By Stefan Jarnason
Industry Research and Durability Standards
A large body of research and testing has been conducted on c-Si technology over the past five decades. This work has demonstrated that, with the correct material selection and good manufacturing quality control, crystalline silicon PV modules are capable of withstanding in excess of 20 years in the field (De Lia 2003). However, this work has also shown that inappropriate manufacturing can produce modules with short life expectancies (Quintana 2002).
Although all photovoltaic modules are tested according to well-developed industry standards IEC 61215/61646 Design Qualification and IEC 61730 Safety Qualification, these standards are not sufficient to ensure that modules will operate for 25+ years in the field. The IEC 61215 and 61646 Design and Qualification standards are respectively applicable to wafer-based crystalline silicon modules and thin-film modules. These standards are almost identical in content, with the thin-film standard including some additional stabilization tests due to the early performance variability of many thin-film technologies. There are three main accelerated environmental exposure tests:
-Temperature Cycle, 200 cycles from -40ºC to +85ºC;
-Damp Heat, 1000 hours at +85ºC and 85%RH; and
-Humidity Freeze, 10 cycles from -40ºC to +85ºC and 85% Relative Humidity (RH).
To meet the standards and achieve certification for a Photovoltaic (PV) module design type (defined by construction materials, manufacturing processes, and power class), a manufacturer subjects a set of modules to these exposures, with each pair of modules subjected to just one of these exposures, not all of them. Hence these standards represent the minimum acceptable level of testing for new or modified PV module designs to be qualified, and are at best equivalent to 3-5 years outdoor exposure (Hoffman & Ross 1978, McMahon et al. 2000). To provide confidence that modules will survive 25+ years in the field, additional testing is essential. Suntech has invested in an extensive testing capability to assess and select incoming module materials and to subject modules and components to extended lifetime tests above and beyond the standard industry certification tests.
One of the key factors in module durability is the selection and control of the raw materials used for the module (Skoczek 2009). In addition to obvious importance of the quality of the photovoltaic cells, key module materials include: backsheet, encapsulant (typically EVA), cover glass, junction box, sealant, and frame. Each of these materials requires in-depth research and testing to understand the lifetime and interaction between each material and the completed PV module.
Material Test Program
Suntech has a dedicated team of materials engineers and scientists that rigorously research, select and test all of the materials used in Suntech’s PV modules. This includes Differential Scanning Calorimeter for transition temperature, heat and enthalpy tests; Rotorless Rheometer for characterising EVA cross-linking; Fourier Transform Infrared Spectrometer for semi-quantitative analysis of material functional groups; Water Vapour Permeability Tester to measure backsheet water resistance; automated adhesion test equipment to measure bond and peel strengths; UV and Environmental Chambers.
This rigorous material selection process eliminates more than 90% of materials tested as being unable to withstand the required 25-year module lifetime. Not surprisingly, it is typically materials selected from global leading suppliers that are able to meet these exacting standards and provide the necessary process control to maintain the required quality levels.
As discussed in ‘Industry Research and Durability Standards’, the standard industry durability testing only provides for the equivalent of less than five years of field exposure. Hence, Suntech conducts an extensive array of additional accelerated environmental tests to simulate the required 25-year lifetime. Suntech has established a world-class test laboratory that has been independently certified to ISO/IEC 17025 and been accredited by the world’s leading test authorities: UL, VDE, and CSA. This includes being the first Chinese organization awarded the Supervised Manufacturer’s Testing for Certification Program.
Extended Accelerated Test Program
Suntech’s 1,800 m2 test facility includes more than 30 sets of advanced environmental test and analysis equipment, and requires 25 full-time staff to run the facility, design the experiments and analyze the modules under test. Equipment includes: AAA-rated IV tester, full spectrum spectrophotometer, outdoor test systems, 14 walk-in environment chambers (refer to Figure 2), mechanical load, hail impact, continuous IV bias, lock-in thermography, electro-luminescence imaging, hi-potential tester, impact load, UV exposure, steady state illumination, and salt mist chamber. In addition to this, Suntech conducts sand storm and wind load testing at both module and system level in conjunction with certified external laboratories and partners.
Realizing the inadequacy of the standard IEC tests, CSG Solar (then Pacific Solar) developed an extended sequence of the IEC 61215 qualification test standard called Combined Cycle. One Combined Cycle consists of 200 Temperature Cycles, followed by 1,000 hours Damp Heat, then 10 Humidity Freeze cycles, each applied sequentially to the same module(s)―refer to Figure 3.
One sequence of such tests takes about 4 months to complete and is called one Combined Cycle. If the module(s) survive, the sequence is repeated until the module power output drops below 80% of its initial measured value. Linear interpolation is used to estimate when output drops to the 80% value, giving a numerical value for the module’s durability performance in terms of number of Combined Cycles withstood (Green et al. 2004).
CSG Solar and Suntech have used this Combined Cycle testing as a key part of their long term durability program for over ten years. Impressively consistent results between module design variations and good correlation with real-time outdoor exposure show that this test is a good benchmark for long-term module durability (refer to ‘Combined Cycle Results’ and Figure 4).
In addition to Combined Cycle testing, Suntech utilizes or participates in several other long term accelerated test programs that have been developed by the PV industry over the past few years, including:
-Double Length―Applies twice the standard duration for each of the standard IEC tests, eg 2000 hours Damp Heat instead of 1000 hours.
-TUV Rheinland Long Term Sequential Exposure test (TUV 2010)―Combination of Combined Cycle and Double Length whereby each module gets double exposure period of each of the three exposures plus some UV exposure.
-Atlas 25plus (Zielnik 2010)―Test regime includes UV, salt spray, condensing humidity, temperature cycle, humidity freeze and outdoor exposure (in Arizona).
-Thresher (Kuhn 2011)―Continues each of the IEC exposures until module failure. Also includes mechanical load testing.
All of these tests take 6-12 months to assess the long term durability of a photovoltaic module design.
Combined Cycle Results
A summary of the historical results for Combined Cycle testing of CSG, Suntech and other commercially available PV modules is provided in Figure 4. The commercial modules are all from global top 10 manufacturers (the few modules tested from 3rd Tier manufacturers have been unable to withstand a single Combined Cycle). Crystalline silicon wafer-based modules from leading module manufacturers are expected to last 25+ years in the field (Skoczek 2009) and combined cycle results show that these modules typically survive 2 Combined Cycles, with remarkable reproducibility between modules from the same batch and manufacturer.
Typically modules from all manufacturers lose some power due to current degradation, however, the primary cause of degradation for both crystalline silicon and thin-film modules is an increase in the module’s series resistance, as determined through lumped circuit modelling of the module under test. The causes of increased series resistance vary between modules, and are caused by factors such as delamination, cell cracking, tabbing connections or local hot-spots. Although the precise cause of the failure of a laminated module can be difficult to diagnose, Combined Cycle testing has been demonstrated to be an essential tool to assess and extend the module lifetime, and is considered a key test to pass to qualify any significant changes in module material or processing.
To improve durability a pro-active approach is required to initiate, investigate and test potential module material and process changes. CSG Solar and Suntech use a range of techniques to evaluate proposed module modifications that are based on the previous durability testing results and process advancements. The result of a typical orthogonal designed experiment comparing two types of EVA in two different thicknesses is given in Figure 5. These results, which were also controlled for material batch, showed that thin EVA B was unable to survive a single combined cycle, while modules with either thickness of EVA type A or with thick EVA type B, were able to achieve 3-4 Combined Cycles before degrading by 20%. Based on this result, EVA type A was the preferred material since it has less susceptibility to thickness and was able to withstand almost four Combined Cycles.
Importantly even the poorly performing thin EVA B combination is able to pass the standard IEC qualification testing, however this combination would be expected to survive less than 10 years in the field. This result demonstrates the advantages of the prolonged Combined Cycle exposure in identifying longer-term failure mechanisms.
In addition to accelerated environmental exposure, CSG Solar and Suntech have been testing modules outdoors. This has enabled a direct comparison between outdoor performance and accelerated testing to be performed for identical module designs that are exposed to both accelerated and real time exposure. This comparison has confirmed the ability of the extended accelerated testing to predict real world durability, and more importantly to identify module designs that will not meet the required 25-year lifetime.
Combined Cycle testing has become an important part of Suntech’s long-term durability testing. Over the previous decade, this exposure sequence has been demonstrated to consistently identify module degradation modes that are not detected by the standard qualification testing. The Combined Cycle test sequence is an essential tool in the kit to assess and extend module lifetime, and is considered a key test that needs to be passed to qualify any significant changes in module processing.
Failure Analysis and Warranty Returns
The precise cause of the failure within a laminated module is generally difficult to diagnose. Using sophisticated equipment and techniques such as Dark Lock-In Thermography (DLIT) (Straube 2010), Electro-Luminescence (EL) imaging, and unique module test structures to identify specific degradation locations, CSG Solar and Suntech have steadily improved the long-term durability of their photovoltaic modules. Refer to Figure 7 and Figure 8 for sample images.
Through the in-depth analysis of modules that have degraded after months of accelerated exposure, it is possible to identify and isolate specific failure mechanisms. This knowledge is then fed back into the research and production departments to increase the expected module lifetime.
With over 20 million modules in the field, Suntech has established a detailed database of module fields failures. Overall, the Mean Time Between Failure (MTBF) for all Suntech modules is 2,352 years. Analysis of these modules has enabled Suntech to identify and rectify specific failure sources and mechanisms, including some early backsheet yellowing, inadequate frame adhesion and hot spot issues.
All long-term accelerated testing, including Combined Cycle testing, requires significant resources and time to conduct, with ‘durable’ modules being expected to survive at least two Combined Cycles and take 8-12 months to test. New product release must take this into account and it is imperative that accelerated lifetime testing is included early in product development plans. Further, it needs to be recognized that accelerated testing cannot simulate the entire spectrum of potential real-world stresses, for example, while more comprehensive than the standard IEC tests, Combined Cycle testing does not include any UV or mechanical load testing, which may accelerate or identify new failure mechanisms that are not evident when solely tested in an unilluminated environmental chamber. The international PV community is currently discussing the relative merits of each of the long-term accelerated test programs, and considering the adoption of a more comprehensive, global, long-term accelerated test standard.
Discussion and Conclusion
With ten years of manufacturing experience, and a longer history in research and development, Suntech recognized early the need to invest in and maintain environmental testing capabilities and to go beyond the required IEC qualification requirements. As a result, Suntech has established facilities to conduct comprehensive tests on incoming materials and to take modules through extensive environmental stress tests and, through its relationship with CSG Solar (now Suntech R&D Australia), has developed the Combined Cycle test sequence described in this article. Modules stressed to failure in these environmental tests and warranty returns over the last decade are assessed for their failure modes using world-class test laboratories. As a result Suntech modules come with a 25-year power warranty and a ten-year warranty on product and workmanship and the number of field returns remains low, with a MTBF in excess of 2,350 years.
In order to provide 25-year warranties with confidence, PV manufacturers need to continue to invest in comprehensive materials research development and large-scale field testing; rigorous selection and testing of module materials; long-term accelerated testing; and in-depth analysis and rectification of identified failure mechanisms and to work with the international PV community to develop and adopt comprehensive extended test protocols.
Stefan Jarnason leads Suntech Australia’s team of engineers in providing technical customer support, training and management of utility and large scale commercial solar plant opportunities in Australia. Jarnason has 15 years’ experience in photovoltaic research, durability testing, manufacturing, product development and commercialization. Highlights include developing the world’s first modular PV system―Plug&Power, designing and commissioning semi-automated module assembly operations for CSG Solar’s German manufacturing facility, and developing the Combined Cycle extended durability test program, which is now being used by photovoltaic organizations around the world. (http://am.suntech-power.com/)
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