PASSIVE COOLING

EVAPORATIVE
COOLING
 NOCTURNAL
RADIATION COOLING WALMI BUILDING, BHOPAL
 PASSIVE DESICCANT
COOLING
 INDUCED TORRENT RESEARCH
CENTRE, AHMADABAD
VENTILATION
 EARTH SHELTERING
 WIND TOWER
 EARTH AIR TUNNELS
 INTRODUCTION
 Passive cooling refers to
technologies or design features
used to cool buildings without
power consumption, by natural
means.
 Using – natural forces, energies
and heat sinks
Sustainable cooling is achieved
 Fans and pumps used -
by the three-tier design approach
HYBRID
 To create thermal comfort
during the summer – Remove
heat from the building by heat
sink for it
 Cool the building or raise the
comfort zone sufficiently to
high indoor temperature –
Hot and dry climates have
Thermal environment so that
buildings with small
the comfort zone shifts to higher windows, light colors,
temperature. People feel more and massive construction.
comfortable even though the
building is not actually being
cooled
TYPES OF PASSIVE COOLING
1. Cooling with Ventilation
A. Comfort temperature – Ventilation during the day and night to increase
evaporation from the skin and thereby increasing thermal comfort.
B. Night flush cooling – Ventilation to precool the building for the next day.
2. Radiant Cooing
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
3. Evaporative Cooling
A. Direct Evaporation – Water is sprayed into the air entering the building. This
lowers the air’s temperature but raises its humidity
B. Indirect Evaporation Cooling – Evaporation cools the incoming air of the
building without raising the indoor humidity
4. 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
5. Dehumidification with a Desiccant removal of latent heat
COOLING WITH VENTILATION
Comfort ventilation brings in outdoor air, especially
during the daytime, when temperatures are at their
highest.
The air is then passed directly over people to increase
evaporative cooling on the skin.
Warm air is actually heating the building.
HOT AND HUMID CLIMATE
Maximize the air flow across the occupants
Use fans to supplement the wind
Lightweight construction is appropriate only in
Air flowing around a building will
climates that are very humid, do not require passive cause uneven positive and negative
solar heating and use little if any air conditioning pressure areas to develop
Use at least a moderate amount of insulation to keep
the mean radiant temperature near the air temperature
Window should be open both during the day and night
Operable window area should be about 20 % of the
floor area, split about equally between windward and
leeward walls. Larger window areas can be used in
tropical climate The pressure on the leeward side of a
Window should be open both during the day and roof is always negative, but on the
during the night windward side it depends on the slope
of the roof.
NIGHT FLUSH COOLING
Night flush cooling works best in hot and dry climates with a daily temperature range that
o o
exceeds 17 C but is still effective humid regions as long as the daily range is above 11 C
Except for areas with consistent night winds, window or whole house fans should be used. Ceiling
or other circulating indoor fans should be used during the day when the windows are closed
2
There should be about 80 lb of mass for each square foot (390 kg/m ) of floor area and the surface
area of this mass should be more than two times the floor area. The mass has to be on the indoor side
of the insulation
The air flow at night must be directed over the mass to ensure good heat transfer
The window area should be between 10 to 15 percent of the floor area.
Window should be open at night and closed during the day.
Humid climate – The night air is cooler the daytime air. This cool night air can be flush out the
heat from a building’s mass. The pre-cooled mass can then act as heat sink during the following day
by absorbing heat. Since the ventilation removes the heat from the mass of the building at night.
o
Hot and dry climate – Large daily temperature ranges – 17oC. Cool night time temperature – 21 C
& Day time temperature – 38oC.
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

With night-flush cooling, night ventilation cools the

mass of the building.
EVAPORATIVE COOLING
 Evaporative occurs whenever the vapour pressure of
water is higher than the partial pressure of water vapour
in the adjacent atmosphere.
 The change in the phase of water from liquid to
vapour accompanied by the absorption of a large
quantity of sensible heat from the air that lowers the dry
bulb temperature of the air while the moisture content of
the air is increased.
 Evaporative cooling is much less energy intensive
and inexpensive.
The main drawback to evaporative cooling is that its
use is limited to dry climates.
The two types of evaporative cooling
• Direct evaporative cooling
• Indirect evaporative cooling

Many traditional courtyard houses have all of their windows

and doors facing the courtyard in large part for thermal
comfort. The courtyard stays relatively cool because it is
self-shading most of the day and because it is protected from
hot winds. Further comfort comes from transpiration from
plants and evaporation from fountains.
DIRECT EVAPORATIVE COOLING
 The direct evaporative cooling system – such as
fountains, pond, pools and wind tower – are very
effective in hot and dry zones where with cooling the
increase in humidity gives additional comfort. Whereas
it is not appropriate in humid climates because the
humidity is too high already.
 In the case of fountains, water sprinkles in the air
with an increased surface area and thus increasing the
evaporation rate.
 This water sprinkled into the air also cleans dust
particles from the air.
In wind tower, beds of wet charcoal are used to purify
the wind from the dust particles.
 Evaporative cooling is
accomplished with the
evaporative
coolers(swamp coolers).
A fan is used to bring
outdoor air into the
building by way of wet
screen.
DIRECT EVAPORATIVE COOLING

WIND TOWER / COOL TOWER

Cool towers are passive evaporative coolers that act
like reverse chimneys.
 At the top of the tower, water is sprayed on
absorbent pads.
 As air enters the top of the tower it is cooled,
becomes denser and sinks.
 The cool air then enters the
building through the opening
that look like fireplaces.
 Instead of hot air flowing up,
cool air flows down the passive
downdraft cool towers.
 Cool air is supplied to the
buildings without the need for
fans.
INDIRECT EVAPORATIVE COOLING
 Indirect evaporative cooling is cooling the indoor air without increasing its humidity
 Roof can be used to indirect evaporative cooling. It acts as a heat sink to cool the interior
FIRST METHOD
 Double roof – One Slope (Insulated roof) and Other Flat roof.
 Openings in the roof enable air currents to pass over the pond during the summer.
 As water evaporates, the water will become cooler and, together with the ceiling structure,
will act as a heat sink for the interior of the building.
 During the winter, the pond is drained and the roof openings are closed.
 Disadvantage - Is the cost of the double roof structure and waterproofing.
SECOND METHOD
 Insulation floats on the roof pond.
 At night, a pump sprays the water over the top of the insulation, and it cools by both
evaporation and radiation.
 When the sun rises the pump stops and the water remains under the insulation, where it is
protected from the heat of the day.
 Cooling occurs at night – very effective – combination of evaporation and radiation
INDIRECT EVAPORATIVE COOLING
THIRD METHOD – WATER SPRAY DESIGN
 This Water Spray design – to store the cooled water at
night and to precool the floor slab at night.
 At night the water is sprayed on a conventional roof,
where to is cooled by both evaporation and radiation to the
night sky.
 The cooled water is then pumped through tubing
embedded in the floor slab and then stored in a tank for the
next day.
 Of course, the tubing in the floor slab can also be used
for the radiant heating in winter. WATER SPRAY DESIGN
 The cooled water stored in the tank can be used with the
fan coil units the next day.

ADVANTAGE OF INDIRECT EVAPORATIVE COOLING

1. Commercially available as packaged units.
2. Outdoor air is used to evaporate water of the surface of tubes. The necessary heat of
vaporisation is drawn in part from these tubes, through which indoor air is flowing
3. Does not increase the humidity of the indoor air.
4. Can be used in climates too humid for direct evaporative cooling.
RADIATIVE COOLING

 Any object emits energy in the form of

electromagnetic radiation. If two elements at
different temperatures are kept facing one
another, a net radiant heat loss from the
hottest element will occur until a state of
equilibrium between the two elements is
achieved.
 Any building surface exchanges heat with
the surrounding systems (Sky) but in order to
have a appreciable net heat flux between the
two bodies, the temperature difference
should be significant.
 Factor that affect radiative cooling
1. Temperature difference between the
sky and surface
2. Emissivity of the surface
3. Emissivity of the sky
4. Radiative coefficient
DIRECT – NOCTURNAL COOLING
 Nocturnal cooling or night sky cooling can be a very low-energy passive
system.
 It can be effectively used in office buildings, institution and residential
buildings.
 The heat storage by the envelope can be removed via nocturnal cooling so that
the envelope is ready to store day heat gain.
 The main design criteria are that the building surfaces possess maximum
reflectivity in the short wave region and maxiumum emissivity in the long-wave
region of the spectrum.
 The thermal storage should be in such a manner that the time lag occurs after
daytime.
INDIRECT – ROOF POND WITH
MOVABLE INSULATION
 Plastic bags filled with water
 During daytime in summer, the roof pond is covered
with insulation by a surface finish of low absorptive
which minimizes the impact of solar radiation on the
roof.
The water in the pond holds the heat gain and further
increases the time lag.
 During night, the heat stored during daytime is
exchanged with the night air.
 In winter the operation of the movable insulation is
reversed to allow the heat gain in the daytime and to
reduce heat lose at night.
 This roof pond system is proven to be very effective
and inexpensive.
Rules for Radiant Cooling
1. Radiant cooling will not work well in very cloudy regions. It performs best under clear
skies and low humidity, but will still work at lower efficiency in temperate regions.
2. This cooling concept applies mainly to one-story buildings.
3. Unless the radiator is also used for passive heating, the radiator should be painted
white.
4. Since the cooling effect is small, the whole roof area should be used.
5. Thermal mass is needed to act as a heat sink during the day.
EARTH COOLING
 Wet earth is a good conductor and store of heat.
The temperature at the surface is the result of solar
gain, radiant loss and heat conduction to form
lower layers of ground
 Throughout the year, the earth’s temperature is
practically constant after a depth of 2.5 m and
remains close to the average annual Temperature.
About 6m depth the fluctuations die and a steady
state temperature exists
 Since the sun heats the soil, shading the surface
significantly reduces maximum earth temperature.
 Water evaporating directly from soil also
reduces earth temperature – During Night
time.
 A canopy of trees - to shade soil.
 Covering the earth surface – 4” light In dry climates, soil can be
coloured stones. cooled with gravel bed, while it
 There are two different types of cooling earth allows evaporation to occur

1. Direct - Earth Berm Structure

2. Indirect - Geothermal Coupling
DIRECT -EARTH BERM STRUCTURE
• When earth sheltering buildings have their
walls in direct contact with the ground. (There Factors that affect earth berm
is little on insulation) is direct earth coupling structures includes
1. Ground water Table
• In an earth berm structure the underground 2. Day lighting
building in contact with the earth benefits from 3. Structural Stability
the huge thermal mass of the adjacent ground 4. Ventilation
and is thus not affected by hot days and chilly
nights.
INDIRECT–GEOTHERMAL COOLING
• Geothermal cooling works on the principles of conduction and convection.
• A building can be indirectly coupled to the earth by means of earth tubes.
• The hot summer winds pass through buried pipes under the earth and gets cooled by
conduction when they come into contact with the cool earth.
• This cooled air cools the structure by convection.
• Similarly, it works during winter by picking the earth’s heat and releasing it into the
structure.
• Sloped tubes and a sump are required to catch the condensation. An open loop system is
seen while a closed loop system would return the air indoors
• To take maximum advantage and to get maximum cooling effect the tubes are buried deep
inside the earth
• The greatest problem with earth tubes is condensation which occurs mainly in humid
climates where the earth temperature is frequently below dew point. The tubes, therefore
should be sloped.
• Factors to be considered include the following –
1. Depth of the buried pipe below the ground
2. Diameter of the Pipe
3. Material of the Pipe
4. Length of the Pipe
5. Wind Speed (minimum 2m/s)
6. Temperature difference (Bet air and ground temp)
DEHUMIDIFICATION WITH
DESICCANT
In humid regions, dehumidifying the air in summer is very desirable for thermal
comfort and control of mild dew.
There are two ways to remove moisture from air.
1. The air is cooled below dew point temperature. Water will condense out of
the air. Conventional air conditioning and dehumidification use this
principle.
2. This involves the use of desiccant (drying agent). A number of chemicals,
such as silica gel, natural zeolite, activated alumina, and calcium chloride
will absorb large amounts of water vapour from air.
3. However there are two serious difficulties in this method.
a. When water vapour is absorbed and turned to liquid water,
heat is given off. This is the same heat which vaporises the water in first place. (If a
desiccant is placed in a room it will heat the air as it de- humidify it – the desiccant
converts latent heat into sensible heat)
b. 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.
REFERENCE

• Manual on Solar Passive Architecture, IIT Mumbai and Mines New Delhi,
1999
• Norbert Lechner., Heating, Cooling, Lighting : Sustainable design Methods for
Architects (United States of America, Permissions Department, John Wiley &
Sons, Inc., 2009) Page No – 267,283-293
• Ignasi Nieto I Magaldi and Ms Anupama Kundoo, Sustainable Building
Design Manual, (The Energy and Resource Institute, New Delhi, 2004) Page
No – 72 - 74
• Mili Majumdar, Energy efficient Building in India, (New Delhi, Thomson Press
(India) Limited, 1997)