ElectroChromic Windows: Change the light transmittance, transparency, or shading of windows
Switchable glazing can change the light transmittance, transparency, or shading of windows in response to an environmental signal such as sunlight, temperature or an electrical control.
Electrochromic windows change from transparent to tinted by applying an electrical current. Potential uses for electrochromic technology include daylighting control, glare control, solar heat control, and fading protection in windows and skylights. By automatically controlling the amount of light and solar energy that can pass through the window, electrochromic windows can help save energy in residences.
A variety of electrochromic technologies and media have been developed. One type is darkened by applying a small electrical voltage to the windows and lightened by reversing the voltage. Light transmittance during operation varies from five to 80 percent. Once the change in tint has been initiated, the electrochromic glazing has “memory” and does not need constant voltage to maintain the tinting. Further, the film can be tuned to block certain wavelengths, such as infrared (heat) energy.
Another switchable technology, the liquid crystal suspended particle device (SPD), contains molecular particles suspended in a solution between plates of glass. In their natural state, the particles move randomly and collide, blocking the direct passage of light. When energized, 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 two types of solar-powered electrochromic devices that vary tinting based on the amount of sunlight it receives. The technology shows promise for controlling unwanted heat in skylights and windows without using an external electric source.
One U.S. window manufacturer has introduced a residential window with a liquid crystal glazing that switches from clear to milky white. Although the windows do not significantly reduce the amount of light transmission, they provide privacy by reducing transparency. This type of glazing requires a steady current to keep the glass in the clear state.
Amid concerns over the durability of electrochromic technologies, the National Renewable Energy Laboratory has been testing electrochromic devices in accordance with ASTM standard E-2141 (Standard Test Methods for Assessing the Durability of Absorptive Electrochromic Coatings on Sealed Insulating Glass Units). At least one manufacturer (Sage Electrochromics) produces a ceramic thin film electrochromic device that has shown excellent durability under test conditions.
Ease of Implementation
Electrochromic films require an electrical hookup that is not required for other types of solar-control window films and therefore requires unconventional wiring at windows and may require coordination of electrical and carpentry trades.
Products are commercially available through various means. For example, SageGlass® is available for purchase by residential window manufacturers and commercial glazing contractors and is available in exterior windows, doors, and skylights. Liquid crystal privacy glazing is available in the United States for residential use but is used primarily for interior office partitions. SPD windows are available through various suppliers.
The cost of electrochromic windows can be from 2 to 3 times that of a standard window. These costs are expected to decrease significantly when manufacturing techniques have improved and quantities have increased. Liquid crystal glazing adds about $90 per square foot to the glazing.
The electricity used for switchable glazing is minimal compared with other energy savings. Low voltage products typically use less power than line voltage products and use virtually no power to maintain the glass in the clear state. PV-powered products do not require purchased electricity. No other operational costs are required for the lifetime of the product.
U.S. Code Acceptance
Part 8 of the International Residential Code and the National Electric Code (NFPA 70) cover electrical requirements in residences. Electrical devices operating on line voltage in residences must be listed with an approved testing agency such as UL. Manufacturers may outline other requirements for installation such as the use of power conditioners, GFCI circuits, or others.
Several performance impact studies on electrochromic windows are available from The Lawrence Berkeley National Laboratory
Installation is more complicated than ordinary windows since electrochromic windows require an electrical source and switch and integration of the electrical and carpentry trades. The installation process for low voltage electrochromic windows does not require a licensed electrician.
The National Institute of Standards and Technology believe that “smart” windows can reduce a commercial building’s energy use by 30 to 40 percent. In the summer months, electrochromic windows can block ultraviolet rays and radiant heat from direct sunlight from passing through windows and skylights to help lower cooling loads. 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). The cost of electrochromic glazing technologies, while currently high, continues to decline as the technology and manufacturing process matures.