Guest Article: Daylighting Controls Q&A / Best Practices

Michael Myer is a researcher at the Pacific Northwest National Laboratory.

Daylighting controls use a photocell to reduce or turn off electric lighting when ample daylight is available. Michael Myer answers questions about the technology and how to use it most successfully for interior projects.

What are the three types of interior daylighting?

1. Sidelighting – daylight enters from vision windows or clerestories (high windows above field of view).
2. Toplighting – daylight enters via penetrations (skylights) in the roof.
3. Roof monitors – vertical or tilted (known as sawtooth) fenestration above the ceiling plane allow daylight into the space.

What type of control loop should you consider?

• Open loop – the photodiode registers the daylight and sends a signal for the electric lighting to dim or turn off. Open loop is low risk and relatively easy to commission. If you do open loop, make sure that the photodiode is directed at the daylight. In toplighting applications, this could involve placing the photodiode on the roof.
• Closed loop – this strategy factors in both the daylight and electric light in the space. In a closed- loop strategy, the photodiode is registering light inside the space and managing a certain desired illuminance. Commissioning and long-term operations may be harder with a closed-loop system because temporary changes in the space could fool the sensor; for example, white papers left all over a desk could increase light reflected towards the sensor, but not actually increase the amount of light in the space.

Are daylight controls required in energy codes?

• Yes, most energy codes require some form of daylighting controls. IECC, ASHRAE/IES Standard 90.1, and California Title 24 all require controls if the installed power (e.g., 150 watts in the primary daylight zone) falls within a boundary defined by the fenestration height, width, length and other parameters. The Washington energy code opts to base daylighting energy code requirements on a certain number of luminaires located within the daylighting zone rather than a power-based approach.

Should I opt for zone-based or luminaire level lighting controls (LLLC)?

• LLLC has a sensor is installed within the luminaire. Many manufacturers make dual occupancy/daylight sensors. LLLC allows for greater coverage by smaller zones. Many utilities offer incentives for LLLC.
• In zone-based daylighting, a photosensor is located on the ceiling and control wires are run to each luminaire in the daylighting zone. In a zone-based approach, more luminaires are dimming together. As a result, sensor placement and commissioning require more review, coordination, and commissioning. Zone-based control is more common in toplighting applications.

What energy savings are possible?

• Daylight harvesting can yield energy savings in many climates and latitudes—whether mainly clear skies or overcast skies—and many building orientations. Toplighting will probably lead to greater savings when sky and building position plays a lesser role. To maximize savings in sidelighted spaces, prioritize south-facing, then west-facing, then east-facing portions of the building. Key sidelighting zones are limited to about 15 feet along the building perimeter, assuming standard U.S. ceiling heights.