WINDOWS OF
THE FUTURE
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, with an ion conductor layer in-between. When
a low voltage is applied across the conductors, moving ions from the
counter-electrode 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 become diffused.
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 offer the next major step in windows that are increasingly
sophisticated and energy efficient.
For more information on this topic, visit the source of this
article at http://www.consumerenergycenter.org/home/windows/windows_future.html
