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January, 2007
A Dawning Day for Energy-Efficient Electrochromic Windows

In 2002 a small, curious building began to rise on a hillside parking lot at Lawrence Berkeley National Laboratory. The exterior walls of the 953-square-foot structure were plain enough, just corrugated sheet metal. But as the building went up, it's most distinguishing feature soon drew the attention of Berkeley Lab employees: the south-facing wall of the structure (with a spectacular view of the greater San Francisco Bay area) had three large picture windows, each 10 feet wide by 9 feet high. And each window was composed of a mosaic of smaller rectangular windows, 15 each, in three columns and five rows.

The Advanced Windows Testbed of Berkeley Lab's Environmental Energy Technologies Division

If you had looked closely enough at these windows after everything was completed, you would have seen sensors of various kinds — some circular and button-like in appearance, some wiry — attached to each of the small windows, right on the glass. If you happened to have lingered there for some time, especially on nice sunny weekdays, you would sooner or later have noticed that some of these windows would gradually darken to a dark blue and then some time later would lighten back to their original clear state. And if you lingered long enough on partly cloudy days, you might see these windows repeat the cycle of lightening and darkening several times over the course of the day.

This building, the Advanced Windows Testbed, was complete in late 2003. Funded by the U.S. Department of Energy and the California Energy Commission's Public Interest Energy Research Program, its initial use was to serve as the lab testbed for a long-term study of electrochromic (EC) windows — windows with coatings that allow them to darken and lighten upon the application of a very small electric voltage.

EC windows are an advanced technology just now appearing on the market in the United States. They hold much promise to be the marketplace's next significant advance in window technology for energy efficiency and comfort enhancement. Early studies at Berkeley Lab suggested they could reduce a commercial building's annual energy use 15 to 25 percent.

As of today they are still in an early stage of technological development. Only a few manufacturers offer commercial products, architects and engineers don't have much experience designing with EC windows, and the technology is still expensive, although costs are expected to decline as companies refine the manufacturing process.

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The three rooms of the Advanced Windows Testbed are equipped with 15-pane windows to test and compare electrochromic and other energy efficient window systems under a variety of conditions.

To help EC windows realize their potential to save energy in California and throughout the U.S., DOE and the California Energy Commission funded Berkeley Lab's Environmental Energy Technologies Division to conduct a three-year field test of EC windows in a realistic office-building setting. This test, along with other tests and computer simulations, was intended to allow the Berkeley Lab researchers to quantify the performance of EC windows and refine the system to improve its performance and reliability.

"The project's aim was to advance EC windows as a viable marketplace solution for energy savings and electricity load management," says co-principal investigator and project manager Eleanor Lee. "Before EC windows can become widely accepted in the marketplace, market movers — developers, facilities managers, architects, and engineers — need to see realistic data on performance and energy impacts. They need to have confidence that these windows will operate properly, save energy, and improve the comfort of the occupants of buildings."

Thus in the fall of 2003, a manufacturer supplied Berkeley Lab with prototype EC windows, then available only in small rectangular sizes, and Lab researchers installed them in the new test facility.

How They Work

Electrochromic windows dynamically control daylight and solar heat gain, the sun's heat passing through the window to the interior, by dimming the window to a dark tint while maintaining a transparent view to the outdoors. The EC coating is a multilayer stack that can be deposited on glass or plastic. The layers include a transparent outer conductive layer, an active electrochromic layer, a passive counterelectrode layer, and an ion-conducting electrolyte layer.

An electrochromic window's layered materials change transmissivity under different applied voltages.

When a small voltage is applied to the outer conductive layer, lithium ions migrate from the counterelectrode layer, across the ion-conducting layer, to the electrochromic layer, tinting the window Prussian blue. Reversing the voltage causes electrons to flow in the opposite direction, making the window transparent. Chemically, the layers act much like those in a battery. The electricity required to switch the window back and forth between transparent and tinted states is orders of magnitude less than the electricity required for commercial lighting systems, typically less than 15 hundredths of a watt per square foot of glass.

With the proper sensors and control algorithms, an EC window system in a large commercial building could automatically darken the windows when the sun is high and its rays are heating the interior, thus reducing the solar heat gain and the need for air conditioning. As the sun sets or clouds cover the sky, the system would move the windows back toward transparency, maximizing daylighting and reducing the use of electric lighting. Dynamic control of the windows can save energy for both lighting and air conditioning.

A properly designed control algorithm could also help improve the comfort of occupants by reducing glare on computer screens and work surfaces, and reducing solar heat gains automatically. One of the goals of Berkeley Lab's research was to test the comfort of the occupants in an office with EC windows, and to develop both the hardware and software for an improved control system.

The Window Testbed

Inside the testbed structure are three identical offices with south-facing window walls, each office thermally isolated from the others. For the tests they were outfitted as typical offices with dimmable fluorescent lighting, furniture, and carpeting. More than a hundred sensors in each room measured interior light levels ("illuminance"), surface brightness ("luminance"), temperatures, plug loads, and EC window and lighting control status, as well as exterior weather conditions from minute to minute.

At first the windows were controlled by a prototype device provided by the window manufacturer; later in the project, researchers and the manufacturer developed a control system that allowed them to control light transmittance more precisely. When the temperature is more than 50° Fahrenheit the 18 x 35-inch windows forming the wall take about six or seven minutes to move from full transparency to full color; the panes switch more slowly at colder temperatures, and the larger the window, the slower the switching. Fifteen such windows formed a single window wall.

Over a period of 20 months the project team engineered, tested, and refined a daylight control system for the facility, with the goal of maximizing energy saved. Using Radiance lighting simulation software developed by Berkeley Lab researchers and the Mathematica program, team members used simulations to determine what window control strategy would result in the most energy savings and best comfort levels.

"Our optimal solution," says Lee, "was to divide the EC window wall into two zones, an upper daylighting zone and a lower view zone." They controlled the upper daylighting zone to minimize the use of supplemental electric lighting. The lower view zone was controlled to allow daylight into the room during diffuse sky conditions, then switched to fully colored during periods of direct sun to reduce glare and brightness on work surfaces like desks and computer screens.

The Field Test

Two of the three rooms in the testbed were equipped with EC windows. A third served as a reference room and was outfitted with energy-efficient, low-emissivity windows, as well as manually operated venetian blinds and daylighting controls. In this way the EC windows were compared against energy-efficient technology that is already widely available; not all buildings use even this accessible technology. The test project demonstrated that EC windows do in fact save energy compared to these state-of-the-art, static, low-emissivity windows.

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Outfitted with a typical office's furnishings, the Advanced Windows Testbed is also wired with scores of sensors.

The two-zone EC windows system saved from a few percent to roughly 25 percent or more on lighting energy over the well-equipped reference room. Using Radiance and Mathematica to correct for manual operation of blinds, the savings over a more typical reference room without existing top-of-the line, energy-efficient windows saved even more, from 48 to 67 percent annually. 

The EC system also produced a reduction of 19 to 26 percent in maximum peak demand power for cooling — potentially a big help to the electricity grid on hot summer days when air conditioning use is high.

Radiance software can simulate a variety of lighting conditions for building exteriors and rooms.

The researchers note that the energy savings from EC windows will depend on differences in climates. Compared to Berkeley's relatively mild climates, the inland areas of California are much hotter. There (and in other areas with similar climates, such as the southwest of the U.S.), the energy and peak power demand savings should be larger, since EC windows reduce solar heat gains significantly. Electrochromic technology would also provide greater savings for large-area windows than for smaller windows and for windows that face south, east, or west than for north-facing ones.

The Test Subjects Speak

In addition to quantifying the performance of electrochromic windows, the researchers also wanted to learn what users thought about rooms with electrochromic windows and dynamic daylighting control. Were these rooms more or less comfortable than rooms with conventional windows and manual shade systems? The input of users is important because EC windows cannot succeed in the marketplace if building occupants don't like them.

To get an indication of user response, the research team brought in 43 volunteers who used the EC window test room for several hours, working at a desk equipped with a PC. They were exposed to three different lighting conditions for 40 to 60 minutes each: a reference episode, during which the user operated lights and shades manually; a semiautomatically controlled period; and a fully controlled period, when both lighting and windows were controlled for daylight and glare. At the end of each period, they filled out a questionnaire.

The majority of occupants preferred the automatically controlled conditions over the reference episode by a significant margin, since it resulted in less use of the blinds and more access to the unobstructed and quite spectacular view. When the EC windows were controlling for glare, the test subjects chose to face the window to do computer-related tasks, reporting less glare, fewer reflections on their computer monitor, and more satisfaction than in the reference case. They did not complain that the room was too hot or too cold.

Thus research indicates that the EC window system does more than save energy. By controlling for daylight and glare it also provides occupants a more pleasant work environment, with year-round access to views and comfortable visibility of computer screens and work surfaces.

Needed: Better Control Devices

Overall, EC windows came off very well in the study. Among a list of recommendations for improving their commercial viability, the research team first suggests that EC windows manufacturers focus on developing an improved control system that would allow a building's staff to diagnose problems and recalibrate window controls more effectively. A better supervisory control system will make the daylighting system more responsive to the needs of occupants and their visual comfort requirements. Better control would allow facilities departments to take maximum advantage of the windows' ability to save energy and peak power and control the room environment for overall environmental comfort.

"This technology promises to help California meet its aggressive energy savings and greenhouse gas reduction goals in the next ten years," says Lee, "if manufacturers can continue to make improvements in the technology and cost of manufacturing."

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