Energy Use Intensity and its Influence on the Integrated Daylighting Design of a Large Net Zero Energy Building
R. Guglielmetti, Scheib, J., Pless, S., Torcellini, P., and Petro, R. ASHRAE Winter Conference Paper NREL/CP-5500-49103, Las Vegas, 2011.Review by Scott Schuetter, Energy Center of Wisconsin:
The authors discuss the daylighting design process associated with the Research Support Facility, a 220,000 ft2 office building on the National Renewable Energy Laboratory campus. From the beginning of design, the project had a goal of low energy use, setting out to be approximately 50% below ASHRAE 90.1-2004. A more specific goal of an EUI of 33.3 kBbtu/ft2/yr was also targeted. Due to this aggressive goal, daylighting was chosen early as an integral part of the project's energy efficiency strategy. In fact, the RFP to the design-build contractors contained language requiring daylighting in all perimeter occupied zones, glare mitigation strategies, automatic, continuous dimming, and commissioning. The early EUI goal had the benefit of empowering the lighting designer to consider advanced controls strategies and motivated the entire team to work together to meet the energy target. This collaborative environment was realized through coupled daylight and energy simulation. The coupling was accomplished by simulating representative spaces in the Sensor Placement Optimization Tool (SPOT), a Radiance based software. For a given set of design parameters, SPOT was used to output an hourly lighting schedule, which was then imported into the energy model in lieu of its own, less sophisticated, illuminance algorithms. In this manner, several design variations were considered and an optimum set was settled upon.
Architecturally, the design team extended the daylit zone into the building by designing tall window head heights. They achieved this by employing daylight glazing, or a set of windows above the vision glazing with high visible transmittance. A light louver system was added to the daylight glazing to bounce the light up the ceiling and even deeper into the building's interior. Further, light reflectances were chosen for the interior surfaces. When coupled with low partition heights, this minimized the amount of daylight that was absorbed and obstructed, thereby allowing a more even and deep penetration of daylight. The lighting design incorporated a task-ambient approach, utilizing direct/indirect, pendant-mounted fixture with 25 watt fluorescent T8 lamps and compact fluorescent downlights for the ambient illumination and LED downlights for task illumination. A target open office illuminance of 25 footcandles was set along with an additional 20 to 30 footcandle contribution from the task lighting. This design led to an average lighting power density of 0.62 watts per square foot. From a controls perspective, the design emphasized energy savings balanced by ease of commissioning. This approach was taken to ensure that the control system would not be confusing to the building occupants and facilities staff, a situation that often leads to the controls being overridden. The controls system itself took a layered approach; time clocks and vacancy sensors then daylighting controls. Closed loop daylighting sensors were used locally to continuously dim the electric lights to off, rather than just their minimum power fraction. Open loop daylighting sensors were used globally to override systems in large, open zones. In these large, open spaces, a north perimeter, a south perimeter, and a core set of fixtures were circuited and controlled separately. Finally, commissioning of the lighting controls was undertaken to check that the photosensor sensitivities and timeclock, vacancy sensor, and daylight controls interoperation are working properly. This final step, coupled with continuous monitoring, will ensure the building's intended level of high performance is realized.