistance aimed at helping individuals
and groups to select and implement technologies appropriate
to their situations.  VITA maintains an international
Inquiry Service, a specialized documentation center,
and a computerized roster of volunteer technical consultants;
manages long-term field projects; and publishes a variety of
technical manuals and papers.
                        by VITA Volunteer Daniel Halacy
Passive cooling systems use simple, low-cost techniques to provide
summer comfort in warm climates for people and animals in
buildings.  Such systems can also be used to keep food, liquids,
and other materials at temperatures that will prevent spoiling or
other deterioration.
Passive cooling is far less costly to operate than active cooling
systems such as air conditioning which typically use vapor-compression
or absorption refrigeration and require complex electromechanical
equipment and a power supply.   Passive cooling
methods use simple mechanisms and require no input of electrical
energy or conventional fuels.
The need for passive solar cooling, and the selection of appropriate
methods for achieving it, depend primarily on the climatic
conditions of a region, the cultural context, and the materials
available locally.
The History of Passive Cooling
Throughout history, humans and animals have learned and benefited
from passive cooling techniques.   Most creatures seek shade for
protection against heat.  Homes are often built in wooded areas.
Favorable breezes are sought.
Historically, building materials have often been chosen for their
effectiveness in tempering solar heat in summer.   Some builders
in temperate regions have adopted the low mass approach, using
walls and floors of wood, which doesn't store much heat.  Others,
needing insulation against winter cold, have learned to use dense
adobe or masonry walls.  In summer these delay the infiltration
of heat until evening, when the structure can be opened and
cooled with night air, breezes, and radiation to the night sky.
An ancient and very effective passive cooling method involves
building in caves of limestone or other workable material.  The
temperature of rock below the surface remains relatively stable(*),
winter warmth as well as summer cooling.
(*) at the mean annual temperature on the surface.
In ancient times the Persians learned to cool their buildings
with thermal chimneys, tall towers that warmed in the sun and
drove warm air up and out (because warm air rises), and thus
pulled cooler air into the building through openings near the
ground on the shady side.
The modern concept of passive cooling is based on these old and
effective methods, plus better knowledge and materials.
Basic Theory
Passive solar cooling uses two basic concepts: preventing heat
gain, rejecting unwanted heat.
The first concept, that of heat-gain control, is of far greater
importance than is generally recognized.   Factors involved
    1.   Site considerations
        Land massing
        Microclimate modification
    2.   Architectural features
        Building exposure
        Surface/volume ratio
    3.   Building component features
        Material type
The second concept, the rejection of unwanted heat, can be
divided into three major categories: (1) Direct loss (see Figure 1);

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(2) Indirect loss (see Figure 2); and (3) Isolated loss (see
Figure 3).
A thermal chimney or mechanical means are required to drive the
air flow as shown in the three drawings above.
These objectives of heat gain control and the rejection of
unwanted heat are accomplished by the following different
     1.   Shading from the sun
     2.   Reflection of solar heat
     3.   Insulation
     4.   Ground cooling
     5.   Wind cooling (natural breeze or induced convection)
     6.   Water cooling
     7.   Evaporative cooling
     8.   Dehumidification
     9.   Night radiant cooling
    10.   Night cooling of thermal mass in buildings
    11.   Exotic passive cooling methods
    12.   Seasonal cold storage
Applications for Passive Cooling
Passive cooling techniques can be applied to residences and other
buildings and to storage areas for food, liquids, and other
materials that may be damaged by overheating.   Passive cooling
obviously is of most value in hot climates, particularly where
conventional active cooling equipment is unavailable or unaffordable.
Availability of passive cooling also depends on such factors as
climate, cloud cover, night sky conditions, and availability of
In arid climates where water is available, evaporative cooling is
a low-cost method of providing comfort in high temperatures.
Yet, this approach is of little value in humid climates where the
air is already saturated with moisture; in such climates dehumidification
may be needed to provide comfortable passive cooling.
Thus, passive cooling differs in different places and situations.
The methods used depend on the specific site and environment.
Not all methods will be useful in every application and set of
The various methods of achieving passive cooling can be used
separately or combined, depending on site, climate, available
materials and skills, and economic considerations.   The
discussion that follows treats the different passive cooling
methods in order of their simplicity and cost effectiveness.
Shading from the Sun
The simplest and most effective passive cooling technique is to
keep the sun's heat from entering a building (Figure 4).  This is

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accomplished primarily by shading, using:
    *   The building itself (roof, walls)
    *   Other buildings, terrain features
    *   Supplemental shade (trees, vines, etc.)
    *   Awnings, shutters, curtains, drapes
When a new building is planned, shading should be included for
effective heat prevention.  With an existing building, benefits
may be constrained by its design and by the amount of money and
labor available for upgrading the building.
The provision of supplemental shading, such as vegetation or
awnings, is only a first step.   Trees must be kept healthy, so
they will continue to provide shade as well as the evaporative
cooling their transpiration of moisture yields.   Movable shades
must be properly maintained and effectively operated to keep
solar heat out of a building during the day but allow circulation
of cooler air at night.
Reflection of Solar Heat
Light-colored roofs, walls, and other shading have the important
advantage of reflecting much more heat than darker materials do.
A white roof may absorb only 25 percent of solar heat, far less
than the 90 percent absorbed by a black one.   This greatly reduces
the amount of heat getting into the building and simplifies
the task of comfort cooling.
Aluminum foil installed in an attic or ceiling (shiny side up)
further reduces the amount of radiant heat getting into the
building.  Reflective films can be applied to windows and other
glass areas to keep out more heat while remaining transparent.

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Insulation usually is considered a means of keeping heat inside a
building, but it can also keep heat out and thus provide cooling
in summer.  If insulation was still not installed in a building
originally because winters are mild, it may be economical to
install it for comfort in summer.
Walls and ceilings may be filled with conventional insulation
materials such as cellulose, vermiculite, rock wool, or glass
fiber.  Various kinds of rigid foam board may be used either
inside or outside of walls.  Potentially toxic materials (including
those that emit toxic fumes when burning) should not be used
inside.  A number of materials that have insulative properties may
be available locally and can serve as home made insulation.  Also
wood fiber, shredded sea weed, etc., can be used for insulation.
Ground Cooling
Like water, earth or subsurface rock reduces extremes of heat and
cold.  Although the surface temperature of soil rises during hot
summer days, soil at a depth of several feet is much cooler and
generally remains constant year-round.   Cool cave habitats date
back thousands of years, and modern versions are being built,
generally for office buildings or for storage.   A new generation
of underground homes is popular as builders seek even temperatures
year round with little or no expense for heating or cooling.
These earth-sheltered homes are excavated and/or bermed
with earth for added insulation.
The temperature of the earth varies according to the seasons.
That is, the highest temperature at each level is reached in the
summer months and the lowest temperature during the winter months
in a given region.
A refinement of underground passive cooling uses subsurface tunnels,
or cool pipes, to provide summer comfort for buildings.
However, caution should be used in this approach.   While good
performance has been obtained with some cool-pipe installations,

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prolonged use can warm the soil to a temperature too high for
comfort cooling.  Unless a large volume of subsurface soil is
available for very little effort and cost, only modest amounts of
cooling can be expected from this technique.   There are other
potential problems as well, including moisture, which can encourage
fungi and insect or animal life, causing adverse health
                   Table 1.  Example of Earth Temperatures
                         (Approximate) at Five Levels
 Depth in meters                                            Range
                                                      (degrees Celsius)
Ground Surface                                             1 - 24
    1.5                                                    6 - 17
    3                                                      8 - 16
    9                                                     11 - 13
Source:  American Institute of Architects
Wind Cooling (natural breezes or induced covection)
The cooling breezes we intuitively take advantage of should also
be used to maximum benefit in passively cooling a building.  See
Figure 7.  If outside air is appreciably cooler than inside it can

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enter open windows and match the cooling power of a small air-conditioning
unit.  Yet it costs nothing to use.  When the sun is
not shining on windows, they should be opened when outside air is
cooler and a breeze is blowing.   They should be opened at night
whenever outside air is cooler than the interior of the house.
Even if there is little or no wind, steps may be taken to induce
a convective flow of air through a building to aid in cooling it.
warm air naturally rises; if outlets in the form of high windows
or vents are provided, this air will flow out and be replaced by
cooler air coming in low openings on the shady side of the building.
See Figure 8.

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Thermal chimneys, an effective form of convective air flow, are
still in use in Iran, and many newer ones have been installed
elsewhere to promote the cooling flow of air through a building.
The upper portion of the chimney is heated by the sun, the warm
air inside rises and goes out the top and cooler air comes into
the building from shaded window openings.
Water Cooling
A stream or pond may provide some passive cooling.   Water can be
piped or pumped through radiators to carry away surplus heat and
thus cool the air inside a building.   The warmed water can then
be returned to its source and not be wasted.
Very cold, underground streams have been used for passive cooling
of buildings.
Evaporative Cooling
Moist air sometimes provides cooling in warm climates.   This
technique has been used for centuries by placing pools and fountains
in courtyards or other areas adjacent to buildings.   Combined
with a breeze from the proper direction, this natural
evaporative cooling provides comfort at little cost (Figure 9).

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Mechanical evaporative coolers using electrically driven fans
provide excellent comfort in and areas.   This cooling equipment
was developed slowly from primitive evaporative coolers consisting
only of a wet cloth or fibrous material hung in a window or
doorway exposed to a breeze.   The material was periodically dipped
into water, or hung so its bottom edge was in a container of
water and a "wicking" action kept it wet.   Such simple coolers
can be improvised today with some effect.
Where water is readily available and expendable, larger applications
of evaporative cooling can be made.   Water can be sprayed
or trickled on a roof to cool it.   In some cases, a pond of water
can be created on a flat, watertight roof.   In dry arid climates,
the evaporative effect of the pool is enhanced by night radiation
of heat from the water to the night sky.
Evaporative cooling depends on a very dry climate to be effective.
When the air is humid and already laden with moisture,
adding more water decreases comfort.   Moreover, pumping systems
may be costly.
Where normal evaporative cooling is not possible because of high
humidity, dehumidification may provide some comfort.   Barrels of
salt were used many years ago in some regions to dry humid air
for human comfort.  Today the concept has developed into
electromechanic active desiccant cooling equipment.   Desiccants
are substances that remove moisture f rom the air.   Such systems
are beyond the scope of are expensive and complex, and thus of
little interest for the cooling applications discussed here.
However, work is also being done on passive desiccant cooling.
Silica gel, lithium chloride, and activated charcoal are typical
desiccants.  Trays of such material are placed in a flow of air
to remove moisture from it.  As with the old-time salt barrels,
however, the desiccant material must be dried periodically so
that it will again absorb or adsorb water.   This can be done
simply by leaving the saturated desiccant in the sun, or the
drying process can be speeded up by using air-type solar collectors.
In either case, two desiccant systems must be used in
parallel, with one in use while the other is regenerated (Figure 10).

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Most desiccant cooling systems use electric or gas heat for
drying the desiccant material.   vHowever, there are active solar-assisted
desiccant systems, and even some rudimentary passive
cooling systems.
Night Radiant Cooling
Even in hot desert regions, the night sky is often quite cool.
This permits the radiation of large amounts of heat from a building.
The Skytherm House, developed by Harold Hay, uses this
principle to stay cool in summer.   The flat-roofed structure is
covered with warm plastic bags covered with insulation during the
day but exposed to the sky at night.   Simpler systems flood the
flat roof to achieve similar but not as effective heat loss at
night (Figure 11).

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Night cooling of thermal mass in buildings
In high temperature climates, low-mass buildings minimize summer
discomfort.  However, many areas are hot in summer but cold in
winter.  Winter comfort demands a well-insulated building and this
is often provided by thick earthen or masonry walls.   With proper
handling, such a building can also promote passive cooling.
The thick walls absorb the sun's heat during the day, keeping it
from reaching the interior of the building.   At night, particularly
with clear skies, the building can be opened up to the
cooler night air and breezes, cooling the walls and roof (Figure 12).

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Cooling is enhanced by wind and radiation to the night sky,
and evaporative cooling can be used also if water is available.
Exotic Passive Cooling Methods
Some work has been done in artificially producing ice, which is
stored and used later for comfort cooling.   This method has been
used on a small scale for air-conditioning office buildings, but
requires special ice-making equipment, and very well-insulated
storage for the long period between winter ice-making and summer
Some experimental work has been done with special solar
collectors and radiators (using zeolite heat-exchange materials)
that operate day and night provide cooling or even ice.   Zeolites
are alumino-silicate minerals (See Figure 14).   Uses have

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included refrigerating foods and medicines and providing cool
water for showers in very hot climates.   Such systems may
technically be classed as passive cooling, because they require
no electric power or fuel energy, but they are complex and
expensive.  Moreover, present passive models require design modifications
to improve performance in areas where there is only a
small temperature change between day and night.
Choice of the appropriate passive cooling method depends on the
application under consideration (residence, school, dormitory,
office building, workshop; dairy or other animal structure; food,
liquid, or medicine storage); on the amount of cooling required;
and on the differing environmental and other conditions at the
site (terrain, soil, temperature, humidity, wind, cloud cover).
The first consideration in any passive cooling project should be
to keep heat generated inside the building to the practical
minimum, thus reducing the need for comfort cooling.   This means
cooking, washing clothes and dishes, ironing, and doing other
heat-producing activities outside if possible or at night.  Proper
dress is obviously important for comfort at relatively high
temperatures.  Clothing of light, absorbent materials minimize
heat retention and discomfort.   Wearing sandals, or no shoes at
all, may be a further help.
Generally Applicable Technologies
Just as the above-mentioned tips for minimizing the need for
cooling apply generally, some passive cooling technologies will
be of benefit in almost all applications and climates.
Use of shade to prevent unwanted heat from entering a building is
the most generally appropriate cooling measure.   It should be
considered first.  Reflection of solar heat is also generally
applicable, whether the sky is cloudy or clear, the air dry or
moist.  Insulation too is an all-around technique, although the
type used will vary with the building construction and climatic
If cool breezes blow, they will cool inhabitants and buildings in
both dry and moist climates.   Induced convection can be used to
vent hot air from practically all structures.   This method is
most effective in buildings with high ceilings.
Arid Climate Technologies
A relatively arid climate makes possible the use of water-cooling
methods (evaporative cooling, roof ponds) where water is
available; rejection of heat to the clear night sky; and ground
cooling.  Large, flat-roofed buildings such as factories,
schools, and hospitals are good candidates for roof-pond cooling
measures.  Clear night skies make this method even more effective
in getting rid of unwanted heat.
Buildings of earthen materials, masonry, and other dense materials
permit the delaying of thermal action that keeps heat from
reaching the inside of a building until it can be cooled at
Underground and earth-sheltered buildings can be built in many
areas where soil is dry the year round.   Underground building is
seldom justifiable solely on the basis of passive cooling, however.
This technique has been most effective in such places as
caves of limestone or other easily worked material.   Such applications
are much more site-specific and thus are limited in
Humid Climate Technologies
In areas of appreciable humidity, dehumidification or desiccant
cooling may be required.  To be truly passive in operation, this
cooling method depends on sufficient wind flow to carry moist air
over a moisture-absorbing desiccant and into the building to be
cooled.  Unless solar collectors are used to continuously regenerate
the desiccant, two desiccant pans must be provided: one in
use while the other is being dried.
The following table is a suggested rough match of passive

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cooling technologies with different applications.   It should
provide a starting point for analysis and planning of a project.
Rudimentary forms of passive cooling have been used successfully
for centuries and much-improved technology is available today.
However, continued research and development suggest that even
greater improvements will be possible in the future.
As population increases in hot regions and as energy becomes
scarcer and more costly, the demand for passive cooling
increases.  Although it is presently only a minor contributor to
human comfort when compared with conventional cooling methods,
the growing demand will create a large potential market.  This
will stimulate better design and more effective systems and
Better materials and equipment for use in passive cooling seem
assured because of advances in allied fields, and the increasing
focus on passive cooling technologies.   Among these advances are:
     o     Improved heat rejecting metals and other materials
     o     Automatic movable insulation and shading devices
     o     Reversible chemical reactions for heat exchange
     o     Selective window glazing for heat rejection
     o     Improved desiccant materials
Those interested in passive cooling should guard against too high
expectations, however.  Passive cooling does not, and probably
will not in the foreseeable future, compare in effectiveness with
conventional electrical and mechanical cooling techniques.  But to
the hot and uncomfortable person for whom such equipment is out
of reach, passive cooling can be a step up in comfort at a small
Publishing Company, 34 Essex Street, Andover, Massachusetts
01810, USA. 197 pp. $8.95. (*)
ASHRA Handbook of Fundamentals. (American Society of Heating,
Refrigeration, and Air Conditioning Engineers, Publication Sales,
1791 Tullie circle, NE, atlanta, Georgia 30329, USA. 748 pp.
Baer, Steve (Zomeworks corporation, P.O. Box 25805, Albuquerque,
New Mexico 87125, USA). "Cooling with Nighttime Air," Alternative
Sources of Energy. Vol. 41, January/February 1980, p.22.
Baer, S. "Raising the `Open U' Value by Passive Means,"
Progress in Passive Solar Energy Systems. (American Solar Energy
Society, Inc., 1983) Vol. 8, pp. 839-842.
Bliss, Raymond W., Jr. "Atmospheric Radiation Near the Surface of
the Ground: A Summary for Engineers," Solar Energy. (International
Solar Energy Sociey) July/September 1961.
Clark, Gene, (Solar Data Center, Box 500, Trinity University,
San Antonio, Texas 78284, USA. "Results of Validated Simulations
of Roof Pond Cooled Residences," Progress in Passive
Solar Energy Systems. op.cit. pp.823-828.
Collier, R.K. (Solar Energy Research Institute) "Desiccant and
other Cooling Systems, "Solar Cooling Applications[paragraph] Workshop. (1
June, 1980, Phoenix, Arizona, USA)pp. 93-109.
Earth Sheltered Housing Design. (University of Minnesota
Underground Space Center, Van Nostrand Reinhold, 1979) 318 pp.
$10.95 (*)
Hay, Harold. "Atascadero Residence,"
Passive Solar Heating and Cooling Conference and
Workshop Processings, May 18-19, 1976 Albuquerque, New Mexico.
(LA-6637-C) $3.00 microfiche domestic, $4.50 michrofiche foreign
pp. 101-107. (**)
McPhee, John. "Ice Pond," New Yorker. 13 July, 1981, pp. 92-95.
Miller, W.C. and J.O. Bradley Energy Systems Center, Dessert
Research Institute, Boulder City, NV 89005, USA). "Radiative
Cooling with Selective Surfaces in a Desert Climate,"
Solar Cooling Applications Workshop). (1 June, 1980, Phoenix,
Arizona, USA) pp. 85-90.
Next Whole Earth Catalog, Second Edition (Random House, 1981) 608
pp. $16.00 (*)
Olgyay, Aladar and V. Olgyay. Solar Control and Shading Devices.
(Princeton University Press, 1967,)
Olgyay, Victor. Design with Climate. (Princeton University Press,
1963) 190 pp. (*)
Passive Cooling Handbook. (Ed. by Harry Miller.) Prepared for the
Passive Cooling Workshop in Amherst, Massachusetts, USA on 20-22
October, 1980. Available from Don Elmer, Passive Cooling Working
Passive Solar Design Handbook DO E/CS-0127/1 US-59. Prepared by
Total Environment Action, Inc. for the US Department of Energy.
March 1980. $3.00 (microfiche) (**)
Rudofsky, Bernard. Architecture Without Architects. (Doubleday &
Company, 1969) 166 pp. $5.95 (*)
Schubert, R.P. and P. Hahn (Environmental Systems Lab, College of
Architecture and Urban Studies, Virginia Polytechnic Institute
and State University, Blackburg, Virginia 24061, USA) "The Design
and Testing of a High Performance Ventilator Cowl: An Element in
Passive Ventilation," Progress in Passive solar energy Systems,
op.cit. pp. 867-872.
Vierira, R.K., (Physics Department, Trinity University)
"Energy Savings Potential of Dehumidified Roof Pond Residences,"
Progress in Passive Solar Energy Systems.