Solar energy use in buildings - Solar Energy Sciences and Engineering Applications - page 601

than 20% of the irradiance reaches the absorber wall. The transmittance of a transpar-
ent insulation system falls, for example, from approximately 50% with perpendicular
incidence to 15% with a sun elevation angle of 60 ◦ .
Simulations for highly insulated low-energy buildings have shown that even when
the south-facing façade is largely covered with a TTI heat insulation system, the excess
heat in summer can be expelled by night ventilation. The number of hours with room
temperatures over 26 ◦ C was below 300 hours per year under German conditions and
only 50 hours higher than a conventionally insulated building.
On the other hand, at angles of incidence of 60 ◦ , transparent insulation with a
glass covering can still have a very high total energy transmission factor of 35%. In
this case, sun protection is unavoidable when the façade is largely covered. If nocturnal
ventilation is not possible, e.g. in office buildings, shading of the TTI surfaces in the
summer should likewise be provided.
The orientation of the transparently insulated façade is crucial both for energy
gain in the winter and for protection against overheating in the summer. Façade orien-
tations between south-east and south-west are suitable. Twice as much energy, some
400 kWh m − 2 , falls on a south-facing façade in the winter heating season than on an
east or west-facing one. In addition, the low winter sun position facilitates good light
transmission by the TTI material. In summer, on the other hand, only the south-facing
façade offers a certain natural sun protection with low transmittance.

18.7 MATERIALS USED AND CONSTRUCTION

TTI capillary or honeycomb structures are assembled from thin-walled plastic tubes
and welded by a hot wire section or manufactured into strips of any width from
extruder nozzles with an almost square cell cross-section. The typical cell diameter is
3 mm.
Two polymer types are in use today: polymethyl methacrylate (PMMA) and poly-
carbonates (PC). PMMA is characterized by high transmittance and by good UV
stability. Due to the brittleness of the material and its poor fire retardance (class B3),
PMMA is bound between windowpanes, which requires a complex mullion-transom
construction costing between 400 and 750 m − 2 . The cost of the 10-12 cm trans-

a

m − 2 . It is the glazing and attachment,

parent insulation is typically only around 50 a

m − 2 , plus shading items such as blinds at around 150 m − 2 ,

at approximately 250 a a

which drive up costs.
Polycarbonates are mechanically more stable and can be processed without the use
of a glass covering; however, they are not very UV-resistant. PC fire retardance is better
(class B1) than PMMA and the material is temperature-resistant to about 125 ◦ C. PC
materials can be used in heatinsulating compound systems. The covering plaster is an
acryl adhesive mixed with 2.5-3 mm diameter glass balls, which is applied in the factory
directly onto the capillary material. Additional UV absorbers can likewise be brought
into the cover plaster. Such heat-insulating compound systems can be manufactured
with substantially reduced costs of around 150 m 2 , since there are no complex glazing

and shading systems. The weight of capillary materials is around 30 kg m − 3 .
Capillaries made of glass are manufactured like the polymer structures, but are
complicated to produce due to high processing temperatures and associated engineering

a