Evaluation of Integrated Lighting System Performance in a Large Daylighted Office Building

The use of daylight for ambient illumination can substantially reduce this energy usage if the electric lighting system is properly controlled in response to available daylight. This paper evaluates the monitored performance of an integrated lighting system in a recently completed 56,000-m² office structure in the San Francisco Bay Area. Natural light serves 3,000 employees in open-plan offices throughout the five-story building. The architectural scheme includes: ceilings that slope from 4.25 m high at the perimeter to 2.75 m at the center; 3.5-m light shelves at the exterior walls; and a central atrium providing light to the interior spaces. The electric lighting system consists of fluorescent fixtures with continuously dimming ballasts controlled by photocells, which supplement available daylight when necessary.

The paper presents a summary of daylighting and electric lighting performance as monitored in several zones of the building. Analysis of detailed measurements on the third floor for four unoccupied days in May indicates that on the brighter south side, the potential for dimming during occupied periods is to 44% of full power. On the dimmer north side of the third floor, the potential for dimming during occupied periods is to 31% of full power based on an eight-day block of data in July. Analysis of detailed measurements during occupied periods indicates that on the third-floor south side the actual average power consumption is 75% of full power over nine-days in May. On the third-floor north side, the actual average power consumption during occupied periods is 50% of full power for eight days in July. Significant potential for daylighting is not being realized.

The paper discusses the potential benefits of daylighting in the context of the overall building electrical energy use. Analysis of annual electricity use indicates that the ambient lighting electrical circuits represent 23% of the total building electricity use. Present operation of the building is consistent with dimming of the ambient lighting electrical circuits to 85% of full power. There are significant opportunities for dimming that are not used. Proper integration of the electric light dimming system is essential for the realization of projected savings in electric power consumption.