The pleasure afforded by a lovely view has always come at the expense of thermal efficiency, glare, and compromised security, a meager tradeoff for the many favorable attributes that windows afford. They frame scenes of the natural and manmade environment, and they let natural sunlight permeate our lives. But they also impose the need for elaborate draperies or blind systems to allow privacy, mitigate glare, and prevent heat from escaping in winter and getting trapped in summer.
Glazing technology, window operability, and thermal performance have improved considerably, especially over the last century. Insulated glass, layered with two or more panes for better insulation and sound absorption, has been in use since the 1960s. Likewise, low-E coatings for blocking bad solar wavelengths have become the norm. But thermal efficiency and the desire to minimize glare are issues that glazing manufacturers and building owners continue to contend with.
Not content with these limitations, a four-year-old window manufacturer in Faribault, Minn., has taken many of these considerations into account in an effort to refine a product known as Sage Electrochromic Glass. Similar in structure to traditional glazing, Sage glass begins to blur the distinction between wall and window, allowing users the possibility of tinting their windows lighter or darker depending on the level and quality of light outdoors. Like low-E glass, the windows modulate the transmission of visible light and solar heat. They change state at the click of a switch, say if you want to see out but don't want too much light to come in.
The light-transmitting properties of smart windows—whether colored, translucent, opaque, or mirrored—typically allow for two opposing states. With electrochromic technology, a temporary burst of voltage triggers a solid state change. Each pane of Sage glass contains five nanoscale layers—less than 1/50 of the width of a human hair—of an inorganic ceramic material derived from tungsten oxide that exhibits a phenomenon in which the color of the material reverses in response to an applied voltage. During a phase change, ions migrate across the five layers, causing the embedded material to tint and absorb light. When the voltage is reapplied, these ions travel back across the layers, and the glass returns to its original clear state. Depending on the size and temperature of the window, the phase change can take up to five minutes, beginning at the outer edges and gradually reaching the center.
Sheets of electrochromic glass going through a large-format glass cutter to be cut down to size for standard or specific applications.
The structural configuration of each pane is similar to that of insulated glass, and it is made by floating the glass on a bed of molten tin. But Sage inserts a thin electrochromic film using a proprietary vacuum-deposition process called sputtering, which places the material between two outer layers before encasing it in an insulating glass unit (IGU). Windows are available in four standard colors: classic, sea green, cool-view blue, or clear-as-day gray. Like most insulated glass, color is added using an inboard lite (an additional layer of glass) that can be fritted, laminated, or custom-colored (though color consistency is difficult to control).
In the year ahead, Sage plans to launch an electrochromic window embedded with strategically-placed swatches of photovoltaic (PV) cells, so that the windows will self-generate the power needed to implement the phase change. “The energy used is very minimal. It only needs three to four volts of DC power to change state. In other words, you can power 1,500 square feet of glass—approximately 100 windows—with the equivalent of a 60 watt light bulb,” says John Van Dine, the company's founder and president. More importantly, the windows can reduce the need for artificial lighting by 60 percent when used optimally, and they can reduce a building's overall energy footprint up to 30 percent, according to studies performed by the University of California, Berkeley's Windows and Day-lighting Group at Lawrence Berkeley National Laboratory.
Van Dine formed Sage Glass in 1989 after working for 12 years as a chemical engineer in thin-film photovoltaics. Under contract with the U.S. federal government—and with grants from numerous agencies, including the Department of Defense, the Department of Commerce, the Department of Energy, and the National Science Foundation—Sage entered into a lengthy research and development phase that lasted through the 1990s. “We were looking for electronic devices and structures that would be appropriate for architectural glazing, with a wide range of performance that would stop glare and heat gain, have dynamic controls, and no moving parts,” Van Dine says. “We were very fortunate that the technology had broad potential—for the military, architectural, and transportation sectors.”