Environmental Engineering Reference
In-Depth Information
Figure 18.6.2 Solar efficiency as a function of the heat transfer coefficient of the opaque wall.
into the room during the day; additional gains from the TTI wall must be stored and
then used in the evening hours. The temporal shift between heat production at the
absorber and the maximum heat flow into the room rises with external wall thickness
and also depends on the density, thermal capacity and heat conductivity of the wall.
Phase shifts of 6 to 8 hours are achieved with a 24 cm brick or lime-sandstone wall
while a little more than 5 hours are possible with concrete walls. With sufficiently
thick wall constructions, the result is typically 100% efficiencies of the heat produced
by the TTI wall within the core months of the heating season; in the transition months
efficiency is around 30%.
The high absorber temperatures of the external wall, which can reach peak values
of between 70 and 80 C, are effectively dampened at wall thicknesses over 20 cm and
are rarely higher than 30 C at the interior surface, even in the summer. The absorber
temperatures are lower with heavy, heat-conducting components than with light. The
heat that develops can penetrate quickly into the heavy external wall and be led into
the interior. Any thermal tensions are thus correspondingly low.
The thermal deformations of an external wall on an experimental house in
Stuttgart were measured on a long-term basis. Compression stresses and slight swelling
of the wall due to the high temperature difference did not pose a problem. Fine cracks
of about 1 to 2 mm in the plaster resulted from accelerated drying of the new building's
brickwork dampness around the absorber; however, these cracks did not influence the
load-carrying capacity of the wall. It is worth planning for defined joints at the edges
of the TTI surfaces for new buildings. The use of shading systems such as blinds or
shutters prevents heating of the external wall during the transition period and in the
summer months, although the use of these systems is complex in terms of construc-
tion and maintenance. In order to avoid exposure to the sun during the transition or
summer period, constructional shadings such as balconies or roof projections must be
planned very carefully. Foregoing shading mechanisms is possible if the transmittance
of the transparent insulation is strongly angle-dependent, such that with a high sum-
mer sun position having angles of incidence over 60
at the south-facing façade, less
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