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.
 |
|
| Table 14. July heat gains1
in typical tract homes2. |
|
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. |
|
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.
 |
 |
| 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. |
|
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.
 |
 |
|
| 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. |
|
|
|
|
 |
|
|
| Figure 26. By using undergrade
air chambers, significant sensible cooling can be obtained. |
|
|
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.
 |
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.
 |
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.
|
|
| 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.
 |
|
| Figure 31. Schematic diagram
of two stage evaporative cooling with rockbed. |
|
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.
|
