Facades and Summer Performance of Buildings - Low Energy Cooling for Sustainable Buildings - page 59

are between 36 and 86Whm − 2 d − 1 , which corresponds to about 2-5 kWhm − 2

room a − 1
of additional cooling loads. Unwanted summer gains from secondary heat fluxes
only occur if no shading system is used and the internal glass pane heats up signi-
ficantly. In all other cases, even with an integrated PVmodule, the inner fa¸ade surface
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. Secondary heat gains are between
17 and 20 kWhm − 2

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

room cooling loads.
Measured air temperature increases in a one-storey-high double fa¸ade were be-
tween 3 and 5 K, with peak values of 8 K. The blind temperaturemeasured in the fa¸ade
was 10-12 K above the air inlet temperature, slightly higher than for the laboratory
measurements. The temperature levels increase by about 4 K when the cross-sections
of the air inlet and outlet are reduced from 44% to 10%.
The total energy transmittance or g -value was determined by a calorimetric method
for single and double fa¸ades in the laboratory. Energy reduction coefficients of 23%
were measured for both a single fa¸ade with external shading and a double fa¸ade
with sun shades in the air gap. The g -values were thus reduced from 64% to 15% for
the single fa¸ade and from 51% to 12% for the double fa¸ade. On the real building,
less precise measurements with pyranometers for optical transmission and heat flux
measurements were taken, which resulted in g -values of 43% for the unshaded double
fa¸ade and a very low value of 3% for the completely shaded fa¸ade.
The position of the sun shades within the double fa¸ade relative to the outer pane
had no measurable influence on the g -value. Also the size of the fa¸ade opening cross-
section does not influence the g -value (within measurement uncertainty), although air
temperature levels increase by a fewKelvins. Apart from the energy transmittance, the
ventilation rate determines the additional cooling load and has to be analysed when
evaluating the summer performance of a fa¸ade.
In conclusion, it can be stated that externally shaded single and internal gap shaded
double fa¸ades can effectively reduce the total energy transmittance to a building in
summer. The highest energetic priority is always the reduction of short-wave solar
irradiance, which is the dominant energy flow. With measured g -values as low as
7% with even slightly open blinds, this condition can be fulfilled by both single and
double fa¸ades. Secondary heat flows only play a role if no shading system is used
and are otherwise negligible. If the fa¸ade is used for providing fresh air to the room,
additional cooling loads of the order of 10-30% of typical office cooling room loads
occur. All results were obtained for moderate German summer climatic conditions
and the increase in cooling load from fa¸ade ventilation and secondary heat flux will
be even stronger in warmer climates.
For the special case of ventilated PV fa¸ades, thermal energy can be produced and
delivered to the building by forced ventilation at high COPs. Temperature increases
are usually between 10 and 15 K and thus mainly useful for preheating the air. If
a building's fresh air supply is delivered through the PV fa¸ade, thermal gains are