Space Technology

By: Marilyn Taylor
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.

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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