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April 30, 2004
Toward a New Generation of Energy-Efficient Windows:
Transition-Metal Switchable Mirrors

The race is on to develop the next generation of energy-efficient windows, and it has a new entrant: transition-metal switchable mirrors (TMSMs). TMSMs are glass panels with a coating capable of switching back and forth between a transparent state and a reflective one. The new coating was developed by Thomas Richardson of Berkeley Lab's Environmental Energy Technologies Division with assistance from Jonathan Slack.

Transition-metal switchable mirror windows vary from transparency to heat and light, like the top half of this sample, to being almost wholly reflective, as at bottom. To see the mirror windows in action, choose from one of the file sizes below (Real Player required):

Controlling the flow of solar radiation through windows to building interiors has already saved billions of dollars in energy costs — $8 billion through the year 2000 from the use of currently available low-emissivity windows, according to a 2001 study by the National Academy of Sciences. "Low-e" windows, the first generation of energy-efficient windows, were developed by Berkeley Lab and its commercial partners during the 1980s. They have coatings that prevent some heat from reaching a building's interior, which reduces air-conditioning use, but they also trap heat inside during cold periods to save heating energy.

A substantial research effort is under way in the U.S. and abroad to develop dynamic window technologies, coatings that reduce light and heat passing through a window by turning darker when the sun is high, then becoming transparent when more light is desired. One type of dynamic technology already being tested at Berkeley Lab and elsewhere is the absorbing electrochromic (AE) window, which switches from a transparent state to a darkened state, usually blue in color.

Thin metal films on glass

The latest dynamic window technology is the switchable mirror, which consist of thin-film coatings on glass that can be converted from a transparent to a reflecting state and back again, by application of an electric field (electrochromic switching) or by exposure to dilute hydrogen gas (gasochromic switching).

"The film used for the Berkeley Lab switchable mirror is made of an alloy of magnesium and one or more transition-metals," says Richardson. "These make a new generation of electrochromic windows possible, superior in many ways to the current generation because they reflect visible and infrared light and heat instead of absorbing it."

Richardson adds that "the use of transition-metals instead of rare earth metals could also significantly lower the cost of these windows." Switchable mirrors based on rare-earth metals were developed in 1996 in Europe. Rare-earth thin films are significantly more expensive and difficult to prepare than the Berkeley Lab transition-metal films and may degrade more readily.

TMSMs perform better than absorbing electrochromic windows in a number of ways. The greater dynamic range of transition-metal switchable mirrors, both in transmission — from 50 percent to 0.5 percent or lower, a factor of 100 — and in reflection — from 75 to 10 percent reflective — gives them considerable advantages over AEs in providing user comfort and energy savings. AEs do not become completely opaque and therefore cannot provide privacy, but TMSMs can become opaque. And while current electrochromic window materials can darken a window from essentially transparent to dark blue, they have little effect on infrared radiation, which accounts for almost half of incident energy. TMSMs reflect both infrared and visible light.

Thin film coatings on the window glass allow TMSMs to change from a transparent to a reflecting state and back again by application of an electric field or by exposure to dilute hydrogen gas. Because they use fewer and thinner coatings than absorbing electrochromics, TMSMs promise to be easier to make and less expensive.

"TMSMs should also be easier to manufacture, because they use fewer and thinner coatings than absorbing electrochromics, which employ thick oxide layers," says Richardson.

TMSMS in your house — and in your car

The primary application of TMSMs is in architectural glass, as dynamic, energy-efficient window coatings. Dynamically controlled windows respond to changes in lighting conditions in real time through the use of light sensors and control technology. When the sun is bright, TMSMs switch to a highly reflective state; in lower-light conditions, such as cloudy periods, or early and late in the day when the sun is low, the window can be switched to a partially-reflective, partially-transparent state to admit some daylight.

The system regulates windows automatically throughout the building, but occupants will have local control over their windows — for example, from their personal computers. The electrical current that accomplishes the switching is extremely small compared to the energy use of lights and air conditioners, so the energy savings are potentially enormous.

Coupled with an automatic sensor and control system, dynamic coatings not only minimize energy use but maximize comfort as well, reducing heat gain and controlling glare. By increasing comfort, the technology has the potential to improve the productivity of people in offices.

"The technology can also be used in transportation, as a dynamic window for automobiles, aircraft, and ships, as well as in helmets for pilots, cyclists, and motorcyclists," says Richardson. Better glare control in motorcycle and flight helmets, airplane and marine windows, cabin partitions, and sunglasses could increase safety for vehicle operators.

And by reflecting some of the sunlight falling on a car's windows and sunroof, the size and weight of its air conditioning unit can be reduced, which in turn reduces fuel use. Previous studies by Berkeley Lab scientists have shown that even conventional thermal insulation can reduce a car's heating and cooling loads by 75 to 80 percent.

Richardson notes that the technology can also be used "in optical displays, electronic data switching, and sensors." In data switching, TMSMs can route signals through fiber optic networks; as electrochromic outer coatings they can modulate the propagation of light through fibers. And, says Richardson, "Engineers may use TMSMs as temperature-regulation coatings for satellites, which need to control solar heat gain while in orbit to protect interior circuits."

Possible energy savings

In the U.S., residential buildings alone currently lose more than 1.7 quadrillion Btus (British thermal units) per year. Berkeley Lab researchers estimated in 1996 that the potential savings in energy from accelerated adoption of existing energy-efficient windows could amount to more than 25 percent of the energy lost through windows.

"Dynamically controlled windows can provide greater energy savings than existing passive window technologies, so it is reasonable to expect that windows based on TMSMs could save more than 25 percent of the energy lost through conventional windows in homes and commercial buildings," says Richardson — savings potentially worth billions of dollars per year.

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