PHOTON rapid test (bronze standard)
This test allows a quick, but at the same time informative assessment of the module quality. It reveals product defects and provides an outlook of the expected decline in performance due to degradation effects. At around €6,000, it is comparatively inexpensive and only takes four to six weeks (depending on the module type).
Four samples of the module type to be tested are required, one of which is used as a reference model. For modules with PERC cells, a further specimen is needed to test for PERC degradation specific to this module type.
At the beginning of the test cycle, all modules are examined. A current-voltage characteristic curve is recorded under standard test conditions (STC) and an electroluminescence image is created. The power is measured in a class AAA sun simulator in accordance with IEC 60904-1, the accuracy of the measurement is plus/minus 2.5 percent.
Procedure of the PHOTON rapid test
After this characterization, one module is set aside as a reference, and the remaining three (PERC modules four) run through different test sequences (see diagram).
Test Sequence Module 1
Module 1, after the electrical properties are determined, undergoes the following tests:
Test for light-induced degradation (LID)
Degradation refers a decrease in performance and therefore lower yield. In the case of light-induced degradation (LID), modules can lose several percentage points of their original power output right at the beginning of their operating life. In order to measure the power loss occurring up until performance stabilizes, the modules to be tested must therefore not have prematurely aged.
The stabilization is carried out according to the industry standards IEC 61215-1-1-1:2016 and 2:2016. The module is connected to a resistor that holds it close to the MPP (Maximum Power Point). Two rounds of exposure with five kilowatt hours per square meter are then performed. To carry out this test, a Class C solar simulator with HQI lamps is used, the module temperature is maintained at 50 plus/minus 10 degrees Celsius and the irradiance is 800 to 1,000 watts per square meter.
The mechanism causing light-induced degradation has been well researched. Today's module manufacturers can almost completely avoid it by choosing the appropriate silicon quality and by implementing process controls.
Measurement of weak light response
Once the module output is stabilized, the weak-light behavior is determined. We determine the complete matrix of module performance according to temperature and light intensity as specified in IEC 61853. This enables us to estimate whether the module will deliver a good yield even in the event of poor irradiation conditions.
Test for potential-induced degradation (PID)
A few years ago, it was discovered that the voltage applied to a module can also cause a power drop. The higher the voltage, the stronger the effect. This potential-induced degradation (PID) has also been understood to the extent that manufacturers can practically avoid it altogether by choosing the most appropriate materials and clean process control.
To find out how susceptible a module is to PID, the maximum system voltage specified by the manufacturer is applied over 48 hours, both as positive and negative voltage. The module is exposed to a temperature of 85 degrees Celsius and a relative humidity of 85 percent. Afterwards, the current-voltage characteristic curve is recorded again within four hours and an electroluminescence image is created.
Test Sequence Module 2
Module 2 is subjected to various stress factors that are intended to reveal production defects and the use of inferior materials:
Electrical safety test (wet-leakage test)
The wet-leakage test is carried out in accordance with IEC 61215. For this purpose, the two poles of the module are connected and an electrical voltage totaling twice the maximum system voltage plus 1,000 volts is applied on the frame. A leakage current test is then carried out in the water bath to determine whether there is a leakage current.
Dynamic load test (shaker test)
During transport, solar modules are often subjected to considerable vibrations. Modules with cells that exhibit microcracking or for which the cells are not properly encapsulated can then sustain damage that leads to a significant loss of performance, particularly in connection with thermal stress. The dynamic load test is carried out according to an international standard used in the logistics sector. The modules are mounted horizontally on a pallet and exposed to vibrations at different frequencies for one hour. This test is one of the most demanding tests, and is therefore one of the most important ones in the entire rapid test cycle, as it provides an indication possible long-term module failures within a short period of time. With a high-quality solar module, it must not lead to a significant deterioration of the STC performance, nor should any damage be visible in the electroluminescence image.
Thermal cycling test
During the temperature cycle test in accordance with IEC 61215, the module is alternately heated to 85 degrees Celsius and cooled to minus 40 degrees Celsius. In contrast to the IEC test, the PHOTON rapid test only runs 50 cycles instead of 200 cycles. However, the test report records whether and, if so, by how much the STC performance has decreased during the test. The IEC certificate does not cover this type of gradation, only specifying the result as “passed” or “failed”. Every module with a performance that has not decreased by more than five percent has passed the test – the bar for the IEC certificate has therefore been set extremely low, and the purchaser of a module tested in this way does not know whether the performance loss was 0.5 or 4.9 percent.
However, a massive loss in performance of up to five percent will also be revealed over 50 cycles. Although much shorter than the IEC test, the PHOTON rapid test is therefore much more informative due to the publication of the test results, even when power losses are lower.
However, the question of interest that remains is what can be inferred from the power losses established during the first 50 cycles of the module's operation. This is the question that the “PHOTON silver standard” durability test examines.
Humidity freeze test
The humidity-freeze test is also carried out in accordance with IEC 61215 specifications. In contrast to the thermal cycling test, the number of ten cycles has been adopted without change. A temperature cycle consists of half an hour at minus 40 degrees Celsius and 20 hours at 85 degrees Celsius with a humidity of 85 percent. This enables the resistance of a module to night frost, followed by hot days with high humidity, to be determined.
Testing the junction box
The test of the junction box is carried out in accordance with the US standard UL 1703. A steel ball is dropped from a height of 1.3 meters onto the junction box. If the damage this causes is severe enough to expose the contacts and there is no longer any contact protection, the module has failed the test. If not, it spends three hours in the climatic chamber at a temperature of minus 37 degrees Celsius. The test is repeated and the junction box is checked again for damage. The module has passed if the contact protection is still intact.
Test Sequence Module 3
Module 3 examines the individual materials. These tests are primarily destructive tests:
Electroluminescence under mechanical load
This test has the same setup as the electroluminescence image does in terms of the currents applied and the camera used. As a special feature, however, the solar module is simultaneously exposed to mechanical stress – the test conditions are therefore largely equivalent to the conditions that prevail during system operation.
For this test, a lifting cushion between the rear of the module and a rigid plate is filled with compressed air, placed at a distance of about 20 centimeters. The critical factor in this case is the pressure in the cushion, as it determines the deflection of the module. However, since it is not possible to define a pressure value in bar due to different geometries and strengths of solar modules, which always results in a uniform deflection, this value must be determined directly. To do so, a digital depth gauge is placed in the center of the front of the module by using a strut, and is zeroed when the module is unloaded. The pressure in the cushion is then increased until the depth gauge indicates a deflection of ten millimeters. The depth gauge and strut are then removed and an electroluminescence image is produced. Its evaluation is based on the same procedure as for a mechanically unstressed test.
Test of the backsheet
Another in-house development is the test used to examine the bonding between the backsheet and the rest of the module. This determines the force required to separate the film from the front glass and cells. This allows the strength of the interconnection to be assessed in relation to other modules. The more solidly the individual components are connected to each other, the lower the risk that they will separate from each other over the course of a hopefully long module service life.
Test of the degree of EVA cross-linking
The testing of the degree of crosslinking of the encapsulation material is also not a standard test, but rather an in-house development. The film made of ethylene vinyl acetate (EVA) which is normally used is available on the market in varying qualities and is sometime also poorly processed. Exceeding the storage period only slightly is sufficient to cause insufficient cross-linking of the material, making the entire module susceptible to delamination. The test aims to uncover exactly these problems.
Test of PERC degradation (Module 4)
For modules with PERC cells, a fourth specimen is tested for PERC degradation. When PERC cells are used, not only is the front of the cells passivated, but also the back of the cells, which results in higher efficiencies. At the same time, however, these cells are also the most capricious of all the cell types, and tend to sustain performance losses that have not been fully understood so far. Current warnings are primarily based on information released by PERC cell manufacturers, who refer to their own measurements showing the corresponding degradation effects on competing products. At the same time, these manufacturers claim that they also have problem under control. The PERC test is designed to reveal how severe the problem actually is.
To run this test, the module is energized with 0.5 amps in the dark at a temperature of 85 degrees Celsius and a relative humidity of 85 percent for 1,000 hours – that is, for a little more than five weeks. The current-voltage characteristic curve is recorded every week and an electroluminescence image is produced.
We will be happy to answer any questions you may have regarding PHOTON module tests.
Oliver B. Ashrafi