The Origins of RETC’s Thresher Test
Developed by and for industry stakeholders, the first consensus reliability and durability test standard for PV modules has been “separating the wheat from the chaff” since 2011.
Reliable in-field operation over a 25- or 40-year warranty period is not an accident; rather it is a testament to rigorous manufacturing quality and product testing programs. In our last article, RETC paid tribute to the Jet Propulsion Laboratory and its decade-long work leading the Flat-Plate Solar Array (FSA) Program, an effort that dramatically improved PV module lifespans by systematically identifying failure mechanisms, accelerated stress testing sequences, and allowable failure levels.
Here, we look back at another milestone in crystalline-silicon (c-Si) PV module reliability and durability testing—namely, the development of the Thresher Test protocol. Much as the FSA Program marks the genesis of the terrestrial solar industry, the introduction of the Thresher Test marks a pivotal moment of market maturation, the birth of a science- and engineering-based approach to bankability.
Though they relied on similar analytical methodologies, the delivery processes could not have been more different. Whereas the FSA Program was a top-down initiative led by government-funded scientists, the Thresher Test was the product of a bottom-up collaboration involving an ad hoc team of industry stakeholders. The effort was led, in part, by a start-up independent testing laboratory.
A SEA CHANGE IN MODULE MANUFACTURING
The rise of comparative PV module testing is best understood in the context of tectonic shifts remaking the global solar market landscape. In 1986, coming out of the FSA Program, U.S.-based PV module manufacturers dominated the nascent terrestrial solar market. However, as the global solar market grew, so did the relative market share of manufacturers in other regions of the world.
According to data published by the European Commission, the U.S. led the world in PV production as late as 1998. In 1999, Japan surged to the front of the pack. Three years later, manufacturing capacity in Japan was more than double that in the U.S., and capacity expansions in Europe dropped the U.S. to third in the global rankings. This tenuous center of gravity—with Japan in the top producer spot, followed by Europe and the U.S.—held steady from 2002 to 2005.
Disruptive change followed. In 2006, China displaced the U.S. as the world’s third-largest PV producer. In 2007, Europe ascended to the top spot; China muscled further up the rankings, dropping Japan to third place; meanwhile, Taiwan surpassed the U.S. to take over the fourth spot. In 2008, China assumed the mantle as the world’s top PV producer, a title it still holds today.
Global PV cell and module production from 1990–2009. [See: PV Status Report 2010]
PAY YOUR MONEY AND TAKE YOUR CHOICE
Though the global proliferation of module manufacturing capacity and PV products was both welcome and necessary, it was difficult for buyers at all levels to navigate the shifting terrain. Purchasers had more procurement options than ever before. But whose product durability and performance claims and warranty could you trust?
The SolarPro magazine archives provide a ground-level view of the dizzying changes taking place from a buyer's perspective. For several years running, the trade publication mined the California Energy Commission’s database of rebate-eligible PV modules and contacted manufacturers to verify which models were currently in production and actively being sold into the North American market.
In 2009, SolarPro’s “c-Si PV Module Specifications Table” included 275 PV modules from 23 different manufacturers. A year later, it included nearly 400 modules from 36 different manufacturers. By 2011, the comprehensive reference guide had grown to include 835 modules from 53 different manufacturers.
SHIFTING THE BURDEN OF PROOF
“Back in 2010, I was frustrated by the lack of available long-term performance data,” reflects Hugh Kuhn. “At the time, I was trying to make informed purchasing decisions for my company, Solar Power Partners, an independent solar power producer that was later acquired by NRG. Due to our business model, I was not buying watts, but rather watt-hours over a 20-year period.
“A few of these firms wanted to charge more, claiming that their products were ‘higher quality’ and would thus supposedly perform much better in the later years of ownership. I suggested that if they could quantify this claim, I would be happy to consider paying more. Sadly, none of the manufacturers could quantify the performance difference, though several tried.”
While doing his due diligence, Kuhn realized that many manufacturers used proprietary accelerated testing regimens to evaluate product designs and benchmark competitors’ products based on long-term performance and reliability. However, developers, engineers, and investors could not use these in-house test data to evaluate long-term investment risk on an apples-to-apples basis due to fundamental differences in test methodologies and the self-reported nature of the data.
Standardized PV module testing allows for apples-to-apples product comparisons.
A CRITICAL MASS IN FAVOR OF STANDARDIZATION
Talking to industry colleagues, Kuhn realized that he was not alone. Many others were concerned about long-term performance and reliability. Moreover, a critical mass of industry stakeholders—including module manufacturers, government-funded laboratories, and independent testing laboratories—favored the adoption of a reliability and degradation testing standard.
“The development of the Thresher Test was very much a joint industry effort,” recalls Cherif Kedir, CEO of RETC. “The stakeholder group included multiple module manufacturers and subject matter experts from NREL and Sandia National Laboratories. Since the goal was to develop a protocol that any independent testing laboratories could implement, the group also included RETC, PVEL, TÜV Rheinland, and TÜV SÜD. We collectively selected Hugh Kuhn to serve as the project leader, since he had an interest in technical due diligence to support bankability assessments.
“RETC played a critical role as the program coordinator due to manufacturer concerns about intellectual property. Module companies were understandably reluctant to share their internal testing protocols with industry competitors. As a workaround, the manufacturers agreed to share testing program details with RETC under a non-disclosure agreement. We could then review these testing protocols against an NREL database of failure modes observed in fielded PV power plants or during product certification testing.”
The “Dream Team” behind the Thresher Test protocol. [See: PVQAT]
ONE TEST PROTOCOL TO RULE THEM ALL
On July 15, 2011, Hugh Kuhn and RETC’s Alelie Funcell introduced the cooperatively developed Thresher Test to the world as part of the International PV Module Quality Assurance Forum in San Francisco. Having grown up in a farming community, Kuhn named the extended test protocol after a type of farm equipment used to separate seed and grain (the good stuff) from straw and chaff (the bad stuff). Much as a threshing machine beats up a plant to separate useful materials from waste, the Tresher Test uses stress sequences to separate good products from bad ones.
As the program coordinator, RETC was the first independent testing laboratory to offer the Thresher Test. Other laboratories, such as TÜV SÜD, followed suit, making the Thresher Test the solar industry’s original commercial test standard for characterizing PV module designs based on long-term performance, reliability, and durability.
In the years since its introduction and adoption as the de facto standard for comparative PV module testing, RETC has adapted or added Thresher Test sequences to better align test outcomes with the latest field-failure data. However, the core extended test sequences—thermal cycling, humidity-freeze, damp heat, and damp heat with system voltage bias—are still used today to characterize product-specific design vulnerabilities, wearout and failure modes, and long-term degradation patterns.
In the next installment in this article series, RETC explores the basic structure of the Thresher Test protocol, reviews its original and contemporary test sequences, and considers how beyond-certification testing benefits industry stakeholders.
Timeline of international efforts to improve PV module quality since the introduction of the Thresher Test. [See: PV Quality Assurance Task Force.]