As building owners and architects take substantive measures toward creating more energy-efficient buildings, "daylighting" has emerged as a consistent design strategy.
Studies suggest daylight-illuminated spaces can enhance student learning, increase workplace productivity, contribute to increased retail sales, and engender a sense of well-being in building occupants. It also spotlights interior space as a primary architectural concern.
But even the brightest interior spaces do not save energy on demand. A careful integration of architectural and electric lighting disciplines must also occur. Saving energy with daylight requires the appropriate use of specialized electric lighting controls.
In the context of electric lighting, "daylighting controls" means reducing light output (and the energy required to power the lighting system) when sufficient daylight is present in a space. Certainly, one way to achieve this is by the users simply switching off unneeded electric lighting during daylight hours. However, this is often unreliable or impractical. Frequently, users who share common space either do not feel "ownership" over the electric lighting system, or they are simply too focused on other tasks to attend to continuously changing daylight levels.
In practice, guaranteeing maximum conservation of electric power use requires automation. This is most often done by using a photocell to dim or turn off lamps when light levels within a space reach a specified level. The challenge lies in designing a system that meets four major objectives.
First, and most fundamentally, the controls system must be located in an area that has been designed to provide enough diffuse daylight to warrant the inclusion of electric lighting controls. Second, the system must adjust light levels in a way that does not disturb or annoy the occupants of the space. Third, it should be designed with a clear understanding of the patterns of daylight distribution within the areas of electric lighting control. This ensures that the system has the capacity to respond to the predominant daylight conditions during times that the space is used. And finally, the system must be correctly calibrated to maximize energy savings while maintaining visual comfort. This process is often called commissioning.
Proper commissioning has been shown to be the difference between a system that works and one that does not.
Two recent studies by
Heschong-Mahone Group indicate a number of challenges facing the design and implementation of electric lighting controls for daylight. The good news is that photo-control systems installed at sky-lit spaces perform very well in the vast majority of cases -- regularly cutting lighting power use in half. One of the main reasons for top-lighting controls success is that when daylight is delivered from above, through light-diffusing skylights, interior spaces are not subject to drastic variations in daylight illumination throughout the course of a day. This makes calibration of photocells much simpler, because they do not have to respond to the wide range of solar exposures associated with areas near vertical windows. Although top-lighting is often the most successful daylighting strategy, it is usually limited to high-bay spaces such as gymnasiums, "big-box" retail stores and areas directly under a roof.
Unfortunately, the second study discovered that for nearly half of the photo-control systems installed in side-lighting conditions (where daylight enters a space through a vertical window) the controls are not working to save energy. Furthermore, roughly half of the functioning systems are not saving energy at the rate expected by the amount of daylight present in their respective spaces. Only 25% of these installed photo-controlled daylighting systems are meeting their energy savings potential! Contrary to expectations, this was not because occupants were disabling the controls or because the systems malfunctioned, but simply because they were not properly commissioned to meet the unique needs of the spaces.
Commissioning controls in side-lighting applications is a challenging task. Windows are subject to constantly changing sun angles and glare sources producing a vast array of daylight conditions that must be interpreted by the photo-sensor and controller. Minimum commissioning of systems under these conditions requires repeated visits to the site, during both daylight conditions and at night, to set the proper range of response. Hiring a properly trained commissioning agent is the only way for owners to get the visual comfort and energy performance they are paying for in a well-designed controls system.
Capturing energy savings through daylight and electric lighting integration is here to stay. Many codes, including the Seattle Energy Code and California's Title 24, require photo-controlled electric lighting within the daylight zone: usually 15 feet of windows or in areas under skylights. This is a good step toward a more energy-efficient built environment.
Now we must ensure that the potential savings -- both in energy and in lower operating costs -- are realized.
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Christopher Meek is a daylighting specialist with the Seattle Daylighting Lab.
This column has been reprinted courtesy of nw current, a publication of the Northwest Energy Efficiency Alliance. It was first published on Jan. 30, 2006.