Testing Quality

Highlights of PV module reliability scorecard report

Between 2010 and 2016, photovoltaic (PV) panel prices dropped by about 80 per cent, and from 2016 to 2017, there was a further drop of 35-50 per cent in panel prices. This has helped accelerate the growth of the solar industry. The cost reductions can be largely attributed to intense pricing pressures, dynamic supply chain behaviours and manufacturers shifting to countries like Malaysia, Vietnam and India, with less stringent manufacturing policies and a low cost of labour. The low module costs have resulted in solar power competing with thermal power and reaching grid parity in many countries.

Although project developers hail the cost reductions, it has led the solar industry to question whether these cost reductions come at the expense of module quality. Out of more than 300 GW of installed PV capacity, 78 per cent has been in the field for less than five years. Modern PV panels are stated to have a lifetime of 25 years. This raises the inevitable issue of lack of market experience regarding module performance at the actual site. In addition, generating field data is time intensive. Constant modifications in cell or module structures are also being made to increase the efficiency of the panels. Therefore, by the time sufficient practical data is accumulated, the technology may have evolved or may even become redundant for new applications.

It has been observed that price, manufacturing capacity or top-tier ranking cannot be taken as legitimate indicators of module quality or performance. Therefore, access to publicly available full-field performance data of modules being many years away, procurement decisions based on the quality and reliability of PV panels from different vendors are difficult.

DNV GL recently released a PV module reliability scorecard as an attempt to address these issues. The scorecard aims to assist investors to develop procurement strategies for ensuring long-term viability of the project.

The following is a synopsis of the report…

Understanding PV module ageing in the field highlights the technology risks and possible failures. It is observed that most PV module failures result from material issues, flaws in product designs and improper quality control during manufacturing. Even though the most accurate method of determining the lifetime of a product is to deploy it on the field for a long period of time, this is not practical for the reasons already mentioned. Laboratory testing, on the other hand, offers an opportunity to test “field-like” conditions over a short period of time to assess the characteristics of the equipment. However, the parameters selected for laboratory testing should correctly emulate real-time conditions and consider all factors to provide a holistic and unbiased view of the equipment’s performance. Therefore, standard testing of different modules in the lab can help provide data on the superiority of one panel over the others for the set parameters.

Module testing

Most PV projects require UL and/or International Electrotechnical Commission (IEC) certification for ensuring a minimum level of robustness and safety. However, the standards lack in testing reliability and consistency of equipment. For example, UL 1703 is conducted for safety so that the panel is not hazardous during operation and IEC 61215 is applied for environmental stress tests, accounting only for module failure within the first few years of field deployment. The two examples reflect that many module characteristics remain untested. For example, resilience to potential induced degradation (PID) is not tested at all. Certification testing is done only for a small number of samples and may not represent the large volume of commercial production of modules produced over time. Moreover, the manufacturer is free to select and send the samples for testing, which restricts transparency in the testing process. Periodic retesting is also not a part of maintaining the certification of the equipment unless there is a change in the material used or the design. In addition, IEC testing fails to report the magnitude of degradation of the modules after they have been given the pass or fail certification and does not address the root cause of performance loss.

Generally, the 25-year warranty of the modules is triggered if they degrade more than 3 per cent within the first year, and at a linear rate down to 80 per cent of their initial nameplate power in the stated period. However, it is difficult to accurately measure small levels of power degradation in the field due to the uncertainty of measurement tools. PV module warranty claims are, therefore, typically only executed for gross underperformance or complete failure. This makes measurement of module resilience to the most common degradation mechanisms essential, prior to module purchase.

The reliability scorecard is based on the specific performance parameter analysis listed below, and the performance of each module over the tested parameter is provided in the report.

  • Thermal cycling: Stresses such as solder joint fatigue can be induced at interfaces in the module because of the varying coefficient of thermal expansion (CTE) of the different materials used to make a panel. These different module components may expand or contract due to the changing levels of ambient temperature as well as irradiance.
  • Dynamic mechanical load: Significant or repetitive pressure on the module can cause deflection of the glass and result in cell cracks or solder joint degradation. The dynamic mechanical load (DML) test determines a PV module’s ability to handle cyclic pressure loads that are often caused by wind or snow.
  • Humidity freeze: Materials such as junction boxes, frame adhesives, backsheets and encapsulants used in PV modules can absorb moisture. This moisture can freeze and expand inside the module package, causing corrosion of the cell metallisation as well as failure of the adhered interfaces, resulting in delamination or other mechanical failures.
  • Damp heat: High ambient temperature and humidity result in conditions that are likely to lead to ageing and damage the different adhered layers of a PV module.
  • PID test: PID occurs when the voltage potential and the leakage current cause a movement of ions within the module between the semiconductor material and other elements like glass, mount or frame. This effect can damage cell properties, thereby resulting in a large reduction in power output.

Conclusion

The report acknowledges the importance of real-world data and the lack of true practical results from laboratory testing. Therefore, laboratory observations should be utilised to accurately assess the impacts of a specific set of ageing mechanisms on the output. Moreover, laboratory data should be leveraged to effectively manage approved vendor/product lists by setting degradation thresholds. Accelerated testing should be used to screen for PV module defects in large procurements.

In general, the report found that most modules submitted to the DNV GL Product Qualification Program (PQP) performed well in the different test legs of the PQP, with the exception of a few notable degradation levels, which may put the financial success of solar projects using these modules at risk. The report highlights certain key takeaways from the test results.

The bill of materials (BoMs), production factory and attention paid by manufacturers to the quality are key factors that affect the reliability and performance of a module. However, the size of the company, volume of shipment or reputation of the manufacturer for procurement decisions is not an indicator of the same.

The report recommends that the degradation levels identified by the PV module PQP should not be used as a direct forecast of yearly degradation rates for on-field modules but as a mechanism to evaluate PV modules and the associated BOMs and factory locations. It can be used as a tool to compare the module’s expected reliability and long-term performance qualitatively. These tests provide information on how vendors, modules, BOMs and factories compare with one another in each set of controlled environmental conditions, simulating failure mechanisms encountered in the field. Therefore, choosing vendors with lower degradation levels increases the likelihood of the technical and financial success of the project.

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