Environmental Engineering Reference
compared with a conventional double glazed fa¸ade (Vollmer, 1999). A conventional
double-glazed fa¸ade is more economical than a PV fa¸ade during the wintertime.
However, in the special case of a library, glare-free light is an important factor and
expensive shading devices would be necessary. Additional cooling loads associated
with double-glazed fa¸ades during the summer also apply to the PV system, which,
however, provides useful electrical gains.
Meyer (2001) analysed the influence of a hybrid double fa¸ade system on the
adjacent room climate. Detailed measurements of external and internal climatic con-
ditions were compared with numerical results from the simulation program TRNSYS.
He found that winter transmission losses may be reduced by up to 50%. The required
electricity for mechanical ventilation of the fa¸ade amounted to approximately 10%
of the achievable gains.
The most effective application for hybrid fa¸ades may be considered in combination
with solar cooling components. For example, the high fa¸ade outlet temperatures may
be used to support a desiccant cooling system (Eicker et al ., 1998; Hofker, 2001).
Despite the controversy, in Germany approximately half of the large office build-
ings are presently being designed with glass double fa¸ades. At the same time, the
planning and realization of double fa¸ades require a considerable effort which is often
economically unjustifiable. The strong arguments between advocates and opposers of
glazed double fa¸ade systems still lead to uncertainties in the planning process.
The thermal situation of glazed double fa¸ades can be summarized as follows:
because of the thermal buffer present in most double fa¸ade types, transmission heat
losses in winter can be reduced. Ambient air can be preheated and if intelligent control
is used, running times of mechanical ventilation systems can be reduced and energy
saved. However, combining air preheating from the fa¸ade and heat recovery from
exhaust air is difficult, as this could only be realized decentrally in every room. It
should be mentioned that in office buildings, high internal gains lead to a diminished
heating demand, so that the energy-saving potential in these buildings is often not
as large as might be expected. In the literature, quoted energy savings range from
'marginal' to over 25, 40 or 50%.
During the summer period, the glazed double fa¸ade increases the fa¸ade gap tem-
perature as well as the indoor temperature, which results in an increasing demand
for cooling. Shading devices within the fa¸ade gap may reduce the incoming solar
radiation with the further advantage that the blinds will be protected from wind forces
and other ambient influences. However, these blinds inside the double fa¸ade gap lead
to an additional increase of the gap temperature. As a result, the heat transmission rate
is higher than for a fa¸ade which has direct contact with the ambient air.
2.1.3 Modelling of Ventilated Fa¸ades
To model the thermal characteristics of ventilated fa¸ades, a simulation program was
developed at the University of Applied Sciences in Stuttgart, which contains a new,