1. Stills are designed to operate with water in them, if they are left dry for a
period of time they may suffer serious damage. It is also necessary to
clean the transparent cover periodically to maintain high transmittance of
solar radiation.
Passive Solar House Heating and Cooling
For solar heating of space, solar energy is to be collected, stored and
distributed properly in the space to be heated. In active solar space
heating system, the solar energy is collected using some kind of solar
energy collectors. Solar energy may stored in sensible heat storage
materials, in latent heat storage materials or in chemical reactions; and
the energy is redistributed in the space using electrically operated pumps
and fans using radiators, etc.
On the other hand in passive solar heating systems all the three functions
of solar energy collection, storage and distribution are done by natural
means and generally, no electrical or mechanical power and electronic
controls are used. In the passive heating system, various elements of the
buildings like walls, roof, windows, partitions, etc. are so selected and so
architecturally integrated that they participate in the collection, storage,
transportation and distribution of thermal energy. Thus in passive solar
house heating system the building becomes an ‘energy machine’ and its
structural and architectural details become integral parts of the
components of the energy system. Passive heating of buildings are
classified as follows:
Direct gain
Thermal storage wall
Attached sunspace
Thermal storage roof
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2. Direct gain
Figure 3.26 shows the simplest passive solar heating concept where the
following concepts are employed.
Summer sun
Overhang roof line
Winter
Sun
Double Glazed
Window
Direct access to
winter radiation
Storage mass in
floor, walls and
roof
Figure 3.26 Direct gain passive solar heating system
A double glazed window facing south or the entire south facing wall is
double glazed through which direct radiation in winter enters and
strikes the floor, walls or other objects in the room. Almost, all the
solar radiation entering the room is converted into useful heat. The
heat loss from the room is reduced by using a double glazed window.
An appropriate overhang above the windows or at the roof level for the
case where south wall is glazed shades the window or the wall during
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3. summer when the elevation of the sun is high. Adequate movable
insulation may be used to reduce heat losses during night.
The floor and / or wall are made massive to increase the thermal mass
which helps in storing the heat during daytime when sufficient heat is
available and releasing the same during night time, thereby reducing
the large variations in the room air temperature. To avoid overheating,
the size of the storage is fairly large.
If some kind of thermal insulation is used to cover the windows during
periods (night) for which heat loss is more than heat gain, then the
performance can be further improved.
Thermal storage wall
In spite of heavy thermal mass provided in the direct gain type passive
heated rooms, there are still large variations in the room air temperature.
A more effective way of heating the rooms and reducing the large
oscillations in the room air temperature is the use of a thermal storage
wall between the double glazing (facing south) and the room. Figure 3.27
shows the thermal storage wall passive solar heating system. In this
category the following concepts are employed.
The entire south facing wall is covered by one or two sheets of glass or
plastic with some air gap between the wall and the inner glazing. In this
air gap, hot air moves from bottom to top generally due to natural
convection.
A south facing thermal storage wall made of masonry or concrete with
the outer side facing the sun is blackened. The solar radiation after
penetration through the glazing and wall gets heated, rises and enters
the room through the upper vent while the cool air from the room
enters in this gap through the bottom vent. This circulation continues
till the wall goes on heating the air. The flow of heat in the room can be
changed by adjusting the air flow through dampers provided at the
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4. inlet and outlet vents. The room is also heated by convection and
radiation from the inner surface of the wall facing the room. Thus, this
thermal storage wall collects stores and transfers the heat to the room.
This wall is also known as Trombe wall. By suitably designing the
glazing and the thermal storage wall, it is possible to get air
temperature difference between glazing and absorber upto 60°C.
°
In some cases the thermal storage wall is made up of drums or barrels
or other suitable containers full of water, stacked over each other
instead of masonry or concrete to collect, store and distribute the heat
and is termed as water wall or drum wall
Overhang roof line
Summer sun
Damper to be opened in summer
to remove excess heat
Warm air to house
Winter
sun
Convection
Double Radiation
Glazing
Air Masonry wall
space
Return cold air
Figure 3.27 Trombe wall or Thermal mass wall
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5. Attached greenhouse (Sunspace)
Solar
radiation
Storage wall
Convection
Double
Glazed Radiation
enclosure
Zone I Zone II
Sunspace Living room
Figure 3.28 Attached green house (Sunspace)
passive solar heating
In this passive solar heating approach the concepts of direct and indirect
gain (thermal storage wall) are combined as shown in Figure 3.28.
There is sunspace (zone I) on the extreme south facing side of the
house covered with single or double layers of glass or plastic sheets
which functions like a green house and can be used either for growing
vegetables or flowers and as a sunny space for living. In the attached
green house there is a large air temperature swing. In summers, the air
temperature in the greenhouse may be very high and therefore, large
vents for air circulation may be used. In some cases heat storage
materials and movable insulation are used in this sunspace or
greenhouse.
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6. There is a thermal storage wall facing south in between the room (living
space, zone II) and greenhouse (sunspace zone I). The thermal storage
wall gets heated by direct absorption of solar radiation coming through
the greenhouse transparent cover. The living room gets heated through
convection and radiation heat transfers from the thermal wall. The heat
loss from the thermal wall to the outside in this case is low because of
the presence of a greenhouse which is further reduced by using a
movable insulation over the walls of the sunspace.
The attached greenhouse or glass house can be integrated into a
building in several ways such as: as an attached green house; as one,
two or three sided integrated lobby; as two storied solarium, as a
climatic envelope such as glass covered courtyard or as a stair case.
Thermal storage roof
The thermal storage roof concept for passive solar heating was developed
by Hay and Yellot in Arizona and is similar to the thermal storage wall
except that the interposed thermal storage mass is on the building roof
instead of a wall. This concept uses the following components:
A metal roof which conducts the heat effectively
Water bags made of transparent or black plastic sheet and filled with
water or any other massive material are put over the metal roof. In
winters, during day time when sun is shining, the water in the bags
gets heated, stores the heat and heats the room below during day time
as well as night time when sun is not shining
Movable insulating shutters are used over the water bags. In winters,
during off sunshine hours these insulating shutters are slid over the
bags, reducing heat loss from the water bags to the outside. During
daytime when sun is shining the insulating shutters are pulled back
allowing the solar radiation to fall on the water bags. Thus, the water
bags keep the room warm in winter during day and night.
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7. Movable insulation
cover during night
SUN
No insulation
cover during day
Night
Day
Roof thermal pond
Metal roof
Hot water
heats building Masonry wall
Figure 3.29 Thermal storage roof (Heating during winter)
The same thermal storage roof system can be used for cooling the room
in summer as shown in Figure 3.30 by simply reversing the process. In
this case the insulating shutters are pulled back during night time
allowing the cooling of water bags and roof by thermal radiation heat
loss to the outside resulting in cooling the room. During daytime, the
shutters are slid over the water bags avoiding the heating of water bags
from direct sunshine. The cool water mass keeps the room below cool
during day and at night
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8. Movable insulation
cover during day
SUN No insulation
Reflected cover at night
heat
Day
Night
Roof thermal pond
Metal roof
Chilled water
cools building Masonry wall
Figure 3.30 Thermal storage roof (Cooling during summer)
Similar to the water bag on the roof, roof pond system in which water is
stored on the roof known as skytherm system can be employed. Here
also, movable insulation plays a significant role and its thermal
resistance is kept as 2 m°C/W. The depth of water is kept as 20 cm.
°
There are several configurations of roof pond system such as: glazed
or unglazed ponds with water in bags, glazed or unglazed ponds with
reflectors, flooded ponds, etc.
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