Elsevier

Energy and Buildings

Volume 66, November 2013, Pages 415-423
Energy and Buildings

U.S. energy savings potential from dynamic daylighting control glazings

https://doi.org/10.1016/j.enbuild.2013.07.013Get rights and content

Highlights

  • We estimate daylighting savings from dynamic prismatic optical element (dPOE) window coatings.

  • Radiance daylight simulations are applied to building stock perimeter estimates.

  • Vertical daylight strategies can save up to 500 TBtu in U.S. commercial buildings.

  • Adding dPOE coatings increases the daylighting savings potential to 930 TBtu.

  • Floorspace >6 m from the building exterior contributes minimally to total savings.

Abstract

Daylighting controls have the potential to reduce the substantial amount of electricity consumed for lighting in commercial buildings. Material science research is now pursuing the development of a dynamic prismatic optical element (dPOE) window coating that can continuously readjust incoming light to maximize the performance and energy savings available from daylighting controls. This study estimates the technical potential for energy savings available from vertical daylighting strategies and explores additional savings that may be available if current dPOE research culminates in a successful market-ready product. Radiance daylight simulations are conducted with a multi-shape prismatic window coating. Simulated lighting energy savings are then applied to perimeter floorspace estimates generated from U.S. commercial building stock data. Results indicate that fully functional dPOE coatings, when paired with conventional vertical daylight strategies, have the potential to reduce 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 vertical daylight strategies alone. Results presented in this study provide insight into energy and cost performance targets for dPOE coatings, which can help accelerate the development process and establish a successful new daylighting technology.

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

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