Tintable Glazing Saves Energy On Demand
As long as humans have been building windows, we’ve been tinkering with ways to let in daylight and provide views to the outdoors while controlling glare and the movement of heat.
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Most windows don’t meet occupants’ desires under all conditions, explains Stephen Selkowitz, head of the Building Technologies Department at Lawrence Berkeley National Laboratory (LBNL). Clear glass often lets in more light or heat than desired, while tinted glass often blocks more light or heat than desired. “In an ideal world, all windows would be able to dynamically control transmittance, glare, solar gain, and daylight at any time to manage energy, comfort, and view,” says Selkowitz. “It’s the holy grail of the glass industry and the technology of the future.”
The only product currently meeting that ideal in the U.S., according to Selkowitz, is SageGlass, produced by Sage Electrochromics.
“SageGlass windows and skylights can be tinted to block solar heat gain without blocking the building occupants’ view,” explains Lou Podbelski, AIA, Sage’s vice president of sales and marketing. In tests run by LBNL, SageGlass has been shown to reduce energy costs by 10 to 28 percent. “Less fossil fuels burned means less pollution, which is good for the environment, and lower operating costs, which is good for the building owner,” he says.
SageGlass is transparent in its neutral state; at the push of a button, however, an electrical current darkens the glass over the course of a few minutes. For a standard SageGlass window, the visible transmittance drops from 62 to 3.5 percent, and the solar-heat-gain coefficient drops from 40 to 1.5 percent. While about 0.28 watts of electricity per square foot are required to darken the glass, only 0.10 watts per square foot are needed to maintain the darkened state.
Sage formally launched SageGlass in early 2006, and it now sells the product through several commercial and residential skylight, window, and curtainwall companies. SageGlass is far more expensive than conventional, low-emissivity glazing—it ranges from $75 to over $100 per square foot, says Podbelski, plus 4 to 12 percent for controls. This premium is limited, however, by the fact that SageGlass obviates the need for shading systems.
“When you take into consideration the cost of other solar-control products, such as exterior sunshades or mechanized shades,” says John Van Dine, president of Sage, “the margin gets close, and SageGlass is sometimes less expensive than the combination of these products.” In some cases, SageGlass could even allow for a smaller heating and cooling system, dramatically reducing or eliminating the cost premium.
While Sage maintains a monopoly on the market for electronically tintable glazing in the U.S., Selkowitz says that similar products are available in Europe and Japan, including one from Saint-Gobain for use in automobiles. “Sage’s challenge, and that of other manufacturers offering similar technology,” says Selkowitz, “will be to deliver that functionality at an affordable price that meets all market needs,” including proven durability and controls integration. “They are still on the early part of the product life-cycle curve, so there’s lots of time for price to come down and integration issues to get sorted out.” He believes that electronically tintable glazing will eventually gain prominence in the U.S. market, but he’s also keeping an eye on related technologies, including photochromic glass, which automatically darkens when exposed to a certain level of UV radiation, as well as motorized shades and blinds.
Van Dine believes that electronically tintable glazing will be mainstream within a decade, once the price drops below $35 per square foot and the marketplace gains familiarity with the product. “Once people start experiencing what it can do for them, they will wonder how they ever got along without windows they can tint on demand,” he says.
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