Civil Engineering Reference
In-Depth Information
times of the day and during different seasons. To benefit from, or compensate for,
this time variability, one would like to have materials whose properties can be
changed, reversibly and persistently, via some external stimulus. Materials which
permit this kind of variability are known as ''chromogenic'' (Granqvist
1990
;
Lampert and Granqvist
1990
).
There are several kinds of chromogenic materials (Granqvist
1990
; Lampert
and Granqvist
1990
; Smith and Granqvist
2010
). The photochromic ones may be
the most well known; their optical transmittance drops when subjected to ultra-
violet light, and they return to their original properties in the absence of such
irradiation. Photochromic glass and plastic are widely used in sunglasses. Ther-
mochromic materials have different optical properties depending on their tem-
perature. Electrochromic materials, which form the basis for the electrochromic
glazings that this chapter is about, change their properties when subjected to an
applied electrical current or voltage. There are also gasochromic materials with
optical properties depending on their gaseous ambience.
1.2 Energy Efficiency and Other Benefits of Electrochromic
Glazing
Energy efficiency in the built environment is a difficult topic. It is closely tied to
user aspects such as thermal comfort (Fanger
1970
), which is perceived differently
among different persons. Nevertheless, the energy efficiency of electrochromic
glazing
has
been
discussed
repeatedly
and
very
positive
effects
have
been
documented.
A very simplistic model for the energy efficiency was put forward several years
ago by Azens and Granqvist (
2003a
). It is based on solar irradiation intensity,
achievable transmittance modulation, and expected use of buildings and argues
that the annual energy savings would be *170 kWh per square meter of (south
facing) window area, i.e., the same as good solar cell panels with 17 % efficiency
would generate if they were oriented in the same way as the windows. There are
several other more detailed, supporting estimates based on elaborate assessments
of the energy performance of commercial buildings; these assessments also
highlight aspects such as downsizing of air-conditioning systems, lighting benefits,
and user satisfaction (Clear et al.
2006
; Lee et al.
2006
,
2012a
; Zinzi
2006
; Lim
et al.
2013
). An unpublished study has indicated that a modern commercial
building in Middle and Northern Europe could function without active air cooling
if it were equipped with electrochromic glazing (Ramezani and Nybom
2009
). A
recent study by Gillaspie et al. (
2010
) argued that the introduction of well-insu-
lated electrochromic glazing in commercial and residential buildings in the USA
would be able to save as much as *4.5 % on the national energy expenditure.
The energy savings possible with electrochromic glazings depend critically on
their control strategy and orientation (Azens and Granqvist
2003a
; Assimakopoulos
et al.
2004
; Jonsson and Roos
2010
) and should account for whether persons are
