Setting U.S. standards for solar PV module performance and durability
Published by Underwriters Laboratories (UL), Northbrook, Ill., ANSI/UL 1703-2004, “Standard for Safety for Flat-Plate Photovoltaic Modules and Panels” covers the design, production, and installation of flat-plate photovoltaic (PV) modules and panels, as well as electrical and mounting components for PV modules and panels, intended for integral or freestanding installation in the United States. The standard works in accordance with the NEC, NFPA 70, and model building codes (see Safe, Reliable, Durable). Yet, the U.S. marketplace currently does not require solar PV panels, modules, or components to undergo third-party, independent testing to back up manufacturer claims of durability, quality, or reliability over time.
“UL 1703 provides only for safety,” says Dr. Govindasamy Tamizhmani, professor in the Department of Electronic Engineering Technology at Arizona State University (ASU), and president of TÜV Rheinland Photovoltaic Testing Laboratory (PTL), Tempe, Ariz., North America’s largest solar and PV testing company. “The module won’t create a fire hazard or electrical shock, but it doesn’t have to keep working. There is no requirement for performance after the stress test.”
Fitness for use
Several U.S. standards organizations have attempted to redress this issue. The International Electrotechnical Commission (IEC) and the Institute of Electrical and Electronics Engineers (IEEE) have published qualification standards for PV products (see Qualification Standards for PV Modules).
In January, the Solar America Board for Codes and Standards (Solar ABCs), a collaborative effort among experts to formally gather and prioritize input from the broad spectrum of solar PV stakeholders, resulting in coordinated recommendations to codes and standards-making bodies for existing and new solar technologies, adopted Solar ABC Policy Recommendations #1. The recommendation includes following IEC 61215 for crystalline silicon flat-plate modules, IEC 61646 for thin-film flat-plate modules, and IEC 62108 for concentrator modules/assemblies. Meeting the requirements of qualification standards is considered to be a minimum requirement for any module procurement.
System engineers can accurately predict energy output over several decades by reviewing PV modules’ performance characteristics at different temperatures and irradiances and ensuring they meet the full requirements of the qualification standards, says Tamizhmani. However, current IEC qualification standards cover only qualification, meaning fitness for use — not reliability or durability over time.
According to the report, “Overview of Failure Mechanisms and PV Qualification Tests,” by John Wohlgemuth, former program manager at BP Solar, a part of BP’s Alternative Energy business, the qualification process includes a set of well-defined accelerated stress tests performed on a strict pass/fail basis. The stress levels and durations are limited so the tests can be completed within a reasonable amount of time and cost, writes Wohlgemuth. The goal for qualification testing is that a significant number of commercial modules will pass — and that all subsequent production modules will be built the same way as the test modules. Passing the qualification means the product has passed the specific round of tests, but doesn’t necessarily predict product lifetime or indicate which product will last longer or degrade in operation. As a result, qualification is not a replacement for reliability testing.
“IEC standards say nothing about the durability of the module or the system or whether it maintains its efficiency,” says David Burns, 3M Weathering Resource Center, 3M Co., Maplewood, Minn., who is chair of Committee E44 for ASTM International (originally known as the American Society for Testing and Materials), one of the largest voluntary standards development organizations in the world. “Solar ABCs adopting IEC is essentially just the start. So now you need that next level of testing, which takes time, unfortunately.”
ASTM Subcommittee E44.09 on Photovoltaic Electric Power Conversion, under the jurisdiction of Committee E44, is now working on a proposed new standard that will cover these long-term issues. ASTM WK25362, “Practice for Photovoltaic Module Reliability Assessment,” includes procedures for accelerating the failure mechanisms of PV modules caused by mechanical, electrical, and environmental stresses. “While a module may meet the initial qualification requirements, this does not provide sufficient information to indicate that the same module will function reliably in the long term,” says Burns. “The intent of ASTM WK25362 is to recommend procedures for conducting accelerated life testing of PV modules to provide a common ground for manufacturers and users to assess durability and estimate functional service life.”
Failure rates recorded by TÜV Rheinland PTL reveal that, as the industry has expanded to include additional manufacturers, failure rates for qualification and reliability testing spiked. Between 2005 and 2007, as a number of manufacturers entered the market, the failure rate for modules increased at an alarming rate, says Tamizhmani. For example, during that time, thin-film modules experienced a qualification failure rate of 20% and an in-the-field failure rate of 70%. Although failure rates for both silicon-based and thin-film modules dropped to more historic norms between 2007 and 2009, according to Tamizhmani, the rates remain “unacceptable” (see Failure to Qualify).
Common failures in the field reported for crystalline silicon modules include broken interconnects and cells, corrosion, delamination, loss of elastomeric properties, encapsulant discoloration, solder bond failures, broken glass, ground faults, hot spots, junction box and module connection failures, and structural failures. As a result, accelerated tests for PV modules include inducing those conditions in the lab. “From the technical end, there’s a science around the service-like prediction of complex electronic systems, which is what PV modules are,” says Burns. “They are pretty complex, with multiple components and multiple modes of failure. Predicting durability out to 20 yr is not simple.”
Using roofing shingles as an example, Burns explains how shingles with a 25-yr durability warranty don’t necessarily have 25 yr of use in the field. The warranty is based on a combination of field experience and high-stress testing in order to speed it to the marketplace.
The organization hopes the results of test protocols described in ASTM WK25362 will help designers and manufacturers identify and quantify the important failure mechanisms that can limit the service life of PV modules, as well as provide methods to evaluate the rate of performance degradation. “When approved, the proposed standard should serve as a technical framework for evaluating improvements to increase the long-term performance of PV modules,” says Burns. “The driving issue behind ASTM WK25362 is how to evaluate whether a PV module will function for an acceptable amount of time. To that end, it could also potentially be used to corroborate warranty claims.”
In the absence of required reliability testing, U.S. consumers and solar contractors depend on manufacturers’ warranties, which usually range between 20 and 30 yr. Most vendors base warranties on their standard test conditions (STC) rating. PV panels and modules are rated according to their maximum DC power output (watts) under STC, defined by a module (cell) operating at 77°F, an incident solar irradiance level of 1,000W/m2, and under air mass 1.5 spectral distribution. However, because these conditions are not always met in the field, actual performance is usually 85% to 90% of the STC rating. “Good manufacturers base their warranties on sound technical assessments of past performance,” says Burns.
Other factors may affect specification of solar panels and modules, as well. Veteran solar contractor and 2006 inductee into the Solar Hall of Fame Tom Lane, president and field engineer for Gainesville, Fla.-based ECS Solar Energy Systems, takes the STC rating under consideration but stays with tested companies and technologies. “I prefer single crystal because it’s the most reliable,” Lane explains. “The technology has been around since the ’50s. You want somebody who’s been around and is a powerful force — not somebody who’s just getting started.”
Therefore, accelerated testing for reliability can actually help emerging technologies. For instance, with a new technology that doesn’t have a history of field use, such as a copper indium gallium (di)selenide (CIGS) module, reliability testing can provide information to potential investors. “If you have a new process — you’re making a module with a new material — and you want to sell them, how is the consumer going to have confidence to buy it?” asks Burns. “But maybe even more important, what makes the venture capitalist want to bankroll the company to stay in business?”
ASTM WK25362 will educate users (including distributors, installers, and building/home owners) on PV property-performance-cost relationships, such as electrical output versus environmental variables, corrective and preventive maintenance, and design factors that influence reliability. “PV modules need to last and perform without significant maintenance for up to 20 yr in order to be an economically viable model,” Burns says. “The question is what makes somebody think that module is going to last that long? The new technology has a lot of potential, but unlike the classic PV modules that have been out there for 20 to 25 yr, they have no history. We’re just trying to find a way to assess the durability potential of a new module design or product based on accelerated testing.”
Sidebar: Failure to Qualify
Testing and product certification are the final hurdles before releasing a product to market. In the testing of photovoltaic (PV) modules, a large proportion of products do not receive certification based on their first testing cycle. According to Intertek, which, as a Nationally Recognized Laboratory (NRTL), provides testing, inspection, and consulting services for a wide range of global industries and markets — including PV module and panel manufacturing — following are five of the most common reasons PV modules do not pass certification the first time:
- Inappropriate/incomplete installation instructions: In more than 85% of the modules Intertek has evaluated, installation instructions were incomplete or contained errors. The installation instructions are considered part of the equipment and must be delivered with the equipment for evaluation. In some cases, up to one-third of the conformity assessment time is spent reading and checking the installation instructions. Installation documentation is particularly important, because numerous sections in ANSI/UL 1703-2004, “Standard for Safety for Flat-Plate Photovoltaic Modules and Panels,” are related to assessing the product after it has been installed according to its intended manner (as specified in the instructions).
- Models provided for testing do not accurately represent the entire production model scheme being listed: The highest watt density modules are required for testing due to their higher failure rate during temperature cycling and loading tests. In some cases, according to Intertek, a manufacturer will add higher-power modules than those available at the beginning of the test process. This problem can be avoided by having a clear definition of the scope of equipment to be included in the evaluation at the beginning of the quote stage. If a manufacturer makes changes to the scope of equipment covered by an evaluation after the testing has begun, it usually results in significant delays and often requires certain portions of the testing to be repeated.
- Testing requested without prior construction evaluation being performed: Because temperature testing test takes several weeks to complete, it’s not uncommon for manufacturers to request starting this test as soon as possible. However, temperature testing without first performing the construction evaluation on the module can lead to having to repeat the test. This is because assumptions must be made as part of the test setup, which cannot be confirmed without the construction evaluation. Key areas are identified during the construction evaluation that will lead to proper sample preparation and ensure that the right samples are chosen with the right components.
- Complete bill of materials with ratings and certification information not provided prior to start of the project: Because of tight time lines and time-to-market demands, manufacturers occasionally provide bills of materials that are missing certification ratings. This leads to tests being performed without checking to make sure that the ratings of the modules’ different materials are within the ratings of the tests being performed. Without the proper bill of materials, it’s not possible to determine the rating of the sample. If any of the materials do not have ratings, it’s not possible to allow modules with those alternate components to be included in the listing.
- Lack of back sheet panel RTI rating: About 95% of substrates in the market today do not have the minimum RTI value as required by clause 7.3 in UL 1703. The practice has been to use the RTI of the outer layer. If the outer layer is a spray-on thin coating, it will not be considered for the RTI requirement. If the middle layer of the substrate has an RTI of less than 90, it will not be acceptable. If the testing lab tries to establish an RTI of 90 for such a laminate, this middle layer will likely fail, and the laminate will destabilize.
To download the complete white paper, “Five Reasons PV Modules Fail Product Certification Testing the First Time,” visit the Intertek website.