by B.J. Novitski

A window is one of the most complex components in a building. It gives us light, views, fresh air, and the sun's warmth. Yet at times trying to balance these benefits works against the goals of comfort and energy savings. Having too few windows deprives workers of psychologically important vistas and increases the need for electric lighting. But too much direct sunlight can cause glare and increase the cooling load. And the delicate balance among all these factors changes throughout the day and year.

Here to help cope with this complexity is the so-called "smart window," which can change its light and heat transmission characteristics.

Researchers at the Lawrence Berkeley National Laboratory (LBNL) have been testing a promising version of smart windows, based on electrochromic coatings. They believe these glazings will be the next major advance in energy-efficient windows, changing windows from an energy liability to an energy source for the nation's building stock.

These glazings are treated with a thin multilayer coating. When subjected to a low voltage, an electrochemical reaction causes the coating to reversibly switch from a clear to a colored appearance. Here's one scenario of how it might work in an office setting:

A photosensor, usually mounted on the ceiling, measures the amount of available light on the workplane (a desktop or a computer screen) and compares it to a specified level. When the incoming daylight is insufficient, the electric lights are tuned to illuminate the room, and the glazing is switched to its clear state if there is no direct sun. As the daylight increases, the glazing switches gradually to its colored state and the electric lights dim down.

When direct sunlight enters the room and threatens glare on the workplane, the photosensor triggers the controller that applies the voltage to the glazing. The glazing begins to darken, gradually diminishing the incoming light. When there is no longer direct sunlight in the room, the process reverses, and the glazing becomes clear again. As the day darkens, the electric lights "dim up" until they alone are once again fully satisfying the need for illumination.

Electrochromic glazing can switch from clear to dark to adjust to incoming daylight and solar radiation. When the technology comes to market, it is expected to be available in more neutral colors. Photo: Eleanor Lee/LBNL Researchers at LBNL conducted their testing in two adjacent rooms in an office building designed by Kaplan McLaughlin Diaz in Oakland, California. Photo: Eleanor Lee/LBNL   Click on thumbnail images to view full-size pictures.