Page 98 - Zero Net Energy Case Study Buildings-Volume 2

CASE STUDY NO. 10
WEST BERKELEY BRANCH LIBRARY
(Right) Daylight Analysis, show- ing footcandle levels in main space BEFORE adjustment of daylight reflector panels below the skylights. (Bands of purple and dark blue colors signify less than 30 footcandles in that area between the arrays.)
Widening the apertures created by the reflector panels erases these bands and distributes
the daylight uniformly across the space. No electric lighting is therefore needed to com- pensate for bands of low light levels.
(Diagrams courtesy of Green- works Studio).
For cost reasons, the design team selected a standard square commercial skylight (4’ X 4’) and standard roof-mounting detail to create the skylight arrays. Rather than being set flat on the roof, the skylights are tilted at an angle of 25° toward the north. This tilt both avoids shading by the adjacent solar panel array and allows the skylight to “see” more of the brightest part of the luminous sky above3,4.
Computer modeling5 was used to adjust the design of the skylight arrays, in particular the geom- etry of the light-diffusing panels of the ceiling of the main room. Daylight level modeling indicated bands of lower footcandle levels at locations between the skylight arrays throughout the year. (See figure below.) As a result, the vertical shafts directly beneath the skylights were changed and the angles of the light-diffusing panels were spread wider to prevent daylight from being focused too much in the downward direction. Modeling of the adjusted design showed that the darker bands were eliminated and the daylight levels at 30 footcandles or above were more continuous across the space.
Modeling of the light levels in the public space confirmed that the standard lighting criteria for libraries6 are satisfied over a large percentage of operating hours (>90%) for the horizontal read- ing surfaces and bookstack aisles, as well as the vertical light level at the lowest shelf in the bookstacks, without any overhead ceiling light fixtures.
3 For this Bay Area location where marine conditions and cloudy skies are dominant, the sky
is typically brighter at the zenith than at the horizon. The daylight collected is also proportional to the amount of sky “seen” by the daylight aperture, which is the solid angle subtended by the aperture.
4 It is interesting to compare the daylight aperture designs of case studies #8, #9 and #10.
5 The daylight models were first constructed in Ecotect then exported to Daysim, a Radiance- based daylight simulation tool provided by National Resource Canada. The results were then imported back into Ecotect for visualization.
6 Illuminating Engineering Society, Lighting Handbook 10th Edition, https://www.ies.org/hand- book/
86
Zero Net Energy Case Study Buildings: Volume 2