TECHNICAL PAPER #48
UNDERSTANDING PASSIVE
COOLING SYSTEMS
By
Daniel Halacy
Illustrated By
George R. Clark
Technical Reviewers
Thomas Beckman
Daniel Dunham
Daniel Ingold
Published By
VITA
1600 Wilson Boulevard, Suite 500
Arlington, Virginia 22209 USA
Tel: 703/276-1800 . Fax:
703/243-1865
Internet: pr-info@vita.org
Understanding Passive Cooling Systems
ISBN: 0-86619-265-4
[C]1986, Volunteers in Technical Assistance
PREFACE
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,
Michigan.
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.
UNDERSTANDING PASSIVE COOLING SYSTEMS
by VITA Volunteer Daniel Halacy
I. INTRODUCTION
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
include:
1.
Site considerations
Location
Orientation
Vegetation
Land massing
Microclimate
modification
2.
Architectural features
Building
exposure
Surface/volume ratio
Screens
Shades
Wingwalls
Overhangs
3.
Building component features
Insulation
Glazing
Mass
Material type
Texture
Finishes
The second concept, the rejection of unwanted heat, can be
divided into three major categories: (1) Direct loss (see
Figure 1);
upc1x3.gif (600x600)
upc2x4.gif (600x600)
upc3x5.gif (600x600)
(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
methods:
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
water.
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
conditions.
II. PASSIVE COOLING
METHODS
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
upc4x7.gif (437x600)
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.
Insulation
upc5x8.gif (600x600)
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,
upc6x10.gif (600x600)
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
conditions.
Table 1. Example of Earth
Temperatures
(Approximate) at Five Levels
Yearly
Temperature
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
upc7x11.gif (486x486)
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.
upc8x12.gif (486x486)
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).
upc9x13.gif (540x540)
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.
Dehumidification
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).
upc10x15.gif (600x600)
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).
upc11x16.gif (437x437)
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).
upc12x16.gif (486x486)
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
cooling.
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
upc13x17.gif (486x486)
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.
III. SELECTING THE
RIGHT PASSIVE COOLING SYSTEM
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
conditions.
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
night.
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
number.
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
upcxtab1.gif (600x600)
cooling technologies with different applications.
It should
provide a starting point for analysis and planning of a
project.
IV. THE FUTURE OF
PASSIVE COOLING
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
equipment.
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
price.
REFERENCES
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.
$53.00
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, et.al. (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
Group.
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., et.al. (Physics Department, Trinity
University)
"Energy Savings Potential of Dehumidified Roof Pond
Residences,"
Progress in Passive Solar Energy Systems.
========================================
========================================
