Rating PV Modules for Field Performance By: Brian GrenkoAdrianne KimberSarah Kurtz As the solar industry has grown and become more sophisticated in recent years, stakeholders have been basing PV system and procurement decisions on total cost of ownership in addition to—or even instead of—installation cost. Ownership cost and asset valuation now often depend on the terms and conditions associated with the power purchase agreement, as well as how much electricity the PV system is expected to generate throughout its intended operating life, which is largely a function of component quality and system design and maintenance. As a result, PV systems are increasingly evaluated based on dollars per kilowatt- or megawatt-hour, rather than on dollars per watt. In this article, we present a perspective on how PV module ratings have evolved to reflect the prevailing design and procurement criteria. Many in the industry agree that current nameplate ratings—specifically the power rating under standard test conditions (STC)—taken alone do not adequately represent PV module performance in today’s competitive marketplace. So what should buyers look for in PV module performance characteristics? Many buyers still rely on module ratings under PVUSA Test Conditions (PTC). Are PTC ratings adequate? We review the motivations for and origin of the PTC module rating. We discuss how PV project valuation practices have evolved away from single-point capacity metrics in favor of more comprehensive measurements and simulation models for predicting PV module performance in the field. In the process, we highlight some promising new energy-based PV module rating methods that industry stakeholders would be wise to adopt going forward. Evolution from STC to PTC Ratings For many years, most PV system designers relied on the nameplate or STC ratings of PV modules as a proxy for expected project performance and the resulting return on investment. These ratings are based on PV module performance at standard testing conditions (STC), which are defined in IEC 61215 as 25°C cell temperature, 1,000 W/m2 global plane-of-array irradiance and a solar spectral irradiance at air mass (AM) 1.5. The problem with STC ratings is that the nameplate power rating of any PV module inherently reveals only one aspect of how that product will perform once installed. The reason for this is twofold: PV module—not to mention PV system—performance is largely a function of operating temperature and irradiance, and temperature and irradiance are highly variable and dependent on site location. History of PTC ratings. Many PV system designers and installers use the PTC ratings system, which traces its origins to the PV for utility-scale applications (PVUSA) research project. The US Department of Energy, as well as local governments and utilities, sponsored this project. A primary goal was to construct, maintain and monitor grid-connected utility-scale projects for evaluation and testing purposes. In 1986, Pacific Gas and Electric Company (PG&E) commissioned the construction of an 86-acre solar farm just outside Davis, California. The large size of the installation (especially for its time) and the scope of its unique monitoring capabilities enabled PVUSA researchers to better understand what happens to PV module and system performance once a system is installed and connected to the grid. The PVUSA research team developed the PTC rating to evaluate overall system performance over time. The team designed this rating for comparing plant performance as a power-output rating to contractual requirements for the PV system. The rating conditions—1,000 W/m2 irradiance, 20°C ambient (not cell) temperature and wind speed equivalent to 1 meter per second—represent the typical peak-production environment for an installed system in Northern California. The research team determined the PTC rating by continuously monitoring the PG&E solar farm systems and performing regression analysis on the collected data, and then calculating the power output at rating conditions. This method eliminates the need to test under an exact condition or to scale a measurement taken under different conditions. Since the exact combination of temperature, wind speed and irradiance occurs only rarely, performing regression analysis on continuously monitored data enables more convenient and timely testing. This method also helps to avoid the larger uncertainties associated with scaling a single measured value under arbitrary conditions to a desired point. System- versus module-level evaluation. The PVUSA research team makes clear its emphasis on system-level performance from its choice of rating condition values. For module-level ratings, direct measurement determines the module or cell temperature. For a system-level rating, measuring temperature is more complex because the operating temperature for cells and modules varies, sometimes dramatically, based on their location in the array and exposure to the elements. In addition, the operation of the inverter and other BOS components is temperature dependent. Since the varying temperature of so many components influences the system’s behavior, directly measuring PV module temperature is not highly effective. For example, where do you place your thermocouple to measure the system temperature? Recognizing this complexity, the PVUSA research team cleverly circumvented the problem by defining the rating condition using basic environmental data: ambient temperature, wind speed and irradiance. These ambient conditions determine temperatures for all PV system components. The PVUSA team realized that it was not necessary to determine device temperatures to rate system capacity; indeed, it was easier to rate capacity based on ambient conditions. Adoption of PTC Rating Methods The research team developed the PTC rating as a tool for the evaluation and contractual acceptance of utility-scale PV power plants at a time when no standards for such testing existed. As the solar industry matured, standards gradually evolved. PV system performance evaluation standards used within the US have drawn heavily on the work of the PVUSA team. For example, both the ASTM International and the California Energy Commission (CEC) have effectively adopted the PTC rating method. ASTM adoption. Two ASTM standards incorporate the PVUSA rating approach: ASTM E2527 (“Standard Test Method for Electrical Performance of Concentrator Terrestrial Photovoltaic Modules and Systems Under Natural Sunlight”) and ASTM E2848 (“Standard Test Method for Reporting Photovoltaic Non-Concentrator System Performance”). In both cases, the E44.09 subcommittee evaluated the PVUSA rating method against other methods for determining the power-output rating of an entire PV system over a relatively short period of time. The subcommittee chose the PVUSA rating approach for its use of ambient environmental conditions rather than device temperature, as well as its relative ease of implementation. CEC adoption. In 1997, PG&E sold the Davis site to the CEC. Building upon the PVUSA team’s conclusions, the CEC subsequently decided to apply the PTC system-rating concept to PV modules to determine a financial incentive structure for PV projects. Since PV module performance drives system performance, assigning incentive dollars in proportion to PV module capacity (kilowatts) seemed both logical and relatively simple to administer. Additionally, the CEC wanted to put in place an incentive scheme that would reward expected performance based on realistic operating conditions. In January 2007, the CEC launched the California Solar Initiative (CSI) program, which addressed industry concerns about the relevance of STC performance ratings by basing incentive payments on PTC ratings. While the industry welcomed the CEC’s decision to adopt PTC ratings, the practice created a new set of issues. Module manufacturers initially treated PTC reporting simply as a box-checking exercise that allowed them to become eligible for rebates in the CSI program. However, they soon realized that a subtle difference in PTC ratings could significantly impact the economics of PV projects in development. To keep manufacturers from gaming the system, the CEC stopped accepting self-reported performance data in July 2009 and instead required data from third-party laboratories. To become eligible for rebates, PV module manufacturers needed to have their products independently tested at PVUSA test conditions. The premise behind the third-party test requirement was that independently verified PTC ratings would better represent real-world performance in Northern California. Thus, in principle, the PTC ratings reported to the CEC allowed PV system designers to make module-purchasing decisions with more confidence, at least for systems deployed in environments similar to Northern California. Growing Concerns with PTC Ratings For the past 7 years, the CEC has maintained a list of incentive-eligible PV modules on the Go Solar California website (see Resources). A large number of PV system designers and installers across North America have relied on the PTC ratings reported in this list to provide a more realistic indication of expected field performance than module nameplate ratings offer. Unfortunately, the future of this list is uncertain. While the CSI program was instrumental in driving PV market growth in California, particularly with respect to distributed generation, it is currently oversubscribed in most utility service territories and the incentive funds are largely exhausted. According to Terrie Prosper, the media contact for the California Public Utilities Commission, the Go Solar California site is funded through 2016. In addition to uncertainty about the CEC list’s lifespan, the PTC data it contains may be suspect: Third-party testing laboratories report very different PTC ratings for modules with similar packaging (such as crystalline silicon PV cells sandwiched between a glass front and a plastic backsheet). PTC ratings can have a significant impact on PV project financing. Do variations in these ratings indicate that similar PV modules will perform differently based on differences in their composite materials, or do they indicate problems with the testing process itself? Deriving PTC ratings. In his article “Changes to the PTC Module Ratings” (SolarPro magazine, October/November 2009), Blake Gleason points out that module PTC ratings are calculated rather than directly measured: “Certain module parameters are measured under specified conditions, and then those parameters are used to calculate the expected performance of the module under different conditions. The PTC rating is the result.” Gleason continues: “Specifically, the nominal operating cell temperature (NOCT) is measured in the nominal terrestrial environment, which is described in ASTM E1036-96, Annex 1 technical standard, and is basically 800 W/m2, 20°C ambient and 1 m/s wind speed. The NOCT is used to calculate the expected module cell temperature under PTC, and then the temperature coefficient of power (measured separately) is used to determine the PTC power rating.” Problems with NOCT procedures. To explain the potential for measurement error, we need to consider the standardized test methods used to determine PTC ratings. Test methods for crystalline silicon PV modules are based on the IEC 61215, Edition 2.0, test standard. Sections 10.4 and 10.5 cover the measurement of temperature coefficients and NOCT, respectively. However, testing labs interpret these sections differently, particularly 10.5, resulting in reproducibility error. For instance, IEC 61215 calls for testers to attach cell temperature sensors by solder or thermally conductive adhesive to the backs of two solar cells near the middle of each tested PV module. Some labs interpret this direction by placing a thermocouple on the outside of the PV module backsheet, so as to not disturb the integrity of the laminate; other labs cut through the backsheet to place sensors as close as possible to the solar cell; and some labs do both, using multiple sensors. Both the test procedures and the type and quality of sensors vary, so the results may vary. A poster report by the CFV Solar Test Laboratory in Albuquerque, New Mexico (see Resources), indicates that even on clear days with low wind, module temperature measurements can vary by as much as 4°C based on thermocouple configuration. To make matters worse, NOCT testing is conducted outdoors where ambient conditions can influence the results. For instance, light spectrum can vary significantly with latitude, altitude and humidity. Identical NOCT tests performed on the same PV module are likely to generate different test results at different locations or at different times of the year, such as in Germany in spring versus in the US Southwest in the middle of summer. This variability is problematic, as there are currently more than 35 CEC-approved laboratories located throughout North America, Europe and Asia. NOCT variability. Industry stakeholders have scrutinized the NOCT variability for several years because it can influence PV project financing and viability. As early as 2010, the CEC reported that third-party NOCT values differed by as much as 10°C for similar rack-mounted crystalline silicon PV modules. This 10°C range represents a coefficient of variation greater than 20% and suggests that module selection alone could effect a 5% improvement in power. Given the similarity of the PV modules, the disparity in the third-party NOCT test results was surprising enough to warrant further investigation. Researchers at the National Renewable Energy Laboratory (NREL) acquired three PV modules representing the range of NOCT values that approved third-party testing laboratories reported to the CEC. They tested these three modules side by side for 1 year in an outdoor environment in Golden, Colorado. The authors of a publication delivered at the 2012 IEEE Photovoltaic Specialist Conference, “Evaluating the IEC 61215 Ed. 3 NMOT Procedure Against the Existing NOCT Procedure with PV Modules in a Side-by-Side Configuration,” detail the test results (see Resources). Table 1 shows that in spite of the 10°C difference in NOCT that independent test laboratories reported, these three modules had very minor differences in NOCT values when tested at the same time. The repeatability and reproducibility errors evident in data reported by different testing labs cast doubt on the value of using the measured NOCT to derive the PTC rating. While heat transfer theory, which suggests that similarly packaged PV modules will have similar NOCT values, can explain these side-by-side test results, some of NREL’s other findings were more surprising. For example, out of a 1-year test period in Golden, Colorado, only 25 days had the necessary environmental conditions to produce the number of data points required to determine NOCT as outlined in the product test standard. In addition, NREL researchers were able to model outdoor NOCT test results predicted under varying environmental conditions—including sky, ground and ambient temperature, and wind speed—and found that the possible range of results for the same PV module varied from 43.3°C to 51.9°C. To put NREL’s findings into perspective, as of May 1, 2014, the CEC’s eligible equipment list included 332 multicrystalline PV modules with a 250 W nameplate rating. The average PTC rating for this population was 224.3 W, while the range of PTC values for this group was 16.5 W. Most of this variability may stem not from differences in the PV module design or quality of manufacturing, but instead from the testing laboratory or the local conditions at the time of the test. The CEC list of eligible equipment does not identify either the testing laboratories or the test dates. To improve the accuracy of measuring module operating temperatures, NREL has investigated methods for improving the outdoor module testing. The decision to adopt this change lies with the committees developing the next editions of the International Electrotechnical Commission (IEC) standards, which proposes replacing NOCT with the metric nominal module operating temperature (NMOT). A key difference is that laboratories measure NMOT with the module operating at the maximum power bias condition instead of at open circuit. In the interim, we encourage PV system designers to consider similarities or differences in module construction when interpreting reported NOCT ratings. Separating fact from fiction. While the PTC rating implies more-realistic device operating conditions, especially temperature, than STC does, it is clear that adapting a system-level metric such as PTC to PV modules can open the door for error and even for gaming the system. The variation in PTC ratings for PV modules made with similar cells and components is sometimes smaller than the measurement uncertainty. Furthermore, a company that measures the module operating temperature multiple times may be tempted to only record the most favorable value. The input values and assumptions for modules have faced an intensifying level of scrutiny because they strongly influence PV project valuation. For example, a subtle improvement in the temperature coefficient for the PV modules used in a system can impact project valuation by thousands or even millions of dollars based on the modeled energy production. Though PV power ratings—such as a module’s or a system’s PTC rating—are valuable with respect to system design or acceptance, the real value of a PV project lies in its long-term energy-generating potential. For this reason, industry experts are currently focused on developing ratings and metrics designed to measure and predict energy generation at both the module and the system level. Moving Beyond Single-Point Performance Metrics While PV modules undoubtedly have a significant influence on overall PV system performance and value, they create value only as an integrated part of a larger system—and design, operations and maintenance factors, such as module tilt, shading or soiling, also influence system performance. To be most valuable, the metrics used to evaluate PV modules must reflect and support the metrics used to evaluate entire systems. The PTC module rating arose from a method for evaluating PV system capacity rating, so a logical place to look for appropriate alternatives to STC and PTC ratings is system evaluation practices, which heavily emphasize energy estimation. The growth of the utility market sector, which is accustomed to more-sophisticated product performance curves, has made the limitations of PV module PTC ratings even more apparent. The PV industry urgently needs to create a practical and credible PV module energy rating system. To make such a system possible, industry stakeholders must continue to define more-comprehensive PV module performance metrics that capture performance over a meaningful range of environmental conditions. Further, the industry needs to adopt new product labeling and datasheet standards to ensure that manufacturers and third-party labs publish and report these PV performance metrics consistently. More-comprehensive performance metrics. In 1982, a team from Arco Solar presented an “am/pm” approach—based on the concept of a standard solar day—for characterizing PV module behavior with respect to operating temperature, irradiance and air mass. In the 1990s, NREL, in conjunction with Endecon Engineering (which later became BEW Engineering, now a renewable energy division of DNV GL), expanded upon this work by developing a model that also considered load type, location and additional weather parameters. At the same time, Sandia National Laboratories developed an empirical PV array performance model based on outdoor PV module testing, known as the “King Method” (named after its creator, David King). In the US, PV designers and simulation software use the King Method to this day. (See “Production Modeling for Grid-Tied PV Systems,” SolarPro magazine, April/May 2010.) IEC Technical Committee 82, Working Group 2 (TC82/WG2), the committee responsible for PV modules, also recognizes the need to standardize PV module performance metrics beyond STC. In January 2011, after more than 15 years of debate and development, the group published IEC 61853-1, “Part 1: Irradiance and Temperature Performance Measurements and Power Rating.” This first part of a proposed four-part IEC 61853 standard describes the requirements for evaluating PV module performance over a 23-element maximum power matrix at different temperature and irradiance conditions (see Table 2). IEC 61853-1 serves as a guide for collecting measurements that labs can use to develop PV module parameters for performance simulation tools. For instance, an independent testing lab may refer to IEC 61853-1 for the sets of conditions under which to measure PV module performance. The lab then uses the resulting measurements to develop a model input file for the prediction of long-term energy production at an installation site. Since PV modules operate under a range of temperatures, irradiances and sunlight spectra, the 23-element performance characteristics matrix in IEC 61853-1 provides more-comprehensive rating information than a single-point value such as STC or PTC. Industry stakeholders will need to fully assess the uncertainty associated with the measurement of the characteristics matrix and module performance variability—within a production bin or under low-light conditions—to ensure that the matrix of measurements provides a more meaningful basis of comparison than the NOCT measurements described previously. Meanwhile, the other three parts of the standard are at different stages of development. IEC 61853-2, approved in draft format, will undergo final committee review in the second half of 2014; it will further develop module parameterizations for PV performance simulation tools. IEC 61853-3 is in the early stages of development; it will present methods and models for predicting PV module energy production based on measurements obtained using Parts 1 and 2, as well as a method for stating a numerical energy rating. Work on IEC 61853-4 has yet to begin; it will define standard time periods and weather conditions for use in the energy rating calculations. Many in the industry speculate that IEC 61853 Parts 3 and 4 will take years to complete, in part because some parties may lobby against them. PV module manufacturers may advocate for different standard weather condition sets to benefit their own technologies or products. Other committee members may advocate positions based on their testing capabilities or their in-house energy simulation models. More-detailed labels and datasheets. Around the time of IEC 61853-1’s publication, DKE, the German nonprofit organization responsible for creating and maintaining standards covering the electronics industry in that country, issued a project progress report entitled “Energy Rating of PV Modules” (see Resources). Noting the slow progress on IEC 61853, the authors recommended integrating the module performance matrix from IEC 61853-1 into EU 50380, “Datasheet and Nameplate Information for Photovoltaic Modules.” According to the DKE progress report, “The most obvious possibility to provide the data is the direct integration into the datasheet of the module in the form of the table.” Integrating the 23-element matrix from IEC 61835-1 into PV module datasheets is an intriguing proposition. Currently, PV module manufacturers typically provide performance data at STC and NOCT conditions, with a note regarding low-irradiance behavior. (The next edition of IEC 61215 is expected to require manufacturers to report module performance at 200 W/m2.) If manufacturers provide all 23 data points, then industry stakeholders will have access to more-transparent and more-reliable performance data. They can use these data to create meteorologically weighted matrices that represent module performance in different climates. Alternatively, the DKE report notes, an energy label that describes performance based on parameters such as irradiance, temperature or spectral conditions could identify PV modules. Such a label would resemble the yellow EnergyGuide label found on appliances in the US, except that it would estimate energy production rather than consumption over the course of a year. The DKE energy label concept appears to have gained traction in some global markets, particularly in South America. While PV module performance at STC dictates the rating itself, labels also include estimated energy production based on typical annual weather conditions in the specific country. Expanded requirements for PV module labeling and datasheet requirements may be coming to the US market as well. The Solar America Board for Codes and Standards (Solar ABCs) has published its power-rating recommendations in a report, “Module Power Rating Requirements,” as well as in “A Proposed Standard for Nameplate, Datasheet and Sampling Requirements of Photovoltaic Modules” (see Resources). Solar ABCs has submitted its proposal as an outline for UL 4703, which is awaiting technical review. If UL 4703 is approved, it would require that manufacturers report PV module performance at five rating conditions specified in IEC 61853-1 (as shown in Table 3), that they take these characteristics from a statistically representative sample population, as defined in ANSI/ASQ Z1.4, and that they measure after module stabilization to account for phenomena such as light-induced degradation. Moving from Power to Energy It is clear that the industry is slowly moving toward integrating more-detailed performance data into PV module datasheets and product labels. As stakeholders come to a consensus on standards for these parameters, PV system designers and installers need to consider how best to put these data to use. The solution lies in part with production modeling tools. The industry requires an accurate energy yield model to effectively rate and compare PV modules on the basis of energy instead of power. This needs to happen before the industry can transpose PV module power performance and standardized weather inputs into a predicted energy production statistic. The rapid increase in utility-scale PV project development has prompted the proliferation and wide-scale adoption of PV system performance models—like those discussed in “Show Me the Model” on p. 66—for simulating energy yield estimates. These production models are based on project design parameters and assumptions, weather conditions and input values for system components, especially PV modules and inverters. Rather than relying on a single power rating to estimate energy generation, these sophisticated production models require a suite of parameter inputs and assumptions to describe PV module behavior. When an advanced performance model determines project value, module inputs and assumptions become the primary indicators of module performance, effectively replacing STC or PTC ratings. Standards for PV system performance evaluation continue to evolve. For example, in 2013 ASTM published ASTM E2848-13 for determining the initial power capacity of a PV system. This standard is of great utility to industry stakeholders for system acceptance because commissioning agents can execute the test over a relatively short period of time (days or weeks). However, stakeholders also need to determine the energy-generating potential of a PV system in its first year(s) of service, as well as its stability over time after accounting for module degradation. Since initial power capacity may or may not correlate with energy-generating potential over time, longer-term PV performance evaluation methods are important, as Timothy Dierauf, et al., discuss in “PV System Energy Performance Evaluations” (p. 22). As the industry matures, we expect that both capacity and energy tests will play crucial roles in predicting and verifying PV system performance. CONTACT: Brian Grenko / Yingli Green Energy Americas / San Francisco, CA / yingliamericas Adrianne Kimber / Incident Power Consulting / Oakland, CA / incidentpower Sarah Kurtz / NREL / Golden, CO / nrel.gov RESOURCES Deutsche Kommission Elektrotechnik (DKE) progress report, “Energy Rating of PV Modules,” August 2011, vde/en/dke Muller, Matthew, et al. “Evaluating the IEC 61215 Ed. 3 NMOT Procedure Against the Existing NOCT Procedure with PV Modules in a Side-by-Side Configuration,” 38th IEEE Photovoltaic Specialists Conference, June 2012, dx.doi.org/10.1109/PVSC.2012.6317705 Go Solar California, “Incentive Eligible Photovoltaic Modules in Compliance with SB1 Guidelines,” gosolarcalifornia.ca.gov/equipment/pv_modules.php Kurtz, Sarah, et al., “Analysis of Photovoltaic System Energy Performance Evaluation Method,” NREL Technical Report (NREL/TP-5200-60628), November 2013, nrel.gov/docs/fy14osti/60628.pdf Sabuncuoglu, Fatih, et al., “Variability in NOCT Standard Test Results as Function of Day, Time of Day and TC Location,” CFV Solar Test Laboratory, February 2012, www1.eere.energy.gov/solar/pdfs/pvmrw12_poster_si_sabuncouglu.pdf TamizhMani, Govindasamy, et al., “Solar ABCs Policy Recommendation: PV Module Power Rating Requirements,” Solar America Board for Codes and Standards, March 2011, solarabcs.org/about/publications/reports/powerratingpolicy/index.html TamizhMani, Govindasamy, et al., “A Proposed Standard for Nameplate, Datasheet and Sampling Requirements of Photovoltaic Modules,” Solar America Board for Codes and Standards, January 2012, solarabcs.org/about/publications/reports/nameplate/index.html
Posted on: Thu, 15 Jan 2015 03:48:33 +0000
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