Facades and Summer Performance of Buildings - Low Energy Cooling for Sustainable Buildings - page 47

Figure 2.22 Secondary heat flux for unshaded and shaded single and double fa¸ades

ventilation with ambient air at a fixed air exchange rate of 0.7 h − 1 on average leads
to heat removal during summer of 13 kWhm − 2

room a − 1 . Unwanted summer gains from
secondary heat fluxes are between 17 and 20 kWhm − 2

fa ¸ ade for the unshaded single and
double fa¸ade, respectively. When related to the room surface area, this corresponds
to 8-9 kWhm − 2

room cooling loads. It should be noted that only for the unshaded fa¸ades
is net energy transferred from the surface to the room air; in all other cases the fa¸ade
remains cooler than room air on average and there are even heat losses between 10
and 17 kWhm − 2

fa ¸ ade during the summer period. It is also clear that effective summer
heat losses from both ventilation and secondary heat fluxes are only possible in a
very moderate summer climate with average ambient temperatures below the room
setpoint temperature. If weather data from a warmer summer (such as in Stuttgart in
2003) is taken, both ventilation heat and secondary heat fluxes are positive and add to
the room's cooling load.

2.4 Energy Production from Active Fa¸ades

Finally, the influence of an active PV ventilated fa¸ade is studied for a library building
with a south-facing PV system (see Figure 2.24). Figure 2.23 shows a schematic view
of the building as well as a cross-section through the fa¸ade and the construction of
the external enclosures.
Measurements were done on a 6.5m high fa¸ade collector element in the build-
ing in Spain (see Figures 2.24 and 2.26) as well as on a similar 2.2m high ele-
ment located at the test site of the University of Applied Sciences in Stuttgart (see
Figure 2.25).
As the total absorption coefficient of the outer PVmodule is about 80% (PVmodule
and glass spacing) and thus significantly higher than a standard sun shading element,