First Solar, a manufacturer of thin-film cadmium telluride (CdTe) photovoltaic modules, recently announced that its latest line of thin-film panels, the Series 6 CuRe, has achieved a 0.2 percent degradation rate, the industry's lowest. The company says that at the end of the panels' 30-year performance warranty, they'll still deliver more than 90 percent of their original power output. This feat should increase the return on investment (ROI) for PV installations and decrease the number of solar panels in the waste or recycling streams.
Degradation rate refers to a solar panel's decrease in power generating capacity over time. Commercial silicon panels typically have degradation rates near 0.5 percent, depending on the climate in which they operate, so they could dip to 90 percent of their original performance in just 20 years. Let's take a look at PV panel degradation, see what's unique about First Solar, and compare it to silicon PV technology.
Degradation
Photovoltaic panels degrade over time due to a number of conditions, such as light, heat and mechanical stress. Light-induced degradation (LID) is largely attributed to impurities in the module's copper, so First Solar launched the CuRe (Copper Replacement) project and developed a proprietary blend of Group V elements to serve as a copper substitute. This is the primary reason that Series 6 the degradation rate went from 0.5 percent to 0.2 percent.
First Solar says that its cells are less prone to cracking under stress than silicon modules, and supports its claim with what it calls "industry’s first and only cell cracking warranty." The company doesn't specify the duration, so I'll assume it's the 12-year limited product warranty and not the 30-year performance warranty.
Thin-Film vs. Crystalline Silicon Modules
While most commercial solar modules are made from rigid crystalline silicon (c-Si), thin-film solar cells consist of materials such as amorphous silicon and cadmium telluride. In general, thin-film cells are less expensive to produce than c-Si cells, and they require less energy to manufacture, but they don't achieve the same efficiency levels. First Solar claims up to 19 percent efficiency for its Series 6 CuRe module, compared to over 20 percent for many of its c-Si counterparts. That may seem like an insignificant difference, but on a utility-scale solar farm, it adds up quickly. As we'll see, though, efficiency doesn't tell the whole story.
First Solar says its modules maintain their efficiency even in hot weather, citing their low temperature coefficient. The company claims that its modules have a lower temperature coefficient than rigid silicon modules, but there are c-Si panels with temperature coefficients that are on a par with First Solar's. One way in which thin-film panels (amorphous silicon as well as cadmium telluride) do outperform crystalline silicon modules is their effectiveness in overcast conditions. In short, c-Si works a little better in direct sunlight, but thin-film delivers more under cloud cover.
Energy Payback Time and Return on Investment
A solar panel's energy payback time refers to the amount of time it takes for the module to produce as much energy as it took to manufacture it. The manufacturing energy (also known as the "embedded energy") includes the energy required to mine, transport, and refine the raw materials as well as the energy used to assemble the module itself. It also factors in transporting the modules to a construction site and building the solar farm. The average embedded energy for monocrystalline silicon modules is 13,428 MJ/m2 (megajoules per square meter), fifteen times higher than that of CdTe, which only requires 894 MJ/m2.
A 2015 study comparing several PV technologies found that CdTe modules have the quickest energy payback time—an average of one year. Monocrystalline silicon modules fared the worst in this respect, with an average energy payback time of about four years. Considering these panels will produce power for 25 years or more, even the "worst" one isn't bad. On the other hand, this is one way in which the CdTe panels make up for their slightly lower efficiency.
The energy return on investment (EROI) is the ratio of a module's lifetime energy output (LEO) to its embedded energy. According to the aforementioned study, the average EROI for CdTe panels is about 34, while the EROI for c-Si is only 8.7. This suggests that a panel's efficiency is less important than its embedded energy when it comes to energy payback and return on investment.
Sustainability
A panel's overall sustainability includes many factors, such as embedded energy, raw materials, recyclability and lifetime greenhouse gas emissions. As we've seen, CdTe is the big winner when it comes to embedded energy. Its materials, however, are more harmful and less abundant than the elements used to make silicon modules.
Cadmium is highly toxic and telluride is scarce, so recycling CdTe panels keeps both out of the waste stream and reduces the need to find and process virgin material. First Solar operates its own recycling facilities and says its modules are 90 percent recyclable. Specifically, the CdTe semiconductor material can be recycled 41 times. With a panel's 30-year lifespan, that's over a millennium of electricity production from the same base material. Glass, which comprises about 97 percent of a CdTe module, is also recycled at the facility, as shown below.
Producing CdTe panels doesn't require as much heat as manufacturing silicon modules, so the energy and carbon footprints are considerably lower for CdTe compared to silicon. A recent study by the European Commission found that CdTe technology is one of the cleanest sources of electricity when it comes to lifetime greenhouse gas emissions, running a close second to wind power. The results of that study are summarized in the following table.
Which Technology is Best?
In terms of energy production, the above report suggests that a system made with CdTe modules and single-axis tracking is the best overall solution for utility-scale PV farms. Silicon still reigns supreme in residential and commercial system performance, however.
In spite of the apparent benefits offered by CdTe technology, it commands less than five percent of the global market share for PV, while Si panels are used in 70 percent of solar installations. Why?
Until recently, the difference in efficiency between thin-film technologies and c-Si was significant. Over the past decade, CdTe efficiency has improved to the point where it's competitive with silicon. Likewise, thin-film technologies have, in the past, suffered from high degradation rates, with many panels surviving only ten years, compared to 20-30 years for c-Si. Again, CdTe has caught up with and surpassed monocrystalline silicon in that respect, but only recently. Meanwhile, as the solar industry burgeoned over the past three decades, silicon had a giant head start over its competitors and its modules are still performing.
Despite its improvements, don't expect the industry to switch to CdTe for all of its new installations, primarily due to the availability of materials. There may not be enough easily accessible cadmium or telluride to meet industry's needs. Silicon, on the other hand, is basically purified sand, so there's an abundance of it that's readily available. The good news, though, is that First Solar is setting an example for the industry to follow by making its panels recyclable and operating its own closed-loop recycling facilities. When panels of any type, including c-Si, are made from recycled materials, it lowers the carbon footprint and the embedded energy of the modules, which improves their sustainability rating.
To learn more about solar panel recycling, check out our article Closing the Loop on Solar Panel Recycling.