Harvest Daylighting's Bright Potential With Ongoing Monitoring

Overview

This feature by contributing editor Rita Tatum ran in the April 2014 issue of Building Operating Management under the magazine's "Close-Up: Daylighting" banner. Its premise is that daylight harvesting usually saves energy but rarely as much as designed, because occupants and facility staff override controls to quiet complaints and because spaces change after move-in. Tatum draws on a field study from the Energy Center of Wisconsin, a Lawrence Berkeley National Laboratory literature review, and the National Renewable Energy Laboratory's own office building to explain how facility managers can keep the savings.

Key Findings

• In 20 commercial and public-assembly spaces studied by the Energy Center of Wisconsin, daylight harvesting was saving a median of 20% of lighting energy as found, plus about 3% more once HVAC effects were counted. Sixteen of the 20 were capturing only about half their potential, and four were saving nothing.
• After a recommissioning visit of roughly two hours per space, average lighting savings rose to 50%, or 57% with HVAC included.
• Diagnosed faults included eight spaces needing major sensor calibration and five needing minor calibration, three with oversized zones, three with heavy shading added to cut glare, two with bad relays, and others with tall cubicle dividers or disabled deadband settings.
• A 2012 LBNL review of about 250 documented installations put the best estimate of daylight harvesting savings at 28% compared with conventional lighting.
• NREL's 218,000-square-foot Research Support Facility in Golden, Colorado, runs at an operating lighting power density under 0.2 watts per square foot against 0.6 installed; NREL lighting engineer Jennifer Scheib attributes roughly half of that reduction to daylighting.
• Practical monitoring tips: use a smartphone light-meter app on walk-throughs, submeter lighting circuits to spot energy spikes, and target about 10 footcandles in some zones and closer to 30 in others rather than a single setpoint.
• A sidebar cites Heschong Mahone Group research finding 20% better student learning rates with daylight and, in a chain of more than 100 stores, 40% higher sales in skylit locations.

Why It Matters for Daylighting

The article is essentially a field manual for keeping a skylight investment paying off. Prismatic skylights over a warehouse floor or retail sales area can deliver most of the needed light for much of the day, but as Lighting Controls Association education director Craig DiLouie says in the piece, daylight alone saves nothing until a control turns the electric lights down. The Wisconsin numbers show that a system left alone tends to settle at a fraction of its design savings, while a short recommissioning pass more than doubles them.

For high-bay spaces the specific fixes are familiar: sensors aimed at the daylight zone rather than at racking, zones sized to the skylight layout, controls left in automatic, and a light-level check each season. The sidebar's retail sales and classroom findings are a reminder that the benefits of toplighting run beyond the utility bill.

About the Source

Building Operating Management is a long-running facilities trade magazine published by Trade Press Media Group and read by owners and facility managers of large commercial and institutional buildings. The article, by Rita Tatum, who has covered facility design and technology for more than 30 years, appeared in the April 2014 print issue on pages 18 to 23 (four editorial pages in the library copy). Online it runs as a three-part series under the title "Use Ongoing Monitoring To Get The Most Out Of Daylighting," linked here.