Thermal performance of the Building elements

1. HEAT EXCHANGE PROCESS IN BUILDINGS
Presentation By-Roopa Chikkalgi
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2. Thermal performance of the Building elements
Heat exchange process in buildings
The focus study is on the understanding of thermal quantities like
–Heat
–Heat flow
–Conductance & resistance
–Sol air temperature
–Solar gain factor
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3. Temperature is actually not a physical quantity but it can be thought of as a symptom-as the outward
appearance of the thermal state of a body. If energy is conveyed to a body, the molecular movement within
that body is increased and it appears to be warmer.
•Outward appearance of the thermal state of the body‐A symptom rather than a physical quantity.
(Unit: degC or degree Celsius)
•Degree of hotness of a body.
–If day time temperature: 36 degC
Night time temp. is 12 degC
Diurnal range is 24 degC
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4. Heat is a form of energy, associated with the motion of atoms or molecules and is
capable of being transmitted through solid and fluid media by conduction, through fluid
media by convection and through empty spaces by radiation.
As such, it is measured in general energy units: joules (J).
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5. Heat flow- Heat energy tends to distribute itself evenly until a perfectly
diffused uniform thermal field is achieved. It tends to flow from high
temperature to lower temperature zones, by any or all of the following
ways:
• Conduction
• Convection
• Radiation
The 'motive force' of heat flow in any of these forms is the
temperature difference between the two zones or areas considered. The
greater the temperature difference, the faster the rate of heat flow.
The rate of heat flow is measured in Watts (W). In most practical
applications, the multiple of watt 'kilowatt' (kW), will be used. (1 kW = 1000
W)
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7. Convection
In convection, heat is transferred by the bodily movement of a carrying
medium, usually a gas or a liquid.
The rate of heat transfer in convection depends on three factors:
 temperature difference (difference in temperature of the medium at the
warmer and cooler points)
 the rate of movement of the carrying medium in terms of kg/s or m3/s
 the specific heat of the carrying medium in J/kg degC or J/m3 degC
These quantities will be used in ventilation heat loss or cooling calculations.
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8. Radiation
In radiation heat transfer, the rate of heat flow depends on the temperatures of the
emitting and receiving surfaces and on certain qualities of these surfaces: the emittance
and absorbance.
Radiation received by a surface can be partly absorbed and partly reflected: the proportion
of these two components is expressed by the coefficients absorbance (a) and
reflectance (r).
The sum of these two coefficients is always one:
a + r = 1
Light colored, smooth and shiny surfaces tend to have a higher reflectance.
For the perfect reflective theoretical white surface: r = 1, a = O.
The perfect absorber, the theoretical 'black body', would have the coefficients: r = 0, a = 1.
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12. Sol-air temperature
For building design purposes, it is useful to combine the heating effect of
radiation incident on a building with the effect of warm air. This can be
done by using the sol-air temperature concept.
Ts = To + [(l x a)/fo]
where Ts = sol-air temperature in ˚C
To = outside air temperature in ˚C
l = radiation intensity in W/m²
a = absorbance of the surface
fo = surface conductance (outside), W/m2 degC.
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13. solar gain factor (θ)
The solar gain factor is defined as the heat
flow rate through the construction due to
solar radiation, expressed as a fraction of
the incident solar radiation.
Its value should not exceed 0.04 in warm-
humid climates or 0.03 in the hot-dry
season of composite climates, when
ventilation is reduced.
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14. Heat exchange process in buildings
Just like the human body, the building can also be considered as a defined unit and its heat exchange processes
with the out-door environment can be examined.
The thermal balance, i.e. the existing thermal condition
is maintained if:
Qi + Qs ± Qc ± Qv ± Qm - Qe = 0
Qi -Heat output from human bodies, lamps
motors and appliances.
Qs -The effects of solar radiations on opaque
surfaces(but through transparent surfaces
window can be considered separately) can be
included.
Qc -Conduction of wall may occur through the
walls either inwards or outwards and is
denoted by Qc
Qv - Heat exchange in either direction may take
place with the movement of air i.e ventilation.
Qm - There may be introduction or removal of
heat (heating or cooling) using some form of
outside energy supply. The heat flow of such
mechanical controls may be denoted as Qm
Qe - If evaporation takes place on the surface
of the building (eg roof pool) or within the
building (human sweat or water in fountain)
and vapors are removed, it produces a cooling
effect.
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15. The existing thermal condition is maintained is cooling if: Qi + Qs ± Qc ± Qv ± Qm - Qe = Negative
the existing thermal condition is maintained is hot if: Qi + Qs ± Qc ± Qv ± Qm - Qe = Positive
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16. The thermal performance of a building depends on a large number of factors. They can be summarised as (i) design
variables (geometrical dimensions of building elements such as walls, roof and windows, orientation, shading devices, etc.);
(ii) material properties (density, specific heat, thermal conductivity, transmissivity, etc.); (iii) weather data (solar radiation,
ambient temperature, wind speed, humidity, etc.); and (iv) a building’s usage data (internal gains due to occupants, lighting
and equipment, air exchanges, etc.). Presentation By-Roopa Chikkalgi
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