Facades and Summer Performance of Buildings - Low Energy Cooling for Sustainable Buildings - page 42

Figure 2.16 Flow velocities, ambient air and mean gap temperature
windows are tilted, the air exchange increases to 1.96 h − 1 . Both sets of measurements
were taken with the internal doors closed. If the internal doors are open, the cross-
ventilation increases the air exchange rate up to 50 h − 1 .
The measured flow velocities during the month were quite constant and reached val-
ues between 0.1 and 0.2m s − 1 (see Figure 2.16). This corresponds to air exchange rates
with the room between 0.5 and 1.7 h − 1 . Average daily heat gains through ventilation
for a fixed air exchange rate of 0.97 h − 1 are 36Whm − 2 d − 1 . The gain was calculated
from the volume flow rate and the temperature difference between the fa¸ade outlet
and room air. As the tilted window starts drawing in air from the centre of the gap
onwards, it might be more realistic to use the mean gap temperature as the room
inlet temperature. The ventilation gain then reduces to 25Whm − 2 d − 1 . If ambient
air were used for ventilation the heat gains for the month considered would be only
11Whm − 2 d − 1 . If both windows are tilted and a fixed air exchange rate of 1.96 h − 1
is used, the daily heat gain from ventilation using the mean gap air temperature is
54Whm − 2 d − 1 . The highest value of 86Whm − 2 d − 1 is obtained if the temperature
difference between the top of the fa¸ade and room air is considered and if the em-
pirically derived regression is used to calculate the air exchange rate for one tilted
window. These values compare with typical daily gains from two persons in the office
considered of 66Whm − 2 d − 1 . Dynamic building cooling loads were then calculated
using the dynamic simulation tool TRNSYS and the boundary conditions of air ex-
change and temperature levels described above. The room loads were calculated for
two persons and 160W of computer equipment. The total daily cooling load strongly
depends on the night ventilation strategy: if no night ventilation is used, the cooling
load more than doubles. As can be seen in Figure 2.17, the heat gain from ventilation
to the total cooling load is significant and contributes at least 30% of the total load.