Dynamic glass technology made a big splash in eyewear in the 1990s. However, the lag in color-transition time and lack of user control often meant that the spectacled were donning shades indoors for several awkward minutes as their lenses adjusted to the change in lighting levels.
Unlike people, buildings have the option of employing electrochromic glass, one of many recently introduced smart-window technologies that offer occupant control while negating the trade-off that occurs with conventional window films, which create a fixed state. For example, a conventional static window coating with a high visible transmittance (VT) and low solar heat gain coefficient (SHGC) may make sense for summer afternoons, but it doesn’t help in the winter, when squeezing out every drop of sunshine can reduce heating and lighting loads while boosting morale.
Commercially available since the mid-2000s, electrochromic glazing changes optical properties when applied with a low, direct-current voltage. (Eyeglasses use photochromic lenses that are activated by UV radiation.) Window panes can go from clear to tinted in minutes.
Used in insulated glass units (IGUs) or triple-glazed windows, electrochromic coatings are sputter-deposited onto the number two glazing face—or the room-side face of the outermost pane. Less than 1 micron thick, which is about 1/100th the thickness of a human hair, the coating comprises multiple layers of transparent conducting ceramic and metal-oxide materials and electrochromic material, which is typically a transition metal oxide.
The stack of materials is then sandwiched between two transparent conductors to create what essentially becomes a battery. When a small amount of voltage—about 3 volts—is applied to the stack, some layers undergo a reduction in polarity while other layers are oxidized. The coloration ions—typically lithium or hydrogen—move back and forth between an ion storage layer to the electrochromic film layer, and create clear or tinted glass.
Though research and development for smart window technologies have been under way for several decades, market-ready products are still limited. Currently only a handful of manufacturers—including Sage Electrochromics in Fairbault, Minn., EControl-Glas in Germany, and Soladigm in Milpitas, Calif.—offer electrochromic glass for the commercial building market; several other companies offer dynamic glazing products, though many are in the development and testing phases.
In the case of Sage’s product, a typical 60-watt incandescent lamp requires more electricity than 2,000 square feet of electrochromic glazing. In its clearest state, Sage’s glass has a VT of 65 percent and an SHGC of about 0.5. Within six to eight minutes, the glass can transition to its darkest state, with a VT of 2 percent and an SHGC of 0.1. The change is subtle, says Phil Newsom, a principal in Concord, Calif., office of tBP Architecture: “It doesn’t automatically go dark.”
In 2011, tBP Architecture completed an early installation of electrochromic glass in Chabot College’s Community and Student Services Center in Hayward, Calif. A spacious, south-facing atrium—the 51,000-square-foot center’s most heavily occupied space—has a two-story, 2,900-square-foot curtainwall that looks out to a plaza. “We needed to strike a fine balance between providing light into the building and not overheating everyone,” Newsom says.
The firm and college selected electrochromic glass over blinds or architectural fins, which were rejected for being visibly obstructive or expensive. The curtainwall was outfitted with dynamic glass in six different control zones to allow distinct portions of the glass envelope to be tinted or clear, based on a custom-designed logic sequence that was integrated into the building-management system. The logic sequence uses parameters such as the time of day, time of year, and indoor and outdoor temperatures to determine when and where the glass is clear or tinted.
The Chabot College installation occurred when the technology had only an on-or-off mode. Now, John Van Dine, founder and CEO of Sage, says that electrochromic glazing offers an infinite number of intermediate states between clear and dark. “Customers have found that four typical states cover the landscape,” he notes.
With this smart technology, Van Dine says, “We no longer have to move shades and blinds up and down, or limit our view and connection to the outdoors by putting them down.”
However, one thing that yards of thick curtains or the dense fins of window shades do provide over coatings is privacy. Even when fully tinted, electrochromic windows maintain some visibility, particularly if the observed environment is brighter than the observer’s environment—at night, for example, pedestrians outside will be able to some activity inside if the lights are on.
In terms of glass durability, only heat-strengthened or tempered glass is used in electrochromic glazing, which reduces the potential of glass breakage due to the thermal stress that other high-performance window films can foster. Manufacturers typically size electrochromic glass to fit into standard IGU frames, which makes electrochromic glazing an option for new construction or renovation projects.
On a cost-per-area basis, electrochromic glass is more expensive than standard low-E coated glass. But Van Dine says that savings realized from not purchasing exterior and interior shading devices makes the cost of electrochromic glass “on parity” with other high-performance window units. The U.S. Department of Energy (DOE) estimates that electronically tintable glass can produce a 20 percent savings in building operating costs and a 25 percent reduction in HVAC sizing. This “annual electrical annuity,” Van Dine says, can reduce the typical return on investment (ROI) of three to five years.
The atrium at Chabot’s Student Center, for one, does not require a conventional HVAC system. Without the high cooling load that the curtainwall could have introduced, the building relies only on radiant flooring and natural ventilation. “The thing that makes it all work is the glass,” Newsom says. “The building is more than 40 percent efficient over the baseline.”
Electrochromic coatings today treat infrared and visible light together; as a result, when the glazing is tinted to limit solar heat gain, visibility is also sacrificed. Though that may soon change. As a forerunner in dynamic glass research, as well as a collaborator with Sage Electrochromics in product modeling and testing, the DOE’s Lawrence Berkeley National Laboratory recently announced a development in smart window coatings that may take the technology to the next level.
Using a transparent semiconductor coating made with indium tin oxide, researchers developed a coating that can selectively block near-infrared energy from the sun while allowing visible light to transmit through. “Our electrochromics [can] modulate heat from the sun while remaining visibly clear—they do not darken,” says Delia Milliron, deputy director of the Berkeley National Lab’s Molecular Foundry. Though, she says, additional shading devices would be required to prevent glare: "The ultimate smart window would be able to selectively block near-infrared light [heat] or block both heat and light on demand."
Such an advancement would allow us to control to a greater degree how our windows perform by allowing just the right amount of natural light in, providing unobstructed views out, and rejecting any unwanted solar heat gain. As this smart technology continues to progress, we may just come one step closer to outwitting the sun while reducing our carbon footprint on Earth.