Passive Cooling Strategies - Low Energy Cooling for Sustainable Buildings - page 73

Table 3.3 Simulated heat fluxes and cooling energy removed for given boundary conditions

Air T amb T amb T room Ceiling Floor Walls Q cool Q cool

change Max Min Min flux flux flux Max eff.

/h − 1 / ◦ C/ ◦ C/ ◦ C / Wh m − 2 /Whm − 2 /Whm − 2 / kWh / kWh

8 36 20 24.2 55 68 50 5.5 4.7

50 36 20 21.0 84 91 68 8.9 6.3

consisting of two 1 . 3 cm gypsum boards on both sides filled with mineral wool. About
equal shares of nearly 30%were provided by the 20 m 2 of heavy 22 cm concrete ceiling
and the 4 cm of mastic asphalt floor covering and a nearly negligible quantity from
the lightweight external wall (Table 3.3).
Concerning the effective cooling energy for the hot days of 36 ◦ C maximum and
20 ◦ Cminimum temperature to the floor surface area of 20 m 2 , this results in removable
loads of 235Wh m − 2 d − 1 for an air change of 8 h − 1 . The simulated ceiling surface
temperature decrease of 1 . 5 Kwas obtained using convective heat transfer coefficients
of 3Wm − 2 K − 1 .
The decrease of room air temperature through higher air exchange rates does not
solve the problem of high surface temperatures, if heat transfer is not significantly
improved at the same time. Doubling the convective heat transfer coefficient from
3to6Wm − 2 K − 1 resulted in an additional ceiling temperature decrease of 0 . 5K.
Increasing convective heat transfer to 18Wm − 2 K − 1 decreased the minimum night-
time surface temperature of the ceiling by another degree (Figure 3.12).

40

ambient temperature
zero air change
4 air changes
8 air changes
15 air changes
50 air changes
50 air changes, 6 W m -2 K -1
50 air changes ,18 W m -2 K -1

surface temperature at
increasing air change

35

30

25

20

15

01 02 03 04 05 06 07

Date in August 2003

Figure 3.12 Ceiling surface temperatures as a function of air exchange rate and heat transfer coefficient