Solar panels aren’t just for rooftops anymore. They are starting to pop up in places—windows, curtainwalls, canopies, and skylights—where they do double duty. While first-generation solar technology—the familiar roof-mounted silicon PV panel—has become a familiar sight, a new generation of solar technologies such as building-integrated photovoltaics (BIPV) has the potential to transform common building materials into renewable energy sources.
Thin-film solar cells, which can measure just a few micrometers thick, are flexible, translucent, and can be made in a variety of sizes, shapes, and colors. Because of their adaptability, these cells can be integrated into rooftop shingles and tiles, curtainwall panels, and window and skylight glazing. Moreover, it is not just PV cells that are being integrated into this new wave of building materials; phase-change materials and radiant energy systems also are being incorporated into windows and walls.
“BIPV is one of the most important and exciting architectural applications to come along in generations,” says Gregory Kiss, AIA, co-founder of New York–based Kiss + Cathcart, Architects, designers of the Stillwell Avenue train-shed canopy in New York City, one of the world’s largest BIPV structures. “For the architectural community, it has a lot of potential to add more depth to the design palette. Done right, it should make for better buildings, even aside from the benefit of electricity being generated.”
Building-integrated solar technologies offer several advantages over traditional roof-mounted PV systems:
Better architectural integration. Unlike roof-mounted solar panels, which can appear tacked on as an afterthought, solar curtainwall panels, skylights, and roof shingles can be truly integrated into a design. Electrical components and connections are similar to those in roof-mounted systems, with the option to tie into the grid or use a stand-alone battery system.
Potential for faster payback. BIPV systems not only generate electricity, they also mitigate heat gain by shading façades or interiors. Further, because integrated solar components replace traditional building materials in new construction, higher upfront cost is offset by savings from replaced materials.
Streamlined financing and permitting. BIPV material costs can be considered part of the overall building budget, rather than treated as an optional add-on. In areas where design review boards might restrict visible roof-mounted solar panels, integrated technologies offer visually unobtrusive alternatives.
Designers, however, should be aware of potential disadvantages as well:
Efficiency and performance trade-offs. Flat horizontal and vertical solar applications are typically less efficient than angled roof panels, due to reduced solar exposure. Further, typical roof-mounted flat-plate collectors are made with thick crystals, which deliver about twice the wattage per square foot (10 to 12 watts per square foot) than thin-film PV panels do (4 to 5 watts per square foot). But there are ways to mitigate these impacts. For example, a translucent bifacial PV skylight or canopy panel absorbs light on both sides, so a reflective surface placed below the panel can boost performance. Or lenses can be used to concentrate solar radiation onto the thin-film cell surface.
High upfront cost. While the cost of typical flat-plate PV panels has dropped steadily in the past decade, BIPV materials are still often priced at a premium. Budget-conscious clients are hesitant to spend additional money upfront.
Potential installation and maintenance challenges. Like any complex building skin, a high-performance solar envelope requires carefully coordinated design and installation. Once installed properly, however, integrated solar materials can perform well. Kiss says that his firm has not seen any envelope failures that were attributed to BIPV in its projects. “PV glass has been just as durable as any other glass in terms of its role in the envelope.”
Examples in the Field
Pythagoras Solar in San Mateo, Calif., is one of several companies working to integrate thin-film PV technology into glazing for curtainwalls and skylights. In the company’s Photovoltaic Glazing Unit (PVGU), thin-film PV strips arrayed like the slats of an open venetian blind are sandwiched between two layers of glass. Transparent prisms direct solar radiation to the PV cells for energy capture, while preserving views to the exterior. Wiring and voltage output are similar to those for standard rooftop PV panels, but the glazing system must be designed to accommodate weatherproof electrical connections between glazing unit and building grid.
With a maximum output of 13 watts per square foot, a PVGU “can produce the same of amount of electricity as a traditional solar panel mounted in the same orientation,” says Pythagoras product manager Brendan Dillon. The system was demonstrated on the 56th floor of Chicago’s Willis Tower earlier this year, and the first commercial installations in the U.S. are slated for this fall. According to Pythagoras, typical payback time ranges from three to five years.
GlassX, another high-performance glazing system, doesn’t contain PV at all—instead, it contains a translucent, crystalline phase-change material (PCM) that stores solar radiation as heat. New to the U.S. market from Switzerland, the translucent, quadruple-glazed GlassX window has the thermal mass of a 9-inch-thick concrete wall, absorbing heat from the sun during the day and releasing it to the interior at night. GlassX can reduce winter heating loads by 30 to 50 percent. Like translucent PV cells, the PCM reduces light transmission significantly; supplementary vision glazing is critical to preserve views and daylighting.
In Los Angeles, Behnisch Architeckten has developed a prototype radiant wall system, dubbed sol.Rad. Kristi Paulson, project architect at Behnisch, says that the system “can offer high performance value for less money than triple glazing.” Sol.Rad starts with a modular façade panel designed for the south wall. The outermost layer is a prismatic glass that transmits low-angle winter light but reflects high-angle summer sun. Admitted radiation passes through a layer of translucent insulation and hits a black-painted precast concrete panel embedded with PEX tubing. The absorbed energy is transferred to water circulating in the tubing to interior wall panels, where heat is then released to the interior; cooled water returns to the south panel to start the cycle again.
Sol.Rad was developed for a university dormitory in California then dropped from that project, but its creators are hopeful that the patent-pending technology will find a home in a future project. “Modularity and upgradability are really important,” Paulson says. “That’s why we designed a panelized system that could be swapped out relatively easily.”
BIPV technology is available in some off-the-wall configurations, too. Solar Ivy, from Brooklyn, N.Y., is a modular, flexible system of miniaturized PV “leaves” that snap into a tensile steel grid mounted to an exterior wall. Each 10-inch leaf generates up to 4 watts and is available in an array of colors; leaves can be arranged loosely to preserve views or densely to maximize production. In the coming year, the University of Utah will sponsor an 800-leaf installation on its Salt Lake City campus.
While BIPV and other solar-integrated building materials are still in their infancy, the clear trend is toward modularity, flexibility, and choice. With so many options available, it is critical for designers and owners to communicate early and often. “While there are technical and economic barriers to BIPV, one of the biggest barriers is knowledge,” Kiss says. “It’s certainly possible to do a highly economical BIPV building—but you have to plan it from the very beginning.”
Peter M. James works as an intern architect and writes about architecture and design from Washington, D.C.