fiber-optic-led-dichroic-glass-wall
radiant color film which can be applied to a surface.
Smart Windows
The emerging concept for the window of the future is more as a multifunctional "appliance-in-the-wall" rather than simply a static piece of coated glass. These facade 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 that 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 20-30 percent in many commercial buildings, increase daylighting benefits throughout the United States, improve comfort, and potentially enhance productivity in homes and offices.
Photochromics
Photochromic materials change their transparency in response to light intensity. Photochromic materials have been used in eyeglasses that change from clear in the dim indoor light to dark in the bright outdoors. Photochromics may be useful in conjunction with daylighting, allowing just enough light through for lighting purposes, while cutting out excess sunlight that creates glare and overloads the cooling system. Although small units have been produced in volume as a consumer product, cost-effective, large, durable glazings for windows are not yet commercially available.
Thermochromics
Thermochromics change transparency in response to temperature. The materials currently under development are gels sandwiched between glass and plastic that switch from a clear state when cold to a more diffuse, white, reflective state when hot. In their switched-on state, the view through the glazing is lost. Such windows could, in effect, turn off the sunlight when the cooling loads become too high.
Liquid Crystal Device Windows
A variant of the liquid crystal display technology used in wristwatches is now serving as privacy glazing for new windows. A very thin layer of liquid crystals is sandwiched between two transparent electrical conductors on thin plastic films and the entire emulsion or package (called a PDLC or polymer dispersed liquid crystal device) is laminated between two layers of glass. When the power is off, the liquid crystals are in a random and unaligned state. They scatter light and the glass appears as a translucent layer, which obscures direct view and provides privacy. The material transmits most of the incident sunlight in a diffuse mode, thus its solar heat gain coefficient remains high.
Suspended Particle Device (SPD) Windows
This electrically controlled film utilizes a thin, liquid-like layer in which numerous microscopic particles are suspended. In its unpowered state the particles are randomly oriented and partially block sunlight transmission and view. Transparent electrical conductors allow an electric field to be applied to the dispersed particle film, aligning the particles and raising the transmittance.
Electrochromic Windows
The most promising switchable window technology today is electrochromic (EC) windows. An electrochromic coating is typically five layers, about one micron thick, and is 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 transparent prussian blue-tinted state with no degradation in view, similar in appearance to photochromic sunglasses.
The main advantages of EC windows is that they typically only require low-voltage power (0-10 volts DC), remain transparent across its 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 0.50-0.70 and a lower range of 0.02-0.25. The SHGC ranges from 0.10-0.50. 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.
Gasochromic Windows
Gasochromic windows produce a similar effect to electrochromic windows, but in order to color the window, diluted hydrogen (below the combustion limit of 3 percent) is introduced into the cavity in an insulated glass unit. Exposure to oxygen returns the window to its original transparent state. To maintain a particular state, the gap is simply isolated from further changes in gas content. The optically active component is a porous, columnar film of tungsten oxide, less than 1 micron thick. This eliminates the need for transparent electrodes or an ion-conducting layer. Variations in film thickness and hydrogen concentration can affect the depth and rate of coloration.
Visible transmittance can vary between 0.10-0.59 with a SHGC range of 0.12-0.46. Transmittance levels of less than 0.01 for privacy or glare control are possible. An improved U-value can be obtained with a triple-pane, low-E system (since one gap is used to activate the gasochromic). Switching speeds are 20 seconds to color and less than a minute to bleach. The gas can be generated at the window wall with an electrolyser and a distribution system integrated into the facade. Gasochromic windows with an area of 2-by-3.5 feet are now undergoing accelerated durability tests and full-scale field tests and are expected to reach the market in the near future.
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