A view of smarter window glass

A new generation of insulated "dynamic windows" that change color to modulate interior temperatures and lighting could become commercial sometime soon thanks to work underway at the DoE's National Renewable Energy Labs.

Buildings consume 40% of the nearly 100 quadrillion Btus (quads) of energy the U.S. consumes annually. Conventional clear windows account for about one-tenth of the buildings' share of that energy load, or four quads. That's because they let heat leak out on chilly days and let sunlight warm a room to uncomfortable levels when it's hot. Using dynamic windows to compensate for some of the electric lighting used inside buildings could save another quad of energy, according to NREL.

But many color-changing window products and prototypes tested at NREL have performed poorly and their color-changing properties have degraded sooner-than-expected. Among contemporary designs, NREL has verified the performance of one technology developed by Sage Electrochromics — which has a cooperative research agreement with the Laboratory. Sage predicts its technology will drop in price by as much as 70% over the next five years as performance improves, volume increases and production becomes more efficient.

That's good: Today's dynamic windows still cost up to $1,000 per square meter of glass.

NREL researchers are experimenting with color-changing glass having a dynamic portion consisting of three layers: active and counter electrodes separated by an ion conductor layer. NREL devices use electrode layers made of nickel and tungsten oxides; the ions are lithium.

The window changes from clear to tinted when a small electric field is applied and the lithium ions move into the working electrode layers. The change can be triggered by sensors in an automated building management system, or by a flick of a switch. Electrochromic windows can block as much as 98% of direct sunlight. Reversing the polarity of the applied voltage causes the ions to migrate back to their original layer, and the glass returns to clear.

NREL uses metal oxides because light does not degrade them. While current manufacturer warranties typically extend for 10 years, NREL is aiming to develop windows that perform for 20 years or more.

Although electrochromic windows add yet another powered device to a modern building, they should save far more energy than they consume. Powering 1,500 square feet of color-changing glass (about 100 windows) would require less power than a 75-W light bulb.

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