Pasive Bldn at Rajasthan

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Solar Passive Cooling/Heating of Building at Bikaner in

Rajasthan, IndiaO.P. Jakhar

1, Dr. A.N. Mathur

2

1Department of Mech. Engg. Government Engineering College, Bikaner, Rajasthan, India2Director, GITS, Udaipur, Rajasthan, India

Email of corresponding author: [email protected]

Abstract: The buildings of Bikaner are reasonably

comfortable indoors because of thermal damping by the

massive roofs and walls. However, the average indoor air

temperature (over 24 hours) is about the same as the

average outdoor air temperature. In a light-weight building

without any external glazing and without any ventilation,

the average indoor air temperature is the same as the area

weighted average of the sol-air temperature of external

surfaces, which is bound to be higher than the average

outdoor air temperature. It is an indication of the

effectiveness of the natural cooling methods used at Bikaner

that the indoor temperatures equal the outdoor air

temperature. The indoor temperatures in Bikaner lie just at

the edge of the comfortable range. In other climates with

higher relative humidity it is possible that inspite of good

building design comfortable indoor conditions cannot be

ensured because the average outdoor temperature is too

high. In such climates it is necessary to adopt measures to

lower the average indoor temperature to a level below theoutdoor. Specially designed mud walls are still poplar in the

hamlets (Dhanies) of western Rajasthan to deflect the hot

winds. These are some of the conventional methods of

passive cooling, typical in hot and dry climatic condition.

India has a very diversified climate heating of buildings in

also required especially in upper latitudes and hilly areas

and cooling of buildings is required in lower latitude and

desert areas. Solar passive architecture provides proper

orientation and design of fenestration i.e. doors and windows

to take maximum advantage of sun and wind. For heating

the aim is to admit the sun’s energy as much as possible and

to reduce he loss of heat in the nights. This is achieved by

direct gain through windows, therm-wall, or solarium and

other such means. The heat loss is minimized by the properdesign of walls, by insulation of walls and roof, by night

insulation on windows, by double glazing of windows etc.

Key words: Vary-Therm Wall, indoor temperature, solar

temperature, summer cooling, winter heating

1. Introduction

In buildings without mechanical cooling or heating, so

long as the outdoor weather conditions are stable, the

daily heat flux entering the building equals the heat flux

out going. In such conditions, the period during which the

building heats up is roughly equal to the period during

which the building cools down (fig. 1). If it is desired thatthe average indoor air temperature be lower than average

outdoor air temperature, it is shown in fig. 2 that the

potential period for natural cooling is reduced while the

period for heating is increased. Conversely, if the indoor

average temperature is to be higher than the average

outdoor temperature (fig. 3), the period for heating is

reduced while the cooling period is increased. This isindependent of the phase lag due to the mass of the

structure. Since the heat flux going out must still equal the

heat flux coming in, it follows that the rate at which heat

flows in through the building fabric cannot be the same as

the rate at which heat flows out. Because the heat flow in

and out of a building element depends upon its thermal

transmittance, a variable heat flow can be ensured only by

a building fabric of which the thermal transmittance

properties can be changed at wall. For hot climates this

means a high resistance to incoming heat and a low

resistance for outgoing heat. Buildings loss and gain heat

by radiation, convection, conduction and evaporation.

Each of these four modes of heat transfer can be used foraltering thermal transmittance.

A i r T e m p e r a t u r e ( C ) 0

Fig. 1 The average internal temperature equals averageexternal temperature

A i r T e m p e r a t u r e ( C ) 0

Fig. 2 The average internal temperature less average

external temperature



A i r T e m

p e r a t u r e ( C ) 0

Fig. 3 The average internal temperature greater average

external temperature

In solar heated buildings, the use of glass permits

short wave radiation to enter the building but does not

allow long wave radiation to go out. Thus solar heated

buildings can maintain indoor an average temperature

higher than outdoor. Movable insulation is sometimes

used along with glass, which also alters the heat flow byconduction. In naturally cooled buildings, selective

ventilation i.e. ventilation only during the cool night

hours, causes rapid convective heat loss while convective

heat gain is much lower. Night time roof surfaceevaporation of such buildings also cause rapid heat loss at

night while during daytime there is no corresponding heat

gain due to evaporation. Several natural heating and

cooling systems have been designed using the idea of

variable thermal transmittance. Most of these systems are

applicable only to very low (one or two storey) buildings

and in the case of skytherm, involve the use of many large

movable panels which are prone to mechanical

breakdown. The idea of using the roof for cooling andheating has many limitations particularly when it is

considered that the roof areas in a building should be

minimized as they intercept a lot more radiation in

summer than in winter. The Trombe wall system can be

used on buildings of any height but its main application is

for heating and not cooling. Therefore, the need to find a

natural cooling system that can be applied to buildings

that are taller than two storeys.

In deep plan buildings, almost the entire outer surface

needs to be glazed to provide adequate day lighting.

However, in the case of less compact buildings, the glazed

part may be only a quarter of the external wall area. In

such buildings the idea of variable thermal transmittancefor natural cooling can be applied to the opaque parts of the external walls. In this paper it presented to use a

specially designed cavity wall for this purpose and to call

it “Therm-wall”.

2. Therm Wall

It is well known that the thermal transmittance of a

ventilated cavity is greater than that of an unventilated.

The design of a wall section that can have high or low

thermal transmittance therefore calls for a cavity wall that

can be ventilated at wall, either to the outside or to the

inside. In its simplest form the therm wall can be made byfixing a thin panel of any weather resistant material

outside a brick wall with an air gap of about 5 cm.

Movable flaps are provided at the bottom and top to

permit or stop air flow through the cavity (fig. 4). It is not

necessary for the flaps to be absolutely air tight. The outer

panel is painted in a light colour of high reflectivity. For

summer cooling, the vents are kept closed during thedaytime and opened at night (fig. 5). The closing of vents

creates an unventilated cavity of high thermal resistance,

in front of the brick wall while shading it from solarradiation. The opening of vents reduces the thermal

resistance of the cavity, allowing rapid dissipation of heat

from the brick wall to the cooler night air.

Fig. 4 Therm Wall Construction

Fig. 5 Summer Cooling

For winter heating, the outside vents are not necessary

but vents are needed towards the room at the bottom and

top of the wall (fig. 6). The vents are kept open during

sunshine hours when the cavity heats up due to solar

radiation incident on the wall. The thermosyphonic of

cavity air brings heat into the room. At night when the

cavity air temperature falls below the room temperature,

the vents are closed to prevent heat loss from the room.

The heat flux brought into the room in this way will be

much less than in the case of Trombe wall, because the

outer panel of therm wall is opaque and not glazed. It

follows, therefore, that this type of wall is useful only in

moderate winter conditions such as prevail in Udaipur.



Fig. 6 Winter Heating

3. Experimental Set-up

Experiments were conducted on a small sized vary thermwall during May 2008 and February 2009 at Bikaner

Latitude, 280 10” N. The experimental setup was created

by fixing a plywood panel ((1.3 m x 1.3 m) in front of a

12 cm thick west facing brick wall of an existing

buildings. The plywood panel was painted a buff colour

while the brick wall was in its natural red colour. The

room measured 2.5 x 4.0 m, and all walls in it were of 12

cm thickness, while the concrete roof was 18 cm thick.

Thermocouples were fixed to the therm wall as well as toan adjoining section of normal brick wall. To measure the

heat flux coming into the room through the two wall

sections, heat flow meters consisting of two copper

constantan thermocouples placed 0.5 cm apart and set into

cement mortar, were installed in the brick walls on the

inner surfaces. The experiment was conducted over a twoweek period in summer and the flow of air through the

cavity was changed as follows:

a) No flow day and night

b) Flow only during nighttime

During the experiment, the temperature of the roomvaried considerably because of heat inflow and outflow

through the thin brick walls. No attempt was made to

maintain the room at a constant temperature but

ventilation through the room was almost completely

stoped.

The winter experiment was conducted in two parts

with a slightly altered set up. The outer ply wood panel

was replaced by a 5 mm thick asbestos cement sheet and

an insulated space was built using expanded polystyrene

sheets behind the experimental wall (fig. 7). The object

was to eliminate the heat gain into the internal space from

other walls and roof of the room. Openings were made in

the bricks wall to allow the cavity air to be circulated intothe room.

Fig. 7 Winter Experimental Set-up

4. Results

The results of the summer experiments with no air flow

and with air flow only during the night are presented in

fig. 8 and 9. In both cases it is seen that the cavity wall

temperatures were much lower than those for the ordinary

wall. This is mainly due to the shading of the outer

surface. Since there was comparatively greater heat loss

and gain through the other surfaces of the room and theroom air temperature was not constant, the internal

surface temperatures of the ordinary brick wall and the

therm wall have been normalized with respect to the room

air temperature. Ordinary cavity wall (with no air flow) ismuch less than that through the ordinary brick wall, but

the outgoing heat flux through the cavity wall is only

slightly greater than that through the ordinary wall. There

is net heat gain to the room through the cavity wall for a

period of 14 hours while the corresponding period of heat

loss is about 10 hours. In the case of the therm wall, the

incoming heat flux is much less and the outgoing heat

flux much greater than for the ordinary wall. The periodfor net heat gain is reduced to 10 hours while that for heat

loss has been increased to 14 hours.

Fig. 8 Measured temperatures with ordinary cavity wallduring summer



Fig. 9 Measured temperature with therm wall during

summer

4. Conclusions

The usefulness of the therm system depends upon the

amount of solar radiation received on the particular

surface. The greater the incident solar radiation, the more

effective is the therm system in protecting in internalspace. In summer the greatest amount of solar radiation

(per sq. metre of surface area) is received on the roof

followed by the west and east walls. The north and south

walls do not receive much radiation in summer. In these

situations, the outer finish would be a light coloured

reflective paint. Two approaches have been tried out, one

in which the outer cover consists of a polythene sheet

which permits emission of long wave radiation from the

roof and the second in which the outer cover consists of a

white painted galvanized iron sheet. In both cases the air

space was ventilated to the inside of the building at night.

The roof surface in the first case and the galvanized iron

sheet in the second case get cooled due to emission of radiation to the night sky and consequently thetemperature of air in the cavity is also lowered. Ventilation

towards the inside brings the cool air into the room. As the

experiments of the thermo system have not been

conducted on a roof, it remains to be seen which approach

works better or if there is any difference between the two.

References

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Pasive Bldn at Rajasthan