Facades and Summer Performance of Buildings - Low Energy Cooling for Sustainable Buildings - page 40

Table 2.4 Measured average g -values and temperatures of a double fa¸ade with different sun shading
positions

Double fa¸ade Double fa¸ade Double fa¸ade

no shading 40 ◦ blind closed blinds

g -value 0.43 0.07 0.03

Tot. transmission τ 0.35 0.04 0.01

Transm. ext. pane 0.69

Energy red. F c — 0.16 0.07

q i / G 0.08 0.03 0.02

Surface temp. ext. 40 41 42

Blind temp. — 43 38

Surface temp. room 36 28 29

Air inlet temp.
Air outlet temp. 36 36 37

Room temp. 30 25 27.5

Ambient temp. 30 27 29

Average irradiance 700 700 470

When comparing measured and calculated g -values, the results are practically iden-
tical. Whereas the simulated surface temperature levels deviate by a maximum of 1 K,
the simulated outlet air temperatures are 2-4 K lower than the measured values. This
again is due to the heat transfer correlations derived from laboratory experiments
with lower turbulence. The total g -value of the fa¸ade with closed blinds is only 3%
compared with 12% in the laboratory experiment. The main reasons are:

a high average incidence angle of 80 ◦ in the real buildingwith 70% less transmission;
13% lower transmission of the thicker external pane;
about 10% less transmission due to dirtying of the real fa¸ade.

In conclusion it can be said that double fa¸ades with integrated sun shading elements
effectively reduce external gains through solar irradiance with measured g -values as
low as 3% for completely closed blinds and 7% for partly closed blinds. If low-e coated
glazing is used to separate the air gap thermally from the room air, the secondary heat
fluxes are also low. However, the ambient air is heated significantly through absorption
in the double fa¸ade. If this air is used to ventilate the adjacent rooms, ventilation heat
gains occur, which reduce energy consumption in winter, but add to the cooling load
in summer.

2.3 Cooling Loads through Ventilation Gains

2.3.1 Double Fa¸ade Experiments

Ventilation heat gains through a double fa¸ade depend on the air exchange rate between
the fa¸ade and the adjacent room and on the absorption characteristics of the glazing