Switchable Glazing Windows - 9/2/2002 - Home Condo Doors Windows

Switchable Glazing Windows

Change the light transmittance, transparency, or shading of windows

Electrochromic windows can block the glare of the sun or provide instant privacy with the flip of a switch. Electrochromic windows are part of a new generation of technologies called switchable glazing-or "smart" windows-which change the light transmittance, transparency, or shading of windows in response to an environmental signal. Electrochromic windows change from transparent to opaque or shaded by an electric signal. Potential uses for electrochromic technology include privacy, daylighting control, and solar control in windows and skylights, making traditional window shades and blinds obsolete.

A variety of electrochromic technologies and media have been developed. One type, developed by Lawrence Berkeley Laboratory, consists of transparent outside layers of electrically conductive film that has inner layers which allow the exchange of ions. When a voltage is applied across the outer conductive layers, ions move from one inner layer to another, causing a change in tinting of the electrochromic material. Reversing the voltage causes the window to become clear again. The windows can have variable light transmittance during operation, from 5 to 80 percent. This type of electrochromic glazing has "memory" and does not need constant voltage after the change has been initiated. Further, it can be tuned to block certain wavelengths, such as infrared (heat) energy.

Another electrochromic technology is called suspended particle display (SPD). This material contains molecular particles suspended in a solution between the plates of glass. In their natural state, the particles move randomly and collide, blocking the direct passage of light. When switched on, the particles align rapidly and the glazing becomes transparent. This type of switchable glazing can block up to about 90 percent of light.

The National Renewable Energy Laboratory (NREL) has developed a switchable glazing based on electrochromic technology that switches from clear to tinted based on the amount of sunlight it receives. This type of window can be used for controlling unwanted heat in skylights and windows without an electrical hookup.

While not considered a true electrochromic window by some, one U.S. window manufacturer has introduced a residential window with a liquid crystal glazing that switches from clear to milky white. The windows do not significantly reduce the amount of light transmission, but only the transparency of the windows. This type of glazing, however, increases the cost of windows by about $90 per square foot of glass and requires a steady current when keeping the glass in the clear mode.

Electrochromic films require an electrical hookup that is not required for other types of solar-control window films. The durability of some electrochromic films, particularly liquids, is still not known. Work continues on development of a compound that improves durability, is less costly, and solves technical problems related to the ion-storage layer. One commercially-available system is used primarily for office interior partitions.

	Switchable glazings such as electrochromic windows can reduce energy costs for cooling by reducing direct solar gains into the home during the summer. In some northern climates this benefit can make it possible to eliminate the use of mechanical A/C systems altogether.
	The environmental benefits include the reduced use of energy and the reduced reliance on window coverings.

The cost of electrochromic windows can be from 4 to 6 times that of a standard window. These costs are expected to decrease significantly when manufacturing techniques have improved and quantities have increased.

The electricity used for switchable glazings is often minimal compared with other energy savings. No other operational costs are required for the lifetime of the product.

In residential buildings, electrical devices operating from line voltage current must be listed with an approved testing agency. One commercially available product requires that wiring be performed by a qualified electrician, a power conditioner be used, a junction box be located within six feet of the window, and wiring pass through a ground fault circuit interrupter. For some electrochromic windows, a low voltage circuit only is necessary, simplifying the installation process.

Not Applicable

Installation is more complicated than ordinary windows since electrochromic windows require an electrical source and switch. When electrochromic windows are introduced, they will likely require installation by a licensed electrician.

Warranty information is unavailable at this time

The Solar Energy Industries Association claims that "smart" windows can save as much as 50 percent of a building's energy use. In the summer months, electrochromic windows can block ultraviolet rays and radiant heat from direct sunlight passing through windows and skylights. This helps lower cooling loads by reducing solar heat gain. They can also help slow the fading of interior furnishings by blocking out the sun's ultraviolet rays. Electrochromic windows offer the flexibility of control not available in photochromic or thermochromic windows-windows that turn opaque when exposed to light or warm temperatures. Electrochromic glazing technologies are expected to be relatively expensive when initially commercialized, but decline in cost as the technology and manufacturing process matures.