Now embraced by top designers, Solar-collecting building components may even influence architectural forms.
By Nancy B. Solomon, AIA

Photovoltaics. For many of us, the term conjures up images of shiny black panels mounted awkwardly and conspicuously on the roofs of remote buildings. Energy-saving, environmentally correct, but not architecturally elegant.

That picture is now changing. The industry has evolved dramatically over the past decade. Photovoltaic panels, which convert sunlight to electricity without consuming fuel or creating pollution, are no longer tacked-on appendages begging to be concealed. Today well-known architectural firms integrate these building components into large commercial buildings, as well as high-rise residential and institutional projects. And instead of being hidden, the hotovoltaics are more often celebrated as part of the overall design.

“This is a new architectural material,” explains Colin Cathcart of Kiss + Cathcart, Architects, in Brooklyn, N.Y. “Yes, it comes from the high-tech and environmental worlds, but it is ready to be used for any building of any function.”

The industry refers to such building-integrated components as BIPV systems. These products are designed to replace more traditional building elements, while also producing electricity. Photovoltaic materials are now available for virtually all surfaces of the building envelope. For example, architects can specify photovoltaic shingles, metal standing-seam or exterior insulation systems for the roof. Solar-collecting spandrels, insulated glass units, and sunshade elements are available for curtain-wall systems. And glazing that produces electricity while allowing various degrees of transparency can be ordered for skylights.

Lower prices, higher demand
This evolution of photovoltaic panels into legitimate building products has been the gradual and logical result of a number of converging factors. Government and utility programs have encouraged the development and installation of PV products that are more in tune with the needs of the architect. This support has helped lower the price of BIPV products, making them more competitive with traditional building materials. The reduced price has also made photovoltaic-generated electricity more competitive with traditional energy sources and, therefore, a viable contributor to the electrical utility grid.

In the meantime, many public and corporate clients have come to recognize the positive educational and public-relations benefits that can be reaped from the new technology. Because BIPV products must be part of the building envelope to receive direct sunlight, this energy-efficient strategy is highly visible. More and more clients are willing to pay an additional 1 or 2 percent of total construction costs to have such an enduring symbol of environmental responsibility, says Steven Strong, president of Solar Design Associates in Harvard, Mass. “It’s not the torn T-shirt crowd anymore. Turner Broadcast, Lucent Technology, Merrill Lynch are considering it now,” he adds. Even British Petroleum, which now advertises that its acronym, BP, stands for “beyond petroleum,” is starting to build PV canopies above its gas-station pumps worldwide.

Technical details
The photovoltaic effect describes the process by which direct-current (DC) electricity is produced when light strikes a semiconductor such as silicon, cadmium-telluride, or copper indium diselenide (see sidebar). Semiconductors are solid-state materials that are treated to allow current to flow through them under certain circumstances. Photovoltaic systems were developed decades ago for the NASA space program.

 

Hamburgische Electricitäts-Werke AG Architect: Kiss + Cathcart, Brooklyn, N.Y. Architect of record: Sommer + Partner, Berlin Cost: $3.3 million Engineer: Ove Arup, N.Y. and Berlin To stop water and air from infiltrating the customer center of this electrical utility company in Hamburg, Germany, the architects proposed draping a second skin of photovoltaic glass over the original curtain wall. The space created between the two would allow for several amenities, including an employee winter garden on the first floor and an outdoor cafe on the ground floor (above).

Photovoltaic-generated electricity can be used as is, stored in a battery for later consumption, or, in the most common building scenario, converted to alternating current (AC) by an inverter. The AC power is used by the building. Excess solar electricity is “sold” to the utility company. In many states, the “sale” takes place by effectively spinning the electric meter backward, providing the PV owner with full retail value for the solar electricity.

 

Defining BIPV products BIPV products fall into one of two categories: crystalline and thin-film. The crystalline technology relies on silicon as the semiconductor material. It is currently available in three types: single-crystal, polycrystalline, and crystalline ribbon. Both single-crystal and polycrystalline are essentially produced by creating solid blocks from molten silicon. The resulting ingot is sawed into wafers about 5 inches square and .012 inches thick. In the production of crystalline ribbon, the molten silicon flows through a die to form a faceted pipe or flat ribbon, which is then sliced with a laser into similarly sized wafers. These wafers are processed into cells, which are soldered together in series to achieve the desired voltage. The series is then laminated onto glass. The laminated cells are covered by a plastic backsheet or another sheet of glass. Single-crystal cells are usually a flat black; polycrystalline cells are a sparkling shade of blue; and crystalline ribbon cells tend toward purple. Custom colors, though possible, decrease the cells’ rate of efficiency—the percent of incident sunlight converted into electricity. The typical crystalline PV module is 18 inches by 4 feet with a 4-by-4-inch junction box on the back for electrical connections. This 6-foot-square module holds 36 cells, which is the number necessary to charge the 12-volt storage battery typically used for off-the- grid applications. When photovoltaic panels are connected to the grid, however, the industry is not constrained by battery-charging requirements. Modules of nonstandard sizes, shapes, and features are now available from second-tier fabricators, who purchase the cells from major PV manufacturers. Working with such fabricators, an architect can tailor the module to suit the design. The transparency can be adjusted by changing the spacing between the opaque cells. The backsheet or interior glass can be tinted to modify the panel’s color. A connector can be installed at the edge of the panel in lieu of the junction box to allow for uninterrupted views. And the two-pane glass module can be inserted into an insulated glass unit. Thin film In thin-film technology, the photovoltaic material is applied as a thin layer to a superstrate or substrate, typically through a process called vapor deposition. One type of thin film, amorphous silicon, has been commercially available for about 15 years. Two others, cadmium- telluride and copper indium diselenide, are just entering the market. Thin film is typically coated onto the interior face of a glass sheet. A laser etches a pinstripe pattern of solar cells that are subsequently encapsulated by another sheet of glass. One PV manufacturer vapor-deposits amorphous silicon onto a thin, flexible stainless-steel substrate and then covers it with plastic. Thin film can also be sandwiched between two layers of plastic. Thin-film modules are dark charcoal to near-black and, from a distance, appear opaque. A computer-guided laser can burn a screen pattern of tiny holes through the material to allow for light and views. The greater the transparency, the greater the manufacturing cost and the lower the electrical output. Currently, equipment and manufacturing processes limit the dimensions of thin-film products to roughly 8 square feet. Like the crystalline version, the double-glass module can be fabricated into an insulated glass unit. Efficiency and costs Generally speaking, single-crystal is the most efficient in terms of electrical output, but it is also the most expensive to manufacture. The sunlight-to-electricity conversion rate is 12 to 15 percent for single-crystal; 11 to 14 percent for both polycrystalline and crystalline ribbon; and 5.5 to 7.5 percent for amorphous silicon. The newer thin-film technologies promise slightly higher efficiencies than amorphous silicon. The average cost for a reasonably sized order of 20 kW (peak) standard factory modules is about $6.50/ watt or $78/square foot for single-crystal; $6.25/watt or $71/square foot for both polycrystalline and crystalline ribbon; and $5.50/watt or $28/square foot for amorphous silicon. Actual costs will vary, depending on the module’s specific features. NBS

The basic building block in a BIPV system is a PV module, which itself is made of PV cells. Modules are linked together in series with conduit to form a PV array. The structural and electrical interface between the PV module and the building itself—including the structural attachments that hold the modules in place, the wiring that emanates from the module, and the inverter—is referred to as the balance-of-system (BOS) hardware. The entire BIPV system includes the modules themselves plus the BOS hardware.

One of the biggest issues facing the industry today is the lack of uniform interconnection standards—the rules that one has to satisfy to plug a BIPV system into a utility grid. Depending on the region, these standards are established by the utility, the state government, local building codes, or some combination of the three. The standards vary from state to state and from utility to utility. Some are relatively easy to follow; however, others are not: According to Paul Wormser, director of technology at Solar Design Associates, one utility company in Massachusetts has 30 pages of interconnection requirements, while the utility for the neighboring district has only 3.

 

University of Wisconsin, Green Bay Architect: Hellmuth, Obata + Kassabaum, St. Louis Architect of record: Somerville, Inc., Green Bay, Wis. Estimated cost: $14.5 million (building, site improvements, and interiors) Engineer: Design Engineer: William Tao & Associates, St. Louis; Engineer of Record: Somerville Inc, Green Bay, Wis. PV consultants: Solar Design Associates, Harvard, Mass. When Wisconsin Public Service heard of the University’s plan for a new campus center that would be a model of energy conservation, the local utility offered funding for a BIPV system in order to investigate this technology. Two types were specified: a PV module laminated on a standing-seam metal roof, and a laser-etched amorphous silicon thin-film glass, to be installed on the sloping roof and upper portion of the south-facing wall of a winter garden (above).

Some regions, for example, may require a particular type of inverter. Others may have special requirements for accessing and disconnecting the electrical system. Some states allow net-metering while others do not. And some utilities require proof that the solar-generated power will meet a certain level of quality. Because of the particular interconnection requirements, a system designed for New York City may not work for Los Angeles. “Uniform standards would make a huge difference, but they are not yet on the horizon,” notes Wormser.

When union workers are on a job, their rules will also have to be considered since photovoltaic panels combine components that are often handled by separate trades. Is a curtain-wall PV module, for example, a glass product installed by a glazier or an electrical device handled by an electrician? Although not an insurmountable problem, this question should be resolved early so there are no misunderstandings once construction begins. Typically, the electrician runs conduit and installs junction boxes at the appropriate locations while the glazier sets the glass in place and plugs it in.

Although the photovoltaic panels themselves are known for their durability—they have no moving parts and some have been producing electricity for over 30 years—the architect must ensure that they will be properly maintained. Overgrown trees or accumulated dirt, for example, will block light rays, thereby reducing the panels’ output.

 

At the winter garden, wiring from thin-film PV glazing modules loops around the structural beam to a junction box.

Maximizing output
The potential amount of electricity produced by a photovoltaic panel is a function of the building location and the panel’s efficiency (see sidebar) and orientation. Different geographic regions receive different amounts of solar radiation, depending on their latitude (the southern half of the United States, of course, receives more sunshine than the northern half) and cloud coverage (North Dakota, for example, is sunnier than New York; Arizona gets more sun than Florida). Such climatic information can be obtained from the Department of Energy’s National Renewable Energy Laboratory in Golden, Colo.

And, of course, energy output will be greatest when the solar panels are oriented to receive the most sunlight. To maximize the annual solar harvest, products installed on buildings north of the equator should face south and be set at a tilt from the horizontal equal to the site latitude less 10 to 15 degrees. Variations from this optimum will reduce the solar harvest by up to 30 percent. The reverse holds true for projects south of the equator.

Such orientations and angles will be modified if the goal of the BIPV system is to maximize electrical output at particular times or seasons. According to Rafael Pelli, aia, principal at Cesar Pelli & Associates in New York, the technology works best as a peak-shaving strategy: “The panels should be situated to obtain the most power when the building has the greatest peak loads and, therefore, the owner is paying the most money for electricity.”

Many of the high-profile buildings being designed today with BIPV systems will only reap a small percentage of their total electric requirements from photovoltaic technology. The photovoltaic system that Cesar Pelli & Associates of New York is integrating into a new residential tower in Manhattan’s Battery Park City, for example, will provide 5 percent of that building’s total peak electric load. And a new academic building designed by Hellmuth, Obata + Kassabaum in St. Louis for the University of Wisconsin at Green Bay will receive 8 percent of its power from photovoltaics. But, says Wormser, depending on the massing, location, and photovoltaic design, it is already possible for certain types of buildings to satisfy all their electrical needs from BIPV systems.

Payback
The cost of electricity plays a large role in determining the cost-effectiveness of a particular BIPV system in a particular geographical region. According to Richard Perez, a research professor at the Atmospheric Science Research Center in Albany, N.Y., the cost of electricity ranges across the country from 4 cents to 25 cents per kilowatt hour (kWh). In the continental United States, the cost of electrical power is highest in the northeast and in southern California. Hawaii is also extremely expensive.

In addition, certain regions of the country receive the most power from the sun when it is most needed, and therefore most valued. This happens to be true for the big cities in the Northeast, the central plain states, California, and Arizona. New Yorkers, for example, benefit from this coincidence since the sun is strongest midday, when commercial energy loads are highest. But Floridians do not because their demand for electrical power peaks on cold winter mornings.

Even in areas with high utility rates, however, the payback derived from solar-generated electricity accrues very slowly. For example, according to Dan Nall, P.E., aia, director of advanced technologies at the engineering firm of Flack + Kurtz in New York, a standard PV module installed on a vertical, south-facing wall in New York generates about 1,000 watt hours (1 kWh) per year per watt of rated module power. If 1 kWh is worth 20 cents and the BIPV system costs eight dollars per watt installed, it would take 40 years of electrical output to compensate for the system’s first costs, financial incentives aside.

BIPV proponents suggest, however, that other variables should be considered in the payback calculation. BIPV products, for example, typically take on the role of other building products, so construction costs are actually reduced by the elimination of both the materials and installation charges for those other products. And in comparison to very expensive building materials, such as polished Italian marble, PV modules may be less expensive.

In addition, the federal government offers two financial incentives to incorporate BIPV products in a building: a 10 percent investment tax credit and a five-year accelerated depreciation. And some states offer “buy-down” programs in which they pay some or all of the BIPV costs.

Most significantly, as demand grows, as more projects are built, and as utilities, insurance companies, contractors, and others in the construction field become more familiar with the innovative technology, the costs of BIPV systems are projected to decrease, resulting in a shorter payback period.

Building design

Photovoltaics can be one of the most visible components of an energy-efficient building. It should not, however, be the first—and certainly not the only—energy-savings measure considered during design. “The building needs to be worthy of a solar array,” explains Strong. Architects should first optimize the project’s site orientation and massing, incorporate passive solar heating and cooling strategies, maximize daylighting opportunities, specify energy-efficient equipment, and consider energy-recovery systems to reduce the building’s overall energy needs. The benefits of solar energy will not be fully realized until the building is as energy-efficient as possible.

The advent of cost-effective, building-integrated photovoltaics has, however, made sustainable designers rethink the nature of energy-conscious architecture. In the past, people were taught that boxy buildings—those with relatively small surface areas in comparison to their internal volumes—were the most appropriate because they can conserve the most energy. “But now that buildings can be energy producers,” explains Cathcart, “that need not be the paradigm anymore. Instead, buildings with BIPV systems should be slimmer and more elongated to maximize their surface areas. They should reach out to the sun with lots of glassy surfaces, canopies, and pergolas to collect solar energy for electricity.” Architectural heliotropism: filled with silicon instead of chlorophyll, buildings mimic plants as they turn and bend toward the light.