A field test shows what can be expected of daylight harvesting techniques.
The idea of dialing down room lights when the sun shines seems like a great way saving energy. Researchers at Rensselaer Polytechnic Institute’s Lighting Research Center (LRC) recently tested out the idea using a daylight-switching device called the DaySwitch. The DaySwitch hooked into existing lighting fixtures and basically reduced the room lighting as light coming through the windows became more intense. LRC research team collaborated with manufacturing partners to build DaySwitch prototypes and recently field tested the devices in a wide range of locations across the Rensselaer campus in Troy, N.Y.
DaySwitch devices were installed in 72 locations across campus, each with varying degrees of daylight access from windows or skylights. To install the DaySwitch, an electrician mounts the device's microcontroller inside the luminaire to switch the lamps on and off. A photosensor connects to the microcontroller via wires and mounts outside the luminaire to monitor daylight levels.
The results of the field test, however, were mixed. Private offices showed little or no energy savings, essentially because they get little sunlight -- blinds, window tinting, and obstructions are problems -- and because they just aren't occupied all that much. Researchers also say some private offices on the campus were over-lighted, which interfered with the ability to switch off the lights because the Dayswitch determines its switching threshold as a function of the measured electric light level.
As you might expect, the DaySwitch provided the most energy savings in large open spaces with plentiful daylight, high luminaire wattages, and long hours of use.The LRC estimates that the five most promising site selections had a simple payback period of two to seven years. They say this period would be less if Rensselaer's electric rates were higher (it pays less than $0.10/kWh) and if summer conditions were included. (The tests took place in the winter time.)
