Passive Cooling Strategies - Low Energy Cooling for Sustainable Buildings - page 69

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34

30

26

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2nd floor
1st floor
ground floor
ambient

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14

01 03 05 07 09 11 13 15

Date in August

Figure 3.9 Measured temperature stratification within the building in summer 2003

external wind speeds of 1m s − 1 . If the wind is not from the north, the air velocity at
the ceiling remains low at about 0 . 12 m s − 1 even at higher external wind speeds.
There is a temperature stratification in the building, which is nearly 2 K per floor
(Figure 3.9).Whereas the top floor is highly glazed and not regularly used, the first floor
has the same types of office rooms as the first floor and stratification is a significant
problem. All air exchange measurements were carried out in the first-floor offices.
Considering the usually suggested literature value of 5 h − 1 for efficient night ven-
tilation, the measured air exchange with an average of 9 . 3h − 1 seems sufficiently high
during summer. However, the room air temperature levels during the hot month of
August only drop by about 3 K from the daily peak temperature and remain 5 to 6 K
above ambient air temperatures.
A detailed analysis of the room and surface temperature profiles showed that there
is a clear temperature stratification within the room despite high air change rates. The
cooler night air enters the room via the top of the window and then drops to the floor.
The floor temperature therefore cools down below the average room air temperature.
The ceilings on the other hand are not effectively discharged, as air movement is very
small at the top of the room and the temperature difference between the ceiling surface
and the temperature measured at 30 cm below the ceiling is very small (less than 1 K).
Only close to the window opening does the ceiling cool down by about 1 . 5 K during
a warm summer night, and at the room centre by only 1 K (see Table 3.1).
The air changes measured during the night varied between 6 and 14 h − 1 and follow
the external wind velocity (see Figure 3.10). Wind speeds in general decreased during
the course of the night, which is very unfavourable for manually opened windows for
passive cooling: in the early evening hours ambient air temperatures are mostly well
above room temperature and the high air exchange rates even increase the room load.