Facades and Summer Performance of Buildings - Low Energy Cooling for Sustainable Buildings - page 26

coloured blind with a 17% absorption coefficient, the temperature level at the same
blind position (close to the inside) decreased to 57.8 ◦ C. These results were calculated
for a closed double fa¸ade. If the fa¸ade is open at the top and bottom, temperature
levels decrease by about 18 K and the cooling energy demand by a further 6 to 9%. The
main question about this work is that no experimental results are given for validation
and the modelling approach for heat transfer and fluid flow is not detailed.
A special double fa¸ade solution is that of the ventilated photovoltaic fa¸ade (PV
fa¸ade). A PV hybrid fa¸ade can be regarded as an architectural design concept,
which might be compared with a representative granite fa¸ade and could be even more
economical (Auer, 1998). By replacing the outer glazing with a semitransparent PV
module, electricity and heat are generated and daylight can still be partially transmitted.
Since the efficiency of PV cells drops with increasing temperature, the heat should
be dissipated as much as possible and warm air can be channelled directly into the
building during the wintertime. In summer natural convection should be used for heat
dissipation, unless the heat can be used for active thermal cooling technologies. Due
to the high absorption of the PV module, high gap temperatures in summer increase
the building's cooling load.
The advantages of a hybrid PV fa¸ade can be summarized as follows:
The transmission heat loss is reduced during the heating period.
Ventilation losses are reduced if the preheated air is fed into the building.
The semitransparent PV fa¸ade forms a natural shading element. The fa¸ade creates
glare-free light.
During the summer period, the building may be ventilated through the fa¸ade gap
(only when there is no or low solar irradiance).
The fa¸ade offers increased sound protection against external noise.
The disadvantages are:
High electricity consumption for ventilation in summer, if the fa¸ade is ventilated
mechanically.
The semitransparency of the PV modules prevents maximum utilization of solar
gains in winter.
High costs.
A methodology has been developed by the author and collaborators (Mei et al .,
2002, 2003; Infield et al ., 2004; Eicker et al ., 1999, 2000) for calculating the thermal
impact on building performance of an integrated ventilated PV fa¸ade. This is based on
an extension of the familiar U -and g -values to take into account the energy transfer to
the fa¸ade ventilation air. For a public library in Spain constructed with a ventilated PV
fa¸ade, the energy-saving and electrical gains for different climatic conditions were