Electrochromic windows for commercial buildings: Monitored results from a full-scale testbed

Electrochromic glazings promise to be the next major advance in energy-efficient window technology, helping to transform windows and skylights from an energy liability to an energy source for the nation's building stock. Monitored results from a full-scale demonstration of large-area electrochromic windows are given. The test consisted of two side-by-side, 3.7x4.6-m, office-like rooms. In each room, five 62x173-cm lower electrochromic windows and five 62x43-cm upper electrochromic windows formed a large window wall. The window-to-exterior-wall ratio (WWR) was 0.40. The southeast-facing electrochromic windows had an overall visible transmittance (Tv) range of Tv=0.11-0.38 and were integrated with a dimmable electric lighting system to provide constant work plane illuminance and to control direct sun. Daily lighting use from the automated electrochromic window system decreased by 6 to 24 percent compared to energy use with static, low-transmission (Tv =0.11), unshaded windows in overcast to clear sky winter conditions in Oakland, California. Daily lighting energy use increased as much as 13 percent compared to lighting energy use with static windows that had Tv=0.38. Even when lighting energy savings were not obtainable, the visual environment produced by the electrochromic windows, indicated by well-controlled window and room luminance levels, was significantly improved for computer-type tasks throughout the day compared to the visual environment with unshaded 38 percent-glazing. Cooling loads were not measured, but previous building energy simulations indicate that additional savings could be achieved. To ensure visual and thermal comfort, electrochromics require occasional use of interior or exterior shading systems when direct sun is present. Other recommendations to improve electrochromic materials and controls are noted along with some architectural constraints