Measured daylighting potential of a static optical louver system under real sun and sky conditions
Introduction
Electrical lighting energy consumption in U.S. commercial buildings accounts for 3.69 quad annually [1]. Of this, a recent estimate by Shehabi et al. found that 2.21 quad (60%) is consumed by electrical lighting in perimeter zones located 0–12.2 m from the facade during typical daytime work hours (8:00–18:00) [2]. Consequently, the delivery of sufficient daylight from windows has the potential to reduce annual electrical lighting energy demand by minimizing the need for electrical lighting in the perimeter zone during daylight hours. However, as a general rule of thumb, conventional windows cannot provide useful daylight beyond approximately 1.0–1.5 times the head height of the window [3], leading to useful daylight for areas within a distance of 0–4.5 m from the facade. Subdivision of the window wall into a lower “view” zone and an upper “clerestory” zone for daylight transmission is a common strategy to extend the daylight zone beyond this distance. Because occupants often reduce the daylight transmission of the view zone with shading devices to maintain visual comfort, the clerestory zone is designed to serve as the primary means of daylight delivery into the space. For single-occupancy offices, this strategy can be effective. Due to the relatively shallow office depth, typically 3.05 m–6.1 m, the occupant is located relatively close to the window wall and the clerestory is not within the occupant's primary field of view. Because many occupants working in open plan offices are located at a greater distance from the facade, the relatively lower ambient light levels combined with a more direct view of the bright clerestory can cause uncomfortable luminance contrasts, leading to the deployment of shading devices to maintain visual comfort. As an example, in a number of case study evaluations conducted in the UK, Bordass et al. [4] noted that tall windows intended to enable greater daylight penetration to open plan workspaces were often found with the shades closed and the electric lights turned on. Bordass reported that the cause of the default state of “shades down, lights on” was the need to resolve the visual discomfort experienced by those who worked the farthest distance from the facade and did not have control over the shades. In addition to the unnecessary use of electrical lighting that can result from the shading of clerestory windows, electric lights may be switched on by occupants in daylit spaces even when sufficient daylight is provided by windows because the contrast in luminance between interior surfaces adjacent to the facade and surfaces away from the facade causes the space to appear dark or “cave-like” to occupants.
Many systems have been developed for the clerestory zone with the goal of increasing the daylit area of the floor plate. A comprehensive description of existing systems can be found in Ref. [5] (see chapter 4). These include light shelves, anidolic daylighting systems, translucent panels, prismatic structures, and Optical Louver Systems (OLS). By reflecting daylight to the ceiling, light shelves have been shown to deliver useful daylight at slightly greater depths from the facade than conventional windows [6]. However, at low sun angles, conventional light shelves fail to block direct sun, which can lead to glare from direct view of the solar disc and, in turn, deployment of interior shading devices. Further, external light shelves of sufficient depth as to be effective in daylight redirection and perimeter shading are rarely implemented in commercial building facades due to additional structuring required for wind loads and to support the system itself. More advanced light shelve designs, such as those developed by Beltran et al. [7], as well anidolic daylighting systems [8], use an arrangement of optical components to redirect daylight and effectively control glare. However, these more advanced designs have failed to be broadly adopted by architects due in part to their large size, complexity, and challenges with integration into otherwise two-dimensional (flat) facade systems popular among architects. Translucent panels with near-lamberitan diffusing properties, such as those characterized by Reinhart and Andersen [9], redirect incident daylight towards the ceiling but also downward into the view zone of occupants. Although these panels can be easily incorporated into a façade system, the downward-redirected light, as well as the relatively high luminance of the panel itself, can become a source of glare. Prismatic structures have been in use for over a century for lighting applications [10] and work (in sidelighting applications) by refracting light to the ceiling plane for a range of incident angles. Recent developments in the application of prismatic structures for sidelighting have led to the development of prismatic window films which can be adhered to both new and existing glazing systems, creating the possibility of broad application at low cost. However empirical assessment has shown than the current film technology produces perceptible levels of glare under clear sky conditions [11]. Optical louver systems utilize reflective mirrored coatings to reflect daylight to the ceiling while blocking direct view of the window and can be incorporated into the clerestory zone as static louvers (e.g. Refs. [12], [13]) or as adjustable lamellas (see systems evaluated in Ref. [14]). Static optical louver systems offer the benefit of not requiring daily or seasonal adjustments in tilt angle, which makes them a potentially more practical and reliable technology for broad application in clerestory zones. However, as a consequence, the profile geometry of static systems must be designed to function effectively over the full seasonal range in incident sun angles.
With the increasing trend of open plan workspaces designed to comply with the daylight illuminance criteria specified in green building rating systems (e.g., Leadership in Energy and Environmental Design (LEED) [15], Building Research Establishment Environmental Assessment Method (BREEAM) [16]), it is important for daylighting strategies to consider human factors issues of visual comfort and luminance uniformity in addition to providing sufficient illumination for performance of visual tasks. When located in the clerestory zone, OLS have the potential to meet daylight illuminance requirements while providing improved visual comfort and light distribution relative to conventional shading systems. Recent developments in computer-based lighting simulation tools, namely the simulation of complex fenestration systems using bidirectional scattering distribution functions [17] and the development of the three-phase simulation method [18] have made it possible to accurately solve for high flux light transport through optically complex daylight redirecting devices such as OLS to produce annual assessments of daylighting performance. These new capabilities are making it easier to assess the application of optically complex fenestration in the design of low-energy buildings. However, the Architecture, Engineering and Construction (AEC) industry is extremely risk averse, and slow to adopt promising technologies without proof of performance in realistic field conditions.
The goal of this study was to compare the daylighting potential of a recently developed OLS installed in the clerestory zone of a facade against a conventional Venetian blind over seasonal changes in sun and sky conditions to test if the optical surface treatment and specific geometry of the OLS consistently resulted in useful daylight illuminance levels and reduced visual discomfort.
Section snippets
Experimental set-up
Experimental tests were conducted in the Window Testbed Facility located in Berkeley, California (latitude 37°4′ N, longitude 122°1′ W) from February 2 to January 19, 2011. The facility consists of three, identical, south-facing, side-by-side, furnished test rooms built to represent a commercial single-occupancy office. The test rooms were unoccupied during this study. Interior surface reflectances of the floor, walls, and ceiling, are 0.18, 0.85, and 0.86, respectively, as measured by a
Workplane illuminance and daylight autonomy
Fig. 6 compares seasonal performance between the OLS and the reference Venetian blind under clear sky conditions using the metric of daylight autonomy (DA). Data represent paired comparisons between systems under identical sky conditions. The performance of both systems was found to be related to the maximum daily altitude angle of the sun where, based on the DA criterion of 300 lx (DA300), the greatest DA outcomes were achieved during the fall equinox and decreased significantly approaching
Conclusions
This study demonstrates that optical sunlight redirecting systems located in the upper “clerestory” zone of the building facade can provide horizontal daylight illuminance levels comparable to a horizontal Venetian blind without the associated window glare, and can deliver a more uniform light distribution across the ceiling plane. Based on side-by-side comparisons of an optical louver system (OLS) and a conventional, matte-white Venetian blind with a horizontal slat angle located inboard of a
Acknowledgements
The authors would like to acknowledge their LBNL colleagues, Dennis DiBartolomeo and Chad Howdy Goudey, for running the experimental test and to Robert Clear, for his insights into the analysis methods. This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Building Technology, State and Community Programs, Office of Building Research and Standards of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
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