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How Smart Windows Work

 

Dynamic glazing—coated plastic or glass that changes tint in response to light, heat, or electricity—has been around for decades as a feature of eyeglasses, mood rings, and digital watches. Similar technologies have been developed more recently to make dynamic glass at a much larger scale for architectural applications. These so-called smart windows can help conserve energy by sometimes blocking unwanted heat gain to reduce the need for cooling and at other times letting in natural daylight and passive solar heat to reduce the need for artificial lighting and space heating.

Smart windows are intended to take the place of louver systems and other shading devices; unlike most shades, however, smart windows let some daylight in and allow views to the outdoors even when fully deployed, and they avoid visual clutter and surfaces that collect dust. Although there are several types of dynamic glazing, the two primary technologies currently in use are electrochromic (responsive to current) and thermochromic (responsive to heat).

With electrochromic glazing, such as SageGlass, building occupants use a switch to manually control when the glass is tinted and when it is not; sensors can also be used to change the glazing automatically based on weather conditions. While these windows require electricity, the power draw for tinting is slight, and the amount needed to keep them in their tinted state is smaller still. By blocking infrared radiation (SageGlass blocks almost 100% of radiant solar heat gain in its darkest state), the windows reduce the need for mechanical cooling and can conserve far more energy than they consume. Electrochromic glazing also blocks visible light—SageGlass at its darkest lets in just 2% of available daylight—which in some situations could increase use of artificial light and offset these savings. However, an electrochromic glazing system can be programmed at intermediate tinting levels so that views and natural daylight are available even when the windows are darkened (but more visible light also brings more heat gain with it).

Electrochromic windows require wiring and electricity, adding extra expense; also, if occupants can control them, then occupants can also forget about them—potentially reducing the energy savings.

Thermochromic glazing, such as Pleotint, is designed to block unwanted solar heat gain on warmer days and allow desirable solar heat in on colder days by automatically darkening when the sun heats the glass and becoming clear again when the glass is cold. While they can provide energy savings, require no wiring, and don’t rely on occupants to operate them, these windows also do not permit the level of control that an electrochromic system provides, so applications are more specialized. These products are also newer and have not been tested extensively in the field.

March 1, 2012

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