Daylight harvesting is an advanced lighting control strategy used to minimize ongoing owner energy costs. It occurs when a sensor measures daylight levels and signals a control to adjust electric lighting system output to maintain a desired task light level. Variable daylight levels are automatically harvested as energy savings through electric lighting reductions.
Because energy savings will be dependent on factors such as type of available daylight, control response and space and task characteristics, actual savings can be difficult to predict, although studies suggest strong potential. A 2003 study conducted by the National Research Council of Canada discovered 40% energy savings in an open office environment and 50% (with manual blinds) to 70% (manual blinds used optimally, or automatic shading) in private offices. Another 2003 study, conducted by Heschong Mahone Group, found that daylight harvesting strategies can produce 50% energy savings in school classrooms.
LEED 2009 encourages providing daylight and views to users. IEQ, Credit 8.1 awards 1 LEED point for providing a minimum 25 footcandles of daylight in at least 75% of regularly occupied building areas. IEQ, Credit 8.2 awards 1 LEED point for introducing views in at least 90% of regularly occupied building areas—that is, a direct line of sight to the outdoor environment via vision glazing 90 in. above the floor. Because strong daylight availability is essential to daylight harvesting, these buildings are well suited to this control strategy. Daylight harvesting, in fact, is favored in LEED projects, not only because of daylight availability, but because energy points are based on exceeding ASHRAE/IES 90.1-2007, and because daylight harvesting is not required by this standard, its energy savings can be captured as LEED energy points. Further, the Green Interior Design & Construction version of LEED further awards 2 points for introducing daylight harvesting controls in all daylighted areas (1 point) and/or on 50% of the lighting load (1 point).
Because of the strong energy savings potential offered by daylight harvesting, coupled with advancing technology, codes and standards are now beginning to address daylight harvesting—specifically, International Energy Conservation Code (IECC) 2009, ASHRAE/IES 90.1-2010, ASHRAE 189.1 and Title 24-2008. In review, IECC 2009 and ASHRAE/IES 90.1-2010 are energy standards offered as model energy codes for states and other jurisdictions. ASHRAE 189.1 is a green building standard. And Title 24-2008 is California’s unique energy code.
All of these codes and standards are different and yet have similar major themes. First, they define daylight availability as zones around sidelighting (e.g., windows) and toplighting (e.g., skylights and roof monitors) daylight apertures. Second, they require separate control for general lighting in these daylight zones. The standard may also specify whether the control must be manual or automatic, switching or dimming, stepped switching or simple ON/OFF. And the standard may reward aggressive daylight harvesting with power adjustment credits that can be used to acquire greater design flexibility with the controlled load.
Let’s look more closely at the daylight harvesting requirements published in IECC 2009 and ASHRAE/IES 90.1-2010. First, what is the daylight zone? After all, daylight harvesting is entirely dependent on daylight availability in the space. Both IECC 2009 and ASHRAE/IES 90.1-2010 define daylight zones using formulas based on size of aperture and whether there are any obstructions blocking the light, with ASHRAE’s approach being similar to ASHRAE 189.1 and California’s Title 24-2008 code. Sidelighted daylight zones are defined as depth x width adjacent to the aperture, and toplighted daylight zones are defined as length x width under the aperture. ASHRAE/IES 90.1-2010 includes helpful drawings detailing daylight zones.
IECC 2009 offers a basic approach to daylight harvesting control by simply stating that general lighting in these zones must be separately circuited and controlled from other general lighting in the space. The method of control is not specified, so the designer has a choice of switching or dimming. ASHRAE/IES 90.1-2010 goes much farther with an approach that is similar to California’s Title 24-2008 energy code:
Sidelighted spaces: If the sidelighted daylight zone is larger than 250 sq.ft., then the control method must be automatic and multilevel (or continuous dimming), providing one step between 50% and 70% of the design lighting power, and another between OFF and 35%. ASHRAE/IES 90.1-2010 encourages more aggressive daylight harvesting strategies in sidelighted office, meeting, classroom, retail sales and public space types with credits that can be used to increase the power allowance for the controlled lighting load. Recognized strategies include continuous dimming control and automatic control of general lighting in secondary (deeper) daylight zones in sidelighted spaces.
Toplighted spaces: In toplighted spaces, if the total daylight area under skylights plus the total daylight area under rooftop monitors is larger than 900 sq.ft., the general lighting must be separately controlled using either a stepped switching or continuous dimming controller, with some exceptions. As with sidelighted spaces, more aggressive daylight harvesting control in toplighted areas is rewarded with power adjustment credits.
Additionally, perimeter lighting in parking garages is required to be automatically reduced in response to daylight, with some exceptions.
Demand for daylight harvesting controls has grown dramatically in recent years, driven largely by sustainability initiatives such as LEED. Since 2005, California’s energy code required daylight harvesting in certain spaces. Now the major energy standards—IECC and ASHRAE/IES 90.1—contain significant requirements for daylight harvesting control, signaling widespread acceptance and adoption of this control strategy in the future.