U.S. energy savings potential from dynamic daylighting control glazings
Introduction
Lighting is a major consumer of electricity in commercial buildings and accounts for about 10% of all electricity use in the United States [1]. Utilizing daylight in commercial buildings to supplant electric lighting has great potential to help reduce this large energy demand. Vertical (i.e., wall portions of the building shell) daylight strategies typically involve coupling photosensors with dimmable ballasts in the near vicinity of windows, allowing the electric lighting output of these ballasts to reduce when daylight is available. Previous studies have modeled such daylighting strategies and estimated a lighting energy savings ranging from 30 to 77% [2], [3], [4]. Lee and Selkowitz [5] empirically observed savings up to 60% in a building equipped with dimming ballasts and automatic shades. While substantial energy savings from daylighting strategies have been modeled and measured, dimming ballasts only make up about 1% of lighting systems found in the U.S. commercial building stock [6], indicating that adjusting electric lighting levels to match available daylight is not yet widely deployed. Two observed limitations with conventional vertical daylighting strategies have the potential to compromise energy saving benefits and may deter the implementation of these strategies. First, previous research has shown that building occupants will close manual blinds when direct sunlight creates uncomfortable glare in their workspace, but rarely re-open the blinds after the glare has passed [7], [8], [9], thus preventing any additional daylight benefit for the remaining portion of the day. Second, even with automated blinds, which increase both initial and maintenance costs compared to manual blinds, potential lighting savings are restricted to building zones adjacent to the daylighting windows. The savings measured by Lee and Selkowitz [5] decreased as interior distance from the building perimeter increased, with lighting energy savings at about 7 m from the windows being approximately one-third of the savings measured at about 3.5 m from the windows.
Recent research in dynamic light redirecting window coatings has the potential to overcome these limitations and increase the potential energy savings available from vertical daylighting configurations. While utilizing panels of prismatic optical elements (POE) to redirect light is an established daylighting strategy [10], window coatings comprised of dynamic prismatic optical elements (dPOE) are now being developed that can alter prism shape in response to an applied voltage, thereby changing the daylight redirection of the coating via Snell's Law [10], [11]. Lab-scale prototypes of the coatings under development are based on electroresponsive polymers, such as polymeric ferric sulfate, that change their dimensionality when polled with an applied voltage [12]. Corrugated structures, supported on conducting glass, can be molded from these polymers with an imprinting process. The polymer has an index of refraction very close to glass and can be modified within a range in the manufacturing process with the addition of optional reflective coatings or high refractive index inclusions to enhance the interaction with light. The active component of this polymer would then be incorporated in an electrochemical cell, not unlike the construction of an electrochromic window coating, to complete the dPOE. This technology could be externally modulated to track the angle of incident sunlight, so that daylight consistently penetrates more deeply into the building interior. Daylight could also be continually redirected toward the ceiling to minimize glare and occupant discomfort while maintaining daylighting benefits. While dPOE window coatings are still in a nascent research and development stage, this study estimates the marginal energy savings possible from the large-scale deployment of this technology. Daylight simulations of an open office floorplan are conducted with a prismatic window coating of multiple shapes to represent an operating dPOE technology. Simulated energy savings are then applied to perimeter floorspace estimates generated from U.S. commercial building stock data. Results provide insight to the technical potential for energy savings available from vertical daylighting strategies and the additional savings that may be available if dPOE coating research culminates in a successful market-ready product. This insight can help streamline the development process and provide much-needed focus on the dPOE performance required for success as a competitive daylighting technology.
Section snippets
Modeled office space
Daylighting simulations in this study are generated using the three-phase method previously validated by McNeil and Lee [13] that utilizes bidirectional scattering distribution functions (BSDF) within the software program Radiance [14]. The three-phase method allows relatively quick annual daylight simulation to model complex fenestration systems [15], [16]. As shown in Fig. 1, the modeled space consists of an open-plan office that measures 12.2 m (40 ft) wide by 14.0 m (46 ft) deep, with a
Results and discussion
Fig. 4 presents modeled estimates of the annual daylighting contribution to a 500 lx average work plane illuminance within each of the four perimeter zones. Results are provided for the model space with south- or east-facing windows, and with the clerestory windows equipped with either static blinds, dynamic blinds, or a dPOE (dynamic) coating. As expected, the dynamic cases consistently provide greater daylighting savings than the static blinds. Relative to the static blinds, the dynamic blinds
Conclusion
The results presented in this study indicate that a fully functional dPOE coating, when paired with conventional vertical daylight strategies (i.e., photosensors with dimmable ballasts in the near vicinity of windows), has the potential to reduce the energy use associated with U.S. commercial electric lighting demand by as much as 930 TBtu. This reduction in electric lighting demand represents an approximately 85% increase in the energy savings estimated from implementing conventional static
Acknowledgements
The authors thank William Morrow for his assistance with ArcGIS and James O’Donnell for his work compiling ASHRAE 90.1 lighting schedules. This work was conducted at Lawrence Berkeley National Laboratory. Portions of this project were carried out at the Molecular Foundry, Lawrence Berkeley National Laboratory, which is supported by the Office of Science, Office of Basic Energy Sciences. D.J.M. was supported by a DOE Early Career Research Program Award. Portions of this project were supported by
References (23)
- et al.
Global energy savings in offices buildings by the use of daylighting
Energy Buildings
(2002) - et al.
Estimation of lighting energy savings from daylighting
Building and Environment
(2009) - et al.
The New York Times Headquarters daylighting mockup: monitored performance of the daylighting control system
Energy and Buildings
(2006) Window blind occlusion: a pilot study
Building and Environment
(1984)- et al.
Evaluation methods and development of a new glare prediction model for daylight environments with the use of CCD cameras
Energy and Buildings
(2006) Energy Information Administration, Office of Integrated and International Energy Analysis, U.S. Department of Energy, Annual Energy Outlook 2012 with Projections to 2035, DOE/EIA-0383, Washington, DC
(2012)A computer tool to evaluate the impact of daylightcontrolled lighting system onto the overall energetic behaviour of buildings
US Census Bureau, Fluorescent Lamp Ballasts: First Quarter 2006, Current Industrial Reports, MQ335C(06)-1
(2006)- et al.
An experimental study for the evaluation of the environmental performance by the application of the automated venetian blind
- et al.
Occupant Use of Venetian Blinds in Offices. Building Research Establishment (BRE), Contract PD233/92
(1992)
Daylight in Buildings: A Source Book on Daylighting Systems and Components, International Energy Agency Solar Heating and Cooling Programme Task 21 LBNL-47493
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