Sarah Kurtz, National Renewable Energy Laboratory

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

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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.

SP: Typically, large solar project developers and EPC firms have more resources and more extensive in-house module reliability datasets for fielded PV arrays than small solar integration companies do. What advice do you have for the latter when it comes to evaluating the reliability of a given manufacturer’s PV modules?    

SK: The intent in creating IEC 62941 and other standards was to make that evaluation easy for smaller companies. Instead of sending an inspector to camp out in the factory during the manufacture of the modules, the customer can ask to see the IEC 62941 certification, which an IECRE-accredited certifying body must issue. We have started developing a similar guide for inverter quality assurance. IEC has just released edition three of IEC 61215 and edition two of IEC 61730, which add very useful requirements and which customers should request.

The draft of the CSA Group standard will be out for public review this spring and should be published by August 2017. Along with IEC 62941, this test protocol, entitled “PV Module Accelerated Testing Protocol for Quality Assurance Programs,” should give smaller companies confidence in PV modules. If customers are installing in very hot climates or on hot roofs, they may want to explore requiring higher-temperature testing that the IEC is developing.

SP: Are there any notable technical developments or advancements in research laboratory settings that you expect to see in the PV module production environment in the near future?

SK: The market is slowly adopting glass-on-glass modules, bifacial modules and modules with smaller junction boxes. These have been on the market for several years, but still have limited market share. Manufacturers are also introducing modules with higher-efficiency cells, including more passivated emitter rear cell (PERC), heterojunction with intrinsic thin layer (HIT) and interdigitated back contact (IBC) designs. Several companies such as NuvoSun and SolarCity are developing building-integrated products. Additionally, manufacturers are integrating solar cells into a range of products, including Toyota’s Prius Prime, drones, hats, backpacks and even roads. Perovskite solar cells have made impressive increases in efficiency to rival those of CdTe and CIGS, and work is under way to make these cells stable in a large area before launching production. Tandem solar cells based on silicon have now passed 30% efficiency, but manufacturers must bring the cost down before implementing production. Regarding the technologies that are higher efficiency than silicon, Alta Devices is to my knowledge the only company moving into significant production.

SP: How important is the research and development that the National Labs conduct in the transition to a 21st-century economy and electric grid? Are these efforts a big part of the US Department of Energy (DOE) annual budget?

SK: Our extensive, reliable power grid has fueled the nation’s growth over the last century; however, the grid we have today does not have the attributes necessary to meet the demands of the 21st century and beyond.  We are seeing significant increases in the percentage of variable generation from wind and PV assets. The proliferation of new smart devices and related goods and services is making consumer demands for electricity more variable as well. A modernized grid will need much more flexibility to accommodate these new technologies without loss of the reliability and affordability of electricity supply we’ve come to expect.

The National Labs, through the DOE-supported Grid Modernization Laboratory Consortium, are working together with industry and academia to create the foundational science and technology innovations needed for a modernized grid.  In over 80 coordinated projects, the National Labs are bridging the early-stage research done by academia with the near-term needs of industry to design, plan and operate a future modernized grid. These efforts are a growing part of the DOE budget and have doubled in size over the last few years with bipartisan support from Congress.

SP: As a percentage of the workforce, women are often underrepresented in science and technology as well as in the building trades. To what extent has this situation changed over the course of your career? Do you have any advice for women seeking to enter or advance in the solar industry?

SK: The statistics for science and technology have changed. When I was in graduate school, I recall finding that each new class was about 10% female. I think that fraction has increased by something like a factor of three, though the statistic varies with discipline and school. Female participation in the PV industry is surprisingly low.

My advice to women is to identify what you’re good at and what you find satisfying, then focus on the work rather than on gender-influenced interactions. Working on renewable energy is satisfying to me because I feel like I can be a small part of the much larger effort to build a better world. Keep in mind that any meaningful career will have its tough moments. Hang in there until you’re successful, comfortable and oblivious to the gender balance. 

When I first started taking science classes in college, I would notice that I was the only female in the room. Recently, my daughter helped me organize a meeting, after which she noted, “Mom, did you realize that you were the only female in that meeting?” I replied, “No, I didn’t. When I was your age I noticed, but now I don’t.” Everyone should undertake a career that matches their capabilities and their interests without regard to their gender, and we’ll end up with a great world.

SP: You have had a long and distinguished career in the PV research field. What are some of the most noteworthy findings or developments during your tenure at NREL?

SK: The most important development in my 35 years of studying PV is definitely the growth of the industry. We cannot find any past projection that PV would grow so fast. Even the most optimistic of the World Energy Outlook projections have grossly underestimated the growth of PV. The costs have also dropped faster than expected, further encouraging growth. Profit margins are at an unacceptably low level for module manufacturers and some other businesses in the supply and project development chain due to a surplus market. However, companies continue to invest, presumably because people want jobs and clean energy, perpetuating the surplus market and lack of profit. Many believed that today’s low prices for silicon modules were not possible to achieve, but module efficiencies have increased while the prices have dropped.

While it’s tempting to say that PV is so successful we don’t need more research, in the lab we see that there are still many more opportunities for improvement: even higher efficiencies, reduced materials use, longer-life modules, lower module operating temperatures to increase the efficiency under operating conditions and so forth. The combination of the low profit margins and the phenomenal success of the industry is making it difficult to sustain the level of R&D that could help implement the remaining opportunities. At the same time, PV has demonstrated that it is on a path to providing low-cost electricity in a way that can bring prosperity to the whole world.

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