Building a Smarter Window

What if you could build a window that changes color the same way you change from clear glasses to sunglasses?

Researchers at the National Renewable Energy Laboratory (NREL) are doing just that. They are testing a new generation of electrochromatic, color-changing windows, which can change from clear to tinted at the flick of switch – or when triggered by sensors.

Once triggered, a small electric field is applied which makes lithium ions move into the working electrode layers. Electrochromic windows can block as much as 98 percent 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.

Buildings use 40 percent of the energy consumed annually in the United States. Conventional clear windows account for about one-tenth of the buildings' share of that energy load because they allow heat to leak out on chilly days or permit the incoming sun to warm a room excessively. A building's climate control system must compensate for these changes in temperature by increasing the heat or air conditioning accordingly. Dynamic windows can also substitute for some of the electric lighting used inside buildings.

"Combined, a broad installation of these highly insulating, color-changing windows could save about one-eighth of all the energy used by buildings in the U.S. every year," NREL research scientist Dane Gillaspie said, "and about 5 percent of the nation's total energy budget."

Electrochromic windows are made with a very thin stack of dynamic materials deposited on the outside pane of a double-paned window. The entire dynamic stack between the glass panes measures about a micron thick, about the same as thin-film photovoltaic cells. NREL researchers are experimenting with electrode layers made of nickel and tungsten.

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 energy than a 75 watt light bulb. And, because the windows modulate the building's interior climate, the rest of the heating, cooling and illumination systems can be smaller, leading to lower construction costs and lower monthly energy bills.

In computer simulations of building performance, the electrochromic windows:

• Reduce electricity consumption for cooling by up to 49 percent;
• Lower peak electrical power demand by up to 16 percent;
• Decrease lighting costs by up to 51 percent.

"The brilliant thing is that not only do you save energy with these windows," Gillaspie said, "but they allow you to scale back the HVAC, so you save money."

Dynamic windows typically are hardwired into a building's electrical system. But in a wireless version, the power for the color change could come from a small Photovoltaic cell installed in the window's casement. When sunlight hits the PV cell, it converts the sunlight to power, which ionizes the electrode layers and darkens the window. Eventually, dynamic windows may produce more energy than they consume so they would be another source of power generation for a building.

Since the 1980s, NREL has tested various window technologies and helped establish technical standards for the industry with the American Society for Testing and Materials. Prototype windows are subjected to extreme simulated conditions to determine their performance and durability. Samples are performance-tested for 20,000 cycles to simulate 20 years of exposure to heat, humidity and other conditions.

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 percent over the next five years as performance improves, volume increases and production becomes more efficient. However, today's dynamic windows still cost up to $1,000 per square meter of glass.

NREL researchers are working to drive down high manufacturing costs by creating the dynamic layers using inexpensive printing technologies and metallic inks similar to research into high-volume thin-film PV manufacturing. NREL research supports the U.S. Department of Energy's goal to deploy the energy-saving windows for residential construction by 2015 and commercial buildings by 2020.

Image courtesy of Flickr