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Reducing field product failures ranks among the top goals for manufacturers of PV modules. When field product failures occur, customers suffer, as they incur property damage or fail to receive their expected yield. Meanwhile, project installers suffer from costly callbacks, and manufacturers suffer from the cost of liability claims. On the whole, the credibility of the industry is undermined.

One method to help ensure product reliability and safety compliance is to test products to universally accepted standards and certify that compliance for the life of the product. The PV industry employs qualification testing to identify in advance the design and construction errors that typically result in the reduction of the module’s performance or cause failure of the module over its lifetime.

Driven by marketplace requirements, this type of testing is mandatory in all of Europe, Japan and parts of Asia and is now a recommended course of action in North America. Qualification testing allows a validated independent laboratory to evaluate a significant number of commercial modules under the industry-established test criteria.

In a competent program, if modules pass the tests, they are certified, and modules produced subsequently are ensured to be similar by frequent inspections of the manufacturer’s production facilities.

Qualification testing is designed to accelerate and induce many of the same failure mechanisms that the module will be subjected to in the field. Performed over 65 to 90 days, these procedures involve a set of well-defined accelerated stress tests - irradiation, environmental, mechanical and electrical tests - with strict pass/fail criteria based on functionality/performance, safety/insulation and visual requirements.

Many common failure mechanisms for PV modules, such as broken interconnects, moisture ingress, delaminations, microcracks, hot spots, ground faults and structural failures, can be uncovered through qualification testing.

The stress levels and durations of these tests are limited to reduce testing time and cost. Qualification testing includes a performance component that validates the module at standard test conditions in a pre- and post-stress modality. Design qualification testing is covered by the following standards: IEC 61215 and IEC 1646 for flat-plate PV modules and IEC 62108 for concentrated PV (CPV) modules.

Testing laboratories also have recognized procedures for regularly monitoring the production output of PV module factories and models under the certification to the standard. Additionally, more robust certification programs include fully independent facilities for testing modules under the criteria. These independent test results and factory inspections help ensure that the module designs that passed the initial test criteria will continue to be manufactured consistently over the life of the module production.

 

Time will tell

However, long-term reliability in the field still remains to be determined after a module has undergone testing. Although qualification testing identifies major catastrophic design issues that could occur, it does not identify all possible lifetime reliability issues that a module can encounter while in service.

When a product passes qualification testing, it means that it successfully withstood a specific set of tests under standard test conditions. Obviously, standard test conditions are only a small subset of the environments and conditions that a module can be subjected to when produced for global markets.

Importantly, the tests do not predict the product’s lifetime performance or indicate which product will last longer or experience faster performance degradation in actual field operation. Real-life conditions vary vastly in different climates and affect both module attrition and module performance degradation in varying degrees.

Overall, based on the analysis of thousands of modules from many global manufacturers, some industry experts believe that the IEC standards represent approximately the first six to eight years of module life in the field.

Safety testing, meanwhile, addresses prevention of electrical shock, fire hazards and personal injury due to electrical, mechanical and environmental stresses in the field. The main safety standards are the IEC 61730 standard for Europe and Asia and the ANSI/UL 1703 standard for North America. Safety testing is widely viewed as an important complement to the IEC qualification testing.

 

 

Banking on solar

As development of large-scale, multi-megawatt PV power plants continues to become more popular worldwide, investors, developers and manufacturers need to consider module reliability in their risk management analysis.

Some testing laboratories have begun providing additional services to investors and project management organizations that plan and operate large-scale power plants, thus supporting the industry’s bankability analysis. Important decisions in the planning phase of power plants include assessing output, evaluating sites, testing the suitability of components, assessing in situ module performance and evaluating the plant design.

Later on, system monitoring - combined with output verification and regular inspections of the technical plant status on-site - will be important during operation to optimize the plant’s financial performance.

PV and CPV module testing evolves continually as the industry conducts research and development of new procedures. Some laboratories, especially those specializing in solar, have added new specific test requirements for modules or systems that are designed to operate in certain climates.

These laboratories might implement tougher requirements than those mandated by industry standards, such as unannounced production inspections, and conduct independent verification of the power output of modules to ensure they perform as indicated. These tests are designed to complement the independent assessment of module performance to industry standards.

A number of initiatives currently under way to enhance product reliability are likely to become actual industry-enforced standards in the future. Here are two credible programs that have gained support in the marketplace:

The National Renewable Energy Laboratory’s (NREL) Terrestrial Photovoltaic Module Accelerated Test-to-Failure (TTF) Protocol. Two stress tests - thermal cycling and damp heat (DH) exposure - are selected for the TTF protocol and are extended beyond the IEC Design and Qualification standards.

Under the procedures for this method, the sample is subjected to multiple sequential exposures and measured for performance along that protocol. This reliability testing also involves determining when and how the unit under the test failed.

The U.S. Department of Energy Office of Energy Efficiency and Renewable Energy’s Thresher Test for Crystalline Silicon PV. The Thresher test is designed to gather and report degradation through the course of the test sequences. Similar to the NREL test, certain simulated environmental stresses are repeated sequentially.

The maximum power output degradation is tracked and should remain within a prescribed window of the initial power rating data. An induced potential is also applied to the modules under the test in the DH sequence. Power drop, leakage current and visual observations are important factors in the final result.

In summary, PV and CPV product reliability is crucial to the success of the solar energy industry and an important component in assessing the renewable energy market. As solar energy technologies evolve, reliability and safety testing are evolving as well. Manufacturers, distributors, engineers, investors and other industry stakeholders can keep up with the latest developments in regulatory compliance by partnering with reputable testing laboratories and following the studies by independent research and development entities. S

 

Richard Bozicevich is vice president of business development at TÜV Rheinland Photovoltaic Testing Laboratory in Tempe, Ariz., where he directs and oversees North American operations for solar technologies. He can be contacted at (480) 966-1700.