advanced building skins 14 | 15 June 2012 - lamp.tugraz.at - Graz ...

6 Load Case Temperature
Advanced Building Skins
The influence of solar radiation on photovoltaic modules compared to transparent façade glazing is
much higher due to the fact that the absorption coefficient increases. Uncoated glasses have a solar
absorption of approximately 8-20 % depending on the glass thickness [12]. A thin film photovoltaic
module (ASI), however, has an absorbance between 60 % and 90 %.
The solar absorption basically leads to two superposed temperature profiles over the module thickness.
A constant temperature load can be superimposed with a variable temperature profile over the cross
section (Fig. 6).
Figure 6: Separation of the thermal stress in a constant and variable part
The constant temperature Tc only leads to compressive stress in the glass for framed modules with an
idealized system (constant linear horizontal support of the edges).
For locally clamped modules and temperatures up to 50 °C (compare [13]) with a 2 mm elastic support
of the glass edge and soft contact materials (Young's modulus E = 5 – 200 N/mm², ν = 0.5, e.g.
EPDM), no design-relevant tensile stresses occur, too. Only for very stiff contact materials (Young's
modulus E = 1 000 – 3 000 N/mm², ν = 0.3 - 0.4, e.g. POM), the principal tensile stresses would be in
a design-relevant dimension.
For back rail supported modules, no significant tensile stresses are caused by constant heating of the
glass sheets due to the flexibility of the overall system.
With a variable temperature profile, the (suppressed) bending of the glass panes leads to tensile
stresses, which may be relevant to structural design.
A parametric study using the above framed module with two types of a variable temperature profile
shows this clearly. In the first variant (symmetric temperature distribution), the upper and lower sides
of the laminate are cooler than the thin film and the lamination foil in the middle due to the ambient
atmosphere. In the second variant (asymmetric temperature distribution), a reduced cooling of the
lower glass panel was taken into account and therefore a constant temperature distribution assumed
(Figure 7).
Figure 7: Investigated temperature distributions (symmetrical and asymmetrical)
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