Facades and Summer Performance of Buildings - Low Energy Cooling for Sustainable Buildings - page 55

Figure 2.33 Flow resistance and pressure drop in a ventilated double fa¸ade

For the rectangular bow, a resistance of ξ 90 ◦ =1 . 3 was assumed. The resistance of
the openings depends on the covers used: the inlet resistance may vary between 1.5
and 300, depending on the opening section of perforated plates or on the density of
the filter pads used. The author's experimental investigations of a single PV fa¸ade
element gave a total resistance of about ξ

=

60 if a perforated plate was used only at
the openings. A total resistance of about ξ = 230 was measured if the original dirty

filter pads from the library in Mataro were used additionally. A flow resistance of
ξ = 200 for the fa¸ade entrance region was assumed when using clean filter pads.

The measured resistances and the pressure drops as a function of air volume flow are
shown in Figure 2.33.
The yearly energy consumption of the fan is obtained from the operating time,
which varies from 8760 hours for Cases 0 to 3 down to 5352 hours in Case 4. When
considering the electrical energy production of the PV modules, a total value of about
15 000 kWh a − 1 (or 58 kWhm − 2 a − 1 ) was obtained for all cases. The calculated max-
imum difference between Cases PV1c and PV3a, where the air gap velocities amount
to 2.3 and 0.2m s − 1 , respectively, is about 400 kWh a − 1 and therefore not visible in
Figure 2.34. Because of the quadratic increment of the flow resistance with the air
velocity, the energy used for ventilation exceeds the generated energy when there are
high-volume flows (PV0c and PV1c). For Case PV4 where the geometry as well as
the flow rate corresponds to Case PV2a, the influence of the lower operating time of
the fan is cancelled out by the lower PV efficiency when the fan is switched off.
The thermal energy gains from fa¸ade ventilation and secondary heat gains from the
internal fa¸ade surface to the room air are higher than the electrical energy produced.
The energy for the fa¸ade ventilation is deducted from this produced electrical energy,
and is important for higher flow velocities. The values which refer to the fa¸ade area
are differentiated into winter and summer situations. As can be seen in Figure 2.35, the