Sidelighting Photocontrols Field Study


Heschong Mahone Group, Inc., Report #06-152, 2005

Review by Scott Schuetter, Energy Center of Wisconsin: 

The Heschong Mahone Group, supported by Southern California Edison, Pacific Gas and Electric and the Northwest Energy Efficiency Alliance, conducted this study of existing daylight-responsive lighting controls in sidelit buildings. The study covered 123 spaces in 49 different buildings mostly consisting of offices and classrooms. During the study, measurements were taken for two weeks. Each space included measurements of the controlled electric lighting's current, the uncontrolled electric lighting's current, and the vertical illuminance entering the window. Additional measurements were made of the illuminance level at the photosensor and on the critical task if possible. Through these measurements, the electric lighting savings for the actual, installed daylight control system was calculated by comparing the measured current to the total current during occupied periods.

An energy model of each space was then constructed in DOE-2.2 using eQUEST as a front end. The simulation used real weather data collected over the same two week period as the measurements. In this way, the electric lighting savings for an idealized, perfectly operating daylight control system was calculated. The ratio of measured savings to predicted savings, or Realized Savings Ratio (RSR), was then calculated for each space. An RSR = 0 means that the photocontrols are saving no energy, while an RSR = 1 means they are working as designed.

Of the spaces studied, 64, or a little over 50%, had controls that were either not functioning or achieving no savings. 35 of these systems were intentionally disabled by setting the sensor illuminance target too high, putting tape over the photosensor, or disconnecting the wire to the photosensor. 9 of the remaining systems had never worked or never been activated. However, older systems were actually found to save more energy than newer ones. Also, it was common that all of the spaces in a building with daylight controls were disabled together, instead of only in problematic spaces.

The daylighting control systems in the remaining spaces were achieving actual savings of roughly half of the predicted energy savings. This equated to a lighting energy savings of approximately 1.1 kWh/sf-yr and a net peak demand reduction of approximately 0.6 W/sf of photosensor controlled area. Higher levels of energy savings were correlated with more uniform daylighting and higher levels of daylight illuminance. These were often accomplished via windows on multiple facades, utilizing glazing with high visible transmittance, ensuring that the interior surfaces had high reflectances, and minimizing partition heights. Dimming controls had higher rates of functionality with only slightly less overall energy savings when compared to stepped systems. Further, the highest performing systems were in spaces with controlled zone depths no greater than 2 times the window head height. Finally, the study concluded that both integrating the design of the architecture, lighting and controls design as well as educating the building occupants were instrumental in the success of designing daylighting controls systems.