Specifying Building-Integrated Photovoltaics
Photovoltaics (PV)-which directly convert sunlight into electricity-have come a long way since their early days as a source of power for spacecraft and remote communications systems. Solar technology is now available in a variety of building materials, including skylights, curtainwalls, canopies, sunshades and rooftops.
Director of Technology
Solar Design Associates, Inc., Harvard, Mass.
Photovoltaics (PV)-which directly convert sunlight into electricity-have come a long way since their early days as a source of power for spacecraft and remote communications systems. Solar technology is now available in a variety of building materials, including skylights, curtainwalls, canopies, sunshades and rooftops. Solar shingles and slates are also available for mounting directly to a conventional roof deck. Incorporating this technology into a building's envelope can offer a highly desirable aesthetic while generating pollution-free electricity.
Solar building components are generally referred to as modules . Several types of modules are available, differentiated by solar-cell technology, size and module construction. The two major categories of solar cells are crystalline and thin film.
Crystalline cells are typically opaque squares, nominally measuring 10 cm to 15 cm on a side. While standard cells range in color-as seen from the outside-from near black to blue, cells with other colors can also be found.
In addition to power, modules with crystalline cells can provide daylighting depending on the distance between cells. Custom modules are available in nearly any size and are available with an insulating-glass (IG) construction.
Thin-film modules generally have a uniform dark appearance, although some manufacturers offer a semitransparent version that relies on laser-etched lines. Thin-film modules are also available as IGs, even from leading IG-producers such as Viracon. Due to manufacturing equipment design, the typical thin-film module is currently available in a 2-foot by 4-foot size.
In the United States, solar modules for building applications are manufactured by roughly ten companies. Many of their names are well known to us, including BP and Siemens; others are quickly developing solid reputations. For the most part, the electrical, mechanical and visual characteristics are company-specific. The warranty period for most modules is 20 years, though the module can often operate for several decades longer.
When PVs are installed as glazing , the curtainwall system or skylight frame should serve as the support for the solar modules as well as the conduit for the electrical wiring between modules. Several curtainwall manufacturers are developing products for this application, following the lead taken several years ago by Kawneer.
Both thin-film and crystalline cell modules can be installed on an existing roof -either horizontally or tilted to the south to enhance solar harvest. When deployed horizontally, the modules might be preassembled onto insulating materials, thereby adding thermal insulation to the roof and, when properly weighted at the perimeter, eliminating roof penetrations.
The Construction Specification Institute's specifications for solar-energy systems fall under section 13650, although it should be cross referenced with Division 16 and other system-specific sections. In addition to local codes and the National Electrical Code, solar modules installed as glazing should also meet appropriate requirements as a glazing product. All PV modules should be listed to Underwriters Laboratories' standard 1703.
All bid documents should incorporate a single-line electrical schematic drawing of the system, with the successful bidder often being required to submit construction documents with the appropriate electrical and structural stamps.
The infrastructure to support building-integrated photovoltaics is growing nationwide, with highly experienced firms that specialize in PV systems working as design-team members alongside the project's lead engineers and architects. If a client wants to incorporate solar energy into a building, the technology is available, the products are available-with more on the way-and the systems are in place for creating a successful solar specification.