Facades and Summer Performance of Buildings - Low Energy Cooling for Sustainable Buildings - page 46

Figure 2.21 Total ventilation gains for a double fa¸ade without shading (F1(a)), with a blind of 10%
absorption (F1(b)) and 30% absorption (F1(c)). The temperature increase of the fa¸ade air is also shown

Without a sun shading element inside the double fa¸ade, the low absorption leads
to a maximum area related heat gain of 50Wm − 2 . With a 10% absorbing blind this
heat gain increases to 90Wm − 2 and to over 150Wm − 2 for a system with 30% blind
absorption (see Figure 2.21).
From this total ventilation heat gain, only a fraction enters the room. At an air
exchange rate with the room of 0.7 h − 1 only 50m 3 h − 1 of the fa¸ade air enters the
room and adds to the cooling load. The heat gain per square metre of fa¸ade is 10% of
the total heat produced in the fa¸ade: that is, between, 5 and 15Wm − 2 . The secondary
heat flux from the room surface temperature to room air is highest if no shading
element is present:

q conv = h c T surface − T room

(2.7)

For the unshaded double-glazed single fa¸ade, a maximum secondary heat flux of
90Wm − 2 is obtained. Only slightly lower values are calculated for the double fa¸ade
without blinds. If sun shading blinds are present, the surface temperatures are reduced
and the secondary heat flux is between 25 and 40Wm − 2 (see Figure 2.22).
The total ventilation heat gain over a summer period varies between 50 and
100 kWhm − 2

fa ¸ ade a − 1 depending on the absorption of the fa¸ade. If all that air were
to enter the room, a very significant additional energy input to the 24m 2 room of
23-46 kWhm − 2 a − 1 would occur. However, as the air exchanges with the fa¸ade are
usually low, only a small fraction of about 10% of the heated air really enters the room
(unless strong cross-ventilation takes place!). Furthermore, the heat gain is not nec-
essarily equal to the required cooling energy, as often room temperatures stay below
the setpoint of 24 ◦ C despite the heat input. Using the standard meteorological data
from Stuttgart, Germany, in the simulation with low average ambient temperatures,