In August 2018, the California Lighting Technology Center (CLTC) published a best practices guide for designing daylight harvesting lighting control systems. Titled Daylight Harvesting for Commercial Buildings Guide, it focuses on compliance with California’s Title 24 energy code, though it has broad application.
Daylight harvesting, or daylight response, is an automatic lighting control strategy in which interior electric lighting adjusts to maintain a target level, reducing energy costs. It is most effective in areas that consistently receive ample daylight, such as lighting adjacent to windows or near skylights.
The CLTC guide begins with daylighting’s benefits and describes various strategies for introducing it to building interiors, including sidelighting (e.g., windows, clerestories) and toplighting (e.g., skylights). After that, it provides a detailed description of design considerations for automatic daylight response:
Establish daylight zones
The first step is to identify lighting in areas of daylight availability, which can be controlled using daylight harvesting. The majority of energy codes require daylight harvesting and establish minimum zoning dimensions.
In sidelighted spaces, there may be two daylight zones: the primary zone directly adjacent to the daylight aperture, and the secondary zone adjacent to the primary zone. This is to differentiate daylight contribution and response.
General lighting in these zones must be controlled independently of other general lighting in the same space, with some form of automatic control responding to daylight contribution to light levels.
Daylight zone in a typical windowed space, defined here as penetrating into the space a distance of 2 times the window height. Image courtesy of CLTC.
Place the lighting
Daylight generally produces ambient illumination, thereby contributing to general lighting. In sidelighted spaces, daylight intensity declines with distance from the daylight aperture, resulting in gradients radiating from it.
For this reason, luminaires may be arranged in individually controlled rows or groups parallel to the daylight aperture. In skylighted spaces, daylight is often distributed fairly uniformly; the lighting can be designed as needed.
Establish the control zones
Control zones are lamps or luminaires controlled simultaneously by a single controller or controller output. All luminaires in a daylight zone may be zoned to a single controller, or the lights may be grouped in smaller zones or controlled individually for greater control responsiveness and typically higher energy savings.
Choose the control method
Next is to decide how the lighting system will respond to rising light levels. Choices include basic ON/OFF, stepped switching, stepped dimming, and continuous dimming.
Switching is well suited for circulation spaces such as lobbies and corridors. With basic switching, the controller turns all luminaires OFF when the light level reaches a target set-point. With stepped switching, some lamps/arrays or luminaires are turned OFF, which may require the reduction be achieved while preserving lighting uniformity in the zone.
Dimming is well suited for spaces with stationary tasks, such as open offices. With stepped dimming, all luminaires undergo a stepped light reduction, which may involve independently controlled lamps/arrays in the luminaires, or dimming to one of several preset light outputs. Continuous dimming is the same as stepped dimming but with at least 10 steps, resulting in more flexibility.
In its guide, CLTC noted that the widespread availability of LED lighting offers dimming capability that can be leveraged for daylight harvesting. For example, the luminaires could be dimmed to a very low level rather than turned OFF, which can reduce potential confusion among users thinking the lights are malfunctioning instead of purposely turned OFF. (Alternately, the owner could educate users about how the controls work.)
Select technology
To respond to daylight, the control system has to “see” it or otherwise predict it will be present. The CLTC guide goes into significant detail about light-detection and prediction methods, notably photosensors (light or daylight sensors) and astronomical time-clocks. Of the two, photosensors are considered more reliable because daylight availability may be highly variable.
For photosensors, important considerations are spectral and directional sensitivity, and open- or closed-loop operation. Sensitivity should be properly matched to the application, which accounts for light levels in different areas types. Closed-loop sensing detects light falling on the sensor from both daylight and electric lighting, while open-loop sensing detects daylight only. Closed-loop sensors typically install indoors facing away from the daylight aperture, while open-loop sensors typically install facing the source of daylight. CLTC points out that both approaches have advantages and disadvantages. Another option is dual-loop, which increases reliability by combining light-detection methods.
Photosensor placement and field of view for open-loop sensors (top) and closed-loop sensors (bottom). Image courtesy of CLTC.
Commission the system
This involves verifying the luminaire types, placement, and zoning to controls; verifying installation and proper operation of controls; tuning the controls; and verifying proper control operation in response to light level changes.
Harvesting daylight
Daylight harvesting remains a viable lighting control strategy and a key mandatory requirement in prevailing commercial building energy codes. The CLTC guide provides useful guidance for compliance.
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