New technologies are producing increasingly energy efficient windows. Already on
the market are "super-windows," boasting triple layer designs, with two low-E
coatings and spaces filled with mixtures of argon or krypton gases.
A new generation of windows, however, is being called "smart windows" because
they adapt to changing conditions.
A few "smart windows" are already commercially available, and others are being
developed in research labs. These windows change properties -- like their shading
coefficients and visible transmittances — in response to either an electric charge or
an environmental signal such as a change in light level.
Depending on the mechanism that initiates the change in the window, these
"switchable glazings" fall into four categories: electrochromic, liquid crystal,
thermochromic, and photochromic.
 Electrochromic Windows
Flip a switch and an electrochromic window can change from clear to fully darkened
or any level of tint in-between.
The technology has been suggested for cars, where with a touch of a switch the
driver can tint the mirror or sunroof. In buildings, the changeable windows allow for
privacy, to cut down glare, and to ward off increases in solar heat.
The action of an electric field signals the change in the window's optical and thermal
properties. Once the field is reversed, the process is also reversed. The windows
operate on a very low voltage — one to three volts — and only use energy to change
their condition, not to maintain any particular state.
To make an
electrochromic
window, a thin,
multi-layer
assembly is
sandwiched
between
traditional pieces
of glass. The two
outside layers of
the assembly are
transparent
electronic
conductors. Next
is a counter-
electrode layer
and an
electrochromic layer,
Electrochromic windows in a fully colored state (left column), intermediate
tinted state (middle column), and clear bleached state (right column).
with an ion conductor
layer in-between.
When a low voltage is applied across the conductors, moving ions from the counter-
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