Silica nanogel for energy-efficient windows - Nanotechnology in Eco-efficient Construction - page 212

still high for cost-sensitive industries such as the building one. In this fi eld,
opaque and transparent or translucent aerogels have been developed
(Baetens et al. , 2011). In the translucent and monolithic form, they have
interesting optical properties, such as high light and solar transmittance and
excellent thermal insulation properties, when compared with conventional
glass. Therefore they are used as transparent walls in solar collectors and
in offi ce buildings (Ackerman et al. , 2001; Reim et al. , 2002).
The opaque aerogels are fl exible blankets obtained by adding fi bres in
the gel before the drying process (Cabot Corporation, Boston, MA, USA;
Aspen Aerogels Inc., Northborough, MA, USA), in order to reduce their
fragility. The thermal conductivity is about 0.13 W/(mK), but their cost is
about 10 times higher than a conventional material with similar perfor-
mance. Nanostructured materials as aerogels could be suitably employed
in highly insulating windows and in the last 15 years windows based on
translucent granular or monolithic aerogels were developed by companies
in cooperation with researchers. Monolithic aerogel panes appear as a very
transparent and lightweight material (Fig. 10.3), but they have a tendency
to scatter the transmitted light, resulting in a hazy picture when objects are
viewed through them.
Twenty years ago, thanks to the International Energy Agency Solar
Heating and Cooling Programme, Task 18-Advanced Glazings and Associ-
ated Materials for Solar and Building Applications (Duer and Svendsen,
1998), several countries (Denmark, Finland, France, Germany, Japan,
Norway, Sweden, United Kingdom) were involved in a project to develop
and set up superinsulating glazings with monolithic aerogel. Five different
samples were realized, characterized by different density, thickness and
transparency: the light transmittance was in the 0.75-0.96 range. Depending
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10.3 View through a monolithic aerogel sample (sample supplied by
AIRGLASS, Sweden, 2010) (Buratti and Moretti, 2012b).