TECHNICAL PAPER #48
George R. Clark
1600 Wilson Boulevard, Suite 500
Arlington, Virginia 22209 USA
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Understanding Passive Cooling Systems
[C]1986, Volunteers in Technical Assistance
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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
Solar Energy Society.
He has served with the Arizona
Solar Energy Commission and the Solar Energy Research
holds three solar patents, and has published eight
books and papers on solar energy.
Reviewer Thomas Beckman is
currently studying artificial intelligence at the
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
in New York City. He
has worked in Asia, Africa, and the
Caribbean on building design, rural housing, and settlement
Reviewer Dan Ingold is a test engineer for
the Hayward Tyler Pump Company in Burlington, Vermont.
George Clark teaches drafting, design, and technical
illustration at Kellogg Community College in Battle Creek,
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UNDERSTANDING PASSIVE COOLING SYSTEMS
by VITA Volunteer Daniel Halacy
Passive cooling systems use simple, low-cost techniques to
summer comfort in warm climates for people and animals in
systems can also be used to keep food, liquids,
and other materials at temperatures that will prevent
Passive cooling is far less costly to operate than active
systems such as air conditioning which typically use
or absorption refrigeration and require complex
equipment and a power supply.
methods use simple mechanisms and require no input of
energy or conventional fuels.
The need for passive solar cooling, and the selection of
methods for achieving it, depend primarily on the climatic
conditions of a region, the cultural context, and the
The History of Passive Cooling
Throughout history, humans and animals have learned and
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
effectiveness in tempering solar heat in summer.
in temperate regions have adopted the low mass approach,
walls and floors of wood, which doesn't store much
needing insulation against winter cold, have learned to use
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
An ancient and very effective passive cooling method
building in caves of limestone or other workable
temperature of rock below the surface remains relatively
winter warmth as well as summer cooling.
(*) at the mean annual temperature on the surface.
In ancient times the Persians learned to cool their
with thermal chimneys, tall towers that warmed in the sun
drove warm air up and out (because warm air rises), and thus
pulled cooler air into the building through openings near
ground on the shady side.
The modern concept of passive cooling is based on these old
effective methods, plus better knowledge and materials.
Passive solar cooling uses two basic concepts: preventing
gain, rejecting unwanted heat.
The first concept, that of heat-gain control, is of far
importance than is generally recognized.
Building component features
The second concept, the rejection of unwanted heat, can be
divided into three major categories: (1) Direct loss (see
(2) Indirect loss (see Figure 2); and (3) Isolated loss (see
A thermal chimney or mechanical means are required to drive
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
Shading from the sun
Reflection of solar heat
Wind cooling (natural breeze or induced
Night radiant cooling
Night cooling of thermal mass in buildings
Exotic passive cooling methods
Seasonal cold storage
Applications for Passive Cooling
Passive cooling techniques can be applied to residences and
buildings and to storage areas for food, liquids, and other
materials that may be damaged by overheating.
obviously is of most value in hot climates, particularly
conventional active cooling equipment is unavailable or
Availability of passive cooling also depends on such factors
climate, cloud cover, night sky conditions, and availability
In arid climates where water is available, evaporative
a low-cost method of providing comfort in high temperatures.
Yet, this approach is of little value in humid climates
air is already saturated with moisture; in such climates
may be needed to provide comfortable passive cooling.
Thus, passive cooling differs in different places and
The methods used depend on the specific site and
Not all methods will be useful in every application and set
II. PASSIVE COOLING
The various methods of achieving passive cooling can be used
separately or combined, depending on site, climate,
materials and skills, and economic considerations.
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
keep the sun's heat from entering a building (Figure
4). This is
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
effective heat prevention.
With an existing building, benefits
may be constrained by its design and by the amount of money
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
cooling their transpiration of moisture yields.
must be properly maintained and effectively operated to keep
solar heat out of a building during the day but allow
of cooler air at night.
Reflection of Solar Heat
Light-colored roofs, walls, and other shading have the
advantage of reflecting much more heat than darker materials
A white roof may absorb only 25 percent of solar heat, far
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
further reduces the amount of radiant heat getting into the
films can be applied to windows and other
glass areas to keep out more heat while remaining
Insulation usually is considered a means of keeping heat
building, but it can also keep heat out and thus provide
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
materials such as cellulose, vermiculite, rock wool, or
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
wood fiber, shredded sea weed, etc., can be used for
Like water, earth or subsurface rock reduces extremes of
cold. Although the
surface temperature of soil rises during hot
summer days, soil at a depth of several feet is much cooler
generally remains constant year-round.
Cool cave habitats date
back thousands of years, and modern versions are being
generally for office buildings or for storage.
A new generation
of underground homes is popular as builders seek even
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
That is, the highest temperature at each level is reached in
summer months and the lowest temperature during the winter
in a given region.
A refinement of underground passive cooling uses subsurface
or cool pipes, to provide summer comfort for buildings.
However, caution should be used in this approach.
performance has been obtained with some cool-pipe
prolonged use can warm the soil to a temperature too high
Unless a large volume of subsurface soil is
available for very little effort and cost, only modest
cooling can be expected from this technique.
There are other
potential problems as well, including moisture, which can
fungi and insect or animal life, causing adverse health
Table 1. Example of Earth
(Approximate) at Five Levels
Depth in meters
1 - 24
6 - 17
8 - 16
11 - 13
Institute of Architects
Wind Cooling (natural breezes or induced covection)
The cooling breezes we intuitively take advantage of should
be used to maximum benefit in passively cooling a
Figure 7. If outside
air is appreciably cooler than inside it can
enter open windows and match the cooling power of a small
unit. Yet it costs
nothing to use. When the sun is
not shining on windows, they should be opened when outside
cooler and a breeze is blowing.
They should be opened at night
whenever outside air is cooler than the interior of the
Even if there is little or no wind, steps may be taken to
a convective flow of air through a building to aid in
warm air naturally rises; if outlets in the form of high
or vents are provided, this air will flow out and be
cooler air coming in low openings on the shady side of the
See Figure 8.
Thermal chimneys, an effective form of convective air flow,
still in use in Iran, and many newer ones have been
elsewhere to promote the cooling flow of air through a
The upper portion of the chimney is heated by the sun, the
air inside rises and goes out the top and cooler air comes
the building from shaded window openings.
A stream or pond may provide some passive cooling.
Water can be
piped or pumped through radiators to carry away surplus heat
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
Moist air sometimes provides cooling in warm climates.
technique has been used for centuries by placing pools and
in courtyards or other areas adjacent to buildings.
with a breeze from the proper direction, this natural
evaporative cooling provides comfort at little cost (Figure
Mechanical evaporative coolers using electrically driven
provide excellent comfort in and areas.
This cooling equipment
was developed slowly from primitive evaporative coolers
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
of heat from the water to the night sky.
Evaporative cooling depends on a very dry climate to be
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
humidity, dehumidification may provide some comfort.
salt were used many years ago in some regions to dry humid
for human comfort.
Today the concept has developed into
electromechanic active desiccant cooling equipment.
are substances that remove moisture f rom the air.
are beyond the scope of are expensive and complex, and thus
little interest for the cooling applications discussed here.
However, work is also being done on passive desiccant
Silica gel, lithium chloride, and activated charcoal are
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
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
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
Night Radiant Cooling
Even in hot desert regions, the night sky is often quite
This permits the radiation of large amounts of heat from a
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
day but exposed to the sky at night.
Simpler systems flood the
flat roof to achieve similar but not as effective heat loss
night (Figure 11).
Night cooling of thermal mass in buildings
In high temperature climates, low-mass buildings minimize
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.
handling, such a building can also promote passive cooling.
The thick walls absorb the sun's heat during the day,
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
Cooling is enhanced by wind and radiation to the night sky,
and evaporative cooling can be used also if water is
Exotic Passive Cooling Methods
Some work has been done in artificially producing ice, which
stored and used later for comfort cooling.
This method has been
used on a small scale for air-conditioning office buildings,
requires special ice-making equipment, and very
storage for the long period between winter ice-making and
Some experimental work has been done with special solar
collectors and radiators (using zeolite heat-exchange
that operate day and night provide cooling or even ice.
are alumino-silicate minerals (See Figure 14).
included refrigerating foods and medicines and providing
water for showers in very hot climates.
Such systems may
technically be classed as passive cooling, because they
no electric power or fuel energy, but they are complex and
present passive models require design modifications
to improve performance in areas where there is only a
small temperature change between day and night.
III. SELECTING THE
RIGHT PASSIVE COOLING SYSTEM
Choice of the appropriate passive cooling method depends on
application under consideration (residence, school,
office building, workshop; dairy or other animal structure;
liquid, or medicine storage); on the amount of cooling
and on the differing environmental and other conditions at
site (terrain, soil, temperature, humidity, wind, cloud
The first consideration in any passive cooling project
to keep heat generated inside the building to the practical
minimum, thus reducing the need for comfort cooling.
cooking, washing clothes and dishes, ironing, and doing
heat-producing activities outside if possible or at
dress is obviously important for comfort at relatively high
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
be of benefit in almost all applications and climates.
Use of shade to prevent unwanted heat from entering a
the most generally appropriate cooling measure.
It should be
Reflection of solar heat is also generally
applicable, whether the sky is cloudy or clear, the air dry
too is an all-around technique, although the
type used will vary with the building construction and
If cool breezes blow, they will cool inhabitants and
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
methods (evaporative cooling, roof ponds) where water is
available; rejection of heat to the clear night sky; and
flat-roofed buildings such as factories,
schools, and hospitals are good candidates for roof-pond
night skies make this method even more effective
in getting rid of unwanted heat.
Buildings of earthen materials, masonry, and other dense
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
areas where soil is dry the year round.
Underground building is
seldom justifiable solely on the basis of passive cooling,
This technique has been most effective in such places as
caves of limestone or other easily worked material.
are much more site-specific and thus are limited in
Humid Climate Technologies
In areas of appreciable humidity, dehumidification or
cooling may be required.
To be truly passive in operation, this
cooling method depends on sufficient wind flow to carry
over a moisture-absorbing desiccant and into the building to
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
cooling technologies with different applications.
provide a starting point for analysis and planning of a
IV. THE FUTURE OF
Rudimentary forms of passive cooling have been used
for centuries and much-improved technology is available
However, continued research and development suggest that
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
it is presently only a minor contributor to
human comfort when compared with conventional cooling
the growing demand will create a large potential
will stimulate better design and more effective systems and
Better materials and equipment for use in passive cooling
assured because of advances in allied fields, and the
focus on passive cooling technologies.
Among these advances are:
Improved heat rejecting metals and other
Automatic movable insulation and shading
Reversible chemical reactions for heat
Selective window glazing for heat
Improved desiccant materials
Those interested in passive cooling should guard against too
Passive cooling does not, and probably
will not in the foreseeable future, compare in effectiveness
conventional electrical and mechanical cooling
techniques. But to
the hot and uncomfortable person for whom such equipment is
of reach, passive cooling can be a step up in comfort at a
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