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Metrology

Review of Performance Testing Requirements for Photovoltaic Modules

Specific performance tests found in IEC 61215:2005 and IEC 61646:2008

Published: Thursday, July 8, 2010 - 08:21

As the push to alternative energy technologies continues, the demand for photovoltaic modules is expected to increase exponentially. This article describes the specific performance tests found in IEC 61215:2005—“Crystalline silicon terrestrial photovoltaic (PV) modules—Design qualification and type approval,” and IEC 61646:2008—“Thin-film terrestrial photovoltaic (PV) modules—Design qualification and type approval.”

IEC 61215 and IEC 61646 set specific test sequences, conditions, and requirements for the design qualification of PV modules. Other standards address the safety qualifications for PV modules, including IEC 61730-1, IEC 61730-2, and UL 1703, and are outside the scope of this article.

Performance testing under IEC 61215 and IEC 61646 requires a total of eight test samples, selected at random from a production batch in accordance with IEC 60410. Figure 1 and figure 2 show the sequencing of individual tests under each standard. Individual samples progress through different test sequences in parallel.

Figure 1: The testing sequence prescribed in IEC 61215. Click here to enlarge the image. 

 

Figure 2: The testing sequence prescribed in IEC 61646. Click here to enlarge the image  

 

If two or more samples fail any of the test criteria detailed in the applicable standard, the design is deemed to fail qualification. Should one sample fail any test, another two samples undergo the relevant test sequence from the beginning. If one or both of these new samples also fail, the design is deemed to fail qualification requirements. If both samples pass the test sequence, the design is deemed to meet qualification requirements.

The sections below provide details on each of the individual PV performance tests.

Visual inspection (subclause 10.1)

Visual inspection is a diagnostic check and is intended to detect any "major visual defects." Visual inspections involve checking the module in a well-illuminated area. It is repeated multiple times throughout all the test sequences and is conducted more than any other test.

Maximum power determination (subclause 10.2)

Maximum power determination (Pmax) is a performance parameter. This assessment is performed several times before and after the various environmental tests.

Although the standard offers the possibility to perform the test for a range of cell temperatures (25ºC to 50ºC) and irradiance levels (700 W/m2 to 1,100 W/m2), it is common practice to perform it at the so-called standard test conditions (STC), which corresponds to: 1000 W/m2, 25ºC cell temperature, with a reference solar spectral irradiance called Air Mass 1.5 (AM1.5).

IEC 61215 and IEC 61646 set several accuracy requirements for the measurement of temperature, voltage, current, and irradiance. In IEC 61215, required repeatability for the power measurement is ±1 percent. There is no mention of such requirement in IEC 61646, probably due to known instability and repeatability issues of the different thin-film technologies. Instead, IEC 61646 recommends that "every effort should be made to ensure that peak power measurements are made under similar operating conditions."

Insulation resistance (subclause 10.3)

The insulation resistance test is an electrical safety test used to determine whether a module has a sufficient electrical insulation between its current-carrying parts and the frame (or the outside world). A dielectric strength tester is used to apply a DC voltage source of up to 1000 V plus twice the maximum system voltage. To pass this test, there should be no breakdown, nor any surface tracking. For modules with an area larger than 0.1 m2, the resistance should not be less than 40 MΩ for every square meter.

Wet leakage current (subclause 10.15)

The wet leakage current test is also an electrical safety test used to evaluate the insulation of the module against moisture penetration under wet operating conditions (e.g., rain, fog, dew, melted snow), to avoid corrosion, ground fault, and electric shock hazard.

The module is submersed in a shallow tank to a depth covering all surfaces, including mating connectors, but not cable entries of junction boxes not designed for immersion. A test voltage is applied between the shorted output connectors and the water bath solution, up to the maximum system voltage of the module, for two minutes. The insulation resistance should not be less than 40 MΩ for every square meter for modules with an area larger than 0.1 m2.

Measurement of temperature coefficients (subclause 10.4)

The measurement of temperature coefficients is a performance parameter, intended to determine the temperature coefficients of short-circuit current, open-circuit voltage, and maximum power from module measurements. Temperature coefficients are performance parameters often used by end users to simulate energy yields of the modules in hot climates. They are valid at 1000 W/m2 irradiance level used in the lab, unless the linearity of the module at different irradiance levels has been proven.

Nominal operating cell temperature (subclause 10.5)

Nominal operating cell temperature (NOCT) is a performance parameter defined for an open-rack mounted module in a standard reference environment. NOCT can be used by the system designer as a guide for the temperature at which a module will operate in the field, making it a useful parameter when comparing the performance of different module designs.

The test setup requires data logging and selection for irradiance, ambient temperature, cell temperature, wind speed, and wind direction. All these quantities shall be within certain intervals to be acceptable for the calculation of NOCT. A minimum set of 10 acceptable data points taken both before and after "solar noon" are used for the calculation of the final NOCT.

Outdoor exposure (subclause 10.8)

The outdoor exposure test is an irradiance test that provides a preliminary assessment of the module's ability to withstand exposure to outdoor conditions. The test involves exposure for only a total of 60 kWh/m2, a relatively short period of time. Nonetheless, this test can be a useful indicator of possible problems that might not be detected by other tests.

IEC 61215 requires degradation of maximum power (Pmax) not to exceed 5 percent of the initial value. IEC 61646 requires Pmax not to be lower than the marked "Pmax – t%."

Hot-spot endurance (subclause 10.9)

The hot-spot endurance test is a thermal/diagnostic test, used to determine the module's ability to withstand localized heating caused by cracked or mismatched cells, interconnection failures, partial shadowing, or soiling.

Hot-spot heating occurs when the operating current of the module exceeds the reduced short-circuit current of a faulty (or shadowed) cell(s). This forces the cell(s) into a reverse bias condition when it becomes a load-dissipating heat. Serious hot spot phenomena can be as dramatic as outright burns of all the layers, cracking, or even breakage of the glass.

Bypass diode (subclause 10.18)

The bypass diode test is a thermal test, and represents an important aspect of module design. It is a critical component that determines the thermal behavior of the module under hot-spot conditions, and directly affects reliability in the field.

The test method requires attaching a thermocouple to the diode body, heating the module up to 75°C + 5°C, and applying a current equal to the short circuit current measured at STC for one hour. The temperature of each bypass diode body is measured, and the junction temperature is calculated using a formula derived from the specifications provided by the diode's manufacturer. The current is then increased to 1.25 times the short-circuit current of the module, as measured at STC, for another hour while maintaining the module temperature at the same temperature.

To pass, the diode should still be operational following the test.

UV preconditioning (subclause 10.10)

UV preconditioning is an irradiance test used to identify materials that are susceptible to ultraviolet (UV) degradation, and is conducted before the thermal cycle and humidity freeze tests. It involves subjecting the module to UV irradiation (UVA + UVB, and UVB), while maintaining the module at 60 °C ±5 °C.

Thermal cycling, 200 cycles (subclause 10.11)

The thermal cycling test at 200 cycles (TC200) is an environmental test that simulates thermal stresses on materials as a result of changes of extreme temperatures. Most frequently, soldered connections are challenged inside the laminate due to the different thermal expansion coefficients of the various encapsulated materials. This may result in failure for major defects, Pmax degradation, or interruption of the electric circuitry.

IEC 61215 requires the injection of a current within ±2 percent of the current measured at peak power when the module temperature is above 25°C. (IEC 61646 does not specify current injection, but the continuity of the electrical circuit must be monitored.) The module is subjected to the cycling temperature limits of –40°C ± 2°C and +85°C ± 2°C with the profile shown in figure 3.

Figure 3: Cycling temperature limits of the TC200 test

Humidity-freeze (subclause 10.12)

The humidity-freeze test is an environmental test designed to determine the module's ability to withstand the effects of high temperatures combined with humidity, followed by extremely low temperatures. The module is subjected to 10 complete cycles as per the harmonized profile in figure 4 (IEC 61646).

Figure 4: Harmonized profile for the humidity-freeze test

After this test, the module is allowed to rest between two and four hours before the visual inspection, and before maximum output power and insulation resistance are measured.

Robustness of terminations (subclause 10.14)

The robustness of terminations test is a mechanical test that determines the robustness of the module's terminations, including wires, flying leads, screws, or PV connectors. The terminations undergo a stress test that simulates normal assembly and handling through various cycles and levels of tensile strength, and bending and torque tests as referenced in IEC 60068-2-21.

Damp-heat, 1,000 hours (subclause 10.13)

The damp-heat 1,000 hours (DH1000) test is an environmental test that determines the ability of the module to withstand long-term exposure to penetration of humidity by applying 85°C ± 2°C with a relative humidity of 85 percent ± 5 percent for 1,000 hours.

The severity of this test is particularly challenging for the lamination process and the edge sealing from humidity. Delaminations and corrosion of cell parts can be observed as a result of humidity penetration. Even when no major defects detected after DH1000, the module has been stressed to the point that it becomes "fragile" for the subsequent mechanical load test.

Mechanical load test (subclause 10.16)

This loading test investigates the ability of the module to withstand wind, snow, static, or ice loads. Mechanical load testing comes after damp heat testing and is performed on a sample that has undergone severe environmental stress.

The most critical aspect of this test is related to the mounting of the module per the manufacturer's mounting instructions. If care is not taken regarding proper mounting, questions will remain as to whether any failure should be attributed to structural problems or to an inappropriate mounting technique.

2,400 Pa is applied (which equates to a wind pressure of 130 km/hour) for 1 hour on each face of the module. If the module must also withstand accumulations of snow and ice, the load applied to the front of the module during the last cycle of this test is increased from 2,400 Pa to 5,400 Pa. At the end of the test, there should be no major visual defects, and no intermittent open-circuits detected during the test.

Hail impact (subclause 10.17)

The hail impact test is a mechanical test that verifies that the module is capable of withstanding the impact of hailstones at a temperature of ~ –4°C. The test equipment consists of a unique launcher, capable of propelling various weights of ice balls at specified velocities, so as to hit the module at 11 specified impact locations (see figure5).

Figure 5: Ice-ball masses and test velocities

After the test, there should be no major defects caused by the hailstones.

Light-soaking (subclause 10.19, IEC 61646 only)

The light-soaking test is an irradiance test that provides a final pass/fail verdict for thin-film modules. The purpose of the test is to stabilize the electrical characteristics of thin film modules by means of prolonged exposure to irradiance after all the tests have been completed, but before checking Pmax against the manufacturer's minimum value. Although a final light soaking test is technically required for all test samples, it is often ignored in practice.

When to stop the testing process

Certain failures can be an indication of serious design problems requiring a comprehensive failure analysis and a complete design review. In such cases, the testing laboratory should stop the test sequence and invite the manufacturer to identify the root cause of the problem, and put into place the necessary corrective actions before submitting the modified samples for retesting.

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About The Author

Regan Arndt’s picture

Regan Arndt

Regan Arndt is part of TÜV SÜDs Photovoltaic Team and graduated from Electronics Engineering at the Southern Alberta Institute of Technology (SAIT) in Calgary, Alberta, Canada. He has more than 15 years of experience in testing and certification in the areas of photovoltaics; information technology equipment; telecommunications; and electrical equipment for measurement, control, and laboratory use. He joined TÜV SÜD America in 2004 as the senior engineer for the Product Safety Services Division in San Diego and went on to become the account executive and business development manager for the Southwestern United States.