Glass gets smarter

Glass panels with their own tint controls look to be among the next big energy efficiency trends.

There are a variety of processes and materials that go into these windows which can instantly tint themselves or switch from transparent to opaque. Applications include architectural and vehicle windows, aircraft windows, skylights and sunroofs.

Electrochromic displays are probably the most widely used technology for large-area building windows and automotive mirrors. They use electrochromism, the phenomenon displayed by some materials of reversibly changing color when a burst of charge is applied. The most widely used electrochromic material in smart windows is tungsten oxide. It only requires power during switching and typically switches with 1 to 5 V. Many designs also have long memory, up to 12 or even 48 h. EC devices use an ion-containing material (electrolyte) close to the EC layer, plus transparent layers for setting up a distributed electric field. Applying a voltage forces ions to shuttle back and forth into the EC layer.

The construction of an EC display has been compared to that of a thin, transparent battery. It consists of two transparent conductors, an electrolyte or ion conductor, a counter electrode, and an EC layer.

Another kind of smart glass technology called suspended particle devices (SPDs),a technology developed by Research Frontiers Inc. in Woodbury, N.Y. Here, a thin-film laminate of rod-like particles suspended in a fluid resides between two glass or plastic layers. When no voltage is applied, the suspended particles lie in random orientations and tend to absorb light, so that the glass panel looks dark (or opaque), blue or, in more recent developments, grey or black. An applied voltage aligns the suspended particles and lets light pass. SPDs can be manually or automatically “tuned” to precisely control the amount of light, glare and heat passing through.

When installed in architectural glass, they can reduce the need for A/C or heating. One of the most recent developments in this area is SPD-Smartglass, billed by Research Frontiers as is the world’s highest performing dynamic glazing. It is actually a second-generation product. The first generation emerged around 2001 when SPD glass was installed in a Lear jet and a few residential applications. In 2007, Hitachi Chemical produced an improved film under license from Research Frontiers. This film is now going into commercial products, says Research Frontiers Market Development Director Greg Sottile.

Among the first uses for the glass is in the sun roof on the Mercedes Benz SLK, announced this past spring. Sottile says the company has licensed the product to firms in the architectural and marine industries as well, but calls the automotive area the "most developed." The SPD glass available today goes from less than 1% of visible light transmittance to above 65% for light SPD film. Switching states takes place in a few seconds, and the glass uses about 1.8 mA/ft² from an ac supply.

(Though commercial applications of SPD glass are a relatively recent development, the company has been around since the 1960s. Edwin Land of Polaroid fame originally conceived the technology but never successfully commercialized it. Research Frontiers took up where Land left off.)

Finally, a third technology in this area is polymer dispersed liquid crystal (PDLC) which has been commercialized since the 1980s. Here a polymer and liquid crystal form a film. The 5-mm liquid crystal droplets are encapsulated within an index-matched polymer matrix that is fabricated between two sheets of transparent conductor-coated polyester or glass that serves as electrodes.

In the off state, the device appears translucent white. An electric field aligns the liquid crystal droplets with the field and the device becomes transparent. Typically these devices operate between 24 and 100 Vac. They generally consume less than 5 W/m², but need continuous power to be clear.

Research Frontiers Inc., www.smartglass.com