Daylighting control performance of a thin-film ceramic electrochromic window: field study results

Control system development and lighting energy monitoring of ceramic thin-film electrochromic (EC) windows were initiated at the new full-scale Window Systems testbed facility at the Lawrence Berkeley National Laboratory (LBNL) in Berkeley, California. The new facility consists of three identically configured side-by-side private offices with large-area windows that face due south. In one room, an array of EC windows with a center-of-glass visible transmittance (Tv) range of 0.05-0.60 was installed. In the two other rooms, unshaded windows with a Tv=0.50 or 0.15 were used as reference. The same dimmable fluorescent lighting system was used in all three rooms. This study explains the design and commissioning of an integrated EC window-lighting control system and then illustrates its performance in the testbed under clear, partly cloudy, and overcast sky conditions during the equinox period. The performance of an early prototype EC window controller is also analyzed. Lighting energy savings data are presented. Daily lighting energy savings were 44-59% compared to the reference window of Tv=0.15 and 8-23% compared to the reference window of Tv=0.50. The integrated window-lighting control system maintained interior illuminance levels to within ±10% of the setpoint range of 510-700 lux for 89-99% of the day. Further work is planned to refine the control algorithms and monitor cooling load, visual comfort, and human factor impacts of this emerging technology.