By Brent Ehrlich
First Solar’s cadmium telluride thin-film modules are made for commercial and utility-scale use and can be found in this 48 MW installation in Boulder City, Nevada.
Manufacturers are improving photovoltaic (PV) module efficiency and bringing down costs, but the most efficient PV systems today use silicon crystal technology that is decades old. At the same time, thin-film technologies offer a cost-competitive solution with some advantages but with lower efficiency. With the hundreds of modules on the market today, how do you choose the best product for the job? This overview of current PV technologies should help sort out what to prioritize in decision-making, and the table shows representative products from our
First made in the 1950s, crystalline silicon is the oldest, most common PV technology. Typically black or blue, these make up the PV arrays that people are accustomed to seeing on rooftops.
PV is made from silicon crystals that are grown under controlled conditions and then cut into individual cells, which are wired together to form modules. Monocrystalline cells have a uniform crystal structure that results in the most efficient PV modules. The modules are relatively expensive due to the time needed to make them, but they are considered the most reliable, proven PV technology, and they produce the most power per square foot.
are cut from blocks of silicon that have been melted and cooled. They have a less organized crystal structure, so the modules are slightly less efficient (13%–15%), but they are also less expensive than monocrystalline.
is produced using a wire run through molten silicon, pulling it into polycrystalline sheets or ribbons. This method requires far less silicon than standard crystalline PV, and the modules don’t require as much processing.
Thin-film PV is made by depositing extremely thin layers of semiconductor material—the exact ingredients are named with each type listed below—onto a plastic, glass, or metal substrate. Production of thin-film is much faster than that of crystalline PV, making the modules less expensive, but it typically has lower efficiencies and requires rare earth minerals whose production has major impacts and is currently limited mainly to China. Available in flexible sheets and rigid modules, these are attractive for use in building-integrated PV (BIPV) like façades, metal roofing, roofing tiles, and windows.
(a-Si) thin-film was discovered in the 1970s and is the oldest thin-film technology. Depending on the configuration, cells typically achieve 6%–9% efficiency, but some can reach 12%.
—for copper indium selenide or copper indium gallium selenide—is the most efficient thin-film at 9%–13% and may compete with crystalline someday, but manufacturing CIS/CIGS is complicated, and the modules are particularly vulnerable to moisture.
(cadmium telluride) is the best-selling and least expensive thin-film PV technology—see the table for typical figures. A downside to CdTe is it is made with cadmium, a toxic heavy metal.
Though there is no universal PV performance standard, in the U.S. Underwriters Laboratories (UL) requires a module to have a power output tolerance +/–10% of the stated manufacturer-supplied STC (standard test conditions) wattage, so a module labeled 220 watts might actually only produce 198 watts. But according to Larry Sherwood, project administrator for Solar America Board for Codes and Standards (Solar ABCs), PV modules should have a power output tolerance of +/–3% for crystalline modules (the tolerances for thin-film are usually higher), which helps ensure out-of-the-box performance. Sherwood also recommends finding modules that meet quality standards IEC 61215 or 61646 for crystalline or thin-film, respectively.
Solar Photovoltaic (PV) Technologies
Two other performance metrics to look for are high
warranted minimum power
—the amount of power the company guarantees the PV system will produce over time, which includes energy lost through wiring and other system components—and high
PTC (PVUsa test conditions). A PV module comes with manufacturer-supplied STC efficiency data, which is based on a quick, laboratory “flash” test of the module only. A PTC rating is a test of an entire working system and is considered more of a “real-world” representation of PV performance. When comparing two modules with the same wattage, the one with the higher PTC rating is more efficient (per watt, a 205 W panel with 182.8 PTC is a better choice than a 200 W panel with 174.6 PTC, because 182.8/205 = 0.89 is greater than 174.6/200 module = 0.87). Typically several points lower than the STC rating, PTC is used by the California Energy Commission to determine rebate eligibility.
To minimize performance problems down the road, look for products with warranties of 25 years or more from established companies or new companies offering third-party performance insurance.
Historically, crystalline has been the efficiency leader while thin-film has cost less, but there are some complexities. Thin-film works better in high temperatures—such as those found in the Southwestern U.S.—and in low-light conditions, producing 5%–10% more power annually per kilowatt of capacity. But you need a larger area of thin-film to have the same capacity, since crystalline is 25%–30% more efficient. During peak hours you can produce more power with fewer crystalline modules in less area, often with lower installation costs, according to Mike Taylor, director of research at SEPA. So if you are installing modules on a commercial rooftop and looking for maximum energy production per square foot, then crystalline could be a good choice; but if you have room to spare, a low-cost thin-film might make more sense.
The cost of crystalline has also been coming down, since there is abundant supply and many of these modules are now made in China (most thin-film is made in the U.S. or Germany), making crystalline more affordable and taking away some of thin-film’s cost advantage. “But thin-film has been making annual improvement in efficiency, so the performance gap is narrowing,” Taylor said. Both technologies are viable, efficiencies are getting better, and the price is dropping, so this is a good time to look more seriously at PV.
July 1, 2011
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