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TECHNOLOGY CORNER: Daylight Harvesting Systems
Mar 11 2010
Daylight harvesting systems reduce artificial or electric light in building interiors when natural daylight is available, in order to reduce energy consumption. These systems can provide energy and demand savings, but have been shown to be expensive to install, commission and maintain. Some systems have had problems with consistency and user satisfaction. Researchers at the California Lighting Technology Center at the University of California, Davis, developed a new approach called the Simplified Daylight Harvesting System (SDH) that is easy to install and provides automatic and continuous calibration. The system uses photosensor readers to set lights to on, off or intermediate levels and gives users the ability to adjust the on and off setpoints to meet their own preferences—a key element in user acceptance of daylight harvesting.
Several studies have recorded a broad range of energy savings through daylight harvesting. Daylight harvesting works best in spaces with access to windows or skylights, and where electric lighting would otherwise be left on for long periods such as offices, atria and schools.
The SDH consists of a photosensor to measure light levels, relays to switch the states of the electric lights, a controller that determines when to change lighting states, and an optional occupancy sensor. Together, these elements create a system that is reliable, user friendly, and cost effective. The SDH control logic uses photosensor measurements to switch among the off, low-output and high-output states in a bi-level electric lighting system.
Differences in photosensor signals are automatically calculated every time the lights are switched and serve to calibrate and govern the system’s response. This process accounts for changes in furniture layout and reflectance of interior surfaces, and also enables the system to adapt to the decreasing levels of electric light that are available as the lamps age. A time delay is programmed so that the system doesn’t respond unnecessarily to transient changes in the daylit environment. A time delay works well for on/off systems because they are generally expected to work once in the early morning and once in the late afternoon. The system has been tested in the bi-level lighting configuration, but is also designed to switch to any number of intermediate light levels.
Several features are aimed at increasing user acceptance of the SDH system. First, the system will allow occupants to adjust the on and off setpoints to match their lighting preferences. Sufficient separation between the setpoints will also help to minimize the on/off cycling that sometimes annoys occupants under conventional daylight harvesting systems. Second, an optional occupancy sensor is available for switching only when the space is unoccupied so that occupants do not experience a sudden drop in light level. In addition, if sudden changes are a concern, conventional bi-level ballasts can be replaced by ramping ballasts that gradually adjust lights from one level to the next.
All of these features will enable users of the SDH to reliably reap the financial benefits of daylight harvesting—significant energy-cost savings and reductions in peak demand charges because peak daylight availability typically coincides with peak electricity demand.
Information excerpted from Daylight Harvesting Made Simple prepared by
California Energy Commission’s Public Interest Energy Research Program
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