Technology Report Photovoltaic Module Reliability Testing Tadanori Tanahashi ESPEC CORP. Solutions Development G., Technology Management Dpt The trend of the recent years can be outlined by the reliability, the longevity etc. of the solar battery module bearing the center part in the solar battery system that will greatly contribute to the achievement of a low carbon society. Currently, the need for long-service life solar battery modules and reliability improvement are considered, which means reliability test methods need to be studied in detail. The test method example introduced in this report is the one presented during RE00 (renewable energy 00 international conference). Introduction The number of photovoltaic (PV) systems being installed is increasing worldwide, and domestic deliveries of PV systems have also greatly increased in Japan. This trend is mainly due to incentives such as installation subsidies and the fall 009 start of the government s Excess Electricity Purchasing Scheme (The delivery volume in the fourth quarter of fiscal 009 was more than.5 times the first quarter volume. ). PV panels on rooftops are no longer an unusual sight in Japan. If PV panels (modules) installed as residential equipment are to last for many years, then as the main components of PV systems exposed to the outdoor environment, they must naturally have high reliability and long service life. But reliability and long service life are also demanded for other reasons, and several reliability test methods have been developed and implemented to ensure them. 3 This paper summarizes why PV modules currently require better reliability and longer life, gives an overview of current reliability test methods and equipment, and examines the future of PV module reliability testing.
Current Need for Better Reliability and Longer Life As shown in Figure, the structure of PV modules could be considered relatively simple. They are currently thought to have a durability of about 0 years (with no major drop in output), but extending their service life to 30 or 40 years could have the following three benefits: Fig. Typical solar module construction and cross-section enlargement ) Peace of mind, safety and long-term maintenance-free operation PV modules are composites comprised of multiple materials and components. Outdoor exposure over many years creates the risk of deterioration and malfunctions due to moisture permeating through component joints or components themselves (Figure ). Since PV modules are made up of components with different thermal expansion coefficients, changes in the temperature of the installation environment may cause thermal fatigue leading to malfunctions (Figure ). According to one report, about % of PV modules that have passed type approval testing needed to be replaced after 5 years due to malfunctions such as output drops or scorching. 4 In April 00, a fire caused by a PV system was reported in Germany. 5 Increasing the service life of PV modules through reliability improvements can reduce maintenance costs and provide greater peace of mind and safety to household users. ) Reducing power generation cost to promote more widespread use Naturally, reducing the cost of the power generated by PV systems will promote more widespread use of them. Increasing service life (making systems last longer) will be a very effective method of reducing power generation cost, along with the more obvious methods of reducing startup costs (such as module, system and installation costs), and
increasing power generation efficiency. 6 More widespread use of PV systems will help reduce carbon emissions. 3) Increasing investment effectiveness by improving long-term yield Currently, most PV systems installed in Japan are for general residential use. In Europe, most PV systems are for power plants, while in the US most are for public facilities. Worldwide, about 60% of PV systems are for general residential use, a far higher percentage than the roughly 5% for power plants (in 00). However, the IEA forecasts that all three sectors will grow at a uniform rate through 050, when PV systems for power plants will provide about,000 terawatt-hours of power. 7 To prevent the end of the current FIT (feed-in tariff) scheme from creating a drop in PV system investment in power plants, the power generation cost borne by PV power plants needs to be reduced, and the investment effectiveness needs to be increased by ensuring long-term stable operation. Improving the long-term yield of PV power plants will enable ongoing investment in them. 3 PV Module Design Qualification/ Type Approval Test There are three main international standards used for PV module design qualification and type approval testing. The standard for crystalline silicon PV modules is IEC 65/JIS C-8990 (Crystalline silicon terrestrial photovoltaic (PV) modules: Design qualification and type approval). The standard for thin-film PV modules is IEC 6646/JIS C-899 (Thin-film terrestrial photovoltaic (PV) modules: Design qualification and type approval). There are also standards for safety qualification items set forth by IEC 6730/JIS C-899 (Photovoltaic (PV) module safety qualification). Figure shows an example of a thin-film PV module approval test sequence. Roughly the same as the crystalline silicon PV module approval test sequence, it consists mainly of () a combination of temperature cycle testing and condensation/freezing testing, () temperature cycle testing, and (3) high-temperature/high-humidity testing to evaluate output deterioration. The procedures used have the following characteristics: () they are PV module design qualification tests, () they do not provide quantitative service life information, (3) test methods are not designed to establish clear failure mechanisms, and (4) they can t guarantee service life. 3
Fig. IEC 6646: Thin-film terrestrial photovoltaic (PV) modules: Design qualification and type approval 8 Circled numbers correspond to the no. of modules. While the applicable standards have been Fig. 3 Equipment for solar module test revised multiple times, their intent of and test area airflow distribution image ensuring rigorous reproduction of temperature/humidity environments has never changed. Creating these rigorous test environments requires test equipment that can provide reliable and precise temperature/humidity control. For example, to enable precise temperature/humidity control when testing a large number of PV modules, equipment is designed to take test chamber airflow volume and direction into account (Figure 3). It precisely controls the test PV module surface temperature, and the relative humidity near the samples. To ensure test equipment can withstand long-duration tests lasting over,000 hours, the design must ensure that refrigeration circuits and other components are corrosion-proof. 4
4 Future of PV Module Reliability Testing Is only evaluating the physical properties of individual PV module components enough to infer the reliability and service life of the entire module? For example, can a module durability of 30 years be guaranteed by combining components found to have no change in physical properties after 5,000 hours of pressure cooking testing or high-temperature/high-humidity testing? It seems this approach has been investigated, but there is currently deeply-rooted resistance to it. For example, it is difficult to confirm the effect that the gases and oxides generated from encapsulant components have on a variety of other components, and difficult to estimate how vapor permeability changes over time when non-uniform tensile and other forces are applied by module creation. It is also impossible to determine the interactions caused by adhesion between components. Therefore, since PV modules are composites comprised of different component types, the actual generation of deterioration and malfunctions needs to be observed and surveyed when temperature/humidity stresses are applied to the PV module itself. But since it takes a lot of time to apply temperature/humidity stresses during approval testing and the relationship between these stresses and failure mechanisms are not understood, researchers need an ultra-accelerated test method to enable short test times and clear failure mechanisms. We have attempted to create this ultra-accelerated test method by examining methods of temperature/humidity stresses applications. An overview of the findings we obtained by applying rapid temperature cycle stresses is presented below. 8 Increasing the test temperature range (by raising the high temperature) used when applying temperature cycle stress ( 40 C/85 C for 00 cycles) for approval testing can adversely affect components such as encapsulant. We therefore investigated a method that uses a greater number of cycles to apply a greater stress to the sample module and accelerate deterioration. It has previously been reported that increasing the number of cycles reduces output by a few percentage points more than the standard temperature cycle test 9, 0,, but since this approach also involves a substantial increase in test time, its value as a practical test method is questionable. We therefore investigated whether module output would clearly change (deteriorate) over a short amount of time by applying a temperature stress greater than the stress used in standard testing. We applied the stress to a PV mini-module created in-house, using a rapid temperature cycle test method that increased the rate of temperature change. By subjecting the sample to a rapid temperature cycle test for 00 to 300 cycles, we 5
found that module impedance increased significantly at high temperatures. The cause of this phenomenon is not understood, but the fact that the same phenomenon is known to occur in rapid temperature cycle tests of solder joints suggests the strong possibility that it is related to damage/deterioration of interconnectors/solder joints. We compared the current/voltage characteristic of a module to which this temperature stress had been applied to its current/voltage characteristic before testing. We found almost no changes to Voc and Isc, but observed a decrease of more than 30% in Pmax/FF. This output deterioration is significantly greater than the deterioration of only a few percentage points caused by increasing the number of cycles of the standard temperature cycle test (rate of temperature change: 00 C/hour or less). We do not know whether this large increase is linked to the large rate of temperature change (about 400 C/hour) of the rapid temperature cycle test. In summary, while pilot studies have reported an inability to detect damage/deterioration to interconnectors/solder joints caused by increasing the number of cycles of standard temperature cycle tests, we found that using rapid temperature cycle tests caused significant module output deterioration. This finding suggests that rapid temperature cycle tests could potentially be used as a new test method for detecting joint damage/deterioration over a short duration. 5 Conclusion This paper has summarized the reasons why PV modules require reliability improvements and longer service life, provided an outline of a reliability verification test method, and described the ultra-accelerated test method we are now investigating. As previously mentioned, PV modules need to be approached as composites that integrate various types of components. As a wide variety of components are developed, the development of new reliability test methods (service life verification methods) should inevitably follow in future. We hope to contribute to these development efforts through our application of temperature/humidity stresses (environmental testing). Acknowledgments We are indebted to Messrs.Segawa, Kuwada, Yoshizaki, Enomoto, Kanazawa and Umehara of Espec for the design examples presented in this paper. The test examples were part of AIST s Consortium for Development and Assessment of Highly-Reliable Photovoltaic Modules project, and were assisted by Espec project coordinators Messrs Aoki and Okamoto, and Dr Masuda and Dr Doi of AIST. We would like to express our 6
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