Building-Integrated Photovoltaics: Putting Power Production Where It Belongs
A lot of finger-pointing is going on in California these days. The rolling blackouts in January were due to deregulation gone awry … or failure to project rapid growth in demand … or permitting delays … or bottlenecks in power transmission. One thing is crystal clear, however: energy is back on the radar screen. People are talking about the oft-ignored commodity of electricity like never before. Some say that California’s problems this past January are a harbinger of things to come—on a much wider level—this summer. Far from unnoticed in all this discussion are the homes and businesses in California that were insulated from the rolling blackouts because they generate their own power with photovoltaic (PV) modules.
While PV power has been around more than 40 years, interest in this power-production technology has mushroomed recently. Production costs have continued to drop, and options have increased for integrating modules elegantly into buildings. This article takes a look at the state-of-the-art with PV power and building-integrated photovoltaics (BIPV) specifically.
An Introduction to PV
Photovoltaics is the direct conversion of sunlight into electricity. To see just how different this form of electrical generation is, you have to understand that virtually all other power generation technologies in use today function with the same basic principle: rotation of a dynamo, or turbine. With coal, gas, oil, and nuclear power plants, the dynamo is turned by high-pressure steam, while hydro plants use falling water, and wind generators use the wind. There are no moving parts in a PV cell—no turbine or bearings to wear out. All that moves are electrons. PV cells are made of semiconductor materials (semiconductors can be made conductive or nonconductive by altering the charge—a property on which the computer revolution was built). In a typical PV cell, two electrically dissimilar semiconductor materials are sandwiched together, separated by a junction. Photons of light excite electrons on the electron-donor side of the cell and cause them to jump across the junction to the electron-acceptor side. By attaching electrical contacts to both sides of the cell and connecting those contacts with a wire, an electrical circuit is created—electrons want to flow back to their source. In a PV panel (or module), many of these cells are wired together—both in parallel and series—to create current that can accomplish useful work.
The photovoltaic effect was discovered in 1839 by the French scientist Edmund Becquerel, but it was not until the early 1950s that scientists at Bell Laboratories in New Jersey figured out how to make a photovoltaic cell that could generate useful electric current. An application for this technology came along just a few years later when NASA needed a way to power its fledgling satellite program. (Vermont-based PV system installer Richard Gottlieb of Sunnyside Solar, in fact, got his start in PVs in 1959 with the Vanguard Satellite program!) The very high cost of PV modules—about $1,000 per peak watt (Wp)—was all right for a NASA satellite, but it precluded terrestrial uses. (PV module output is typically measured in peak watts, indicating the output under optimal sun conditions.) By the time of the first energy crisis in 1973, prices of PV cells had dropped tenfold. While still far from cost-effective for most uses, at $100/Wp, PV modules began to find cost-effective uses for hard-to-reach applications, such as remote radio transmitter stations and Coast Guard signal buoys. Prices continued to drop. By the early 1980s, with prices at around $10 per watt, PVs became cost-effective for remote homes. (For an energy-efficient home more than a quarter- to a half-mile from power lines, the savings from not bringing utility power to the site can offset most or all of the capital cost of the PV system.) Today, the least expensive modules are selling for below $3 per watt.
PV Technologies Today
The highest-efficiency fully commercialized PV cells today are made by growing large crystals of silicon (typically about 4 inches, 100 mm, in diameter), sawing thin wafers from those crystals, doping those wafers with either boron or phosphorous to give them different charge properties, laminating the dissimilar wafers together into PV cells (to create the photovoltaic junction), attaching contacts and wires to both sides of each cell, connecting these cells together, and sealing them into a protective, weathertight module. This single-crystal or Czochralski manufacturing process requires very pure silicon and is expensive, but modules are produced commercially that convert 15% of the sunlight striking them into electricity. (In the laboratory, efficiencies up to 24% have been achieved.) Single-crystal silicon PV module manufacturers include Siemens Solar, Kyocera Solar, and AstroPower.
Much of the research in photovoltaics over the past 30 years has focused on how to reduce these costs. In the early 1970s, Solarex Corporation (now folded into BP Solar), developed a manufacturing technique in which molten silicon is cast into forms where it solidifies into a
polycrystalline form. Because of the casting process, these cells are less expensive to produce, can be square instead of round for more compact arrangement in modules, and are only slightly less efficient than single-crystal cells: up to 14% in commercially available modules and up to 18% in the laboratory. But they still require slicing into wafers. Manufacturers include BP Solar and Kyocera Solar.
Other PV technologies provide even greater cost savings by significantly reducing the quantity of semiconductor material required, increasing the speed of crystal growth, or eliminating the need to slice wafers from solid blocks of silicon. Mobil Solar Corporation developed a process in the 1980s for crystallizing silicon into ribbons as the molten silicon is pulled through an octagonal die—a process called edge-defined film-fed growth or EFG. Modules made from these cells—which are still fairly thick (about 250 microns)—provide efficiency of about 11% (up to 13% in a laboratory). These modules are produced today by ASE Americas, which purchased the solar division of Mobil in 1994. The company has a manufacturing capacity of 20 peak megawatts (MWp) per year at its Billerica, Massachusetts plant and in 2000 produced 4 MWp. Another company, Evergreen Solar, which was formed from previous Mobil Solar employees, is also trying to commercialize a silicon ribbon PV technology (see. AstroPower Corporation has developed a process of growing thinner (40–80 micron-thick) crystalline silicon cells on a stainless steel substrate. Although the modules are up to 10% efficient and just as stable as single-crystal cells, they promise to be significantly less expensive to produce. While AstroPower has been refining this technology, they have built up manufacturing and distribution experience by manufacturing single-crystal silicon modules. Based mostly on their single-crystal PV technology, the company is now the world’s fifth largest PV manufacturer. Another approach for cutting the cost of PV manufacturing is to eliminate the crystal-growing step altogether. Amorphous (noncrystalline) silicon has been used as a PV material since the 1980s, though initially only for electronic devices (watches, calculators, etc.) rather than power generation. Atoms of silicon are deposited in a very thin layer (just a few angstroms thick) on a glass or stainless steel substrate to produce thin, often-flexible modules. The efficiency of PV cells made with amorphous silicon is inherently lower than those made with crystalline silicon, and there have been problems with stability (a drop in efficiency over time known as the Staebler-Wronski Effect), but much lower manufacturing costs compensate for the lower performance. To improve efficiency of amorphous silicon, manufacturers have stacked multiple layers—producing tandem-junction or triple-junction cells. All of the power-generating amorphous silicon PV modules on the market today are multi-junction products, though electronic devices are still using single-junction amorphous silicon. With a stacked design, semiconductor materials of different bandwidth can be used to optimize the absorption of different wavelengths of light. BP Solar produces translucent, tandem-junction, laser-etched, amorphous-silicon modules under the tradenames Millennia™ and PowerView™ (see photo, page 14). Energy Photovoltaics (EPV), based in Princeton, New Jersey, also produces tandem-junction amorphous-silicon modules (at an overseas plant and soon in a new plant in Sacramento, California), but these are deposited onto stainless steel rather than glass. United Solar, in Troy, Michigan, meanwhile, is producing triple-junction amorphous silicon modules on stainless steel substrate. All of these products offer module efficiencies in the range of 4–7%, with laboratory efficiencies as high as 10%. The drop-off in efficiency appears to be “taken care of pretty well,” according to industry analyst Paul Maycock of PV Energy Systems, although we still only have three to four years of actual product performance experience in the field with these products. Amorphous silicon power ratings already factor in this reduction in performance during burn-in (meaning that for the first year or so of operation the modules will produce significantly more power than rated). Most amorphous-silicon products on the market carry 20-year warranties but allow for some reduction in performance below rated specifications (20% in the case of BP Solar).
Instead of silicon, PV cells and modules can be made from other semiconductor materials—most commonly cadmium-telluride (CdTe) or copper indium diselenide (CIS).
BP Solar is operating a pilot CdTe module plant in California, and First Solar, LLC (seehas built a large, 100 MWp/year-capacity CdTe module factory in Toledo, Ohio that has just begun to ship modules and is expected to ship about 5 MWp of product this year. Commercialized CdTe modules offer efficiency of about 7%, with laboratory efficiencies as high as 16%. (See the sidebar on page 9 for more on CdTe PV manufacturing.)
Siemens Solar, until recently the largest PV producer in the U.S., has built a pilot plant producing CIS modules, but very little, if any, product has been shipped, and the status of the plant is uncertain. (At the beginning of March, the German company Siemens announced the sale of their photovoltaic operations to Royal Dutch Shell.) EPV is also planning a CIS manufacturing facility at its Sacramento plant. Efficiencies of CIS modules are expected to be in the range of 7–10%, and laboratory efficiencies as high as 18% have been achieved.
Integrating PVs into Buildings: BIPV
Just as exciting as the technical developments with PV modules have been recent developments with integration of PV modules into building elements. Several different levels of building integration can be achieved with PVs. At one end of the spectrum, the building can simply provide the mounting infrastructure for the PV array. At the other end of the spectrum, the PV modules can serve a building function—for example, as the exterior curtainwall skin or skylight glazing. When we consider integrating PV into any building project, it should be within the context of energy-efficient design. Steve Strong, president of Solar Design Associates and a pioneer in BIPV, is fond of saying that “a building has to be worthy of PV.” The higher the performance of a building into which PVs are integrated, the greater the fraction of electricity use the PV system will satisfy. Put another way, it does not make sense to invest in PV until rapid-payback energy-efficiency features like energy-efficient lighting and high-performance glazings have been included. Most of the action with BIPV today is with grid-connected systems—systems in which the PV-generated DC power is converted into AC and fed into the utility grid. Battery storage is not required because the grid provides the “storage.” Grid-connected PV systems are one example of
distributed power. Utility companies have become extremely interested in distributed power in recent years as a way to quickly boost generation capacity near the points of use, eliminating transmission bottlenecks. (See Capstone Microturbine review in EBN .) Integrating PVs into a building has a number of important advantages, as well as a few drawbacks. We can consider the two levels of building integration separately: placement of PV array on a building; and true integration of the PV array with other building functions. As with many aspects of a building, how the design is carried out will be an important determinant of success with an integrated PV system. These advantages and disadvantages to BIPV are described below:
Placing PV Array on a Building
The real estate is “free.” Because a building is used for the PV array, valuable land around the building that might be used for a stand-alone array can be dedicated to other uses. A roof-mounted PV array is also less likely to be taxed than a stand-alone array. (Some municipalities specifically exempt renewable energy equipment from property taxes.)
Permitting usually not required. A stand-alone PV array would typically require special permitting or zoning approvals; that is rarely the case with building-mounted PV systems.
Minimal “site-development” costs. Site development for the PV array is the building—which is built or being built anyway.
Reduced costs and electrical losses associated with transmission. If the PV array is 100 yards (91 m) from a building where the power is being used, electrical losses can easily exceed 10%; mounting a PV array on the roof usually puts it in much closer proximity to the balance-of-systems equipment.
Utility connection in place. If the building on which the PV array is to be installed is grid-connected, the basic utility connection already exists (though additional components will still be required to complete the interconnection).
Most power produced is worth retail value. If net-metering laws are in place (as they are in 30 states), most of the excess PV power generated by the system can be sold to the utility company at retail rates, essentially “running the meter backwards.” (Usually if there is a net flow into the grid for an entire billing cycle, that excess will be purchased by the utility company at the lower “avoided cost” per kWh.)
Potential for capturing waste heat. Typical PV modules are less than 15% efficient, and most of the remaining solar energy striking a module is converted into heat. That waste heat can be captured for heating water. That’s the idea with photovoltaic/solar-thermal (PV/T) systems. This sort of integration makes sense only when the PV modules are located on a building where that waste heat could be used. Innovative Design has built and installed prototype PV/T systems, and Solar Design Associates has an exciting PhotoTherm PV/T system under development and prototype testing.
Permitting delays. Few building inspectors are familiar with PV systems, and locating a PV array on a building may result in permitting delays. This is more likely to be a problem with residential systems.
High cost. While costs of PV systems have dropped dramatically in the past few decades, they are still expensive. On a green building project with a limited budget, installing a PV system may not be the best use of funds—in terms of minimizing the building’s overall environmental footprint.
Aesthetics. While PV modules can be discreetly integrated into buildings—and even enhance a building’s appearance (see below)—they are still often perceived as being a detriment. Some municipal zoning regulations (particularly in historic districts) or design covenants governing private developments will not allow accessories, such as PV systems, on buildings.
PV array serves dual role. If the PV array also serves as a skylight system, exterior curtainwall system, light shelf, or shade device, the “cost” of the PV system can be figured as the PV system cost minus the cost of the component that it replaces—this makes the PV system much more economically attractive.
The PV system can be justified on more than economics. We generally don’t require “payback” analysis on things that don’t return value over time—like a polished granite façade. By having the PV system serve another function, perhaps we can eliminate the need to justify the cost based on return-on-investment or payback.
Easier financing. The PV can be financed as part of the overall building financing; separate justification to the lender may not be required.
Cleaner architectural integration. Rack-mounted solar arrays can stick out like sore thumbs. By integrating the PV system into the building’s architecture, the aesthetics are often more acceptable. In fact, the PV elements can provide interesting architectural detailing.
Public relations value. A company incorporating PV into its building is providing a highly visible statement of environmental commitment. This can generate good will among customers and the community, which may translate into improved financial performance.
Capabilities of trades. Particularly with residential PV roofing systems, it may be unclear just who does what. Does a roofing contractor or an electrician install PV shingles (each one of which requires an electrical connection), for example, and who is responsible for leaks? (One PV installer EBN spoke with quipped that with earlier-generation products they used to measure rooftop PV installations in “buckets of water per watt.”)
Orientation and tilt not optimized. Rack-mounted PV modules on a flat roof can be installed at optimal orientation and tilt. If the PV array is the roofing or skylight glazing or vertical façade elements, PV performance is much harder to optimize.
When combining functions, performance of both may be compromised. With PV glazing systems, for example, it may be very difficult to optimize both the glazing performance (daylighting, passive solar heat gain, cooling load avoidance) and the PV performance. Some designers argue that it’s better for each element to function well on its own than to compromise the performance of both.
Separate functional building elements makes adaptation easier. As Stewart Brand has argued in How Buildings Learn, keeping services separate from the building skin and the building skin separate from the building structure simplifies future reconfiguration of the building. If a PV system is part of the glazing on a commercial building, replacement of those modules may be more difficult than when the elements are separate.
Growth in PV Markets
Market growth in the PV industry has been dramatic over the past decade. Worldwide PV module shipments were up 43% in 2000 compared with 1999, according to the March 2001 issue of PV News from PV Energy Systems. Since 1993, growth has averaged 26% per year! Somewhat ironically, the growth in demand for PV has had some downsides when it comes to BIPV. “It’s so strong that it’s hurting everyone,” Steve Strong told EBN. To keep up with demand, PV plants are operating around the clock, and companies are reluctant to tailor their manufacturing to produce the often-specialized modules required for optimal building integration, according to Strong: “Customers are standing in line to buy the plain vanilla. There’s no way they will produce the special stuff because it slows down production of standard panels.” Paul Wormser of Solar Design Associates said that one could easily expect a six- to nine-month delay in getting orders filled. That should change as new capacity comes on-line over the next two years—from plants currently under construction or planned; manufacturers are expected to become more responsive to designers’ needs.
Prices of PV modules dropped steadily until the late 1980s. Since then, high demand and limited production capacity have slowed price drops. The least expensive modules available in the U.S. are made in Hungary by Dunasolar using tandem-junction amorphous-silicon technology. Maycock reported that these modules have sold for as little as $2.25/Wp to the Sacramento Municipal Utility District (SMUD) for a large project that utility is funding. First Solar™ is signing contracts for delivery of CdTe modules from their new Toledo plant for as little as $2 to $2.25/Wp, according to Maycock. (That plant has a 10 MW capacity currently, but the thin-film cell line is much larger, allowing rapid ramp-up to 100 MW.) Interestingly, as BIPV applications grow, it’s becoming increasingly common to see prices (and electrical output) quoted on a per-square-foot (square-meter) basis—which makes sense as these products replace conventional building materials that are priced by area coverage. Strong uses the rule of thumb that thin-film modules typically cost about half that of thick-crystal on a per-square-foot basis and deliver about 30–50% of the performance. Amorphous silicon modules he is specifying today cost $20 to $25/ft2 ($215 to $270/m2) and produce 4.5 to 6.0 Wp/ft2 (50–65 Wp/m2). Thick-crystal modules (single-crystal, polycrystal, and crystalline ribbon) he is specifying today cost $50 to $60/ft2 ($540 to $650/m2) and produce 12 to 15 Wp/ft2 (130–160 Wp/m2). These prices are for materials only and do not include installation.
While still expensive, BIPV systems have some special economic advantages on many commercial buildings, due to “net-metering” laws and the way electricity charges are billed. Some 30 states require utility companies to buy back PV-generated power at the retail price by “running the meter backwards.” This means that the PV-generated power is worth the retail price of the electricity being displaced. In commercial buildings with “time-of-day” billing, electricity displaced during the sunniest hours of the day (when air conditioning loads are the greatest) is worth the most. PV prices are expected to continue gradually dropping, though projections from a decade ago have proven overly optimistic. Paul Maycock is currently predicting thin-film PV prices of $1.25/Wp in 2010 and crystalline PV prices of $1.50/Wp.
Building-integrated photovoltaics is still in its infancy in the U.S. Even in Europe, which is far ahead of the U.S. in this area, BIPV accounts for less than 5 MWp of PV installations per year, according to Maycock (out of 288 MWp shipped last year). In the U.S., our 1.4 million commercial buildings have roughly 40 billion ft2 (3,700 million m2) of low-slope roofing, and each year a half-billion ft2 (46 million m2) of architectural glazings are installed. By contrast, last year only 20 million ft2 (1.9 million m2) of PV products were installed worldwide. Add to this the projections for continued shortages of electricity, the growing arguments for distributed power generation, continued high prices for natural gas (which most new power plants are burning), and heightened concerns about global warming—and the opportunities for building-integrated PV appear virtually boundless.
For more information:
PV Energy Systems
4539 Old Auburn Road
Warrenton, VA 20187
540/349-4497 (phone and fax)
Publishers of PV News, a monthly industry newsletter, including assessment of module sales and manufacturing capacities worldwide.
Kiss + Cathcart, Architects
44 Court Street
Brooklyn, NY 11201
718/237-2786, 718/237-2025 (fax)
The firm has been involved in a number of highly integrated, high-profile BIPV projects for the past 10 years.
Solar Design Associates, Inc.
Harvard, MA 01451
978/456-6855, 978/456-3030 (fax)
This pioneering design firm, specializing in BIPV for over 20 years, authored Photovoltaics in the Built Environment: A Design Guide for Architects and Engineers (1997), published by the National Renewable Energy Laboratory.
Solar Electric Power Association
1800 M Street NW, Suite 300
Washington, DC 20036-5802
202/857-0898, 202/223-5537 (fax)
Previously known as the Utility Photovoltaic Group, SEPA is the leading association advancing the use of PVs. Members include utilities, PV manufacturers, government agencies, and educational organizations. SEPA manages the DOE-funded TEAM-UP program, which awards matching grants to PV demonstration projects.
Building-Integrated Photovoltaic Designs for Commercial and Institutional Structures: A Sourcebook for Architects, an excellent resource by Patrina Eiffert, Ph.D. and Gregory J. Kiss, is available at no charge from the Imaginit LLC Web site at(9.7 MB download).