Energy-Efficient Electrochromic Windows on the Threshold

A small, curious building began to rise on a hillside parking lot at Lawrence Berkeley National Laboratory (Berkeley Lab) in 2002. The exterior walls of the 953-square-foot structure were plain enough-corrugated sheet metal. But as the building went up, a distinctive feature appeared: the south-facing wall, with a spectacular view of the San Francisco Bay area, held three large picture windows, each 10 feet wide by 9 feet high and composed of a mosaic of 15 smaller rectangular windows in three columns

Those who looked closely enough once the building was complete in late 2003 saw numerous sensors-some circular and button-like, some wiry-attached to the glass of each small window. Curious passersby who lingered, especially on sunny weekdays, sooner or later saw some windows gradually darken to a deep blue and later lighten to their original clear state. In partly cloudy conditions, the windows might repeat the cycle of lightening and darkening several times a day.

This mysterious building was the Advanced Windows Testbed, funded by the U.S. Department of Energy (DOE) and the California Energy Commission (CEC) Public Interest Energy Research Program. It hosted a long-term study of electrochromic (EC) windows, which have coatings that allow them to darken and lighten in response to low electric voltage. EC windows are an advanced technology that is just appearing on the U.S. market. They hold much promise to be the next significant energy-efficient and comfort-enhancing window technology. Early studies at Berkeley Lab suggested these windows could reduce a commercial building's annual energy use by 15 to 25 percent.

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

To help EC windows realize their potential to save energy in California and throughout the U.S., DOE and CEC funded Berkeley Lab's Environmental Energy Technologies Division (EETD) 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 designed to quantify the performance of EC windows and help researchers improve the windows' 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 2003, a manufacturer supplied the lab with prototype EC windows, which were then available only in small rectangular sizes, and lab researchers installed the windows in the new testbed facility.

How EC Windows Work

Electrochromic windows dynamically control daylight and solar heat gain (the sun's heat passing through the window to the interior) by dimming to a dark tint while maintaining a transparent view to the outdoors. The multi-layer EC coating can be deposited on glass or plastic windows. The coating is made up of a transparent outer conductive layer, an active electrochromic layer, a passive counter-electrode layer, and an ion-conducting electrolyte layer (see Figure 3).

When a low electric voltage is applied to the outer conductive layer, lithium ions migrate from the counter-electrode layer across the ion-conducting layer to the electrochromic layer, tinting the window Prussian blue (see Figure 4). Reversing the voltage causes ions to flow in the opposite direction, making the window transparent. Chemically, the layers act much like a battery. The electricity required to switch the window back and forth between transparent and tinted states (e.g., 0.07-0.15 Watts/ft²-glazing) is orders of magnitude less than the electricity required for commercial lighting systems.

With the proper control algorithms and sensors, an EC system in a large commercial building would automatically darken windows and reduce solar heat gain and thus the need for air conditioning when the sun is high and its rays are heating the building's interior. As the sun sets or clouds cover the sky, the windows would shift back toward their untinted state, maximizing daylight passing through the window and reducing the need for electric lighting in the building's interior. This dynamic control of the windows would save both lighting and air conditioning energy.

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

The Window Testbed

Within the test structure at Berkeley Lab are three identical offices, each thermally isolated from the others, with south-facing window walls. Each room is outfitted as a typical window office might be, with dimmable fluorescent lighting, office furniture, and carpeting. More than a hundred sensors in each room measured interior light levels ("illuminance"), surface brightness ("luminance"), temperatures, plug loads, EC window and lighting control status, and exterior weather conditions from minute to minute. The windows were controlled at first by a prototype device provided by the window manufacturer; later in the project, researchers and the manufacturer developed a system to more precisely control light transmittance.

The 18 x 35-inch window panels forming the window wall take about six to seven minutes to switch from clear to full color when the temperature is more than 50°F. The windows switch more slowly at colder temperatures (the larger the window, the slower the switching.

Over 20 months, the project team engineered, tested, and refined a daylight- control system for the facility with the goal of maximizing energy saved. Using the Radiance lighting simulation software (developed by Berkeley Lab researchers) and the Mathematica computer program, team members performed simulations to determine the window control strategy that would maximize energy savings and interior comfort.

"Our optimal solution," says Lee, "was to divide the EC window wall into two zones, an upper daylighting zone and a lower view zone." Researchers 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 or to switch 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; the third was a reference room, outfitted with energy-efficient low-emissivity (low-e) windows, so the EC windows were compared against a state-of-the-art energy-efficient technology that is widely available. The reference room was equipped with manually operated venetian blinds and daylighting controls.

The results of this effort, which are now available and posted on the project's website, demonstrate that EC windows save energy compared to state-of-the-art static low-e windows. Compared to the reference room case with the static, energy-efficient low-e windows and the blinds down, the two-zone EC windows system saved +/-10 to 15 percent of daily lighting energy use during the test period. Simulations of these test results using Radiance and Mathematica to correct for manual operation of blinds and conditions that change with the seasons indicated that the total annual lighting energy savings would be 48 to 67 percent compared to the reference case.

The EC system also reduced peak demand power for cooling by 19 to 26 percent-potentially a big help to the electricity grid on hot summer days when air-conditioning use is high. The researchers note that energy savings from EC windows will be affected by climate differences. Compared to Berkeley's relatively mild climate, inland areas of California are much hotter. In those and other areas with similar climates, such as should be larger than in Berkeley because EC windows significantly reduce solar heat gains. EC technology would also save more in large-area windows than in smaller ones, and in south-, east- and west-facing windows than in north-facing ones.

The Test Subjects Speak

In addition to quantifying the performance of EC windows, the researchers also wanted to learn what users thought about rooms with EC windows and dynamic daylighting control: were occupants more or less comfortable than they would be in rooms with conventional window and manual shade systems? User input 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 sat in the EC window test room and worked for several hours at a desk equipped with a PC. Occupants 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 the lighting and windows were controlled for daylight and glare. At the end of each period, occupants filled out a questionnaire (see Figure 5).

The majority of occupants preferred the automatically controlled conditions over the reference episode by a significant margin. The automatically controlled EC system resulted in less use of the blinds and more access to an unobstructed outdoor view. Subjects chose to face the window to do computer-related tasks when the EC windows were controlling automatically for glare. The subjects reported less glare, fewer reflections on their computer monitors, and more satisfaction than in the reference case. They also did not complain about thermal discomfort in the room.

These results indicate that, in addition to providing energy savings benefits, EC window systems, when controlling for daylight and glare, provide occupants a more pleasant work environment than rooms with conventional windows; EC windows have the advantages of year-round access to views and comfortable visibility of computer screens and work surfaces.

Needed: Better Control Devices

Overall, EC windows performed very well in the study. However, first on a list of recommendations to EC manufacturers for improving the windows' commercial viability, the research team suggests focusing on developing an improved control system, to allow facilities staff to manage recalibration and diagnose problems effectively.

A better "supervisory control system" for the windows will make the daylighting system more responsive to occupants' needs and visual comfort. Better control of the windows will also allow facilities departments to take maximum advantage of EC windows' ability to save energy and peak power and regulate interior spaces 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 if manufacturers can continue to make improvements in the technology and cost of manufacturing," Lee says.

— Allan Chen

More on Advancement of Electrochromic Windows.

The final report, "Advancement of Electrochromic Windows," LBNL-59821, is available online.

More on Radiance simulation software.

This research was funded by the California Energy Commission's Public Interest Energy Research Program, and the U.S. Department of Energy.

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