Natural
Cooling
Passive
cooling techniques can be used to reduce, and in some
cases eliminate, mechanical air conditioning
requirements in areas where cooling is a dominant
problem. The cost and energy effectiveness of these
options are both worth considering by homeowner and
builders. Contained within this section are rules of
thumb and an explanation or the essentials of passive
cooling systems.
In
many parts of the southwest, summer cooling is as
important as winter heating. In the arid part of the
country, cooling is the primary design consideration.
Thermal
comfort in summer means more than keeping the indoor air
temperature below 75°. High temperatures, or high
humidity (or both) can lead to excessive discomfort.
Fortunately, the regions of high summer temperatures are
quite arid (relative humidity is usually low). The only
regions of fairly high humidity, the coastal regions,
are also among the coolest parts of the region in
summer.
There
are three major sources of unwanted summer heat: direct
solar impacts on a building and through windows and
skylights; heat transfer and infiltration, of exterior
high temperatures, through the materials and elements of
the structure; and the internal heat produced by
appliances, equipment,
and inhabitants. Of the three, the first is
potentially the greatest problem in the southwest, but
it is usually the easiest to control.
Table 14 lists approximate heat gains from each
source for typical single-family detached homes in a
climate where the temperature averages 75° F on a July
day. The homes are built to local energy codes and are
oriented east-west, and have two-thirds of the total
glazing facing south. The remaining glass is located on
the east and west walls, and all glass is completely
unshaded. Even assuming that sunlight could be excluded
from the interior in summer (a difficult feat), these
homes would experience excess heat loads of 250 to 450
thousand BTU per day in July. Worse yet, the houses
would require about 4-8 tons of air conditioning each to
handle peak heat gains and keep the rooms comfortable in
the afternoon.
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| Table 14. July
heat gains1 in typical tract homes2. |
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HEAT
GAIN CONTROL
Many
of the principles and techniques of passive solar
heating are adaptable to natural cooling. Insulation and
weather-stripping that prevent heat loss in the winter
will also retard heat gain during summer. Movable
insulating shutters for winter nighttime containment of
heat gain can also be used to reduce summertime daytime
heat gains. Inside the house, thermal mass such as
masonry walls and floors, act as "heat
sponges", absorbing heat and slowing internal
temperature rise on hot days, and can be cooled down by
nighttime ventilating (at the beginning and end of the
summer season) and by use mechanical cooling during
off-peak cost hours (nighttime). Suitably placed near a
window, skylight, or vent, the same thermal mass can be
exposed to cool night air to release the heat absorbed
from the space earlier in the day. Finally, earth
integrated buildings, embedded into the ground, benefit
from the lower difference between interior and exterior
surface temperature.
For
optimum summer cooling, a building's surroundings should
be designed to minimize summer sunlight striking
external surfaces, and to prevent surrounding area heat
re-radiation and reflection. Great temperature
differentials between desert exterior conditions of 110+
degrees and 78 degrees required for interior comfort can
be tempered using "thermal decompression"
zones that become increasingly more effective as one
nears the building. Mitigation of undesirable summer
direct sun and thermal impacts is achieved through use
of vegetation i.e. deciduous trees which interrupt the
summer sun's direct path, and ground covers which
prevent ground reflection as well as keep the earth's
surface cooler thereby preventing re-radiation.
One moves out of intense direct sun and heat
through vegetation that filters sunlight and shades the
ground; then through a more densely filtered zone with
ground covers; then through a patio area with
vegetation, trellises and water features; into a
tempered building entry
("thermal lock"); and finally into the
building proper. This movement, 110 degrees stepping
down in stages to 78 degrees, allows the body to adjust
properly, and provides the best means of arriving at a
lesser differentiation between the building's perimeter
wall interior and exterior surface temperatures. It is
this difference, between interior and exterior surface
temperature, that exacerbates the amount and rate of
heat flow through the material. Glazing should be
minimized on the roof and the east and west walls where
summer sunlight is most intense.
Intense
direct solar impacts from the sun rising in the east are
equal to those of the setting west sun. The reason we
feel the setting sun impact more is due to the added
thermal impact of the earth reradiating the heat it has
gained during the day. The summer sun is much higher in
the sky and has a negative impact on skylights and roof
windows and lead to enormous solar heat gains. They
should not be used in hot climates unless they are
insulated and/or shaded. Vertical south facing glass
(windows, clerestories, etc.) with overhangs or shades,
present fewer problems but are still adversely affected
by exterior air temperature. A horizontal overhang or an
awning above a south window is an inexpensive, effective
solution. If it protrudes to half the window height
(Fig. 21), such an overhang will shade the window
completely from early May to mid-August, yet allow for
winter sun access. A trellis with deciduous vines can be
used. Another good strategy is the use of deciduous
trees that shade the south face and roof during the
summer. All these shading methods work equally well with
Trombe walls, water walls, greenhouses, and other
south-wall passive solar collector strategies.
Figure 21. Shading devices
should be sized using the above graphic method. |
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Mitigation
for the roof and the east and west walls requires a
different approach. Since the sun is low in the horizon
during sunrise and sunset, overhangs are not effective
for solar mitigation and vertical shading is in order.
Vegetation is perhaps the most effective way of keeping
the intense morning and afternoon sun off the east and
west walls and windows, but care must be taken to avoid
blockage of nighttime summer breezes that can be part of
the diurnal cooling strategy. If vegetation is
impractical, a combination of tinted or reflecting glass
and exterior shades or shade screens that roll down over
east and west windows are an effective strategy.
Additionally, light-colored paints and materials on the
roof and the walls are effective in reflecting away most
of the sunlight that makes it past your shading.
CONVECTIVE
COOLING MODELS
The
heat gain control methods discussed above should suffice
to keep room temperatures comfortable in houses built
where mild summer temperatures are the rule. But there
are many other regions of the southwest, particularly
the desert areas, where additional cooling will usually
be necessary. The next step in natural cooling is to
take advantage of "convective" cooling methods
- those which use the prevailing winds and natural,
gravity-induced convection to ventilate a house at the
appropriate times of the day.
The
oldest, straightforward convective method admits cool
night air to drive out the warm air. If breezes are
predominant, high vents or open windows on the leeward
side (away from prevailing breeze) will let the hottest
air, located near the ceiling, escape. The cooler night
air sweeping in through low open vents or windows on the
windward side will replace this hot air and bring
relief. To get the best cooling rates, leeward openings
should have substantially larger total area (50% to 100%
larger) than those on the windward side of the house
(Fig. 22).
If
there are only light breezes at the site, natural
convection can still be used to ventilate and cool a
house as long as the outdoor air is cooler than the
indoor air at the peak of the house. Since warm air
rises, vents located at high points in the interior will
allow warm air to escape while cooler outdoor air flows
in through low vents to replace it (Fig. 23). The
coolest air around a house is usually found on the north
side, especially if this area is well shaded by trees or
shrubs and has water features. Cool air intake vents are
best located as low as possible on the north side. The
greater the height difference between the low and high
vents, the faster the flow of natural convection and the
more heat mitigation can occur.
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| Figure 22. Locate
cool air inlet vent on the building side which
receives predominant cool summer breezes. |
Figure 23. To aid
in natural ventilation, during summer use high
ceiling vaults, and thermal chimneys to promote
rapid air changes. |
There
are two basic ways to enhance the convective cooling
rate: 1) increase the volume of air escaping per minute,
or 2) bring in cooler air. If Delta T is the temperature
difference between exiting indoor air and incoming
outdoor air, the overall cooling rate in BTU's per hour
is given by the simple equation:
Cooling rate = 1.08 x V x DT
where
V is the volume of air escaping in cubic feet per
minute. Table 15 contains sample values of the cooling
rate for selected air flow rates and temperature
differences. For example, an air flow velocity of 1-2
feet/sec. through a vent of 10 square feet will result
in air flow rates between 500 and 1000 cubic feet per
minute. If incoming air is 10 degrees cooler than the
indoor air, the overall cooling rate will be about 5 to
10 thousand BTU's per hour.
Table 15. Convective cooling rates. |
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To
a point, increasing the vent area will increase the
airflow rate by natural convection. Turbine vents at the
roof peak are one way to enhance airflow and improve the
cooling rate. Even gentle breezes flowing up and over
the roof peak create an upward suction that draws out
warm interior air (Fig. 24). An even better approach is
to use solar radiation to induce a more rapid flow. One
of the many possible approaches, shown in Figure 25,
uses a Trombe wall vented to the outside. Sunlight
striking the concrete wall will heat the air in the
space between glass and wall to temperatures above 150°F.
This very hot air rises quickly and escapes,
drawing cool air into the house through low vents on the
north wall. Additionally, specifically constructed
"solar chimneys", composed of passive air
heaters with seasonal dampers can be incorporated where
solar heated air can be dumped into the building in the
winter, and used as a "ventilator driver" in
the summer to draw outdoor air through a house and
ventilate it. Frequently, they can induce air velocities
of 1-2 feet per second.
Another
convective cooling strategy is the drawing of outdoor
air is drawn through tubes buried in the ground and
dumped into the house. Made of material that allows easy
thermal transfer, these tubes are buried several feet
deep to avoid the warmer daytime surface temperatures.
Warm outdoor air entering the tube gives up its heat to
the cooler earth, and cools substantially before
entering the house
(Fig. 26). Thermal saturation of the surrounding
earth must be addressed, by means of surface landscaping
and watering, thereby removing the gained thermal energy
from the tube/earth transfers. Though condensation is
rarely a problem in dry climates, such tubes should be
sloped slightly and have adequate drainage to insure
that water build-up doesn't block the passage of air.
The intake end should be screened and placed in a shady
spot away from foot traffic. When properly built and
sized, these underground tubes can supply cool air
during the peak load daytime even in the hottest
climates.
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| Figure 24. Wind
turbines can be used to increase the ventilation
rate of rooms. |
Figure 25. An
indirect gain mass wall can be used to
significantly increase ventilation rates in
adjoining spaces. |
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| Figure 26. By
using undergrade air chambers, significant
sensible cooling can be obtained. |
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RADIATIVE
COOLING METHODS
The
exterior water wall and roof pond systems mentioned
earlier are also very effective summer cooling
strategies. In the cooling cycle (Fig. 27), insulating
panels remain closed by day to reject unwanted solar
heat. The cool ponds act as "thermal sponges",
absorbing room heat conducted through the interior
ceiling ceiling (metal deck) supporting them. At night.
panels are rolled back,
exposing the ponds to the black body of the night
sky and to
the cooler night air and breeze. The ponds lose heat by
radiation to the night sky and by natural convection to
the air. Roof pond systems are particularly effective in
regions of low humidity and clear summer nights. The
conditions exist in most of the southern tier, where the
cooling demand is greatest. If conditions are less than
ideal, augmented heat dissipation by evaporation can be
integrated (see following section).
For
best cooling results, ponds can range from
6-12 inches deep, depending on location and local
conditions, and should cover as much of the roof as
possible. An average tract home in the southwest, with
good heat gain control, can easily gain 200,000 to
400,000 BTU's on a hot July day.
A 6
inch deep roof pond covering the entire roof, will rise
in temperature by only 4-8°F from this heat gain, and
with nighttime cooling rates of 25-30 BTU/hr/ft' (Fig.
27), all this excess heat can be released to the outside
by daybreak.
A
few considerations for roof systems and ponds. They
should have an unobstructed path toward the zenith
(directly overhead). Adjacent trees, walls and other
buildings can impact the cooling rate by reducing
radiation to the night sky. Trees and walls also absorb
solar heat by day and radiate this energy into the ponds
at night. Cloud cover can interfere with the cooling
performance of a roof pond system. For this reason, roof
ponds are less effective in coastal areas where a low
cloud layer or dense fog frequently rolls in off the
ocean in the evening. Fortunately, however, such coastal
areas need little if any cooling during the summer.
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Figure 27. Roof
bonds utilizing cool, clear nightskys can provide
total cooling in many sections of the state. TOP -
panels are kept closed during the day. BOTTOM -
panels are opened after dusk to radiate out the
absorbed day time interior heat. |
EVAPORATIVE
COOLING METHODS
When
water evaporates it absorbs a large amount of heat from
its surroundings (about 1000 BTU per pound of water
evaporated). The most familiar example of this is the
cooling effect of evaporating perspiration on the human
skin. In arid, hot climates body temperature is
partially controlled by the rapid evaporation of
perspiration from the surface of the skin. In hot
climates with high atmospheric moisture the cooling
effect is less because the high moisture content of the
surrounding air. In both situations, however, the
evaporation rate is raised as air movement is increased.
Both of these facts can be applied to natural cooling of
structures.
Evaporative
methods can be used to enhance the cooling rates in
convective cooling systems. One way of doing this is to
bring the outdoor air into the house through a moist
filter or pad as shown in Figure 28. The familiar
evaporative cooler, precursor to the air conditioner, is
a mechanical system which uses these principles with a
motor to force air movement and distribution. Passive
cooling strategies with earth tubes and/or cool towers
use the same principles but utilize natural systems for
air drivers and distribution. If underground intake
pipes are made from a porous material, and ground above
them is well cool and watered, some evaporation will
occur at the inner surface of the pipe.
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Figure 28. Swamp
cooler - drier outside air is drawn through a
moist pad. As moisture is picked up by the air
heat is absorbed by the water, the result is
cooler air. |
Cool
towers utilize wet cooling pads, and the force of
gravity. Heavier, cooled air "falls", via
gravity, into the building and its momentum floods the
habitable area. This
cool tower action, as well as that of the earth cooling
tubes, can be enhanced and distribution extended, by the
placement of thermal chimney "drivers" which
can pull the cooled air through the building with an
increase in both air quantity and velocity. In either
case, the cooler air now has a higher relative humidity,
but this is not usually a problem and can even be a
benefit in arid climates.
In
some areas, there may be a time of higher humidity
(desert monsoon season). While sensible heat
continues to be mitigated by passive cooling techniques,
the latent heat contained in the humid air is more
difficult to dissipate, which renders evaporative
cooling less effective. The integration of a air
dehumidification system easily corrects this short term
problematic condition.
Evaporative
cooling strategies are well suited to those areas of the
southwest with the most severe cooling requirements. In
the desert areas of the South, the warm night air (80
degrees+) may impede natural convection heat dissipation
from a roof pond cooling system. That is one of the
reasons why the cooling rate falls to about 25
BTU/hr/ft^2 in the extreme southeast corner of the state
(Fig. 29). Simple introduction of a thin water layer
over the water containment surface can increase the
overall cooling rate of the roof by 50-100 percent due
to the resulting evaporation.
In
the most severe climates where nighttime air
temperatures often remain above 90°F in summer, sprays
can be used to achieve maximum natural cooling, at
standard roofs and roof cooling systems like the roof
pond strategy. In the summer, sprays can be used to
achieve optimum natural cooling. In the approach shown
in Figure 30, water is pumped to sprinklers along the
peak of a house and allowed to trickle down a sloping
roof. The rate of evaporation is greatly enhanced in
such a system because a much larger surface area is
exposed to the night air. Roof sprays rely on a little
external power to get the water to the roof and hence do
not qualify as completely passive systems. But the total
amount of energy consumed for pumping is very minimal
compared to the energy saved by the added cooling rate
attained. Excess water can be captured and reused or
used elsewhere on the site.
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| Figure 29. Roof
top sprinkler - combined radiative and evaporative
cooling can be integrated together to increase the
rate of cooling |
Figure 30. Open
pond with water wall - combined systems can be
devised to provide direct cooling. For all
interior spaces. |
A
passive evaporative system developed in California is
shown in Fig. 31. An open pool of water located above
the living spaces on the north side of a house is shaded
from the summer sun but exposed to the cool north sky
both day and night. Evaporation from the pool surface,
aided by radiation and natural convection, keeps the
water in this pool 30°F below the outside air
temperature on a hot summer day, without the use of
movable insulating panels. Natural convection brings
this cool water into the house and draws heat back up to
the pool as shown.
With
all evaporative cooling methods, it is important to
maximize airflow across the exposed water. Fresh air
must be continually available to replace the humid air
being built up near or over the water. Failing this, air
will be quickly saturated with water vapor, and the
evaporation and cooling rates will decline abruptly.
Lips, edges and other structures or buildings that could
block or deflect prevailing winds away from the water
surfaces should be studiously avoided. Sometimes, a
small fan to disturb the air over a pond will greatly
aid the evaporation rate on a hot, sultry day or night.
Even
with direct, active evaporative cooler systems,
provision of interior thermal mass combined with direct
evaporative cooling is a combination that works
effectively. During the day, the structure can utilize
the stored coolth in the walls and floors, and maintain
an improved level of comfort while reducing power
requirements of direct evaporative cooler system. In
many areas of the southwest which are considered hot,
arid zones, periods of higher humidity renders
mechanical evaporative cooling unsatisfactory even when
optimized techniques are used. A solution to this is the
two-stage evaporative cooling system, which has been
shown to be an effective alternative to direct
evaporative cooling or refrigerated air-conditioning.
While
not a passive system, two-stage evaporative cooling is
an important element to be considered as part of passive
cooling strategies. Cooling is accomplished by
pre-cooling ambient air without humidification before
further cooling by evaporation. The cool air entering
the structure is then exhausted, typically through areas
of heat gain such as windows or the attic. The
pre-cooling may be accomplished by a combined cooling
tower, heat exchanger unit, or by nocturnally cooled
rock bed through which air is drawn. The second stage,
evaporative cooling, is accomplished by a standard
commercial evaporative cooling device, or by passive
cooling elements of earth tubes or cool towers. Rock bed
mechanical cooling has been used extensively in
Australia with high degrees of effectiveness.
Two-stage
evaporative system can also be combined with active and
hybrid solar heating systems using the same storage
(rock bed) system for both seasons. Working systems have
been developed and demonstrated. This type of system is
necessarily suited for new construction because of the
requirement for the rock bed, which is most effectively
located beneath the structure. It works well during hot,
humid periods in the southwest using only slightly more
power than direct evaporative cooling and the comfort
attained is similar to that of refrigerated
air-conditioning.
A
typical system consisting of two evaporative coolers and
a large rock bed is shown in Figure 31. At night, one
evaporative cooler cools the rock bed while the other
cools the house using a one-stage evaporative cooler.
During the day, hot outside air is drawn through the
night-cooled rock bed where it is pre-cooled before
entering the main house evaporative cooler. Since no
moisture has been deposited in the rock bed, the
pre-cooled air has not had moisture introduced into the
house. An attractive feature of this type of system is
the combining of heating and cooling systems in order to
make the best possible use of components during the
entire year. An air heater may be used to provide hot
air during the heating season to the rock bed where the
rock bed, fans, ducts and many of the control systems
are used both during the heating and cooling season.
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| Figure 31.
Schematic diagram of two stage evaporative cooling
with rockbed. |
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Recuperative
and regenerative evaporative cooling options are other
methods to produce greater comfort using evaporative
cooling. These techniques use the relatively cool air
exhausted from the structure to improve the performance
of the evaporative cooling device. Evaporatively cooled
water reduces in temperature the ambient air in the heat
exchanger without humidification as it enters the
structure. The cool, dry air warms a few degrees as it
passes through the structure and exits through the
evaporative cooling device or a cooling structure. Since
the exiting air is cool and dry, the wet bulb
temperature is lower and the water produced by the
evaporative cooling device is cooler than if ambient air
were used. The rock bed heat exchanger and the
evaporative cooling device could be combined into a
single unit. If the rock bed is used to store heat in
the winter, the cost effectiveness of the system is
improved.
NOTE:
The psychometric chart should be used at all times to
analyze the effect of changing air conditions in these
systems. As a rule of thumb, pre-cooling the air ten
degrees will cause a three degree decrease in the output
temperatures of an evaporative air cooler. The improper
use of this rule can lead to errors of judgment when
analyzing the results of changing conditions.
Because
of the large volumes of air that are moved in an
effective evaporative system, the ducts must be large
and appropriately sized. Typically, evaporative cooler
ducts are at least three times the cross-section area of
ducts refrigeration; ducts should be laid out using the
shortest route possible and a minimum of turns.
Evaporative cooling has been shown to be an effective
alternative to refrigerated air-conditioning throughout
the desert regions of the southwest. The selection of
the particular evaporative cooling techniques must be
made carefully through analyzing the local climatic
conditions. These cooling systems should be integrated
into the design of the home and where possible, with the
design of the solar heating system. By integrating these
systems at the design stage, greater efficiencies and
more attractive economics can be obtained.
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