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Transition-Metal Switchable Mirrors Win 2004 R&D 100 Award

Transition-metal switchable mirror window: top portion is transparent, revealing a plant located behind the window; while the lower portion is reflective, showing a shell.

Figure 1. Transition-metal switchable mirror windows vary from transparency to heat and light (top half of this sample), to being almost wholly reflective (bottom).

In July, R&D Magazine announced that transition-metal switchable mirrors, a technology developed by researchers in the Environmental Energy Technologies Division (EETD) at Lawrence Berkeley National Laboratory (Berkeley Lab), had won an R&D 100 award. The following article, first published on Berkeley Lab's Science Beat website, describes the technology and its potential for next-generation energy-efficient windows as well as other applications.

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 that can switch back and forth between a transparent state and a reflective one. The new coating was developed by Thomas Richardson of EETD with assistance from Jonathan Slack.

Controlling the flow of solar radiation through windows to building interiors by use of existing low-emissivity (low-e) technology has already saved billions of dollars in energy costs-$8 billion through the year 2000, according to a 2001 study by the National Academy of Sciences. Low-e windows are the first generation of energy-efficient windows, developed by Berkeley Lab and its commercial partners during the 1980s. The coatings on these windows prevent some heat from reaching a building's interior, which reduces air-conditioning use; 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 the next generation of efficient windows using dynamic technologies, which change characteristics in response to climate conditions, e.g., reducing transmission of light and heat through a window by turning darker when the sun is high and 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, (Figure 1) which uses thin-film coatings that can be converted from a transparent to a reflective 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." Current electrochromic windows have little effect on infrared radiation, which accounts for almost half of incident energy. Figure 2 illustrates the structure of absorbing and reflecting EC windows.

TMSMs perform better than AE windows in a number of other ways as well. 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 user comfort and energy savings. In addition, unlike AEs, TMSMs can become completely opaque to provide interior privacy.

Richardson adds that "the use of transition-metals instead of rare-earth metals" in the Berkeley Lab TMSMs "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 and may degrade more readily than the Berkeley Lab transition-metal films.

"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 as dynamic, energy-efficient window coatings for architectural glass. Dynamically controlled windows respond to changes in sunlight 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 windows switch to a partially reflective, partially transparent state to admit some daylight. Dynamic windows can be regulated automatically throughout a building, but occupants can also have local control over their windows, from their personal computers, for example.

Schematic drawing of absorbing and reflecting electrochromic window structure

Figure 2. Schematic drawing of absorbing and reflecting electrochromic window structure

The electrical current that accomplishes the switching in TMSMs is extremely small compared to the energy use of lights and air conditioners, so the potential energy savings from these windows are 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.

"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. The technology could improve glare control in motorcycle and flight helmets, airplane and marine windows, cabin partitions, and sunglasses which would increase safety for vehicle operators. By reflecting some of the sunlight falling on a car's windows and sunroof, TMSMs can lead to a reduction in the size and weight of car air-conditioning units, which in turn reduces fuel use.

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 British thermal units (Btus) per year. Berkeley Lab researchers estimated in 1996 that the potential energy savings 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. These savings could be worth billions of dollars per year.

— Allan Chen

For more information, contact:

  • Thomas J. Richardson
  • (510) 486-8619
  • Jonathan Slack
  • (510) 486-4148; Fax (510) 486-6099

TMSMs have received U.S. patent #6,647,166. Contact Pam Seidenman at in Berkeley Lab's Technology Transfer Department for information about licensing transition-metal switchable mirror technology.

More information about Transition Metal Switchable Mirrors.

Transition-Metal Hydride Electrochromics

This research is funded by the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy.

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