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 July 2007
Feature
GOVINDASAMY (MANI) TAMIZHMANI is the director of the Arizona State University Photovoltaic Testing Laboratory, an associate professor in the electronic systems department and a member of ASTM, IEEE and IEC. He has over 25 years of R&D experience and over eight years of testing and certification experience in the area of photovoltaics.

ASTM International Standards for Photovoltaics The Foundation of Qualification Standards

ASTM International Committee E44 on Solar, Geothermal and Other Alternative Energy Sources develops standards relating to the methods and applications of solar and geothermal energy conversion. This article focuses on how the standards developed by Subcommittee E44.09 on Photovoltaic Electric Power Conversion form the foundation of qualification standards developed by other national and international standards developers.

As of this writing, there are 23 active standards under the jurisdiction of E44.09. The ASTM test methods do not establish pass or fail levels; however, a great number of these test methods serve as the foundation for photovoltaic qualification standards that do. These qualification tests are a set of well-defined, accelerated stress tests (irradiation, environmental, mechanical and electrical) with strict pass/fail criteria developed by the Institute of Electrical and Electronics Engineers, the International Electrotechnical Commission and Underwriters Laboratories.

The qualification standards and resulting certifications are heavily used by industry for procurement specifications. For example, the Arizona State University Photovoltaic Testing Laboratory, an accredited laboratory, has been providing photovoltaic testing services since 1992 and has issued more than 300 design qualification and type approval certificates.

Between 1997 and 2005, about 1,200 modules (87 percent crystalline silicon modules and 13 percent thin-film modules) were subjected to various qualification tests at ASU-PTL. In addition to performance test methods (see the article written in this issue by Keith Emery, page 30), there are 18 stress/non-stress test methods (Figure 1) used in IEEE 1262, Recommended Practice for Qualification of Photovoltaic (PV) Modules, and IEC 61215, Crystalline Silicon Terrestrial Photovoltaic (PV) Modules — Design Qualification and Type Approval.1

Fourteen of the test methods performed in these qualification standards are addressed by ASTM International standards (shown by the asterisks in Figure 1). One of the most extensively used ASTM standards at ASU-PTL is E 1799, Practice for Visual Inspections of Photovoltaic Modules. Many visual inspection results are subjective. This standard practice identifies about 19 quality checks in addition to several environmental stress checks and inspections to reveal potential module safety and performance problems before and after the stress tests of the qualification standards used by ASU-PTL.

The PTL has adopted E 1799 in the form of a checklist that identifies about 46 features of a PV module that must be individually inspected. As part of the PTL’s quality assurance program, we have developed a standard operating procedure based on E 1799 and other qualification standards that minimizes qualitative differences arising from personal judgment.

Electrical Biasing During Thermal Cycling

The lifetime of PV modules depends on their ability to withstand repeated thermal cycling in the field. The investigations conducted by the National Renewable Energy Laboratory2 and others indicate that the actual field failures of PV modules were not fully identified in the previous accelerated thermal cycling test method (200 cycles) without current injection but the failures were caused with current injection.

In the new thermal cycling test method in ASTM standard E 1171, Test Methods for Photovoltaic Modules in Cyclic Temperature and Humidity Environments, the effectiveness of this test method is improved by injecting peak power current through the module when the module experiences room temperatures above 25°C. The current injection is intended to stress the interconnections and solder bonds in ways similar to those that are believed to be responsible for the degradation of field-deployed modules. Following the release of this new ASTM test method, the 2005 edition of IEC 61215 now cites this testing procedure for its pass/fail qualification testing requirements.

Hot Spot Protection

Hot spot heating occurs in a PV module when its operating current exceeds the short-circuit current of a shadowed or faulty cell in the string. This shadowed/faulty cell can overheat due to reverse bias and become a combustion or electrical hazard. Currently, there are two test methods used in qualification standards to identify and address this issue. These two methods are based on the intrusive method in UL 1703, Flat-Plate Photovoltaic Modules and Panels, and the non-intrusive method in IEC 61215.

Comparing these methods and identifying the best test method (in terms of the real hot spot issue, test duration, testing cost and equipment complexity) is of great value to consumers, PV module manufacturers and test laboratories. The IEC 61215 method was found to be a less expensive, less complicated and shorter duration testing method, but it selects only the cell with the highest shunt resistance for the stress testing. The highest shunt resistance cell has the characteristics of uniform heating, and this minimizes the real localized hot spot failure during the stress testing duration warranted by the IEC 61215 standard.

The UL 1703 standard calls for a good cell selection method to identify the low, median and high shunt resistance cells, however, the cell selection is done only on 10 randomly selected cells. From the 10-cell sample, the standard does require selection of both the highest and lowest shunt resistance cells, but there is a reasonable likelihood of not selecting the potential small population of cells with localized shunts that can cause hot spot problems if used in high voltage systems.3 Moreover, this is an intrusive method requiring special test module construction, longer test duration and a complex test setup.

ASTM standard E 2481, Test Method for Hot Spot Protection Testing of Photovoltaic Modules, is a simple, non-intrusive testing method that selects cells of both types (highest and lowest shunt resistances) to be stressed. This newly developed test method is being considered by the IEC 61215 qualification standard committee as a potential replacement of the existing hot spot test method.

Electrical Performance of Concentrator PV Modules and Systems

Developing a recognized test procedure for the electrical performance measurements of concentrator photovoltaic modules and systems is extremely challenging for the qualification standards where measurement repeatability tolerance must be very low. The newly developed test procedure in ASTM standard E 2527, Test Method for Rating Electrical Performance of Concentrator Terrestrial Photovoltaic Modules and Systems Under Natural Sunlight, is expected to be used in the IEEE and IEC qualification standards.

Surface Cut Test

Existing safety qualification standards (UL 1703 and IEC 61730, Photovoltaic Module Safety Qualification) that require the cut test do not specify the radius of the cut test blade. This radius has recently been found to be critical; if it is too large, failures are never seen, and if it is too small or the tip has sharp burrs, the test always fails. Because neither the UL nor the IEC standard places any restrictions on the cut test blade, a proposed new standard is being developed by Subcommittee E44.09, WK9920, Specification for Steel Blades Used with the Photovoltaic Module Surface Cut Test. This draft specifies the recommended physical characteristics of the steel blades required for the surface cut test described in UL 1703 and IEC 61730.
In summary, ASTM standards not only provide stand–alone test methods, but also serve as foundations to photovoltaic qualification standards used around the world. //
References
1 B. Li, T. Arends, J. Kuitche, W. Shisler, Y. Kang and G. TamizhMani (2006): IEC and IEEE Design Qualifications – An analysis of test results acquired over nine years, European Photovoltaic Conference, Dresden, Germany.
2 C. Osterwald, J. Pruett, S. Rummel, A. Anderberg, L. Ottoson (2000): Forward-Biased Thermal Cycling: A New Module Qualification Test, NCPV Program Review Meeting, Denver, CO.
3 J. Wohlgemuth and W. Herrmann (2005): Hot spot tests for crystalline silicon modules, IEEE Photovoltaic Specialists Conference, Orlando, FL.