The sun is an important resource for architects and designers who wish to promote energy efficiency and environmental quality in built spaces. Daylight can increase light levels, provide good color rendition, and positively impact immune system health. Numerous research studies indicate that daylight can enhance satisfaction, performance, learning, and retail sales. Studies conducted by Heschong Mahone Group, for example, found an increase of up to 40 percent in retail sales in stores with skylights compared to those without any daylighting, and a 21 percent improvement in learning rates, as well as 7 to 18 percent higher test scores in classrooms with daylighting compared to those without. Another study of 100 workers in a call center found that employees with the best possible views processed calls 6 to 12 percent faster than other colleagues.
Daylight offers physiological and psychological benefits, and is encouraged in green-building rating systems such as LEED, but what makes it truly green is leveraging higher levels of daylight to reduce electric light use, thus saving energy and reducing carbon emissions. This is often accomplished by using a photocontrol system that adjusts electric light levels based on daylight availability to maintain an overall target light level, reducing energy use as daylight increases.
How much energy can be saved? The Lawrence Berkeley National Laboratory (LBNL) analyzed 240 lighting controls’ energy savings estimates from 88 papers and case studies. Filtering the data to focus on the savings in energy use for lighting produced by lighting controls in actual field installations (as field simulations were found to overestimate savings), LBNL estimated that the average lighting energy savings from harvesting daylight is 28 percent. Actual savings may be higher or lower depending on many factors, such as daylight availability and interior reflectances.
Daylight harvesting controls can be effective in virtually any type of facility where the lights operate most of the time and where daylight is ample. Spaces with skylights and corridors, and private offices and open cubicles near windows—particularly those that are equipped with task lighting—are good candidates for daylight harvesting.
The potential for energy savings has led to daylight harvesting being featured in the latest generation of energy codes and standards such as the International Energy Conservation Codes (IECC) of 2009 and 2012, ASHRAE/IES Standard 90.1-2010, and California’s Title 24. It is also required in the International Green Construction Code (IgCC) and ASHRAE Standard 189.1. These codes and standards share two major requirements. The designer first needs to identify areas of daylight availability and, second, ensure that the general lighting in these areas is separately controlled from other general lighting in the space. In some cases, a more aggressive application of daylight harvesting can be employed to earn power-adjustment credits.
During the conceptual design phase, it is helpful to develop a written control narrative, which defines the basic functionality of the system. (For more information on the process of writing a narrative, see the first part of this lighting series, “Spell It Out.”) During the design development phase, control equipment is selected, specified, and placed on plans. During these steps, daylight zones and control zones must be defined.
Daylight zones indicate areas of high, consistent daylight available around toplighting, such as that provided by skylights and sidelighting apertures that include windows. Again, codes and standards define the dimensions of daylight zones around specific aperture types. If the entire space is uniformly toplighted, energy savings can accrue across the entire lighting load. In comparison, daylight harvesting more commonly applies only to the perimeter zone of a sidelighted application.
The next step is to define the control zones, dividing the lighting load into groups of luminaires controlled by individual controllers. The more control zones there are in the daylight zone, the more responsive the control system will be, which may increase energy savings. The control method and system can then be selected and developed as a finished design.
It is helpful at the outset to visualize the daylight harvesting system in terms of inputs and outputs. The input is typically an automatic signal from a photosensor measuring light at the daylight aperture (creating an open loop), over the task area (creating a closed loop), or a combination of the two (employing a dual loop). The output is either dimmed or switched. Any automatic control effects in occupied spaces should be nonintrusive and reasonably transparent, so continuous dimming is recommended in regularly occupied spaces, while step dimming and switching is recommended in spaces such as lobbies and transition zones. In some cases, the choice of manual or automatic controls and the use of switching or dimming is made by applicable building codes.
Daylight-harvesting control schemes can benefit from digital communication systems. Digital control systems allow control zoning to be as small as individual luminaires and zoning (and rezoning) can be created using software. Similarly, field sensor calibration can also be performed remotely using software (or onsite using handheld remotes), without tools or ladders.
Selecting the right photosensor, which is the “eye” of the control system, and placing it properly in the space is critical to achieving desired results; software such as SPOT (available for free at archenergy.com/SPOT) can help. In some applications, wireless sensors may be appropriate, particularly for existing buildings; these sensors provide flexibility in that they can be easily moved as needed during startup. In some applications, such as closed-loop sensors that use suspended indirect luminaires, luminaire-integrated sensors may provide more accurate results.
As daylight harvesting control systems are sensitive to error, careful design should be supported by good commissioning practices. At a minimum, this should include verification that all devices are properly installed; performance testing to ensure they work as intended; personnel training for system operators; and delivery of an operations and maintenance manual, a written control narrative, and final device settings.
These practices, combined with good design, modest goals, and consistent follow-up to ensure user satisfaction—all geared toward properly applying control to buildings with high, consistent daylight availability—should result in significant energy savings and a satisfied client.
ECO-STRUCTURE contributing editor Craig DiLouie is a lighting specialist and principal of Zing Communications. This is the last of a three-part series focusing on lighting. The other two parts are "Spell It Out," which looked at the process and importance of the written lighting-controls narrative, and "Protecting Your LED Investment," which guides you through managing the risks associated with choosing LEDs.
