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.

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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.

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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.

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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.

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.

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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.