Switchable windows that maintain view out

• Electrochromic windows (described in the previous section) are the most promising switchable window of today. The main advantages are that they are efficient at rejecting solar heat gains and admitting daylight, require low-voltage power to switch, can require no power to maintain the EC in any switched state for several days (depending on material composition), and preserve a clear view out. Independent tests have indicated that electrochromic windows can have long-term durability.
• Photochromic materials slowly change their tint in response to light intensity. They are used in eyeglasses that change from a clear appearance when indoors to a tinted appearance when in the bright outdoors. Large photochromic windows are not commercially available.
• Suspended particle device (SPD) windows switch instantly from a dark blue to a clear slightly hazy state with an applied AC voltage. When unpowered, the window is tinted. When powered (100 V AC or 0.05-0.5 W/ft² of glass), the window can be set to any intermediate state between clear and fully colored. The SPD window is laminated, can be fabricated in up to 4x8 feet sheets, and is offered in curved and flat shapes. Long-term durability (e.g., greater than 3-5 years) has not been independently verified so this window type is not promoted in this guide. This product is commercially available (e.g., ThermoView AlterLite windows).

Switchable windows that don’t maintain view

• Thermochromic materials slowly change from a clear state when cold to a more diffuse, white translucent state when hot. Prototype windows have been tested but are not commercially available.
• Liquid crystal device windows are translucent when in an unpowered state and become instantly clear (with a noticeable haze) when power is applied. Power must be applied continuously for the window to remain clear (24-100 V AC or 0.5 W/ft² of glass). The window has a high daylight and solar heat gain transmittance and is therefore of limited use in commercial buildings. The window is commercially available. While typically used for high-end interior applications, ultraviolet (UV)-stable formulations now permit exterior applications but the cost remains high.
• Reflective hydride window coatings are a relatively new type of electrochromic device that switches from a transparent to a reflective appearance with the injection of hydrogen gas. This coating is still in the R&D stage. More information can be found at: http://windows.lbl.gov/materials/chromogenics/default.htm

This website focuses on tungsten-oxide (WO₃) EC windows such as that produced by SAGE Electrochromics, Inc. At the time this website was produced (Spring 2006), this type of EC window was only just introduced by SAGE Electrochromics, Inc. (an all solid state lithium based WO₃ device) to the commercial market in sizes of up to 42.5 by 60 inches with on-off control (fully bleached and fully colored states only). Products from other manufacturers may enter the market shortly. An EC window product was offered in Germany by Flabeg GmbH & Co. (polymer-laminated WO₃ device) in 1997 in sizes up to approximately 3x6 feet with intermediate-state controls (ability to modulate the EC window to any tinted state) but is no longer available (for unknown reasons).