Switchable glazing technology for eco-efficient construction - Nanotechnology in Eco-efficient Construction - page 235

energy effi ciency can be taken if the glazings are 'switchable' (or 'smart'),
i.e., are able to vary their throughput of visible light and solar energy
(Lampert, 1984; Svensson and Granqvist, 1984). This functionality makes
good use of the 'chromogenic' materials (Granqvist, 1990; Lampert and
Granqvist, 1990; Smith and Granqvist, 2010) which are characterized by
their ability to respond persistently and reversibly to external stimuli.
There are four kinds of chromogenic materials of primary interest for
glazings in buildings. They are referred to as 'photochromic', 'thermochro-
mic', 'electrochromic' and 'gasochromic'; their transmittance depends on
irradiation intensity of ultraviolet light, temperature, application of an elec-
trical voltage or current, and exposure to reducing and oxidizing gases,
respectively. The largest energy savings can be accomplished with electro-
chromics (Selkowitz and Lampert, 1990; Granqvist et al. , 2010), and a recent
report indicates that highly insulated electrochromic windows used in com-
mercial as well as residential buildings would be able to save as much as
4.5 per cent on the annual energy use in the USA (Gillaspie et al. , 2010).
The user acceptance of this technology appears to be very good (Clear et
al. , 2006; Zinzi, 2006; Lee et al. , 2012). Thermochromics does not have an
equally vast savings potential, but thermochromic devices can be based on
a single thin layer or nanoparticle composite and are simpler than electro-
chromic devices which typically employ fi ve superimposed layers (Gran-
qvist, 1995). It should also be noted that electrochromic and thermochromic
devices may be combined with optimized thermal insulation in future 'super
fenestration' (Granqvist et al. , 2010).
Parts of this chapter are based on recent scientifi c papers by the author
and his coworkers (Granqvist, 2012; Li et al. , 2012), but the text has been
integrated and adapted. Throughout the text there are references to various
coating technologies, and Chapter 8 of the present book, on the manufactur-
ing of thin fi lms and nanostructured coatings for eco-effi cient constructions,
can serve as a parallel source for some background material.

￿ ￿ ￿ ￿ ￿ ￿

11.2 Electrochromics: materials and devices

Electrochromic materials are characterized by their ability to change their
optical properties, reversibly and persistently, when a voltage is applied
across them (Deb, 1973; Granqvist, 1995). This functionality clearly is of
great interest for numerous applications, and device aspects rather than
fundamental science have been leading the development of this technology
ever since the discovery of electrochromism. Display devices were the focus
during the early years, but applications to energy-effi cient windows have
been the driving force ever since the mid-1980s when it was realized that
huge energy savings were feasible with electrochromic glazings (Lampert,
1984; Svensson and Granqvist, 1984; Granqvist, 2012).