Facades and Summer Performance of Buildings - Low Energy Cooling for Sustainable Buildings - page 58

Figure 2.37 Mean annual thermal efficiencies from building simulation (points) compared with results
from the fa¸ade thermal model

this effect, the thermal gains were calculated for day and night separately. With the
assumption that the thermal influence of the building is identical for day and night,
the remaining collector energy may be estimated from the difference between the two:
Q col = Q day − Q night . In Figure 2.37 the thermal efficiencies calculated by the new
simulation tool TransFact (solid lines) were compared with the averaged results from
TRNSYS which were calculated from the day and night differences. These calculated
efficiencies correspond to a stationary thermal model of a ventilated double fa¸ade
(Eicker, 2003). At the same flow velocity the thermal efficiency increases with gap
size, as the total volume flow increases, the average temperature level reduces and the
heat losses to ambient also decrease.

2.5 Conclusions on Fa¸ade Performance

The summer performance of single and double fa¸ades with sun shading systems or
active solar elements was analysed both experimentally and by computer simulation.
Experiments on one-storey-high fa¸ades were done both in the laboratory and in a
real office building. It was shown that the summer thermal energy production in a
ventilated fa¸ade is significant and ranges between 50 and 100 kWhm − 2

fa ¸ ade a − 1 for
typical absorption coefficients of sun shades between 10 and 30% and can double if a
highly absorbing element such as a PVmodule is used. From this thermal energy, only
a fraction usually enters the room and adds to the cooling loads. If the air exchange
with the double fa¸ade only occurs by tilting one or two windows and not allowing any
cross-ventilation, then the fa¸ade to room air exchange rates stay below 1-2 room air
exchanges per hour. Measured heat gains per square metre of office space in August