Sarah Kurtz, National Renewable Energy Laboratory

Making the World a Better Place, One Solar Cell at a Time

Sarah Kurtz is a principal scientist at the National Renewable Energy Laboratory (NREL), and has served as a group manager for the PV Module Reliability Test and Evaluation Group since 2008 and the co-director for the National Center for Photovoltaics since 2015. Kurtz co-founded and is a leader of the International PV Quality Assurance Task Force, which seeks to do collaborative research as the basis for creating international standards that will be the foundation of the next stage of growth for the PV industry. She holds a PhD in chemical physics from Harvard University. Since her graduation in 1985, Kurtz has worked at NREL, which she considers a special place, with colleagues who share her dedication, commitment and vision of improving the human experience and reducing its impact on the global environment.

SP: Are comparative indoor accelerated-testing methods progressing to allow results that better represent degradation in outdoor environments over long periods of time?

SK: Yes, they are. As a community, we have found ways to identify many of the causes of degradation. For example, the fraction of modules that are reported to discolor has dropped for newer modules, presumably because the manufacturers learned that modules needed more UV exposure in the test environment to ensure an encapsulant wouldn’t discolor. Whether for this reason or another, the degradation rate of the photocurrent—often associated with discoloration— also has been dropping. Similarly, methods for detecting susceptibility to potential-induced degradation are allowing customers to avoid products that may degrade because of the voltage that the system itself generates.

SP: Have downward cost pressure and manufacturing changes such as the use of thinner cells significantly impacted long-term cSi module stability? 

SK: An NREL senior reliability engineer, Dirk Jordan, recently published a paper, “Compendium of Photovoltaic Degradation Rates,” summarizing reports in the literature. He and his team found that the failure rates for modules deployed before 2000 were about twice those for newer modules. Although the types of problems reported have changed, we do not yet see a major increase in the reported failure rates for the modules deployed after 2000.

In 2012, profit margins became painfully slim; modules manufactured after that time have been in the field for less than 5 years, so it is too soon to judge whether cost-cutting measures will significantly impact their long-term stability.

According to the International Technology Roadmap for Photovoltaic (ITRPV), cell thicknesses have been steady at approximately 180 µm between 2010 and 2015. However, inspectors report cracked cells in the majority of array fields. It’s unclear whether the damage is originating during cell fabrication, module lamination, transportation or installation, and whether the cracked cells will present a significant problem. Nevertheless, because cracked cells often lead to reduced power output and can create safety risks, this is an area of concern that is attracting the attention of those who inspect fielded PV arrays.

SP: Some industry stakeholders contend that glass-on-glass module designs afford increased structural and electrical reliability over a module’s lifetime compared to modules with polymer backsheets. What’s your perspective?

SK: Glass-on-glass construction is less likely to suffer from cracked cells compared to glass-on-polymer modules, since the cell is at the neutral position when the module flexes. However, glass-on-glass construction can have problems with delamination if a gas forms in the encapsulant faster than it can diffuse out of the module. The glass-on-polymer designs are able to breath better, which some assume is a bad thing since diffusion of moisture into the module can contribute to corrosion. The corollary is that glass-on-polymer designs are better at allowing reaction products to diffuse out, preventing gas buildup and bubble formation. We need to understand this phenomenon better and learn how to test for it to avoid the problem in the field as glass-on-glass modules become more widely used.

SP: What is the status of the International PV Quality Assurance Task Force (PVQAT) efforts to develop a comparative module rating system? 

SK: PVQAT has undertaken three goals. The first is a rating system to ensure durable design of PV modules for the climate and application of interest. The second goal is a guideline for factory inspections and quality assurance during module manufacturing. The third goal is a comprehensive system for certification of PV systems, verifying appropriate design, installation and operation.

The standards committees are reviewing multiple test methods to develop the rating system. In particular, they are discussing methods to test for hotter use conditions. In addition, partnering with a standards development organization that the CSA Group formed, we at NREL are working toward an international standard for more-stringent testing that will give project stakeholders increased confidence almost everywhere in the world. We plan to publish this standard by August 2017.

The International Electrotechnical Commission (IEC) published the IEC 62941 standard, “Terrestrial Photovoltaic (PV) Modules: Guideline for Increased Confidence in PV Module Design Qualification and Type Approval,” in January 2016. The IEC System for Certification to Standards Relating to Equipment for Use in Renewable Energy Applications (IECRE) is handling the implementation and is accepting applications from organizations wishing to issue these certificates. NREL hopes that customers will consider requiring IEC 62941 certification because this should increase confidence that a vendor has manufactured its modules with adequate quality assurance.

IEC created the IECRE to issue certificates for PV plants at each stage of development and each business transaction. Customers may consider asking for these certifications starting now. It includes certifications for wind, PV and marine systems. For PV plants, certifications are now available for the completion of the system (Conditional PV Project Certificate) and for an annual performance check (Annual PV Plant Performance Certificate). IECRE is developing additional certificates for the design phase and for a complete assessment of a PV plant at the time of sale.

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