General Concepts Flashcards
What does OTTV refers to? And what is it?
Overall thermal transfer value. Its a measure of heat transfer through the external envelope of a building.
Formula of OTTV
Heat gains through the building envelope / external surface area (W/m2)
Smaller the OTTV…
… less heat gain, less energy use for cooling in air-conditioned buildings
Specifications of OTTV in HK
Commercial buildings and hotels
Window conduction not considered
35W/m2 for tower 80W/m2 for podium (1995)
First revision 70 for podium
Last revision (2011): 24 W/m2 tower, 56 podium
Where is the most energy consumed in HK
By building stocks
Arise of electricity consumption from 1970 to 2014 in HK
16000 TJ to 160000TJ increase rate of 5.5% each year
Percentage consumption through the years by domestic, commercial and industrial
Commercial : 38 to 75%
Domestic : 20 to 30%
Industrial : 40% to 6%
Two major electricity consuming components in office buildings
AC (50-60%) and electric lighting (20-30%)
OTTV classification
For external walls including windows
For the roof including skylight
What does OTTV aim for?
Achieve adequately insulated building envelope
Cut down external heat gain intro air conditioned buildings
Reduce the cooling load of air conditioning systems
Is OTTV an indicator of the total energy that will be consumed for cooling?
No, its just a measure of comparative performance or how close to the optimum a particular envelope design comes
Daylight factor
The ratio of the internal illuminance Ei to the illuminance simultaneously available on a horizontal plane from the whole of an unobstructed CIE overcast sky excluding direct sunlight and expressed as a percentage.
What is easier to compute roof or wall OTTV?
Calculation of roof is much simpler because does not contain large amount of glazing
Which of the three components of heat gain is the most significant one by far?
The solar radiation through window
Shading coefficient of reflective glass
0.2 to 0.4
