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Smart Windows | Switchable Glazing | Electrochromic Windows Print Bookmark and Share

In recent years, new advances in window technology have resulted in better performing windows. Warm edge spacers, special coatings, and gas fills are just a few of the innovations that have reduced heat transfer and improved energy efficiency in windows, historically one of the biggest drains on a house's thermal performance.

One of the more recent and novel innovations in windows is electrochromic windows. Electrochromic windows are one of the new wave of modern technologies referred to as "switchable" glazing, or "smart" windows, that can help homeowners save money and energy.

What is "switchable" glazing, and why is it useful?

Just the flip of a switch, and electrochromic windows can block the glare of the sun. Switchable glazing can change the light transmittance, transparency, or shading of a window. The flipped switch doesn't have to be literal electrochromic windows can respond to a variety of cues, such as an environmental signal as well as an electric current.

Electrochromic windows can automatically monitor and control the amount of light and solar energy that can pass through a window, and this can translate into cost and energy savings for consumers.

Electrochromic windows do this by responding to external triggers such as an electrical control, or to natural stimuli such as sunlight and temperature. Once they receive a signal, they can change from transparent to tinted, just by applying an electrical current.

Homes with electrochromic windows and switchable glazing can practice daylighting control, glare control, solar heat control, and fading protection.

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How does switchable glazing work?

Although new, there are already multiple methods for switchable glazing to work. One way is to darken windows by using a small electrical voltage on the window, and to lighten them by reversing (and removing) the voltage.

The electrochromic glazing has "memory," which means that once the tint has been initiated, the voltage doesn't have to be maintained. (An important point for the energy conscious.) During operation, light transmission can vary from 5% to 80%, and the film can even be fine-tuned to block out select wavelengths, such as infrared (heat) energy.

Another method uses something called a liquid crystal suspended particle device (SPD). This fancy-sounding gizmo contains molecular particles suspended in a solution between glass plates. These particles zoom around rapidly and collide in their natural state, and this activity effectively blocks the passage of about 90% of the light. When the particles are energized, they align swiftly and render the glazing transparent, letting the light in.

For those with energy (electricity) concerns, there are two types of solar-powered electrochromic windows, created by the National Renewable Energy Laboratory. As you would expect, these windows vary the tinting based on the amount of sunlight shining on them. Controlling the tint controls the amount of heat that enters through a window, so this development is probably particularly interesting to homeowners with skylights, or who are trying to reduce their electrical demands.

Another residential window with electrochromic glazing switches from clear to milky white via a liquid crystal glazing. This glazing is more about providing privacy by reducing transparency. It does not dramatically reduce the light transmission. However, this glazing requires a constant current to keep the glass in a clear state.

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What are the benefits of switchable glazings?

Switchable glazings can help reduce energy usage. Switchable glazings can lower energy costs for cooling a home by reducing direct solar gains, especially in the summer. They can block UV rays and radiant heat (particularly useful for skylights) and lower cooling loads. In cooler northern climates, electrochromic windows can effectively eliminate the need for mechanical air cooling systems altogether.

By decreasing the solar heat gains, electrochromic windows can also help eliminate the need for window coverings such as shades or blinds. That means that homeowners can not only enjoy their surroundings more (keeping a room comfortable, and not too hot), but they can also enjoy their views during the day.

Switchable glazings can also protect a home's interiors from damaging UV rays, so that carpets, furniture, and artwork don't experience fading from the sun. Electrochromic windows also offer more control than windows that are photochromic or thermochromic (these are windows that turn opaque when exposed to light or warm temperatures).

While many might think that the electricity required to operate the switchable glazings is a negative attribute, manufacturers and supporters point out that the energy required to run the windows is minimal. Low-voltage products use less power than line voltage products. Plus, they use virtually no energy to maintain a clear state. Moreover, the energy that is saved by reducing cooling needs and energy expenditures far outweighs the energy use by electrochromic windows.

Solar powered windows go a step further, requiring no purchased electricity to operate the windows whatsoever. In either case, electrochromic windows do not make any other energy demands during their lifetimes just what's necessary to make the switchable glazings functional.

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What are the downsides to switchable glazings and electrochromic windows?

Electrochromic windows can reduce energy usage and sun damage, but they are complex windows that aren't exactly a piece of cake to install. Electrochromic windows that utilize electricity require a non-standard electrical hookup and unconventional wiring (including a switch). That means that homeowners may have to hire an electrician as well as a window installer, or a general contractor or other tradesman who can handle all facets of installation.

Switchable glazings are commercially available, but inconsistently. Few can be found at major retailers such as home improvement stores. Some are for residential use only, some are only available through the manufacturer or select vendors, and so on.

Probably the biggest disadvantage of all is the cost. Electrochromic windows cost significantly more than a standard window, as much as 2 to 3 times more. Liquid crystal glazing can add as much as $90 per square foot to the glazing! As with most 'new' technologies, the price point for these high-tech windows is expected to drop to more reasonable levels as supply and demand increase, technology improves, and manufacturing becomes less involved.