光伏组件长期可靠性研究(BP)-

1、Long Term Photovoltaic Module ReliabilityJohn H. WohlgemuthBP Solar630 Solarex CourtFrederick, MD 21703ABSTRACTThe reliability of crystalline silicon PV modules has improved dramatically over the years. Module warranties of 25 years are now common. Extension of the warranties to 25 years was based o

2、n excellent field results for modules with 10 year warranties and on extensive accelerated testing. Since none of the 25 year warranty modules have been in the field that long, we do not know how or when they will eventually fail. It is important for the PV industry to know this, because it impacts

3、the ultimate useful life of our PV systems, it provides critical input for future improvements in module reliability and it provides important data on the long term wear out or failure of todays crystalline silicon PV modules.1.IntroductionPublications on photovoltaic (PV system reliability often st

4、ate that the PV modules are the most reliable part of the system. 1,2 This good PV module reliability is a result of industry government collaboration in the late 70s and the 80s. So today most PV modules carry long term warranties. BP Solar crystalline silicon power modules typically have a 25 year

5、 warranty. This is a remarkably long time. What else can you purchase that comes with a 25 year warranty on performance?Module reliability as well as the length of the module warranty offered by PV module manufacturers increased rapidly during the late 80s and the 90s. Table 1 shows the length of th

6、e standard warranty offered by Solarex on crystalline silicon power modules during this time period. In the 12 years from 1987 to 1999 the warranty period increased from 5 years to 25 years.Table 1: Module Warranty PeriodDate Length of Warranty Before 1987 5 years1987 to 1993 10 years1993 to 1999 20

7、 yearsSince 1999 25 yearsThe increase in warranty length was based on data indicating that the modules could survive for this length of time. Some of the evidence came from the performance of modules in the field. However, because of the long time frames involved the bulk of the data had to come fro

8、m accelerated testing. To understand the module reliability today, it is valuable to review the same two sources, module field data and accelerated stress testing.2.Crystalline Silicon Field ExperienceBP Solar has a data base that includes all field returns of Solarex crystalline silicon modules ove

9、r the time period1994 to 2002. During this time nearly two millionmodules were in the field under warranty. The totalnumber of returns over this nine year period was 0.13%*.This represents one module failure every 4200 moduleyears of operation.Besides measuring the overall rate of failure it is also

10、 very important to learn why modules fail so that you candevelop accelerated tests for those failure mechanisms.Then it is possible to develop materials, designs andconstruction practices that ultimately eliminate thosefailures from occurring. Table 2 lists the observed fieldfailures as a percentage

11、 of the total number of failuresobserved.Table 2: Types of Field Failures ObservedTypes of Failures % of Total FailuresCorrosion 45.3 Cell or Interconnect Break 40.7Output Lead Problem 3.9Junction Box Problem 3.6Delamination 3.4 Overheated wires, diodesor terminal strip1.5Mechanical Damage 1.4Defect

12、ive Bypass Diodes 0.2The top two items, corrosion and cell/interconnect breakage are materials, design and process issues. They areevaluated in the standard accelerated tests discussed in thenext section. The middle items are more related toworkmanship on individual modules and having theappropriate

13、 quality system. The last three items may haveas much to do with how modules are mounted andelectrically connected in the system as to the constructionand design of the module itself.One important observation from this field data is the fact that as the modules with 10 year warrantiesapproached that

14、 time, there was no increase in the returnrate. This clearly indicates that the lifetime of thesemodules was well in excess of 10 years.3.Accelerated Stress TestsSince the time scales in PV are so long it is critical to develop accelerated stress tests to estimate future* Returns from one single occ

15、urrence, know cause early inthe period have been removed to give a clearer picture ofnormal operations.1performance. We cant make a change and then wait 15 or 20 years to see what impact the change had. For the accelerated stress tests to be meaningful, they must test for known failure mechanisms. M

16、uch work was done in this area in the late 70s and 80s lead by the JPL effort. JPL developed a series of “qualification tests for modules that had to be met before product could be shipped to them. 3,4 Specific examples of identified field failures, the mechanism causing the failure and the accelera

17、ted test developed for that mechanism are given in Table 3.Table 3: Examples of Identified Field Failure Mechanisms and the Accelerated Test Developed for It Field Failure Failure Mechanism Accelerated TestInterconnect Breakage Thermal expansion& contractionThermal CycleDelamination of Encapsula

18、ntMoisturePenetrationHumidity FreezeCorrosion of Cell MetallizationMoisturePenetrationDamp HeatToday most commercial PV modules have been qualified to IEC 61215, “Crystalline silicon terrestrial photovoltaic modules Design qualification and type approval” or IEC 61646 “Thin-film terrestrial photovol

19、taic modules Design qualification and type approval”. These test sequences include three major branches (200 thermal cycles, 1000 hours of damp heat and UV/50 thermal cycles/10 humidity freeze cycles as well as specific tests for mechanical loading, hail, and hot spot. Qualification of a module type

20、 to either of these test sequences likely indicates that this type will survive in field deployment for a significant amount of time. However, these test sequences are not specifically designed for evaluation of long term reliability. Some of these tests are too short to adequately evaluate a module

21、s ability to survive for 20 or 25 years. Solarex and now BP Solar have developed an in-house test procedure to qualify modules for 20 and more recently 25 year warranties 5. All BP Solar module types with a 25 year warranty have been subjected to a sequence similar to IEC 61215, but with the followi

22、ng changes:500 thermal cycles with peak power current flow.1250 hours of damp heat.Wet high pot before and after the test sequences. While all BP Solar modules successfully pass this extended series of test, modules manufactured by some of competitors do not. While these modules have also been quali

23、fied to IEC 61215 and carry a 25 year warranty, the results of our tests indicate that they may begin to experience significant failure rates in the 15 to 20 year timeframe.4.Why Continue to Study Module Reliability?If modules are the most reliable component in the PV system with low failure rates,

24、why should we be interested in continued study and improvement? There are three main reasons why the PV industry should continue to be interested in studying the long term reliability of PV modules.1.None of the modules with 20 year (nor 25 yearwarranties have been in the field for that length of ti

25、me so we do not know if the failure rates will increase toward the end of the warranty period.2.Since the return rates are so low, we really do notknow what aging mechanism(s will finally result in module failure. Since we dont know what mechanism will lead to failure we do not know for sure what ac

26、celerated stress tests to use to study module failures.3.When modules fail will they maintain their dielectricintegrity? This becomes even more important as PV modules are used in larger, high voltage systems with more potential exposure to the public as they are integrated into buildings.5. Reliabi

27、lity TestingWhile most PV module manufacturers do qualification tests on their modules, there have been a lot fewer reports of reliability testing. So what is the difference? A qualification test is a proscribed set of tests with defined duration and pass/fail criteria. When we do qualification test

28、ing, we want our modules to pass. Reliability testing is designed to evaluate failures to help develop new more reliable products. Reliability testing is usually run until the product under test fails. In doing reliability testing it is important that the same failure mechanisms as seen in the field

29、 are the ones being evaluated. For example it would be a mistake to use a 200 °C temperature during a thermal test as a means of accelerating the failure mechanism since modules will never see any temperature near this during terrestrial operation. So the reliability tests must use longer durat

30、ions or accelerations within the normal bounds of operation.One approach to reliability testing is to continue the same accelerated stresses from the qualification test sequence until the modules fail. For example a module can be thermal cycles from 40 to +85 °C until it starts to experience fa

31、ilures. BP Solar multicrystalline silicon modules have been thermal cycled in excess of 1000 times without failing completely, although at this level some measurable changes in performance were observed. 6 The damp heat exposure at 85 °C and 85% relative humidity can also be extended beyond the

32、 normal 1000 hours. In this case however, care must be taken to be sure that any failures observed are typical of failures seen in the field. In the field modules run hotter than the environment when the sun is out and they are producing electricity, so they dry out. A sealed module will not let moi

33、sture in so will be better in an accelerated damp heat test, but it will also not let moisture out so may be worse in the real world.Another approach is to combine multiple stresses. Several examples of this are current flow during thermal cycling and applied high voltage during damp heat. These are

34、 both realistic combinations as they occur together in operational PV arrays.26. Field Stress TestingAnother approach to stress testing is to use the outdoor environment, but to artificially enhance one or more stresses on top of the natural environment. Examples of this approach include:Applying hi

35、gh voltage between the module frame and its active circuit to evaluate the added influenceof higher systems voltages on module performance.Insulate the back of the module so that it runs at a higher temperature. Higher temperatures almostalways accelerate chemical reactions that can lead tomodule de

36、gradation and failure.Put prestressed modules outdoors. For example set up to monitor the outdoor performance of modulesthat have already been through a qualification testsequence.Typically PV modules are only monitored for output power. We can learn more about their behavior and potential failure m

37、echanisms by monitoring other parameters during outdoor exposure. One important parameter to monitor over long time periods is leakage current. Besides providing valuable information on water vapor transport and encapsulant conductivity, long term monitoring will provide us with the best evidence th

38、at our modules can continue to operate safely up to and beyond their warranty lifetime.Some systems problems have been caused by degradation of the electrical contacts between modules, not by degradation of the modules themselves. We are now beginning a long term program to directly monitor the cond

39、uctivity of several types of commercially available PV connectors to determine whether they maintain consistent conductivity levels over the expected lifetime of the module.7.Safety TestingToday most commercial PV modules sold in the United States have been safety certified to UL Standard for Safety

40、, 1703 Flat-Plate Photovoltaic Modules and Panels. In addition many have been qualified to IEC 61215 or IEC 61646 that also include some important safety tests. In the near future IEC 61730, “PV module safety qualification” will expand on the IEC safety requirements for construction and testing of P

41、V modules. These tests are capable of determining if a new PV module is safe for use in most applications. They also subject the modules to accelerated stress tests (thermal cycling, damp heat, humidity freeze before performing the final safety tests. These tests are adequate as long as the stress t

42、ests result in failures similar to what causes these modules to wear out at the end of their useful life. As stated earlier, however, how they are likely to fail is exactly what we dont know at this time.So in our reliability and field stress testing, we must determine whether some of the likely fai

43、lure modes compromise the modules dielectric integrity. Will the modules insulation resistance fail or be compromised to the point where a high voltage can cause a dielectric breakdown? If the module superstrate and/or substrate break can arcing occur in any of the current carrying components? Does

44、the contact resistance in connectors increase to the point where an arc is possible?These potential module failures highlight why it is so important for large, high voltage PV systems to be designed and built with the appropriate safety concerns including providing the necessary creepage and clearan

45、ce distances and utilizing active and passive circuit protection.8.Other PV TechnologiesOther PV module technologies, most notably thin films, are not as far along as crystalline silicon in terms of understanding and eliminating failure mechanisms. However, to compete commercially with crystalline s

46、ilicon the manufacturers of these new technologies must offer product warranties similar to those available on the more mature crystalline silicon modules. Even if a new thin film module type can pass the thin film qualification sequence, IEC 61646, we may still see field failures due to failure mec

47、hanisms that are not evaluated in the test sequence. We may also see higher field failure rates than for crystalline silicon because the newer technologies do not have the years of experience (field and accelerated stress testing that crystalline silicon has.Those involved in newer technologies shou

48、ld be following the same approach that got crystalline silicon to the reliability level it is at today. The methodology of using field results to develop accelerated test methods to verify improvements in the product should be applied to any new PV module technology. However, it will be a mistake if

49、 newer technologies try to skip the process and use the tests and packaging solutions developed for crystalline silicon rather than developing their own based on their unique properties and requirements.9.SummaryOur study of crystalline silicon PV module field returns indicated a rate of failure of one module per 4,200 module years of operation. The largest fraction of failures (>85% was due to known failure mechani