From the Lab to the Marketplace Ten Years Later, Energy Efficient Technologies from Research at the Lawrence Berkeley National Laboratory Berkeley Lab logo (left) with six rows of gray dots transitioning to a line art drawing of a cityscape and residential houses.

Transition-Metal Switchable Mirrors

Transition-Metal Switchable Mirrors (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.

This dynamic window technology consists 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 transition-metal switchable mirror, from left to right, switching from the mirrored to the transparent state.

Thin film coatings forming the surface of switchable mirrors are deposited on glass using a cosputtering process in chambers such as this.

Thin Ni-Mg films, on exposure to hydrogen gas or on reduction in alkaline electrolyte, the films become transparent. The transition is believed to result from formation of nickel magnesium hydride, Mg2NiH4.

The film used for the Berkeley Lab switchable mirror is made of an alloy of magnesium and one or more transition-metals. 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.

TMSMs have a great dynamic range in light transmission — from 50 percent to 0.5 percent or lower, a factor of 100 — and in light reflection — from 75 to 10 percent reflective. This gives them considerable advantages over absorbing electrochromic windows in providing user comfort, privacy and energy savings. 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 have a greater potential to save energy because they both reflect infrared light (the heat of the sun) and can help manage interior visible light levels by allowing daylight to pass to the interior when it is available.

Windows made of dynamically controlled switchable mirrors could save more energy in commercial buildings than current state-of-the-art efficient, low-e windows.

(Left) Switchable mirrors could be used to regulate solar heat gain through car sunroofs.

(Right) Regulating the internal temperature of satellites—another possible application of switchable mirrors.

The primary application of TMSMs will be 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.

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.