RESULTS OF 12 YEARS OF MODULE QUALIFICATION TO THE IEC STANDARD AND CEC SPECIFICATION 503

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1 May -8,23 Osaka. Japan 6-B8- RESULTS OF 2 YEARS OF MODULE QUALIFICATION TO THE IEC 625 STANDARD AND CEC SPECIFICATION 53 H. Ossenbrink and T. Sample European Commission, DG Joint Research Centre Institute for Environment and Sustainability, Renewable Energies Unit, Ispra, Italy ABSTRACT Since 98 the European Solar Test Installation of the European Commission s Joint Research Centre id performing qualification tests on terrestrial photovoltaic (PV) modules. Since 99, the test standard applied is IEC 625, or it s direct predecessor, Specification 53. The paper describes the results of more than 25 module types tested, focusing on the reliability and lifetime.. INTRODUCTION Since 98, Terrestrial Photovoltaic (PV) Modules are rigidly tested at the European Solar Test Installation (ESTI) of the European Commission s Joint Research Centre, to a progressive set of standards which evolved along increasing experience with applications and manufacturing methods. Originally, the test standards developed at ESTI served to accompany the first pilot- and demonstration programmes funded by the European Commission, and was to ensure that such publicly funded systems are made from highest-quality PV products. The useful lifetime of a PV module is one of four factors determining the cost per unit of generated electricity (the other being operational and capital costs, solar irradiation, and investment cost per unit of power). In economic terms, estimating this lifetime is as much an important task as determining the power output or efficiency of a module. In developing standards for reliability and lifetime testing, an essential motivation was always to build up user s confidence in a product, which incorporates quite advanced and complex technology, and had not yet the opportunity to demonstrate it s real world lifecycle. Almost all lifetime testing methods are based on test schedules, which accelerate the test-time by simulating environmental conditions in such way, that possible failure mechanisms develop earlier. The acceleration factor is in first order unknown, until comparisons with normal outdoor exposure allow better estimates. However, PV technology applies often already known materials and features, were such factors could be often derived from testing experience of other products. Acceleration methods are for instance to increase exposure temperature andor humidity, Ultraviolet (UV) radiation. to perform temperature cycles with extreme high and low cycle temperatures and increased cycling speed 2. THE IEC STANDARD 625 The International Electrotechnical Commission (IEC) has developed within it s Technical Committee 82, Solar Photovoltaic Energy Systems, which was published in 993 [2]. This standard is based on previous testspecifications in use in the US, Japan and Europe. The standard lays down IEC requirements for the design quallfication and type approval of terrestrial photovoltaic modules suitable, for long-term operation in general openair climates,.... The standard contains test levels and a test sequence, and specifies it s purposc as...to determine the electrical and thermal characteristics of the module and to show, as far as is possible within reasonable constraints of cost and time, that the module is capable of withstanding prolonged exposure in climates described in the scope. The actual liyetime expectancy of modules so qualified will depend on their design, their environment and the conditions under which they are operated.... For a complete programme of qualification tests, see Figure. It requires eight modulcs taken at random from production, which undergo four test sequences. The first sequence characterises electrical performances; the other three subject a pair of modules to environmental and mechanical tests. One of the modules is used as a control. Test levels and pass /fail criteria are described in the standard [2] The defects, which fail a module type, are:. Visible defects, checked for during the pre- and posttest Visual Inspection, such as cracked cells, bubbles or delamination or loss of mechanical integrity. 2. Open-circuit or ground faults detected during or after test execution. 3. Performance losses following each test or sequence of tests. 4. Insulation failures. The Thermal Cycling, Humidity Freeze, Twist and Load tests require that the module be monitored for open circuits and ground faults (conduction between the circuit and the module frame). The pasdfail criterion for output is: failure if the loss in power output following a test is greater than 5% of the value measured before the test. For sequences of tests, the loss may not exceed 8% of the initial value. 882 Plenary, Oral

2 May Osaka, Japan After each qualification test, modules undergo an re-test, however has to be met by two new modules of the insulation test applying twice the anticipated system same type voltage plus V. Several Manufacturers have requested tests at higher levels to meet national codes or the requirements for double-insulated products. If two or more modules do not meet the qualification criteria, the type does not meet the qualification criteria. I test can be repeated, if it is failed by a single module, the The test levels of the IEC 625 aim to simulate 5..2 years of lifetime in a quite moderate climate ( general open air climate ). The climatic categories referred to in the IEC standard do not include tropical climates such as in Florida, for instance, or desert areas. I Initial Visual Inspection (VI) I Performance Measurement (PS) / Insulation Test (IN) I NOCT VI I PS I IN VI / PS / IN Performance at NOCT Load Hail Resistance VI IPS I IN VI I PS I IN Fig.. IEC 625 Module Qualification Test Sequence. Abbreviations: NOCT: Performance at Nominal Operating Cell Temperature; VI: Visual Inspection; PS: Performance at Standard Test Conditions; IN: Insulation Test 3. TEST RESULTS SINCE 99 ON CEC SPEC 53 which was omitted in the first edition of IEC 625, but was included in Specification 53. Accordingly, modules For reference purposes, Table repeats the distribution of having achieved prior type approval according to Spec 53, major defects provoked per qualification test, executed were later considered as being to be certified according to between 99 and 995 on 8 modules [I]. In this period, the IEC 625. This is particularly important, when the European Solar Test Installation was performing tests according to the in-house specification 53, a predecessor of the IEC standard later published. Results can be The UV-Test was later published as IEC 6345: 998, comdared as both test-seauence and test-levels are not test for (pv) different from IEC 625, apart from the UV exposure test, Plenary, Oral 883

3 3rd World Conference on Photovoltaic Energy Conversioti May -8, 23 Osaka, Japan certification is requested for design changes, as only a part of the tests needs to be repeated. ltotais Circuit Visual Power Test IFaults li)pfects /Loss Failure "" per Test Outdoor Exposure (OE) Hot-Spot Endurance Ultraviolet Exposure (UVE) Irradiation 5 thermal cycles (TCY 5) 2 Thermal Cycles (TCY2) Hail Resistance WAR) s- l l l l l o Humidity- Freeze(HUF) I I ' I * Damp Heat (DAH) mental Terminations Twist(TW) I Load (MEL) I I TEST RESULTS SINCE 996 ACCORDING TO IEC Test Summary Immediately after publication of IEC 62 5, the European Solar Test Installation issued type approval certificates according to the new international standard. In parallel, the laboratory aligned with the laboratory standard EN 45, the European implementation of IS Guide 25. This standard lays out requirements for test laboratories, such as a Quality system, equipment calibration schedules and documentation. Accreditation was achieved 996 by the French accreditation committee COFRAC. The International Standards Organisation IS published later a more stringent standard as IS 725, and EST was accredited to this new standard by Failure Summary In Table 2 a summary of the number of passed resp. failed module types is given, for the two above described periods. From this data, about two thirds of the modules pass all tests on the first run. It is worthwhile to note, that in the period 996 to 22, the number of full test programmes decreased in favour of more qualification extension tests. Manufacturer were more often modifying already approved products rather than developing entire new modules. The risk involved can be drawn from the fact that only 6 of the 3 new module types undergoing full qualification tests passed the sequence immediately; 4 needed to repeat tests to achieve type approval and 3 were rejected, which corresponds to a reject rate of 23%, much above the average Total Modules Tested Full Programme 37% 22% 3% Extension 63% 78% 7% Passed 6% 7% 65% Passed after 3% 9% 25% Re-Test Rejected % % % Table 2. Summary of modules tested from 99 until 22. It is remarkable, that all modules subjected to the tests passed the insulation test requirements, clearly indicating progress in encapsulation and careful design of frames. 5. DISCUSSION Table 3 shows the distribution of major defects detected during the qualification tests in the period By far most of the failures were provoked by environmental tests. 5. Irradiation Most of the failures within this test sequence occur after the UV exposure test, which causes mainly power loss, but also yellowing of the encapsulation material. Outdoor exposure subjects the modules as well to irradiation, in a correlation with the UV-exposure test can be in fact observed for a number of cases, as depicted in Figure 2, and initial photon induced degradation is most probably the cause. Still, other degradation mechanisms can be the cause for the power degradation of Figure 3, which are probably 884 Plenary, Ora\

4 May II Osaka, Japan temperature and humidity induced, and from Figure 2 one can conclude for a correlation in about 6 cases. Test Outdoor Exposure (OE) Hot-Spot Endurance (HW Ultraviolet Exposure (WE)., Irradiation 5 thermal cycles (TCY 5) P Humidity- Freeze (HUF) 2 Thermal Cycles (TCYZOO) Damp Heat (DAH) Environmental Robustness of Terminations (ROB) Twist (TW) Load (MEL) Hail Resistance (HAR) s- Table 3. Distribution of major defects Provoked per Qualification performed between 996 and 22. Total number of module types tested was 58. Hot-Spot Failures occurred only in one case, showing that protection by bypass-diodes is well understood for current module technology. Typically the number of cells in a sub-string protected is Outdoor Exposure Power Loss ["h] Fig.2. Correlation of power degradation after UV Exposure and Damp Heat with Outdoor exposure. - m -2 ' c..l L L n -4 5, oor Exposure (34 Samples) ~ ~ _ ~ _ -5-6 Module Code Fig.3. Distribution of power loss after Outdoor Exposure for different modules Plenary, Oral 885

5 Ma-v -8,23 Osaka, Japan 5.2 Environmental The great majority (75%) of failures occur after the environmental tests, and in particular after the Damp-Heat and Thermal Cycles tests. This indicates that PV modules need a very carefully designed encapsulation system to pass the Damp-Heat test. Typical defects provoked are delamination, corrosion, leaking junction boxes. The high failure rate after 2 thermal cycles indicates fatigue, which cannot yet observed after 5 cycles. Good cell tabbing, stringing and placement of cells are required to pass this test. With six failures, the Humidity freeze test is the next severe, and addresses deficiencies in the choice of polymer materials for j-boxes and encapsulation. From the tests performed, no correlation can be proved with the Damp-Heat tests. 6. QUALIFICATION EXTENSION Most manufacturers offer a series of module types, which are different only by size or the number of cells. EST is offering tailored re-testing schedules, which focus only on design features, which may be affected by changes in the construction. After good experience with a proposed retest schedule in [I], further refinements have been proposed to WG2 of the Technical Committee 82 of the International Electrotechnical Commission, within the frame of a future quality assessment system for PV products. They are shown in Table REVIEW OF THE STANDARD Currently the Standard IEC 625 is revised under the normal maintenance schedule, which shall allow adjusting for developments and change in practice. Two new tests are introduced in the current draft. One is a Wet Insulation Test, which shall replace the previous Insulation test. It foresees to perform a high-voltage test under wetted conditions, which in particular shall reveal weaknesses of junction-boxes and framed modules, which incorporate an Aluminium-Backsheet. The other additional test is a Bypass Diode Test, which addresses problems related to sizing and thermal cycling of Bypass diodes during partial shadowing periods of modules. The method of the Hot-Spot test may undergo revision as well. 8. CONCLUSIONS -8.I... i Module Code Fig. 4. Power loss of 3 samples after h damp heat test 5.3. There is a slight increase of mechanical failures observable, when comparing with the previous phase of tests. One of the reasons is certainly that more frameless modules for building integration have been tested. Generally speaking, the mechanical features required to pass the mechanical tests are well understood, however, special typologies such as roof-shingles or non-glass superstrates need careful design to pass the mechanical tests. The IEC Module Type Approval tests have been proven to address design problems of commercial modules very well. Also, feedback from operational PV-installations confirms acceleration factors, and defects detected already during the qualification tests. The initial failure rate of 54% of new module types is still relative high, and 23% needed to be rejected. This probably reflects the increasing number of new module manufacturers in the past 5 years, which either need to gain experience in manufacturing quality control, or embark with new technical design features. REFERENCES J. Bishop, H. Ossenbrink, Proc. 3 h European Photovoltaic Solar Energy Conference, 995: Results of Four Years of Module Qualijkation Testing to CEC Specijkation 53. IEC 625: 993; IEC Central Office: Crystalline Silicon Terrestrial Photovoltaic Modules - Design qualification and Type approval. 886 Plenary, Oral

6 Muv -8, 23 Osaka, Japan Modification OSP HSP UVE TCY5 HUF TCY2 DAH ROB MEL HAR Change of Cell Technology X X X X X Modification of Encapsulation X X X X X [I Superstrate Modification PI x X X 3 X X Module Size X X X X Backsheedsubstrate X X X [3 [4 [4 Frame or Mounting System [5] [5 [5 [5 X J-BOX X X X X Cell Interconnect X x [6 X Change of Circuit or X Power>+l% Power Change <- % X X Removal of Frame X X X X [ Non-tempered Glass Codes: [2]Material/Thickness changes OSP: Outdoor Exposure DAH: Damp Heat [3]Non-Glass HSP: Hot Spot Test ROB: Robustness of Terminations [4]Supporting Substrate only UVE: UV Exposure MEL: Load [5]if plastic TCY5O: 5 Temperature Cycles HAR: Hail Resistance [6]For change of Material TCY2: 2 Temperature Cycles Table 4. required for extension of qualification testing following design changes Plenary, Oral 887