Daylight is one of our most abundant natural resources. Farmers have timed their waking hours to daylight’s availability. However, most of us are usually indoors and dependent on artificial lighting, even in daytime. The need to reduce energy consumption is fueling interest in using daylight as a somewhat free resource. However, getting daylight to the inside of buildings where it can be used has real costs.

According to the Lighting Research Center at Rensselaer Polytechnic Institute, Troy, N.Y., (LRC), “Energy cost savings in practice are highly dependent on the behavior and cultural values of occupants—if occupants are provided with opaque aluminum blinds and leave them closed all the time to ensure privacy, then no lighting energy savings are possible. If occupants value daylight and the view out or are motivated to minimize energy costs, substantial savings are possible.”

Daylight harvesting seems like a good way to go. You put up a measuring sensor near a window and it automatically lowers the electric lighting to compensate when the window light is bright enough. However, it’s not that simple. The LRC, in its survey of daylight harvesting in commercial buildings, distinguishes three different scenarios:

• In a small office, an automatic system that turns off or dims electric lights in response to daylight can save up to 50 percent of energy costs if used with a manually operated window blind system. The savings could rise to 70 percent if the blinds were optimally controlled.

• A system that progressively dims lights in a large open-plan office could save 40 percent if used with manually controlled window blinds.

• In large partitioned offices, an automatic system would only save energy in the first row of offices with high partitions.

A few of the available systems suggests a wide range of sophistication.

At the simpler end is the LS-101 daylighting controller and newly released LS-102, both by WattStopper/Legrand, Santa Clara, Calif. These are single-zone, on/off devices that can be installed in an open or closed-loop application to turn lights off automatically when sufficient natural daylight is present.

Lutron Electronics Co., Coopersburg, Pa., makes EcoSystem—which balances inputs from multiple sensors and controls and can perform dimming functions for multiple fluorescent ballasts—with plug-and-play components that communicate over low-voltage wires. The company’s Quantum system can integrate EcoSystem with Sivoia motorized shading and Grafik Eye QS, which provides a wall controller that manages it all.

Ciralight Global, Irvine, Calif., offers a skylight system that uses a GPS unit to track the sun. A lens and mirror design tracks, captures and magnifies the natural sunlight at a level of intensity and clarity that the company claims makes electric lighting unnecessary during daylight hours.

Will it work?

Consider an article by Craig DiLouie for the Lighting Controls Association, “Why Do Daylight Harvesting Projects Succeed or Fail?” Problems included poor system design that didn’t properly match control zones to light patterns and improper calibration. Success required proper design, installation and calibration, but also training, maintenance and recalibration over time to account for experience with the installation and changing conditions.

In February 2009, this column featured the New York Times office building—a project that used energy efficiency and daylight harvesting throughout. Lutron’s DALI-based dimmable lighting control system works with automated motorized shades. Performance data issued by Glenn Hughes, director of construction, said the design goal was 1.28 watts per square foot for lighting. Since opening in November 2007, the lighting load has been 0.38 watts or less per square foot, a yearly energy savings of 72 percent.

What made the Times building a success? First, the structure was designed around daylight harvesting, under collaboration with the Lawrence Berkeley National Laboratory (LBNL), the New York State Energy Research and Development Authority, the U.S. Department of Energy and the California Energy Commission. A full-scale model of a corner of the building helped builders evaluate available systems. Once the building was up and the systems were installed, commissioning began before the new occupants moved in. LBNL wrote a test procedure that used a commissioning cart assembled by Lutron, a digital camera with a fisheye lens and other special equipment. The final commissioning report said, “The system was not used by the Times to simply pass/fail the manufacturer as initially intended. Instead, it was used to enable a productive evidence-based discussion between the Times and the manufacturer so that the two parties could collaboratively diagnose and tune the system.”

You can conclude the choice of successful daylight harvesting approach should be based on a realistic task appraisal. Start with the budget. If cost is limited, use a simple on/off system, with less pricey and more effective equipment, installation and commissioning. Energy can be saved regardless of the system’s sophistication as long as it is carefully designed and then installed and commissioned at a level appropriate to the design.

BROWN is an electrical engineer, technical writer and editor. For many years, he designed high-power electronics systems for industry, research laboratories and government. Reach him at ebeditor@gmail.com or at www.writingengineer.com, an independent professional writing service.