Environmental Engineering Reference
In-Depth Information
Table 3.2
Temperature profile during a cool summer's night (22 to 23 August 2003). The ceiling
temperatures were measured near the windows, at the centre of the room and between the centre and the
windows
Time
Ambient
Room
Floor
Ceiling
Ceiling
Ceiling
air
air
to corridor
centre
between
at windows
/
◦
C
/
◦
C
/
◦
C
/
◦
C
/
◦
C
/
◦
C
20:00
26.0
25.5
24.2
25.0
25.0
25.0
23:30
18.0
25.1
24.1
25.1
24.2
23.1
00:10
16.8
25.2
24.1
25.1
24.2
23.2
00:50
16.2
23.7
24.0
25.0
23.5
22.9
03:00
15.6
22.2
22.5
24.3
22.7
20.8
05:00
14.2
20.5
22.0
24.1
22.1
20.8
07:10
13.2
22.3
21.6
24.0
22.6
21.6
08:50
17.6
22.7
21.8
23.8
22.4
21.4
temperature. Due to the always decreasing temperature difference between ambient
and inside air during the course of the night, about 6 kWh should be removed. These
results indicate that effectively the air change between ambient and inside is less than
8h
−
1
. Further research is necessary to quantify the amount of two-directional flow,
which up to now could only be visualized qualitatively by fog experiments.
When the night air temperatures were significantly lower, ceiling temperature
decreases of 3-4 K were measured for the surfaces (Table 3.2). Again the heat transfer
is significantly better on the ceiling near the windows. Also during that night the air
exchange rates were much more favorable for passive cooling: early in the evening
with high ambient temperature levels, the air exchange rates were low, but increased
to high values in the middle of the night.
3.2.3 Simulation of Passive Cooling Potential
Building simulation studies using TRNSYS were then carried out to investigate the
effect of parameter changes, which could improve the passive night cooling strategies.
As air change rates were not measured during the whole operation time, the simulation
used constant air exchange rates whenever the windows were open. The measured
room and surface temperature data during August was very well reproduced for air
change rates between 4 and 8 h
−
1
. External sun shading was set constant at 50%, as
the external shutters consist of two parts, of which the upper part is mostly open for
daylight use. All other boundary conditions were taken from measurement data.
First the influence of air change on the nightly room temperature dropwas evaluated,
while keeping all convective heat transfer coefficients constant. The simulated roomair
temperature only approaches ambient temperature during the night if air exchange rates
are very high. For a constant air change of 8 h
−
1
the room temperature after night venti-
lation stays at 4 K above ambient air temperature. Even at 15 to 20 air changes per hour,
the room temperature stayed at 2 K above the minimum ambient air temperature. At 50
air changes per hour, the difference between room air and ambient air decreases to 1 K.
Search WWH ::
Custom Search