Silica nanogel for energy-efficient windows - Nanotechnology in Eco-efficient Construction - page 214

was made, with the required barrier properties against atmospheric air and
water vapour, in order to achieve a low thermal bridge effect and to ensure
a theoretical lifetime of the glazing of about 30 years. The fi nal assembly
and evacuation process was set up and a fi nal pressure in the aerogel of
500 Pa was reached. The solar and daylight transmittance of the aerogel
glazing were optimized by means of low-iron glass covers, with an antirefl ec-
tion coating. The optical quality had a minimal disturbance in the view
through, except if exposed to direct non-perpendicular radiation, when the
diffusion of the light becomes signifi cant. The centre U-value, measured by
means of a hot-plate apparatus, was equal to 0.66 W/(m 2 K), which corre-
sponds to an estimated thermal conductivity of 0.010 W/(mK) for the
aerogel pane (average aerogel thickness 14.8 mm) (Schultz et al. , 2005).
Granular silica aerogels were also investigated, in order to integrate them
into highly-insulating translucent glazing (Reim et al. , 2002; 2004). Within
the R&D project ISOTEG pursued by the ZAE Bayern (Bayerisches
Zentrum für Angewandte Energieforschung, Germany), a daylighting
system was developed by inserting aerogel granules between a double skin
sheet made of polymethyl-methacrylate (PMMA). Two types of granular
aerogel were used in prototype windows: semi-transparent, consisting of
rather regular spheres, and highly translucent granulates, consisting of irreg-
ularly fractured spheres (Fig. 10.5). The optical properties (transmittance
and refl ectance) of the aerogel granulates between two highly transparent
glass panes, for a 10 mm packed bed, were measured with an integrating
sphere arrangement in the 400-2000 nm wavelength range. Scattering at
structural inhomogeneities causes the decreasing of transmission with
decreasing wavelengths below 600 nm (Fig. 10.6) and the investigated frac-
tured aerogel samples show a higher transmittance than the more regular
ones, both in the visible and solar range (see Section 10.4 for more details).
Different daylighting systems in PMMA were manufactured with a thick-
ness less than 50 mm: in order to optimize the thermal insulation, the sheet
was mounted between two low-e coated glass panes (emissivity equal to
0.03 or 0.08) and argon or krypton were used as fi lling gases (Fig. 10.7).

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1 cm

1 cm

10.5 Granular aerogels: semi-transparent, consisting of regular
aerogel granules (left) and highly translucent granulates, consisting of
fractured aerogel pieces (right) (Reim et al. , 2002).