Identifying and Addressing Underperforming Solar Assets: Page 3 of 3

CASE STUDY 1: SYSTEMIC ISSUES

For the first 6 years of its operational life, this site was subject to condition-based maintenance, and project stakeholders used a remote analytics platform to appraise its health. In 2017, an aerial inspection provider conducted an aerial thermographic site inspection for the first time. This inspection revealed a number of faults with a similar infrared (IR) signature, a linear submodule anomaly corresponding with one or more cell strings. Based on the portfolio-level data in Figure 2a, the percentage of faults per site fell within a normal distribution; however, the percentage of linear submodule faults per site in Figure 2b raised red flags.

To gather more information, the owner dispatched an IE team to the site with the full-field IR scans in hand to conduct detailed tests. This team performed a variety of in-field tests on a random sampling basis, including visual inspection, IV-curve traces, IR thermography and electroluminescence (EL) imaging. The results of these tests largely confirmed the aerial IR results. The team determined that the most prominent defect was open-circuit submodule cell strings. Closer analysis revealed that a defective off-cell solder joint was the root cause of this defect.

In the most extreme cases of overheating, this defective solder joint was identifiable via visual inspection. However, visual inspection alone would not have captured the true scope of the problem, as the defective solder joints did not always show evidence of visual discoloration. Remote analytics based on KPIs also did not detect this issue. System availability was solid. The performance index showed no cause for alarm. The peer-to-peer yield appeared normal because these submodule faults were evenly distributed across the site.

Data gathered during the aerial inspection and the follow-up in-field tests support an ongoing warranty claim to address the underperformance issues at this site. Those modules with open-circuit submodule faults—whether due to a solder-joint failure or a shorted bypass diode—clearly meet the criteria for a valid performance warranty claim as they reduce output power by at least 33%. Since this is a systemic defect, it is also important to consider its impacts on energy generation. Given that we know the number and location of defects and have module- and string-level IV-curve test results, we can use component-level modeling software to accurately assess these losses based on the number of modules per string and the number of paralleled strings per maximum power tracker.

CASE STUDY 2: SUBTLE ISSUES

Remote analytics had flagged this site for active investigation based on availability and performance index. In spite of the ongoing CM investigation, the nature of the underperformance issues remained unknown until the owner conducted an aerial IR inspection. As shown in Figure 3, this inspection revealed a unique spatial distribution of modules presenting with some type of hot spot. This issue not only was localized to one subsection of the site, but also specifically impacted the last module in almost every eight-module source circuit. The visual imagery showed no corresponding anomalies.

While hot spots are usually low on a remediation priority list, the aerial inspection provider recommended further ground-based investigations to determine the root cause of this persistent defect pattern. These revealed that the inverter associated with this section had a nonfunctional ground-fault circuit, which meant that the array was operating in a condition susceptible to potential-induced degradation (PID). Cell degradation that starts at the beginning or end of the string of modules not only is consistent with PID, but also has effects that progress in severity over time.

After repair of the inverter issue, the IE firm conducted ground-based IV curve and EL measurements to determine that the remediation efforts were successful. Because project stakeholders caught the PID problem at an early stage of development, its effects were still reversible. If the problem had persisted, hot spots would have spread to subsequent modules in every string. If this development had gone undetected long enough, it could have caused permanent damage to the impacted modules.

Continuous Feedback

While we have focused on the operational aspects of defect identification and mitigation, it is also important to avoid repeating costly issues in the future. The best time to maximize long-term system performance is before building a project, as system design decisions and equipment selection correlate strongly with performance and reliability. One way to reduce risk is to have a robust due diligence program in place throughout the design, procurement, installation and commissioning processes. Equally important is continuous feedback from operations back to the design, construction and commissioning teams, as this is what allows stakeholders to improve future projects based on lessons learned in the real world from fielded projects.

Facilitate continuous improvement. Performance issues are inevitable. The key to long-term success, therefore, is continuous improvement in how you identify, address and learn to avoid the issues that impact performance. Accelerating this learning process is vital to the success of the solar industry. Even as we are racing to reduce the levelized cost of energy, we must maintain the internal rate of return for solar projects. This is a challenging dynamic as it requires a circular flow of information between project stakeholders while looking for opportunities to streamline costs throughout the value chain. Only by sharing lessons learned can we continue to evolve and mature.

The authors would like to acknowledge Rob Andrews from Heliolytics, Mason Reed from Core Energy Works, Rob Chatelain and Steve Wheeler from Constellation, and Magin Reyes and Stan Tehee from Exelon Power Renewables for their contributions to this article.

Kristine Sinclair / Heliolytics / Toronto, ON / heliolytics.com

James Rand / Core Energy Works / Newark, DE / coreenergyworks.com

Robert Flottemesch / Constellation / Baltimore, MD / constellation.com 

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