Windows of the Future

The emerging concept of the “window of the future” is more of a multi-functional “appliance in the wall” rather than simply a static piece of coated glass.

These façade systems include switchable windows and shading systems such as motorized shades, switchable electrochromic or gasochromic window coatings, and double-envelope window-wall systems that have variable optical and thermal properties which can be changed in response to climate, occupant preferences, and building-system requirements. By actively managing lighting and cooling, smart windows could reduce peak electric loads by 20 to 30 percent in many commercial buildings, increase daylighting benefits throughout the United States, improve comfort, and potentially enhance productivity.

Among the most promising switchable window technologies today is the electrochromic (EC) window, which has the ability to change from clear to a colored transparent state without compromising views and consists of an electrochromic coating (typically five layers, totaling about 1 micron in thickness) deposited on a glass substrate. The electrochromic stack consists of thin metallic coatings of nickel or tungsten oxide sandwiched between two transparent electrical conductors. When a voltage is applied between the transparent electrical conductors, a distributed electrical field is set up. This field moves various coloration ions (most commonly lithium or hydrogen) reversibly between the ion storage film through the ion conductor (electrolyte) and into the electrochromic film. The effect is that the glazing switches between a clear and a transparent, Prussian-blue-tinted state with no degradation in view, similar in appearance to photochromic sunglasses.

The main advantage of EC windows are that they typically only require low-voltage power (0 to 10 volts DC), remain transparent across their switching range, and can be modulated to any intermediate state between clear and fully colored. Switching occurs through absorption (similar to tinted glass), although some switchable reflective devices are now in research and development. Low-emittance coatings and an insulating glass unit configuration can be used to reduce heat transfer from this absorptive glazing layer to the interior. Typical EC windows have an upper visible transmittance range of 0.50 to 0.70 and a lower range of 0.02 to 0.25. The solar heat gain coefficient (SHGC) ranges from 0.10 to 0.50. A low transmission is desirable for privacy and for control of direct sun and glare, potentially eliminating the need for interior shading. A high transmission is desirable for admitting daylight during overcast periods. Therefore, the greater the range in transmission, the more able the window is to satisfy a wide range of environmental requirements.

Excerpted from (www.commercialwindows.umn.edu) and the book “Window Systems for High-Performance Buildings,” W.W. Norton & Co., 2004, authored by John Carmody, Eleanor S. Lee, Stephen Selkowitz, Dariush Arasteh, and Todd Willmert. Carmody is director at the Minneapolis-based University of Minnesota’s Center for Sustainable Building Research (CSBR) in the College of Design.