This is an extract from Renewable Energy Test Center’s (RETC’s) PV Module Index Report 2022. This year’s PV Module Index Report,
explores three interrelated topics—n-type PV modules, field forensics and extreme weather—that demonstrate some of the inevitable technical risks associated with solar project development. These timely topics also elucidate the value of a data-driven approach to risk management.

Evaluating new N-type PV modules

The solar industry’s continued ability to drive down costs while improving performance is a primary reason solar accounted for the largest share of new US electricity generation capacity in 2021. This trend is best exemplified by continual changes to module designs and cell technologies. Last year, for example, RETC explored the benefits and challenges of developing and deploying large format modules, which many analysts expect will dominate the market in the coming years. This year, RETC is closely monitoring another technology trend that is quickly gaining market traction and acceptance, the rise of next-generation n-type PV cells with passivating contacts.

Rise of TOPCon

Many industry analysts and material scientists believe emerging n-type PV cell designs are the next logical progression on the PV technology roadmap. In 2013, researchers at Germany’s Fraunhofer Institute for Solar Energy Systems presented a method of producing high- efficiency n-type silicon solar cells with a novel tunnel oxide passivated contact (TOPCon) structure. Thanks to excellent surface passivation and effective carrier transport, this novel cell design achieved high marks for open-circuit voltage (Voc), fill factor and efficiency. Less than a decade later, TOPCon is the buzziest word in solar. The largest module manufacturers in the world are beginning volume production of PV modules with TOPCon cells. While LONGi Solar is betting big on p-type TOPCon, many other leading module companies—such as Jinko Solar, Jollywood Solar Technology, JA Solar and Trina Solar—are making substantial investments in modules with n-type TOPCon cell designs. This collective pivot in the market is primarily due to flattening efficiency curves for the p-type passivated emitter and rear-contact cell (PERC) modules. Although these have dominated the market in recent years, manufacturers are starting to reach the physical limits of p-type mono PERC cell designs. Transitioning to n-type TOPCon cells will allow module companies to boost cell efficiencies further in the laboratory and in mass production.

Benefits of N-type cells

Solar manufacturers have long recognized the potential efficiency benefits of n-type PV cells. For example, Sanyo began developing n-type heterojunction technology (HJT) PV cells in the 1980s. In addition, SunPower has built its interdigitated back contact (IBC) PV cells upon a base of high-purity n-type silicon. Due to the manufacturing complexities involved, high- efficiency PV modules based on n-type HJT and IBC cell designs are relatively expensive to produce and remain a niche part of the market. By comparison, n-type TOPCon cell manufacturing is very similar to thePERC process. As a result, manufacturers can produce these next-generation high-efficiency TOPCon modules on upgraded PERC production lines.

Though today’s n-type TOPCon modules cost slightly more to produce on a per-watt basis than p-type mono PERC modules, the efficiency gains result in a lower levelized cost of energy (LCOE) in large-scale field deployments. Best of all, leading experts expect n-type TOPCon to benefit from an accelerated learning curve. A primary material advantage of n-type TOPCon cells relative to p-type mono PERC cells is a lower degradation rate due to a decreased susceptibility to both light-induced degradation (LID) and light- and elevated temperature –induced degradation (LeTID). Additional advantages may include a higher bifaciality factor, as well as improved performance under both low-light and high-temperature conditions.

Risks of early adoption

Most analysts expect modules with n-type TOPCon cells to quickly increase market share based on these performance advantages. However, emerging PV cell technologies—even ones that ultimately prove successful in the field—invariably carry more risk than mature and proven technologies. Until products are deployed at scale, the potential exists for as-yet-undiscovered degradation mechanisms. Today, for example, independent engineers and financiers consider p-type mono PERC PV modules to be a stable and low-risk technology. This assessment was not always a consensus opinion. Early versions of mono PERC modules had issues with stability, especially LID and, in rare instances, LeTID. These unexpected mono PERC degradation modes demonstrate the performance risks that early adopters face with new technologies.

While n-type TOPCon PV cells have proven resilient to LID and LeTID, some evidence exists of susceptibility to ultraviolet-induced degradation. For example, researchers at the SLAC National Accelerator Laboratory and the National Renewable Energy Laboratory (NREL) have documented front-and back-side power loss in advanced solar cell technologies after artificially accelerated UV exposure testing. These data do not point to a single degradation mechanism but suggest that different cell designs degrade via different pathways.

Forensic analysis of field performance

Forensic analysis is a detailed investigation that seeks to establish the root cause of PV system underperformance. In many cases, inverter failures or inaccurate production estimates are to blame for real or perceived system underperformance.

Baseline assessment

One of the best ways for project stakeholders to reduce project risk is to engage a qualified third party to conduct a baseline module health assessment during project commissioning. By capturing high-quality measurements prior to commercial operations, a baseline forensic assessment provides both short- and long-term benefits over the operating life of a PV power system. In the short term, a baseline commissioning assessment improves the accuracy of system performance estimates.

Daytime EL testing

Electroluminescence (EL) testing uses a special camera system to document the light emissions that occur when an electrical current passes through PV cells. EL testing has a long history in the laboratory, where it is used to detect a wide range of hidden module defects. Once relegated to controlled indoor environments, EL testing is increasingly common in field forensic investigations. Daytime EL imaging provides two distinct benefits over earlier approaches. First, our EL testing methodology allows technicians to test modules in situ, which expedites the testing process and eliminates cell damage due to module removal and handling. Second, daytime EL testing eliminates the need to test modules in the dark of night, further improving safety and throughput.

The results of in-field EL testing are valuable for identifying major manufacturing defects, off-site shipping and transportation damages, on-site material handling or installation damages, or damages resulting from severe weather events such as hail, wind or snow. These EL images allow project stakeholders to identify cell damage that can lead to thermal nonconformities, hot spots and future module underperformance. When adequately documented and reported, third-party EL images can help settle warranty and insurance claims. Unlike aerial infrared (IR) imagery, which identifies only the potential locations of performance issues, daytime EL investigations elucidate the root causes of underperformance. These findings benefit project stakeholders by expediting issue resolution and minimising production losses.

Predictive maintenance

Third-party field performance forensics are especially practical when coupled with a robust monitoring platform and predictive maintenance protocols. As PV modules age, fielded assets are at increased risk of underperformance. Cell microcracking often does not impact module performance when modules are new, but that is not necessarily the case as systems age. After 5 or 10 years in the field, some modules continue to perform as expected, whereas others suffer from accelerated degradation.

Differentiating between “good” modules and “bad” modules is not a simple matter, especially in systems deployed after the US Department of Commerce enacted its AD/CVD policies. Large projects that appear to have a single module supplier may in fact integrate modules manufactured using cells sourced from a dozen different vendors. Given that each bill of materials (BOM) is unique, each has a different risk profile.

Mitigating extreme weather risks

No one understands the natural perils associated with solar deployments better than renewable energy insurance specialists such as GCube Insurance. According to the company’s 2021  market report, “Hail or High Water: The Rising Scale of Extreme Weather and Natural Catastrophe Losses in Renewable Energy”, weather-related insurance claims have grown in frequency and severity as solar projects have increased in frequency, size and geographic distribution. Given the rapid growth of the solar market globally, a commensurate rise in solar insurance claims is not entirely unexpected. However, the root cause of solar insurance claims has surprised some insurance industry insiders. Specifically, since 2015, insured losses associated with extreme weather events are roughly twice the magnitude of those stemming from natural catastrophes.

While extreme weather events result in more insured losses than natural catastrophes do, insurance claims associated with the severe weather loss category are not unavoidable. Project stakeholders can prevent or mitigate many extreme weather losses by exercising reasonable care and foresight in product selection and system design. Moreover, risk mitigation specialists can help tax equity investors and insurance companies understand the financial risks associated with severe weather.

Comparative testing

Strategic product selection is an essential first step for mitigating the leading causes of extreme weather losses. RETC’s bankability and beyond-certification testing results demonstrate how different PV module designs or combinations of modules and racking resist these different types of environmental stresses. These differences are mission critical in the context of extreme weather risk mitigation.

Examples of preventable extreme weather perils include wind, hail and snow. Based on claims frequency, high wind events are a leading cause of insured losses in fielded solar assets. Based on the severity of losses, a widely publicised hailstorm in West Texas damaged some 400,000 PV modules, resulting in the largest single solar insurance claim to date. Snow is a relatively lesser hazard overall but presents significant risks at specific elevations or latitudes.

The goal of comparative and accelerated testing is to empower project stakeholders to identify and specify the best products and system designs for specific applications and environments. Modules that perform well under dynamic mechanical load testing are well suited for deployment in high wind environments. Modules that perform well in RETC’s Hail Durability Test (HDT) sequence are well suited for deployment in hail-prone regions. Modules that perform well in mechanical load tests are best suited for resisting the loads associated with ice and snow. Modules that do not perform well in these two tests are not “bad” products, especially in the proper application. Modules hardened against wind and hail often incur higher manufacturing costs. The conditions for an installation in California’s Central Valley, which rarely experiences high winds, hail or snow, may not justify these additional costs.

To mitigate supply chain risks, developers often evaluate and source a variety of PV module models and vendors. Extreme weather susceptibility will vary across this portfolio of selected PV modules. By paying attention to these differences, developers can direct wind-, hail- or snow- hardened modules respectively to wind-, hail- or snow-prone sites. This type of selective deployment is a relatively simple and cost-effective way to reduce extreme weather risks.

Defensive stow strategies

After filtering and selectively deploying modules based on resistance to site-specific conditions, project stakeholders can implement weather-responsive software control strategies to reduce extreme weather risks further in large utility applications. Many large-scale PV systems integrate intelligently controlled single-axis trackers that use software to follow the sun while avoiding self-shading. As weather-related insurance claims have increased, industry-leading tracker manufacturers have implemented novel software-control responses, such as threat-specific defensive stow or load shed modes.

Due to the highly localised and fast-moving nature of high wind events and hailstorms, severe weather alerts often give plant operators little advance warning. Moreover, the types of storms that produce high winds and large hail often result in downed power lines and loss of AC power. Active software controls can address these challenges and provide effective risk mitigation with product features such as local or remote initiation, rapid response times and failsafe battery backup. It is also important to consider coincident weather risks.

Though the insurance industry has long relied on probabilistic risk assessments to provide coverage sustainably, the challenge posed by solar projects is twofold. First, limited historical data is available to understand extreme weather risks, especially considering the rate of technological change and market expansion. Second, the natural catastrophe data that insurers typically rely on do not capture “uncategorised” extreme weather events.

Module quality

Products that appear similar on paper may perform very differently in the real world. A manufacturing commitment to quality often accounts for these differences. Fielding increasing numbers of higher-capacity solar projects in locations around the globe is not without risk. Mitigating site-specific risk requires the strategic application of products and technologies. A one-size-fits- all approach to product design and project development invariably increases project risk profiles. Strategic product differentiation improves project resilience.

Hail-hardened module and system designs mitigate project risk in hail-prone regions like West Texas. Product and system designs that resist dynamic wind effects reduce project risk in high-wind locations worldwide. Product and system designs that resist high static mechanical loads lessen catastrophic failure risks in extreme-snow locations. Corrosion-resistant products extend operating lifetimes in coastal areas.

Testing laboratories use calibrated and certified equipment under audited and controlled test conditions. Characteristics captured under these rigorous conditions represent the proper measure of PV module performance and provide value to multiple project stakeholders. While factory testing according to standard test conditions (STC) parameters is ideal for establishing module nameplate ratings, factory test results do not characterise typical module operating conditions. To accurately model system performance in the real world, it is essential to understand how modules perform under low-irradiance conditions or in relation to changing sun angles. Moreover, it is crucial to characterise module performance under test conditions that reflect the operating conditions under which PV systems typically produce optimal energy yields. It is also critical to understand how short-term sun exposure and the resulting degradation impacts in-field PV performance.

Throughout the 2022 edition of the PV Module Index Report, RETC has recognized 9 different manufacturers and showcased 61 instances of high achievement in manufacturing. To identify the best of the best, it reviewed and ranked the overall data distributions across all three disciplines: quality, performance and reliability. The Overall Results Matrix highlights six top performers based on overall high achievement in manufacturing: JA Solar, JinkoSolar, LONGi Solar, Hanwha Q CELLS, Trina Solar, and Yingli Solar.

The complete report can be accessed here