Daylighting: Measuring the Performance of Light Shelves and Occupant-Controlled Blinds on a Dimmed Lighting Systems

The design of a day lighted space is both an art and a science. The biggest challenge facing the lighting designer is to admit only as much light as necessary and distribute it evenly throughout the space without introducing glare or heat. In warm climates such as Florida, it has become common practice in windowed spaces to specify blinds and glazing with high shading coefficients to control glare and minimize heat gain. However, this practice reduces the effectiveness of lighting systems that dim automatically. Improved systems are needed to capture natural daylight and distribute it uniformly throughout a space while controlling heat gain and glare. One such system is the light shelf. Light shelves shade the space from direct sunlight and reflect this sunlight onto the ceiling for a deeper and more uniform distribution. While this is not a new idea, littIe unbiased empirical data has been collected, outside the laboratory, that compares the performance (energy savings, uniformity, and level) of an automatic daylighting system. This study measures the effectiveness of light shelves and manually controlled horizontal blinds in an automatic daylighting system. Power consumption and interior work-plane lighting levels were compared in four essentially identical private offices. Two offices were configured with an interior light shelf, one with a white diffuse top surface and the other with a specular surface. The third office had no window treatment and the fourth office had horizontal blinds, which were manually adjusted by the user. All offices had two lamp fluorescent luminaires with dimming ballasts (min. 20%) controlled by a ceiling mounted photosensor. The study showed that daytime savings ranged from 29% to 46%, with the largest savings from the office with the light shelves. The office with horizontal blinds showed the poor savings (32%) and also the poorest light uniformity and level. INTRODUCTION Recent introduction of electronic dimming ballasts have excited utilities, facility managers, and engineers in their search for new technologies to reduce energy. Previous use of these dimming ballasts was primarily for manual dimming in areas such as conference rooms, although now they are increasingly being used to capture savings automatically in areas where daylight is available. The most common application of these ballasts appprs to be in areas daylighted by windows. Typical daylighted offices in warm climates such as Florida often have internal or external shading devices to reduce heat and glare. Common shading devices are window film, vertical or horizontal blinds, drapes, and overhangs. While these devices often control heat and glare they do so at the expense of visible light. The reduction in visible light reduces the overall effectiveness of the dimming ballasts. For instance, in three large commercial buildings in Canada (5 1 ON latitude) it was found that daylighting systems functioned poorly due to the use of glazings with low visible transmittance and low interior reflectance (Love, J. A., 1995). A literature review yielded few studies that examined the interaction that shading systems have on daylight-linked fluorescent dimming in the field. Schrurn et al. (1996) researched the effects of window orientation on savings, however the blinds were fixed in horizontal position. Savings were found to be greatest on the southern exposure (37%). Another study conducted in a Florida school cafeteria, measured savings due to dimming at 27% although many commissioning difficulties were encountered (Floyd et al. 1996). This study expands the knowledge by examining different shading systems in "real world" offices side lighted from the south. In the study, four very similar offices were instrumented to measure power and illumination to determine how different shading devices effect power savings and illumination uniformity. The reason for conducting the study was two fold, first to select a suitable shading device for the southern offices at the Florida ESL-HH-98-06-42 Proceedings of the Eleventh Symposium on Improving Building Systems in Hot and Humid Climates, Fort Worth, TX, June 1-2, 1998 Solar Energy Center's newly constructed building and second to evaluate each devices performance, primarily, energy savings, illumination uniformity and glare control.