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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 counterelectrode to the electrochromic layer cause the assembly to change color. Reversing
the voltage moves ions from the electrochromic layer back to the counter-electrode
layer, restoring the device to its previous clear state. The glass may be programmed
to absorb only part of the light spectrum, such as solar infrared.
Early research indicates that the technology can save substantial amounts of energy
in buildings, and electrochromic glazings may eventually replace traditional solar
control technology such as tints, reflective films and shading devices.
Liquid Crystal Windows
The first commercially available "smart window," liquid crystal windows are used for
privacy control. They do not provide energy savings.
In this window's normal "off" condition, the glazing is a translucent milky white.
When an electric current is applied, however, it turns slightly hazy clear. The switch
between the two states is nearly instantaneous.
The technology works this way: two layers of film enclose a layer of tiny liquid
crystals. This assembly is laminated between two pieces of heat-treated glass. Both
faces of the film are covered with a transparent, electrically conductive metal
coating. These conductive coatings are wired to a power supply.
When the power is off, the liquid crystals are randomly scattered. Light entering the
glazing does not have a clear path out, and the window is a translucent milky white.
When an electric current is applied between the two conductive coatings, the liquid
crystals align neatly and you can see through the window.
Other than the diffusion of light, the optical properties of the two states are nearly
identical -- the window lets in nearly the same amount of light and solar heat
whether it's on or off. Because there is little change in performance properties and
because it requires constant energy to maintain its clear state, this liquid crystal
window provides no energy saving benefits.
Thermochromic Windows
As the prefix thermo- implies, heat causes thermochromic windows to alter their
properties. In response to changes in the ambient temperature, clear thermochromic
glazings becomes diffused.
Time lapse luminance images of the tested on a clear sunny winter solstice day with a south-facing EC
window at Tv=0.05. View of LCD VDT from the seated position of the occupant. Note that the maximum
value on the false color luminance scale differs for each image. The sun orb is visible at 13:00 and 14:00.
Several thermochromic technologies are being explored, but gel-based coatings
seem to be the most promising. "Cloud Gel, " a product now on the market, is a thin
plastic film that can be incorporated into almost any window assembly. The response
temperatures of "Cloud Gel" can be adjusted depending on need and location.
In addition to automatically changing from clear to diffused in response to heat, the
glazings also turn white and reflective, reducing the transmission of solar heat. That
can reduce air conditioning costs significantly when it's hot outside. Because you can
no longer see through the window once it loses its transparency, this glazing is
probably best suited for skylights rather than view windows.
Photochromic Windows
Still in the development stage, photochromic windows respond to changes in light,
much like sunglasses that darken when you move from a dim light to a bright one.
While this type of technology may seem like a good idea, it has its drawbacks for
saving energy. Photochromic windows work well to reduce glare from the sun, but
they don't control heat gain. That's because the amount of light that strikes a
window doesn't necessarily correspond to the amount of solar heat it absorbs.
Because the sun is lower in the sky during the winter months, for example, its rays
may strike a window more intensely in the cold season than in the summer, when
the sun is higher in the sky. In this case, a photochromic window would darken more
in the winter than in the summer, although winter is the time when solar heat would
be beneficial.
Another problem is that, while this technology works fine on small, eyeglass-sized
pieces of glass, it has yet do be done successfully on a large-scale, commercial level
for window-sized pieces.
Despite some problems, "smart windows" hold the promise of reducing energy
demand and cutting air conditioning and heating loads in the future. They are the next major step in windows that are increasingly sophisticated and energy efficient.
Various information courtesy of the Department of Energy and ENERGY STAR www.energystar.gov,
and the California Energy Commission www.consumereneravcenter.org
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