Harvesting Light: Benefits and Strategies

Lighting 0319 Photo Credit: Shutterstock / Bullstar
Photo Credit: Shutterstock / Bullstar

In 2018, the California Lighting Technology Center (CLTC) published “Daylight Harvesting for Commercial Buildings Guide,” a best practices guide to designing daylight harvesting systems. While focused on compliance with California’s tough energy code, the information has broad application.

In review, daylight harvesting is a lighting control strategy in which interior lighting automatically adjusts to maintain a target light level, thereby saving energy. It is most effective in areas receiving consistent, ample daylight, such as locations adjacent to windows.

The CLTC guide describes the benefits of daylight and various daylighting strategies, notably sidelighting (e.g., windows and clerestories) and toplighting (e.g., skylights). It then dives into lighting and control considerations.

Identify lighting for control: The first step is to identify areas of daylight availability—called daylight zones—that are suitable for daylight harvesting control. For sidelighted spaces, the daylight zone may be primary (directly adjacent to the daylight source) or secondary (adjacent to the primary zone). Energy codes define their minimum dimensions, and lights in these zones are controlled independently from the rest of the general lighting in the space.

Arrange the lighting: Daylight generally contributes to ambient illumination, making daylight harvesting most suitable for general lighting. Daylight may penetrate a deep sidelighted space as a series of gradients, with daylight levels reducing with distance from the daylight aperture.

As a result, luminaires are typically arranged in individually controlled groups parallel to the daylight aperture. In spaces with skylights, daylight is typically distributed with a fair degree of uniformity throughout the space, meaning the lights can be arranged by lighting need.

Assign the lights to controls: Called control zoning, this defines which luminaires are simultaneously controlled by a single controller. Though not covered by the CLTC guide, this step is implicit in the design process.

A single lighting controller may control all lights in a given daylight zone, though for more responsive control, the lights can be grouped in smaller zones all the way down to single luminaires.

Determine the control method: Choices include simple on/off, stepped switching, stepped dimming and continuous dimming. With on/off, all luminaires are turned off when light levels reach a target set-point. With stepped switching, certain lamps/arrays or luminaires are turned off. Stepped dimming involves a stepped light reduction in all luminaires, which may involve separately switching lamps/arrays or dimming to one of several preset outputs. Continuous dimming is similar but includes at least 10 steps.

Though not in the CLTC guide, switching is more suitable for circulation spaces such as lobbies and corridors, while dimming is better suited for spaces with stationary tasks, such as offices and classrooms.

Determine the available daylight: Daylight can be detected or predicted using photosensors (also called light or daylight sensors) and astronomical time-clocks, which signal a connected lighting controller that determines if the lights should be changed and, if so, by how much. Of the two, photosensors are considered more reliable because daylight availability may be highly variable.

Key considerations for photosensors are spectral and directional sensitivity and whether they are open or closed loop. The sensitivity should be matched to the application, accounting for typical daylight levels encountered in different areas. Closed-loop sensing monitors the light falling on the sensor from both daylight and electric lighting. The sensor typically is installed indoors facing away from the daylight source. Open-loop sensing monitors daylight only. The sensor typically is installed outside or at an aperture facing the daylight source.

Each has advantages and disadvantages. According to the CLTC, the main disadvantage of open-loop sensing is that it doesn’t account for indoor conditions, which may result in overdimming, while the main disadvantage of closed-loop sensing is it can be more challenging to place the sensor and determine its direction and field of view. Another option is dual-loop, which increases reliability by using closed-loop sensing to monitor light levels, while the open-loop sensor helps determine if a change in signal was produced by daylight changing.

Conduct commissioning/acceptance testing: The steps of commissioning daylight harvesting controls include verifying luminaire types, placement and zoning to controls; confirming installation and proper operation of controls; tuning the controls for the application; and verifying proper operation of the controls during daylight changes.

Daylight harvesting remains a viable lighting control strategy and a key mandatory requirement in prevailing commercial building energy codes. The CLTC free online guide is available at bit.ly/2x4FLJ4.

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