2. Introduction
The term "passive" implies
that energy-consuming
mechanical components
like pumps and fans are not
used.
To achieve thermal comfort
in the summer in a more
sustainable way, one should
use the three-tier design
approach.
5. In urban settings and other places with little wind,
wind scoops are used to maximize ventilation.
The wind towers in Hyderabad, Pakistan,
all face the prevailing wind.
The wind towers in Dubai, United Arab Emirates,
are designed to catch the wind from any direction.
(Photograph byMostafa Howeedy.)
6. Some wind towers in
hot and dry areas cool
the incoming air by
evaporation.
A mashrabiya is a screened bay window p
opular in the Arabic Middle East.
It shades, ventilates, and provides evapo-rative
cooling. Cairo, Egypt.
(Photograph byMostafa Howeedy.)
In the evening, the orchestra
platform provided entertain
ment while the water cooled
the air at the Panch Mahal
Palace at Fathepur, India.
(Photograph by Lena Choudhary.)
7. Small domes made of sun-dried
mud bricks work well
in very hot and dry climates,
such as those found in Egypt.
Narrow alleys enable buildin
gs to shade each other. Small
courtyards provide outdoor s
leeping areas at night.
The cliff dwellings at Mesa
Verde, CO, benefit from the
heat-sink capacity of the stone
walls and rock cliff.
Dwellings and churches are
carved from the volcanic tuffa
cones in Cappadocia, Turkey.
(Photograph by Tarik Orgen.)
8. Five methods of passive cooling:
I. Cooling with Ventilation
A. Comfort ventilation: Ventilation during the day and
night to increase evaporation.
B. Night flush cooling: Ventilation to pre cool the building
for the next day.
II. Radiant Cooling
A. Direct radiant cooling: A building's roof structure cools
by radiation to the night sky.
B. Indirect radiant cooling: Radiation to the night sky cools
a heat-transfer fluid, which then cools the building.
9. III. Evaporative Cooling
A. Direct evaporation: Water is sprayed into the air entering
a building.
B. B. Indirect evaporative cooling: Evaporation cools the
incoming air or the building without raising the indoor
humidity.
IV. Earth Cooling
A. Direct coupling: An earth-sheltered building loses heat
directly to the earth.
B. Indirect coupling: Air enters the building by way of earth
tubes.
V. Dehumidification with a Desiccant: Removal of
latent heat.
For example, in the South the earth might be too warm for
cooling unless its temperature is first lowered by
evaporation.
10. I A. Comfort ventilation
Air Flow through Buildings:
> Site Conditions: Adjacent buildings,
walls, and vegetation on the site.
>Window Orientation and Wind Direction
Usually indoor ventilation is better from
oblique winds than head-on winds because the
oblique air stream covers more of the room.
Acceptable wind directions for the
orientation that is best for summer
shade and winter sun.
Deflecting walls and vegetation can be
used to change air-flow direction so that
the optimum solar orientation can be
maintained.
11. > Fin Walls
Cross-ventilation between windows on
opposite walls is the ideal condition.
Ventilation from adjacent windows can be poor
or good, depending on the wind direction.
Some ventilation is possible in the
asymmetric placement of windows because
the relative pressure is greater at the center
than at the sides of the windward wall.
12. > Horizontal Overhangs and Air Flow
The greater positive on one side of
the window deflects the airstream
in the wrong direction. Much of the
room remains unventilated.
A fin wall can be used to direct the
airstream through the centre of the
room.
The solid horizontal overhang causes the
air to deflect upward
13. A louvered overhang or at least a
gap in the overhang will permit
the airstream to straighten out.
A solid horizontal overhang placed
high above the window will also
straighten out the airstream.
> Roof Vents
The design of roof ventilator has a great effect on its performance.
Percentage show relative effectiveness.
An air speed of 0.5m per second equates to a 3 degree
drop in temperature at relative humidity of 50 per cent.
14. > Comfort Ventilation
A completed air-flow diagram.
Air flow should also be checked in section.
Comfort ventilation is most
appropriate when the indoor
temperature and humidity are
above the outdoor level.
The Mayan Indians of the hot and humid
Yucat an Peninsula build lightweight,
porous buildings although mud and
rocks are available.
15. I B. Night flush cooling
Since the ventilation removes the heat from the
mass of the building at night, this time-tested
passive technique is called night-flush cooling.
Night-flush cooling works in two stages. At night,
natural ventilation or fans bring cool outdoor air in
contact with the indoor mass, thereby cooling it.
The next morning, the windows are closed to
prevent heating the building with outdoor air.
16. With "night-flush cooling," night ventilation cools the mass of the building.
During the day, the night-flush cooled mass acts as a heat sink. Light colors, insulation,
shading, and closed windows keep the heat gain to a minimum. Interior circulating fans
can be used for additional comfort.
17. II. Radiant Cooling
At night the long-wave infrared radiation from a
clear sky is much less than the long-wave
infrared radiation emitted from a building, and
thus there is a net flow to the sky. On humid
nights, the radiant cooling is less efficient but a
temperature depression of about 7°F is still
possible. Clouds, on the other hand, almost
completely block the radiant cooling effect
18. Direct & indirect radiant cooling
On clear nights with little humidity,
there is strong radiant cooling.
Humidity reduces radiant cooling, and
clouds practically stop it.
Potentially the most efficient approach to radiant cooling
is to make the roof itself the radiator. For example, an
exposed-concrete roof will rapidly lose heat by radiating
to the night sky. The next day, the cool mass of concrete
can effectively cool a building by acting as a heat sink.
19. At night, the movable insulation is in
the “open” position so that the
buildings’ heat can be radiated away.
During the day, the insulation is in the
“closed” position to keep the heat out.
The specialized radiator cools air, which then
blown into the building to cool the mass.
During the day, the radiator is vented
outdoors, while the building is sealed
20. III. Evaporative Cooling
When water evaporates, it draws a large amount
of sensible heat from its surroundings and
converts this type of heat into latent heat in the
form of water vapor. It works best when relative
humidity is lower (70 per cent or less during
hottest periods) and the air has a greater capacity
to take up water vapor.
21. A. Direct Evaporative Cooling
When water evaporates in the indoor air, the temperature
drops but the humidity goes up. In hot and dry climates,
the increase in humidity actually improves comfort.
Misting the air has
become quite a popular,
direct evaporative-cooling
strategy. Water
under high pressure is
atomized into tiny
droplets, which then
readily evaporate to cool
the air.
Evaporative coolers (swamp coolers)
22. B. Indirect Evaporative Cooling
This indirect evaporative cooling
system uses a roof pond. Note no
humidity is added to the indoors.
This indirect evaporation cooling
system uses floating insulation
instead of a second roof.
Indirect evaporative coolers
reduce the indoor air temperature
without increasing its humidity.
23. IV Earth Cooling
Before one deciding for
earth-cooling
techniques one should
check the thermal
properties of soil must
be considered. The
temperature of soil near
the surface follows the
air temperature and
may change from region
to region.
Soil temperature varies with time of year and depth below grade.
Cooling the Earth
In dry climates, soil can be cooled significantly
below its natural temperature by shading it and
by keeping it wet for evaporative cooling.
24. A. Direct coupling
In dry climates, soil can be cooled with a
gravel bed, which shades the soil while it
allows evaporation to occur.
In earth-sheltered buildings in cold
climate, the earth should be insulated
from the cold winter air.
When earth-sheltered buildings
have their walls in direct
contact with the ground (i.e.,
there is little or no insulation in
the walls), one say that there is
direct earth coupling. In regions
where the mean annual
temperature is below 60°F,
direct coupling will be a
significant source of cooling. To
limit excess heat loss in winters,
insulate the earth around the
building from the cold winter
air but not from the building.
25. B. Indirect Earth Coupling
Indirect earth cooling is
possible by means of tubes
buried in the ground.
Sloped tubes and a sump
are required to catch
condensation. An open-loop
system is shown, while
a closed- loop system would
return the air from indoors.
Tubes must be absolutely
tight to prevent radon gas
or water from entering.
26. V. Dehumidification with a Desiccant
In humid regions, dehumidifying the air in
summer is very desirable for thermal comfort
and control of mildew. Two fundamental ways
to remove moisture from the air exist.
• With the first method, the air is cooled below
the dew point temperature. Water will then
condense out of the air. For example, in humid
climates, water will often condense in earth
tubes.
27. • The second method involves the use of a desiccant
(drying agent). A number of chemicals, such as silica
gel, natural zeolite, activated alumina, and calcium
chloride, will absorb large amounts of water vapor
from the air. However, there are two serious
difficulties with the use of these materials. First,
when water vapor is absorbed and turned into
liquid water, heat is given off. The second problem
with the use of a desiccant is that the material soon
becomes saturated with water and stops
dehumidifying. The desiccant must then be
regenerated by boiling off the water.
28. Biblography
• Lechner, Norbert., Heating, Cooling, Lighting :
Design Methods for Architects.
• Daniel Halacy, Understanding Passive
Cooling Systems.
• AIA Research Corporation, Passive Cooling,
Designing natural solutions to summer cooling
loads
• Brown, G. Z., and M. DeKay. Sun, Wind, and
Light: Architectural Design Strategies.
• N. B. Geetha, R. Velraj / EEST Part A: Energy
Science and Research
29. Topic of research area
The idea is to explore further for temperature reduction
techniques by allowing indirect evaporation to take place
through building’s outer skin. The phenomenon is similar
to that of sweating/ earthen pot, where water will
evaporate through a multi layered structure resulting in
reduction of temperature due to the loss of latent heat of
evaporation. It can be further explored with respect to the
factors like porosity of the layer, temperature of water/ air,
wind speed, sunlight exposure, rate of water flow, humidity
level etc. Some of these can be controlled and for rest,
natural range may be taken into consideration.
