Page 27 - Zero Net Energy Case Study Buildings-Volume 2

CASE STUDY NO. 7 DPR CONSTRUCTION OFFICE BUILDING It is noteworthy that the east side of the skylight glazing is operated differently from the west side,
which allows tuning to the brightness of the sky in different directions.
The cost of this glass technology was relatively high, but because so many design objectives were achieved with it, the company opted to make the investment in this case.
In addition to these two significant shaped skylights, there were eleven old standard flat rectan- gular skylights that had excessive heat loss via single glazing. Seven of these were removed in the area planned for the solar panels and four existing skylights along the north side of the roof were simply replaced with new openable residential-type skylights, equipped with insulated glazing.
These four smaller skylights are by design only manually openable, making them unsuitable for night flush operation to pre-cool the building during the cooling season. This is an example of an- other design decision made based on the financial parameters of the project: not enough energy was saved according to the energy modeling via night flush operation to justify the added cost of having the skylight opening controlled by the BMS3.
Not so incidentally, the San Francisco Fire Department requires a four-foot clear walking area around skylights, which greatly impacts the overall size of the possible solar panel array. This requirement eliminated the possibility of distributing new standard skylights over the entire roof to provide daylight to the occupied space below. Thus, the four replacement skylights on the north side of the roof are the only ones used in this project for daylighting. Fortunately, however, the SF Fire Department does not have the same requirement for tubular skylights4, which can provide substantial daylight when the use of skylights is limited, making this daylighting device a good solution for tight urban sites with limited access to sun and sky.
Given their relatively small diameter, the tubular skylights could be located in the middle of a solar panel array without significantly reducing the available area of that array. For this urban renovation project, therefore, a distributed set of tubular skylights became an integral part of the daylighting solution. The number and layout of the tubular skylights were determined by a day- lighting analysis5, which indicated the right distribution to achieve a minimum of 20 footcandles at the work surface most of the time.
The tubular skylights in this project are not adjustable via dimming. To add a dimming feature would have added cost and internal visibility to the tubular skylight hardware, both of which were rejected by DPR for the financial and historic reasons. Light levels in the tubular skylight area of the second floor have been measured at a glare-free 80 footcandles in summer and 20 footcandles in winter. In the latter case, the task level lighting is raised by electric LED fixtures operating at a very low level. This overall lighting design solution is rated as very comfortable by the building occupants.
3 BMS: Building Management System—the computer-based control system for the mechanical and electrical equipment in the building.
4 Tubular skylights are tubular daylighting devices that use internally reflective metal tubes to transmit daylight to interior spaces from the transparent end of the tube at roof level.
5 Daylighting analysis was done using AGI32 software
PHOTO: TED VAN DER LINDEN
Solar photovoltaic arrays with tubular skylights at roof (Photo, below left; section drawing below right.)
Zero Net Energy Case Study Buildings, Volume 2
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PHOTO: TED VAN DER LINDEN