Reliability of PV and CPV modules is a key requirement for sustainable consumption of solar energy. To ensure their reliability and safety, PV modules must be tested to and comply with a universally accepted set of standards. Additionally, several initiatives are currently underway to enhance product reliability, some of which are likely to become industry-enforced standards in the near future.
Many globally recognised testing laboratories currently employ qualification testing as a means to identify initial short-term reliability issues in the module’s design and construction. The demand for this testing is driven by marketplace requirements and is mandatory in all of Europe, Japan and parts of Asia. Qualification testing attempts to identify in advance the design and construction errors that can result in reducing the module’s performance or causing failure over its lifetime. Many common failure mechanisms, such as broken interconnects, moisture ingress, delaminations, microcracks, hot spots, ground faults and structural failures can be uncovered through qualification testing.
This type of testing is a 65- to 90-day process designed to accelerate and induce many of the same failure mechanisms to which the module is subjected in the field. Qualification tests are a set of well-defined accelerated stress tests developed out of a reliability programme. They incorporate strict passing and failing criteria. The stress levels and durations of these tests are limited so that they can be completed within a reasonable amount of time and at a reasonable cost. From the manufacturer’s point of view, the end goal for this type of qualification testing is to evaluate a significant number of commercial modules and, after they pass the tests, produce all subsequent modules in the same way as the test modules.
A few of the major laboratories that perform and certify to these requirements typically have recognised procedures for regularly monitoring the production output of the factories and models under certification to the standard. If done properly, these factory inspections help ensure that module designs that pass the initial test criteria will continue to be produced in a robust manner over years in the module production environment.
At the Front Line
As we look into the future at a world with substantially more solar power, it is important to take a long-term and sustainable approach to module and device implementation as well as the performance analysis of these devices. In the solar industry, qualification testing is often confused with reliability testing in the minds of many customers. However, it is important for them to understand the distinction between these two tests.
For a product to pass the qualification tests means it has successfully undergone a specific set of tests under what are called Standard Test Conditions. These tests do not predict the product’s lifetime performance, nor do they indicate which product will last longer or experience faster performance degradation in actual field operation. The real-life conditions in humid tropical environments are much different from arid elevated regions with large swings in temperature. This can impact both module attrition and module performance to varying degrees.
According to a principle known as the Bathtub Curve, performance failure drops from a higher rate for out-of-the-box and start-up field failures, then stabilises to a more predictable and stable operating period during the usable life of the product, then upticks again as end-of-life failures begin to occur. For PV modules, the total time axis is generally depicted as 30 years of usable product life.
When reviewing this information, it is important to understand that what appears to be an approximately linear degradation projected at the belly of the curve is variable for different module designs. In tandem, this degradation profile has a second order of variance based on the climatic conditions in which the modules are installed. These impacts, while relatively minor in comparison to the cost capital of a small project, can accumulate to a considerable financial cost for medium and large sized installations.
Limitations of the Current Standards
Currently, the International Electrotechnical Commission (IEC) design and qualification standards, coupled with robust factory inspections by PV or CPV knowledgeable engineers, represent the first positive steps toward ensuring the integrity of the product design and its initial quality.
However, these standards have their limitations. Based on the analysis of thousands of modules from many global manufacturers, some in the industry believe that these standards represent approximately the first six to eight years of module life in the field.
To be able to test products for a longer period of field performance, many in the market have tended to make a simple logical multiplication of the IEC programmes: if one round of IEC testing approximates six or eight years, then three rounds would equal 18 to 24 years. However, at this point engineers, scientists, module developers and market analysts alike met with the far greater influence of variance considerations.
Looking ahead – Bankability in Solar
Many organisations currently address the need to improve PV and CPV module reliability standards via research and development of new testing procedures. In their turn, some laboratories offer independent tests to manufacturers. Even though these tests help evaluate they do not replace a truly independent assessment.
While many industry protocols are currently being proposed and conducted, thus far no one organisation has developed a completely fair and balanced method to determine how effectively a module is going to work throughout its lifetime.
Improving the reliability of PV modules benefits everyone – from manufacturers looking to offer the best product possible and investors underwriting projects to governments welcoming alternative energy sources and protecting consumers. All these forces combined are pushing reliability standards further to deliver a product that will serve as designed for many years. Manufacturers, consumers and other interested parties will do well to follow the latest developments in regulatory compliance.
The performance and reliability standards trace their early origins back to the 1970s in the NASA Jet propulsion laboratory. Later, the product certification of crystalline PV modules for open-air climates was converted to standards from the series of International Electrotechnical Commission (IEC) 68 ‘Environmental Test Procedures.’
The Research Centre of the European Commission in Ispra, Italy, laid the groundwork for defining special test procedures for PV modules. Test specifications no. 503, ‘Terrestrial Photovoltaic Modules with Crystalline Solar Cells – Design Qualification and Type Approval’ were adopted as the standard IEC 61215 in 1993 and ratified as the European standard EN 61215 in 1995. In April 2005, a second edition of IEC 61215 was published with changes in testing conditions and pass criteria. In 1996, a comparable standard was developed for thin-film PV modules. In 2008, a second edition to this standard, IEC 61646, ‘Thin-Film Terrestrial Photovoltaic Modules – Design Qualification and Type Approval,’ was released addressing new developments in the thin-film technologies and reducing testing efforts.
In 2001, the IEEE 1513 standard first specified criteria for the design qualification and type approval of CPV modules and assemblies. In 2007, a comprehensive CPV standard IEC 62108 was issued. Programmes which have also gained support in the marketplace include the NREL ‘Terrestrial Photovoltaic Module Accelerated Test-to-Failure Protocol’ (TTF) and the DoE’s Office of Energy Efficiency and Renewable Energy’s Thresher Test for Crystalline Silicon (c-Si) PV.
Richard Bozicevich is vice president of business development for TUV Rheinland PTL.