PV Module Testing Laboratory

The PV Module Testing (PVMT) Laboratory at SERIS is a national laboratory accredited under ISO/IEC 17025:2005 for the provision of high-precision PV module testing, including “Golden Module” measurements. A core activity in the PVMT Laboratory is secondary certification testing in accordance to the international standards for module qualification (IEC 61215) and for module safety (IEC 61730 and UL 1703). In addition, the laboratory conducts various industry relevant or customer specified research activities to further improve PV module reliability with additional, pre-normative testing. In 2017, a world-class measurement uncertainty of 1.6% for the maximum power measurement of crystalline silicon PV modules has been achieved in the laboratory. Consolidation of measurement procedures and uncertainty calculations was carried out in 2017, through acquisition of new know-how as well as equipment and system refinements and optimisation.

  • Current-voltage (I-V) characterization provides the maximum power and thus the energy conversion efficiency of a PV module. Reaching a low measurement uncertainty is an important goal in the PVMT Laboratory. The laboratory has the rare advantage of hosting two Class* A+A+A+ large-area pulsed solar simulators: (i) Pasan SunSim 3B (short pulse) and (ii) h.a.l.m. cetisPV-Moduletest3 (long pulse). The major technical specifications of the two simulators are:

    Pasan Sun Sim 3B h.a.l.m. cetisPV
    Light source 4 (linear) Xe lamps 1 (circular) Xe lamp
    Pulse duration Up to 10 ms Up to 10 ms
    Spatial non-uniformity < 1% < 1%
    Temporal stability Better than 1% Better than 1%
    Spectral mismatch < 12.5% < 12.5%
    Light intensity range 200-1200 W/m2 (standard 1000 W/m2) 200-1100 W/m2 (standard 1000 W/m2)
    Dark I-V curve No Yes

    Both simulators are equipped with a thermal chamber located in a dark room, allowing measurements at several temperatures (typical range 20-75°C). The power output of crystalline silicon modules can be measured on the Pasan SunSim 3B with our best expanded (k=2) uncertainty of ±1.6% at Standard Test Conditions (STC). The mounting flexibility of the systems allows the testing of modules with sizes of up to 2.2 m × 1 m. An important application are calibration measurements of “Golden Modules” for industry partners. The h.a.l.m. solar simulator also allows I-V measurements in the dark, a useful tool to evaluate module parameters such as the series resistance. The equipment set is presently being upgraded to enable the measurement of bifacial PV modules.

    * Although Class A+A+A+ is not defined in the current version of the IEC 60904-9 standard for solar simulator classification, it is already widely used in the PV community when referring to simulators with spatial uniformity, temporal stability and spectral match, fulfilling more stringent requirements than Class AAA solar simulators, thus allowing more accurate measurements. This new classification is expected to be introduced in the next edition of the standard to be published in 2018.

    Electrical characterisation on Pasan SunSim 3B. The testing PV module is located at the centre of the target plane where spatial non-uniformity is better than 1% (Class A+). A calibrated reference cell placed below the testing module controls the desired value of total irradiance from the Xenon large area pulsed solar simulator.

  • Module degradation is often caused by defects at the cell level (cracks, broken interconnections, shunted cells, etc). Optical characterization is an important and straightforward method to investigate such defects and their influence on power generation. In the PVMT Laboratory, electroluminescence (EL) images can be captured for PV modules with a variety of commercial sizes. Two EL systems are available:

    • In-house EL system, integrated in the h.a.l.m. tester (full flexibility regarding device size, power and camera control)
    • Pi4 EL inspection system (fast and automatic feature detection and performance evaluation for standard c-Si modules)

    Before analysis, EL images are corrected for camera based and perspective distortions. EL analysis can be applied to single EL images as well as a series of EL images. It includes image quality metrics (signal-to-noise-ratio, sharpness, saturation) in compliance with the current IEC TS 60904-13 draft standard for EL imaging. Furthermore, average cell intensities and cell features (such as crack length and distribution, disconnected fingers, PID and high series resistance areas) can be identified. A series of EL images furthermore allows the estimation of performance degradation over time.

    Optical characterisation: EL measurement of a commercial PV module on the Pi4 EL inspection system

  • STC measurements are performed under the AM1.5g spectrum, using 1000 W/m2 total irradiance and 25°C cell temperature. Even the class A+A+A+ solar simulators described above may give rise to spectral mismatches of more than 1% for modern c-Si modules (with enhanced response in the UV and in the IR) and even 2-3% for thin-film technologies and multi-junction modules, overall decreasing the measurement accuracy. Spectral responsivity measurements allow spectral mismatch corrections according to the international standard IEC 60904-8. Additionally, the equipment can be used to assess the optical properties of the encapsulant (absorbance and transmittance), the spectral performance of bifacial modules, the spectral responsivity of defective cells or the extended spectral responsivity in the UV or in the NIR. The spectral responsivity can be measured in the PVMT Laboratory with the following testing tools:

    • Filtered Pasan SunSim 3B
    • Enlitech MSR-2011-P

    Spectral characterisation: inside Enlich MSR-2011-P spectral responsivity measurement equipment. The bias light system on the top of the picture (a set of QTH lamps) provides uniform illumination on the testing module, while the 10 × 4 mm2 chopped monochromatic beam detects the spectral responsivity of a specific cell in the module via lock-in technique.

    The following table illustrates the main technical specifications of the tools. The opportunity of having two systems available with different specifications allows a great range of products to be tested and technical issues to be investigated.

    Filtered Pasan Sun Sim Enlitech MSR-2011-P
    Monochromatic source Xe lamp + optical filters (50 nm FWHM) Xe lamp + monochromator (17 nm FWHM)
    Illumination area 2.4 m × 1.8 m 10 mm × 4 mm
    Monochromatic beam target Full module Cell in module
    Pulse duration 10 ms/filter < 5 min (full scan)
    Wavelength range 400-1100 nm, in 50-nm steps 300-1800 nm**, in 10-nm steps
    Light bias No Yes (QTH)
    Voltage bias No Yes
    Detector Si (300-1200 nm) Si (300-1000 nm) + Ge (900-1800 nm)
    ** Typically up to 1300 nm. Although conventional c-Si has a bandgap-related limit of about 1200 nm, some other PV technologies may require extended spectral characterization in the IR.

  • While characterization at STC is important for comparing different PV modules, only the outdoor exposure in real operating conditions can take into account important effects of the environment (wind, elevation, tilt, etc) and allows to investigate a variety of degradation effects. The Outdoor test facility of the PVMT Laboratory, which is located on the roof of the Cleantech One building, is used for the following tests of the module reliability standard IEC 61215:

    • Module stabilisation
    • Light soaking
    • Measurement of the nominal module operating temperature (NMOT)

    Outdoor degradation studies, for example light-induced degradation (LID) and potential induced degradation (PID), are performed as well. The outdoor facility is also used for monitoring the environmental conditions for energy rating purposes. Weather variability in the tropics is particularly challenging for many of these tests, highlighting the need for a dedicated PV module testing centre in South East Asia.

    Outdoor characterisation: the outdoor test field on the roof of PV Module Testing Laboratory at CleantechOne. A set of PV modules of various technologies are subject to light soaking for stabilisation against light-induced degradation (LID).

  • Module qualification:

    The commercial success of any PV technology is based on the reliability of its modules. Almost all commercial PV modules now have a 25-year warranty. It is therefore of great importance for PV module manufacturers to appropriately assess the reliability of their products (module qualification and type approval).

    Beyond the tests described above, the PVMT Laboratory is further equipped according to the PV module qualification standard IEC 61215. This includes the following tests, some of which are described below in more detail:

    • Visual inspection
    • Insulation test and wet leakage test
    • Hot-spot endurance and UV preconditioning
    • Thermal cycling, humidity freeze and damp heat tests
    • Bypass diode tests
    • Mechanical stress tests: robustness of termination, mechanical load and hail test
    In 2016 the IEC 61215 standard was revised, with some minor changes and several major ones. The PVMT Laboratory is working on the required modifications of the equipment and the measurement procedures.

  • These tests are performed to assess the electrical insulation resistance of a module. The insulation test is a “dry test”, aiming to assess module electrical insulation between active parts and accessible parts. A voltage of either 500 V or the maximum system voltage (whichever is greater) is applied to the module for 2 minutes and the insulation resistance is measured. The wet leakage test is designed to assess the module’s electrical insulation in a “wet” environment, to verify that moisture from rain, fog, dew or snow does not enter the active parts of the module, and is thus particularly relevant for the tropics. The test is carried out essentially as for the insulation test, except that the module is tested while immersed in a water solution of given resistivity. Both tests act also as “control tests”, as they are often performed after many of the other stress tests listed below.

  • The purpose of the hotspot endurance test is to determine the module’s ability to withstand hot-spot heating. Defective cells are first detected by electrical characterization, then the worst ones are shaded for 1-5 hours to determine if there is an excessive cell temperature increase. The UV preconditioning test is an ageing test aiming to identify any susceptibility to UV degradation. The module under test is irradiated with at least 15 kWh/m2 of total UV irradiation in the 280-350 nm wavelength range. In both cases the tested module is then checked against damages (visual inspection, insulation, wet leakage, electrical characterisation if required).

  • Climate chamber tests are commonly used to study the effect on the module reliability of environmental factors such as temperature and relative humidity variations. The PVMT Laboratory is equipped with five thermal chambers to conduct the following tests: Thermal cycling test, humidity-freeze test, damp heat test. The thermal cycling test involves a cyclic repeat of temperatures of 85°C and -40°C. The same temperature range is used in the humidity-freeze test, but the relative humidity is kept at 85% while at 85°C, to assess the module’s ability to withstand the effect of freezing at sub-zero temperatures. The same humidity level occurs in the damp heat test (typically 85°C and 85% relative humidity for 1000 hours), to assess effects of long-term penetration of humidity. Control tests (visual inspection and wet leakage) are performed to detect failures at the end of each test. Besides standard reliability tests, the climate chambers are also used for potential-induced degradation (PID) tests (see below).

  • The standard static mechanical load test is performed to determine the ability of PV modules to withstand mechanical loads, such as that of snow load, which is relevant at higher latitudes and thus part of the international standard. The system applies a pressure of +2400 Pa and -2400 Pa alternately for one hour each, and then repeats this cycle two more times. If desired, the load is increased to +5400 Pa during the last cycle. The equipment can also operate in the “dynamic mode”, where the module is subjected to 1000 cycles of ±1000 Pa.

  • Module Safety:

    IEC 61215 does not cover most aspects of electrical safety. These are covered instead by IEC 61730-2 “PV module safety qualification – requirements for testing”.

  • Safety qualification tests range from evaluating basic components - such as the adhesion of the junction box - to analysing the safety level of the composite module. IEC 61730-2 comprises basically all tests of IEC 61215, plus several additional tests that can be performed at the PVMT Laboratory, as follows:

    • Continuity test of equipotential bonding (was termed "Ground continuity test" in the previous version of the standard)
    • Dielectric withstand test
    • Cut susceptibility test
    • Impulse voltage
    • Module breakage

    In 2016, the IEC 61730-2 standard was strongly revised. The PVMT laboratory is updating the procedures and (where needed) the equipment to fulfil the new requirements. As an example, the photo shows the carriage used in the cut susceptibility test, to assess whether the PV module still meets the insulation requirements after defined cutting of the backsheet.

  • Tightened PV module testing for the tropics:

    Due to both historical and practical reasons, many international standards were originally developed to fulfil the environmental requirements of mainly Central Europe and North America. In recent years a big effort has been made by IEC TC82 to continuously update the old versions, while taking into account the requirements of a wider market, including the tropical sunbelt. This is needed by PV module manufacturers aiming at deployment of their products for the tropics.

    SERIS is actively investigating the impacts of high humidity, salt mist, high irradiance and high weather variability on a variety of PV installations in the tropics, including building-integrated PV, open rack installations and floating PV systems. Two examples of the tightened PV module testing available at the PVMT Laboratory are:

    • Potential induced degradation (PID) testing
    • Salt mist testing

  • PID is a degradation effect that has received strong attention in recent years. It is due to the large electrical potential difference (up to 1500 Volt) that arises between the solar cells in a PV module in a string and the module’s metallic frame. As a consequence, certain ionic species (for example Na+) can migrate through the module’s front glass sheet to the cells and cause cell degradation, which in turn reduces the performance of the PV module. PID is more severe in humid and hot environments, and is therefore particularly relevant for PV modules deployed in the tropics.

    Standard PID test
    The currently available standard (IEC 62804-1) describes how to detect PID in c-Si modules (a dedicated standard for thin-film PV modules is still being developed). The PID test for c-Si modules requires

    • 1000 V potential difference between the cells and the module frame
    • 60°C ambient temperature
    • 85% relative humidity
    • 96 hours exposition
    This test is carried out in the PVMT Laboratory in a dedicated thermal chamber. PID tests can also be performed outdoors at the PVMT Laboratory.

  • This test, according to the IEC 61701 standard (“Salt mist corrosion of PV modules”), determines the resistance of PV modules to corrosion from salt mist containing Cl– ions (NaCl, MgCl2, etc). The PV modules are heated to up to 60°C and exposed to salt water mists in various repeat cycles and holding intervals. Such tests are relevant for PV modules to be installed in coastal locations.

    Extended PID test in salt mist:
    Tropical environmental conditions may be more aggressive. Thus, a new protocol for an extended PID test in a salt mist environment was developed, as follows:

    • 1000 V potential difference
    • 50°C ambient temperature
    • > 95% relative humidity
    • Salt mist chamber for 112 hours

For further information, please contact:

Kenneth GOH
kenneth.goh@nus.edu.sg