Facades and Summer Performance of Buildings - Low Energy Cooling for Sustainable Buildings - page 22

Furthermore, internal loads should be as low as possible. This can be mainly
achieved through optimized daylight use and energy-efficient electrical equipment.
Low glazing fractions certainly help to reduce the problem of unwanted external
heat gains, but the reality is often highly glazed buildings. The role of the fa¸ade on
external heat gains through transmission, and also through ventilation heat gains and
secondary heat flows, is the subject of this chapter.
The current argument is that highly insulated buildings perform worse in summer,
as the heat is trapped inside the building and cannot be dissipated by transmission
through the building skin. For a start, this is only true if the external temperature is
lower than the indoor temperature, so that there is a net heat flux from inside to outside.
In hot climates, this is usually not the case. Also, under European moderate climate
conditions, the mean summer temperature difference between the inside and outside
is not significant, so that transmission losses are small anyway. Furthermore, highly
insulated buildings are much more suited to control heat fluxes in summer: during
the daytime with high external temperatures, the insulated building skin prevents
unwanted transmission heat gains. During the night, heat removal can then be much
more efficiently controlled through ventilation rather than transmission.
If the building is therefore highly insulated, the main external gains occur through
the glazed fa¸ade part. The gains consist of two parts. The dominating part is usu-
ally related to the short-wave solar irradiance gain transmitted directly through the
fa¸ade or indirectly as secondary heat flux from absorbed irradiance. This contribu-
tion can be efficiently reduced by shading devices. Experimental results on the total
energy transmittance of different fa¸ades are presented in the experimental section.
To analyse the main influences on the total energy transmittance, a dynamic fa¸ade
model including new Nusselt correlations derived from particle imaging velocimetry
and CFD (Computational Fluid Dynamics) simulations was developed and validated
using temperature and heat flux measurements carried out in the laboratories of the
University of Applied Sciences in Stuttgart (Fux, 2006).
The second part of external gains concerns ventilation gains, if the hygienically
required fresh air is taken directly via the fa¸ade system. In naturally ventilated build-
ings or in buildings with only mechanical exhaust systems, this is nearly always
the case. Air intake from the fa¸ade is also an issue in double fa¸ade constructions.
The temperature increase through absorption on shading devices inside such dou-
ble fa¸ades depends on air volume flow rates and the optical characteristics. The
energy input to the building is a function of the air exchange between fa¸ade and
building.
The main objective is to quantify the total summer energy input to the building
through the fa¸ade system. To achieve this goal, laboratory experiments have been
carried out using a large solar simulator to investigate flow and temperature conditions
in different fa¸ades. The derived theoretical model was then validated on measurement
results from office buildings with double fa¸ades.