Daylight Harvesting: Riding Beams of Sunlight To $avings and Efficiency

March 22, 2005
Soaring electricity prices have pushed the quest for energy efficiency from the rarified realm of tree-hugging environmentalists straight to the bottom line of companies’ balance sheets. Lighting companies have responded not only with a new generation of power-stingy light sources but with increasingly sophisticated automation devices that encourage entirely new ways of looking at the how and when and where of lighting.Among the more efficient new illumination sources are low-power light emitting diodes (LEDs) and ever-better ballast technologies. Just as significant, though, are a bevy of rapidly evolving automation controls, including:Simple timers that automatically turn off lights after business hours.More sophisticated timers that switch lights on and off in patterns that precisely match the varied routines of a building’s occupants. Internet-based (TCP/IP) devices that tie the control of lighting to other elements of a building automation system, including HVAC and security. (This also helps enable the concept of load shedding to reduce kilowatts consumed by electrical lighting in conjunction with HVAC and avoid peak utility rates.)Ever more sophisticated and inexpensive occupancy sensors that detect the presence of people, either in zones of a building or in individual offices, and provide light when they arrive or turn it off when they leave.Manual A-B switching that enables occupants to manually select desired lighting levels from full off to half, two-thirds or full on.Local overrides that free occupants from total reliance on fixed time-of-day schedules.But perhaps the most effective of all trends in energy-efficient lighting is not a product at all but a complex system that blends the best of new lighting technologies with intelligent design strategies and ties them both to building automation schemes. At its most basic, this system, known as “daylight harvesting,” simply employs light-level sensors to detect available daylight and then to modulate the output of electric lights to compensate for light coming into an architectural space from the outside. But that is a deceptively simple description.Daylight harvesting is beneficial from two standpoints: sunlight is good for people, and electricity is expensive, both financially and environmentally. Yet most lighting systems in schools, offices and retail spaces operate at full output during all hours of operation regardless of how much sunlight is available. The amount of natural light available to any given building differs by geography and the building’s design, but on average, the sunlight available to interiors through windows and skylights can provide sufficient light for most educational and business activities.The financial costs of not dimming electric lights include unnecessarily high electric bills for lighting and for the air conditioning required to remove heat created by lights. But the total costs go far beyond economics to include eye strain because of excessive brightness and even a lessening of emotional and intellectual well being. Combining good building design with automation to create the process known as daylight harvesting is the preferable way to deal with these problems because, as any facilities manager will tell you, counting on occupants to manually dim lights is highly unreliable. “When properly designed and effectively integrated with the electric lighting system, daylighting can offer significant energy savings by offsetting a portion of the electric lighting load,” the U.S. Department of Energy asserts in the introduction to its Energy Efficiency and Renewable Energy Building Technologies Program. And the DOE goes on to note a non-economic benefit that has been proven time after time in decades of well documented research: “In addition to energy savings, daylighting generally improves occupant satisfaction and comfort. Recent studies are implying improvements in productivity and health in daylighted schools and offices. Windows also provide visual relief, a contact with nature, time orientation, the possibility of ventilation, and emergency egress.”In a 1999 report titled “Daylighting in Schools—An Investigation into the Relationship between Daylighting and Human Performance,” the Heschong Mahone Group found that children being taught in naturally lighted classrooms achieved 13% to 26% higher scores on standardized tests than those being taught in artificially lighted classrooms.Nor is natural light important just for keeping workers smiling and students alert. “Consumers prefer shopping in stores that use natural light to bathe aisles and render colors accurately,” Venture Lighting proclaims in its marketing literature. “Studies have reported an increase in sales as high as 40% as a result of daylighting in big box retailers.”Venture Lighting is among a growing number of fixture and control manufacturers that are cashing in on the daylight harvesting trend. For example, Lighting Control & Design and The Watt Stopper each produce lighting integration technology that ties together the various sensing devices and building automation networks that make daylight harvesting possible, while Lutron’s EcoSystem technologies interface with those integration devices and others to enable individualized control of lamps and fixtures. Venture Lighting produces a lamp that closely mimics natural light so that when clouds roll in or darkness falls, the transition from sunlight to artificial light retains consistent brightness and colors. “The type of lamp incorporated with daylight harvesting is critical,” Venture states. “The lamp’s light quality should blend well with natural light and provide good color rendering and contrast.”Taken together, these lighting and control technologies must be aggregated to produce hybrid systems for daylight harvesting. While payback time may make the cost of such systems prohibitive for retrofits of older buildings, the cost benefits of their use in new construction is a simple economic algorithm: With daylight harvesting, bright sunlight can provide up to 100% of illumination during the times of the day when energy costs are highest.In most instances, daylight harvesting is part of a larger strategy for cutting costs and improving interior lighting. For example, the Brighton Charter school in Colorado was given lower rates by the local utility in return for allowing the utility to dim the school’s lighting during sunlight hours. The utility was able to dim the lights remotely by using controllable ballasts made by Electronic Lighting Inc. (ELI) of Newark, Calif., which designed and engineered the retrofit. Coupled with daylight harvesting and demand limiting, the project achieved a kilowatt reduction of about 54%, or 130,035 kilowatt hours per year. With savings like that, the reduced utility rates were almost incidental.Incorporating daylight harvesting into designs of new construction by emphasizing “fenestration” — the choice and placement of glazed openings — goes well beyond sensing and control issues, enabling  architects to deal intelligently and creatively with the induction of daylight. Top-lit designs using skylights or clerestories are the most obvious means of funneling light into a building and dispersing it evenly. But even the use of large view windows can make a huge impact, with sensors and automation controls compensating for ever dimmer portions of an interior as the distance from windows increases. The trick with view windows is getting wanted light without unwanted glare, and it is important to keep in mind that thermal comfort can also be reduced by poor fenestration.The National Best Practices Manual:  Daylighting and Windows offers the design criteria for six different fenestration devices. This Best Practices Manual and several informative case studies on daylight harvesting are available on the Daylight Harvesting page of TRIATEK Lighting’s Web site ( compensation of daylight can be controlled either by dimming (varying the light output to provide the desired brightness) or by switching (turning individual lamps or fixtures in different areas of a building or room off or on as necessary). Just as a typical two-lamp fluorescent fixture can be switched to illuminate both lamps, one lamp or neither lamp, multiple fixtures can be turned on or off individually to bathe each part of a room in just the needed amount of light. In addition, dimmable ballasts can adjust the output of individual lamps to achieve even greater control. Dimmable ballasts are more expensive than switched systems, but they also offer greater energy savings, and the light they produce is often easier on the eyes than the output of switched systems. According to, a Web site supported by a consortium of government and private organizations, dimming offers two advantages beyond energy savings over switching systems: First, conventional lighting systems are typically designed to over-illuminate rooms to account for the 30% drop in lighting output over time. Electric light dimming systems automatically compensate for this reduced output to give a constant light level over time. Second, daylighting controls can be adjusted to give the desired light level for any space. Thus, when floor plans are changed, it is easy to adjust the light levels to meet the lighting needs of each area (provided the system is zoned properly and has sufficient lighting capacity).What is most important to keep in mind is that daylight harvesting is a system, and all the elements of that system must be considered. Whether dealing with an existing building or a new design, system design begins with fenestration. Next, light compensation must be achieved with gradations of illumination, produced either through switching or dimming/brightening to maintain balanced light levels that illuminate without generating glare. And finally, to maximize savings, the glazing and HVAC system must be designed into the daylighting system. Glare should be reduced through fenestration design, so that heavily tinted windows can be avoided. The cooling system can be reduced in size when solar gains are at their highest and lights are dimmed to their minimum. advises that daylighting works best with indirect lighting because occupants are less likely to notice changes in electric light output. Conversely, daylighting compensation does not work well with spot lighting. Maximum energy savings (up to 75%) are achieved when the lighting system is controlled by both daylighting and occupancy sensors.Without question, daylight harvesting is a complex subject that requires a great deal of planning and the evaluation of multiple lighting products and control systems. But the savings that can be realized are well worth the effort, and the enthusiasm of lighting and control companies for the concept promises that the economics will only continue to improve in years to come.

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