Electrochromic Window Tests in U.S. Office Show Promise
Electrochromic glazings promise to be the next major advance in energy-efficient window technology, helping to achieve the goal of transforming windows and skylights from an energy liability in buildings to an energy source for the nation's building stock. The glazing can be reversibly switched from clear to a transparent, colored state by applying a low voltage, resulting in dynamically controllable thermal and optical properties ("smart windows"). Incorporating electrochromic glazings could reduce peak electric loads by 20 to 30% in many commercial buildings and increase daylighting benefits throughout the U.S., as well as improve comfort and potentially enhance productivity in our homes and offices. These technologies will provide maximum flexibility in aggressively managing energy use in buildings in the emerging deregulated utility environment and will move the building community toward the goal of producing advanced buildings with minimal impact on the nation's energy resources. Customer choice and options will be enhanced further if the customers have the flexibility to dynamically control envelope-driven cooling loads and lighting loads.
Large-area electrochromic windows have recently become available in limited quantities. These windows have been installed in two side-by-side private office test rooms, enabling researchers to conduct full-scale monitored tests (Figures 1 and 2). Full-scale tests bring laboratory devices one step closer to commercialization by solving key design problems in a short test-evaluate-test iterative cycle of development within a realistic building environment. At this time, large-area windows (90x200 cm) are technically viable but can be produced only in small quantities and at substantial cost (~$1,000/m2). Volume production facilities are under development and several glazing developers expect new electrochromic window products to emerge in the marketplace by 2001-2002. With volume, glazing costs are expected to drop to about $100/m2. Material performance, optical characterization, coloration efficiency, durability, and fabrication research remain major foci of the electrochromic R&D community.
Testbed Description and Objectives
Large-area electrochromic windows were installed in two side-by-side test rooms in the Federal Building, Oakland, California, and operated from November 1999 through February 2000. Test objectives included developing control systems, monitoring energy use, and evaluating visual comfort. Each test room was 3.71 m wide by 4.57 m deep by 2.68 m high and furnished with nearly identical building materials, furniture, and mechanical systems to imitate a commercial office-like environment. The southeast-facing windows in each room were simultaneously exposed to approximately the same interior and exterior environment so that measurements between the two rooms could be compared.
A laminated electrochromic glazing was combined with a low-E glazing to form a double-pane window with a visible transmittace (Tv) range of 0.14 to 0.51. Each electrochromic double-pane window was then mounted on the interior side of the building's existing monolithic green-tinted glazing (Tv=0.75). The overall composite Tv range was therefore 0.11 to 0.38. Electrochromic windows were placed in an array of five upper and five lower windows to cover the full area of the window opening (3.71 m wide by 2.29 m high) as shown in Figure 1.
Results and Future Directions
Recent material advances have resulted in large-area electrochromic devices with good performance properties. The electrochromic window system tested had excellent optical clarity, no coating aberrations (holes, dark spots, etc.); uniform density of color across the entire surface during and after switching, smooth, gradual transitions when switched; and excellent synchronization (or color-matching) between a group of windows during and after switching. The windows had a very slight yellow tint when fully bleached and a deep prussian to ultramarine blue when fully colored. The glazings were not reflective. To all outward appearances, the electrochromic windows looked exactly like conventional tinted windows with the exception that one can change their coloration. Architecturally, the windows impart a high-tech, spare appearance without the usual intervening window shades.
Electrochromic glazings save energy by reducing cooling loads and reducing electric lighting energy consumption when dimmable lighting systems are used. In tests conducted during the winter, the focus was on the lighting energy impacts. Ceiling-mounted photosensor controls were used to modulate the glass transmittance and maintain a light level of 510 lux at the work surface. When insufficient daylight was available, the electric lights provided the additional required illuminance. When comparing the electrochromic glazings to a static dark glass (Tv=11%) on sunny and overcast days, the daily lighting energy consumption for the room with the electrochromic windows was on the order of 6 to 24% lower. Whenever direct sun enters the room, the electrochomic window switches to its darkest state (11%), so there are no savings relative to the static glazing. But much of the time in the afternoon, there is no direct sun on these facades, and under most overcast conditions the electrochromic window switches to a clearer state, allowing the lights to be dimmed, saving energy (Figure 2). However, when the electrochromic glass is compared to a higher transmittance glass (Tv=38%), the lighting energy use is actually 0 to 13% greater. This is because the static glazing always transmits as much light as or more light than the electrochromic, which will often be switched to control direct sunlight, thus requiring some added electric light. Overall, however, the high-transmittance static glass is likely to have higher cooling loads and result in more glare problems. And in an occupied space, people would likely have added blinds or shades to control glare, further reducing the apparent advantage of the clearer static glazing.
Two strategies can improve lighting energy savings with electrochromic glazings: increase the upper Tv limit and decrease the lower Tv limit for glare control. For this test, the upper Tv limit could have been increased if the existing building glazing had been removed. This work also suggests that it may be advantageous for electrochromic devices to have a larger contrast ratio and higher transmission in the bleached state; for example, a device that can switch between Tv=0.06-0.85 will have greater daylight efficacy and control over intense sunlight than the device tested in-situ. Additional field tests will be conducted to better understand electrochromic glazing properties, the relationships between these properties and lighting savings, cooling savings, and occupant satisfaction and methods to integrate dynamic control of the window system with whole building energy management systems.
For more information, contact:
- Eleanor Lee
- (510) 486-4997; fax (510) 486-4089
- Dennis DiBartolomeo
- (510) 486-7402; fax (510) 643-7957
- Stephen Selkowitz
- (510) 486-5064; fax (510) 486-4089
This work is sponsored by the U.S. Department of Energy's Office of Building Technology, State and Community Programs.