Civil Engineering Reference
In-Depth Information
is a composite of a silica aerogel and fi brous reinforcement, turning the
brittle structure of a silica aerogel into a durable, fl exible, and hydrophobic
material. The resulting blankets have a thermal conductivity
k
between
0.013 and 0.014 W/(mK). The aerogel insulation material consists of amor-
phous silica instead of crystalline silica, reducing possible health risks at
exposure. Aerogel blankets can be used in the entire building industry
similar to the use of traditional thermal insulators. However, their current
high economic cost means that they are only used where limited space is
available and where the use of vacuum insulation panels is not possible due
to their drawbacks.
Aerogel is especially very interesting as a translucent or transparent
insulation material because of its combination of a low thermal conductivity
and a high transmittance of daylight and solar energy. At present, there are
two commercial types of such aerogel-based daylight systems, i.e., Scoba-lit
and Okagel windows. The aerogel product has a thermal conductivity of
0.018 W/(mK) and the fabricator offers skylights with a heat transmittance
coeffi cient between 0.6 and 0.3 W/(m
2
K) for layers of 30 and 60 mm Okagel
respectively. The visible light transmission
T
vis
is 0.40 and the sound reduc-
tion is 52 dB. Research has been conducted in the last decade on the devel-
opment of highly insulating windows based on both granular aerogel and
monolithic aerogel. Two types of granular aerogel are used in prototype
windows: semi-transparent spheres with a solar transmittance
T
sol
of 0.53
for a 10 mm packed bed and highly translucent granulates with a
T
sol
of
0.88. The granular aerogel is stacked in a polymethylmethacrylate double
skin-sheet, between two gaps and glass panes. Increasing the aerogel
thickness to 20.0 mm will lower the U-value further to approximately
0.5 W/(m
2
K), while the solar transmittance will still stay above 0.75.
9.4.2 Partial vacuum thermal insulators
The areas of application of vacuum insulation panels are strongly linked to
their physical properties and so these will be discussed fi rst.
Properties
The main benefi t of vacuum insulation panels is the reduction of the
required thickness of the insulation layers. With a pristine center-of-panel
thermal conductivity
k
of 0.004-0.005 W/(mK), equal thermal resistances
are achieved within a thickness 5-8 times lower than traditional thermal
insulators. The way this low thermal conductivity is achieved determines at
the same time its main drawbacks, i.e., degradation through time of the
thermal conductivity, thermal bridging at the panel edges, and strong limita-
tions for installation and its resulting areas of application.
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