Passive Cooling Strategies - Low Energy Cooling for Sustainable Buildings - page 78

Figure 3.19 Temperature levels of room surfaces and globe thermometer and heat fluxes to and from
the surfaces (positive = heat flux from surface) from 1 to 6 July 2006 in a ground floor office

which external irradiance on the east fa¸ade decreases (see Figure 3.19). The operative
temperature has a maximum of 27 ◦ C for external temperatures up to 35 ◦ C. During the
heating-up phase of the room the average heat flux of the ceiling is 2Wm − 2 ; that is,

there is a net heat flux into the ceiling, which is not fully discharged during the night.
During a cooling-down period of the room (e.g. 15 to 17 July 2005) the average heat
flux is positive at 4Wm − 2 ; that is, there is a net discharge of the ceiling at night.
The use of phase change materials (PCMs) in the gypsum boards of the top floor
ceiling and wall (16.2 and 19m 2 surface area) did not significantly improve the situa-
tion. The melting point is rather high (26 -28 ◦ C) and the latent heat capacity limited to
about 80Wh m − 2 . The main problem, however, is again the low heat flux for discharg-
ing the PCM boards during the night, with a maximum night air exchange of 2 . 6h − 1 .
The average nightly heat flux from the ceiling is less than 2Wm − 2 , corresponding
to a total energy removed of 30Wh m − 2 . The reference gypsum board without PCM
showed a slightly higher temperature swing (0 . 5 K) and a slightly lower heat removal of
25Wh m − 2 per night. Detailed measurements during a hot week in July 2006 showed
that the charging of the PCM works well with heat fluxes up to 6Wm − 2 , especially
during the morning hours, and that the PCM plates can be charged for a longer time
period during the day than the conventional gypsum board. At mid-day a reduction
in heat flux can be observed due to lower internal loads during the lunchtime break.
After three warm days, the additional charging capacity is exhausted and the PCM
ceiling boards behave exactly the same as the gypsum board (see Figure 3.20). This
can be attributed to the very inefficient night discharging of the room surfaces. The
supply air temperature levels never drop below 20 ◦ C, although ambient air night tem-
peratures are often below 16 ◦ C. From ambient air up to the air distribution box after
the air handling unit, there is already a temperature increase of 2 K. A further 2 K

−