Smart windows are based on innovative composite glasses and allow to
automatically adapt the characteristics of light and heat transmission to
different external circumstances. For energy efficiency smart windows are a
promising technology since they may shield direct sunlight through the windows
and thus save energy for air conditioning.
Technical details:
Smart windows are composed of a multi-layered glass that change colour in a
continuously variable fashion. This is obtained by two different effects:
gasochromism and electrochromism.
Electrochromism:
Smart windows using electrochromism are made of two glass panes which are
laminated together with an ion conducting polymer foil. The glass panes are
coated with an electrochromic film. These thin films (e.g. Wolfram-Oxides like
HXWO3, LiXWO3) change color in response to an applied electric field due to
changes in the light-absorption spectra (especially for visible light and heat
in the near infrared spectral range). This effect is triggered by
voltage-induced ion transport in the ion-conducting foil situated between the
glass panes.
Figure 1: Layers of a smart window
Source: GESIMAT
This way light and heat transmission as well as reflection properties can be
altered. This process is regulated automatically. Light transmission may be
reduced by up to 70% by means of smart windows.
The energy necessary for the coloration of electrochromic glazing is low
(between 20 and 150 Ws/m2). The DC voltage for switching the electrochromic
window is between 0.5 and 2.2 V.
Depending on the choice of these parameters full coloration or bleaching of
the electrochromic glazings can be achieved in time ranges between 2 seconds and
10 minutes. A multitude of different colors (e.g. blue, green, red, brown,
violet, grey) can be realised.
Usually the WO3-films are evaporated onto the glass layer and must therefore
be available in a nano-crystalline state, with grain sizes between 1 and 20 nm.
This sometimes leads to the categorization of smart glasses as a
nanotechnology.
Gasochromism:
Gasochromism is closely related to electrochromism. Instead of an
ion-conducting foil, there is an interspace between the two WO3-coated glasses
filled by gases. If hydrogen concentration in the interspace is increased the
WO3-film turns blue. The backwards process of achromatisation is obtained by
regulating oxygen concentration. The gas concentration between the glasses is
controlled by a small electrolyser and a pump, which are installed at the
window’s balustrade. The gas-circle is closed. The admixing of hydrogen is so
small, that there is no danger, even in case of window breaking.
Figure 2: Transmission and reflection on a smart window
Source: GESIMAT
Fields of application:
Buildings, cars (electrochromic glass already in use for car mirrors)
Manufacturer:
Electrochromic windows: Fraunhofer-Institute; GESIMAT-Berlin (Gesellschaft
fuer Intelligente Materialien und Technologien)
Gasochrome windows: Interpane in cooperation with Fraunhofer-Institut fuer
Solare Energiesysteme (ISE) |