Facades and Summer Performance of Buildings - Low Energy Cooling for Sustainable Buildings - page 57

The thermal gains in summer and winter are approximately within the same range.
However, the secondary heat transfer from the inner fa¸ade surface to the room air
is very low in summer because of the compensation between day and night where
the heat flow directions are different. Case PV4 directly shows the influence of the
operating time of the fan in comparison with Case PV2a. Because of the reduced
operating hours in Case PV4, the thermal gains are proportionately lower.
Higher flow velocities for lower gap sizes lead to higher thermal yield at the expense
of higher pressure drops. The coefficient of performance (COP) describes the rela-
tion between the thermal energy gain of the fa¸ade collector Q Fa ¸ ade and the energy
consumption of the fan E vent :

˙ Vρc T gap,out − T gap,in t
E vent

Q Fa ¸ ade
E vent

COP = = (2.10)

For the widest gap of 0.2m with the lowest flow velocity within the air gap, and
thus the lowest resistance (Case PV3a), the COP reaches a maximum value of 50.
The dominating influence on the COP is clearly the low flow velocity, which reduces
electrical energy consumptionwithout greatly reducing the thermal efficiency. At more
realistic operation modes (such as in Case PV4 with a 10 cm air gap) the COP reaches
values of 12 (see Figure 2.36).
The thermal efficiency η of the fa¸ade air collector can be calculated directly from
the relation of thermal gains Q Fa ¸ ade to the incident irradiance G :

Q Fa ¸ ade
GA Fa ¸ ade =

˙ Vρc air ( T outlet −

T inlet )

η = (2.11)

GA Fa ¸ ade

Afa¸ade collector, however, also interacts thermally with the building, as the trans-
mission losses through the fa¸ade increase the air gap temperature. To compensate for

60

50

40

30

20

10

0

PV1a PV2a PV3a PV4

Figure 2.36 Ratio of produced thermal energy to electrical fan energy