Testing of Modules for PID Susceptibility
In the earlier editions of this newsletter, we discussed the mechanism of potential induced degradation in poly-crystalline and thin-film modules and how it is impacting the performance of the existing solar power plants. In the current article, we will talk about the different component in cells and modules that could impact the PID susceptibility of solar modules in the plant. We will also discuss how we can test for PID susceptibility and what does “PID free” tag actually mean
Certain types of cells or modules may be more susceptible to PID than others. The structure of the cell itself, like the density of the charge carrier of the silicon used and the chemical composition of the ARC (anti-reflective coating) layer on the cell, has an influence on the PID process. The chemical composition of the used glass and the EVA in which PV cells are encapsulated also affects the possibility and rate of PID.
How to check the PID susceptibility of solar cells?
When solar cells are procured, it is imperative to check the PID susceptibility of the cells besides checking the electrical parameters including current, voltage and efficiency. There is no IEC recognized standard test set-up to check for PID susceptibility at cell level. Individual cell manufacturers may have their own set-up to check PID susceptibility. There are two methods to check PID susceptibility in cells.
Corona discharge technique
In this technique, positive ions are generated by the tip of a thin wire due to the high potential applied and are deposited on the surface of the cell. An electrical field is then induced by these positive ions which creates ‘PID like conditions’. It is important to note here that these positive ions are not Na+ ions which are mainly responsible for creating PID effect in cells and modules. Further, this method can also damage the underlying SiN, which is another major drawback of using this method.
In this method, a solar cell is placed on a temperature-controlled chunk of Aluminium to get uniform temperature distribution. On the front side of the cell, encapsulant foil and glass sheet is kept. High voltage is applied between this stack and Al-chunk. This way, the PID effect can be simulated and shunt resistance can be monitored. This method is closer to the real situation and hence has better chances to give an estimate of PID susceptibility. Set-up for the above tests can also be created by the module manufacturer to monitor the quality of incoming cells. There are off-the-shelf cell testers available to test PID susceptibility of solar cells.
A point to be noted here is that the AR coating, which is the coating of SiN, has a significant impact on the PID susceptibility at the cell level. Multiple research papers discuss that PID is greatly affected by the refractive index of SiN film. Higher is the refractive index, higher is the solar cell’s resistance to PID. The higher refractive index is achieved by increasing the Si/N ratio during the deposition process. So the PID susceptibility can be designed to the desired level by controlling the Si/N ratio during the deposition of SiN.
Which other factors impact the PID susceptibility of solar modules?
Apart from solar cells, other components in the modules which can influence the PID susceptibility include glass, encapsulant, and laminate, of which the former two are more important.
Most modules used Soda-lime silicate glass due to its relatively lower cost and more suitable physical properties. However, the Na+ ions in this glass play a major role in increasing PID susceptibility. Some of the alternatives to Soda-lime glass are
- Aluminum Silicate glass, which has less 3% (w/w) of Na20 and bulk resistivity 2-3 times higher than the soda-lime silicate glass
- Boro-silicate glass or quartz glass is also superior to Soda-lime glass in terms of containing the PID effect
However, the cost of these is significantly higher than that of the soda-lime glass which will increase the overall module cost. Another, relatively low-cost alternative, can be to coat the soda-lime glass with a layer of SiO2 or TiO2 to prevent the migration of ions and reduce leakage current. However, these coatings will reduce the optical performance of the modules by absorbing, scattering and reflecting a part of the incident light.
As strings voltages are moving to 1500V, glass manufacturers are developing glasses that contain lower sodium percentages and these kinds of glasses are now being regularly used by the module manufacturers.
The most commonly used encapsulant is EVA ( Ethyl Vinyl Acetate ) and its bulk resistivity plays an important role in determining the extent of PID susceptibility. Higher bulk resistivity variant of EVA (in the order of 10e15 ohm-cm) shows improved PID resistance. The operating condition of solar modules in the plant can sometimes exceed even 60oC in hot climates, which can lead to a reduced bulk resistivity of EVA, dropping to a range of 10e11-10e13 Ohm-cm. This condition can accelerate the rate of PID, and hence, must be checked for.
Besides bulk resistivity of the encapsulant, another factor that plays an important role is Moisture Vapor Transmission Rate (MVTR). It is understood that moisture ingression will promote the progress of PID in PV modules. Encapsulation materials with lower MVTR offer better protection against moisture penetration and are, therefore, more favorable for reducing PID. This is particularly important when the modules are being placed in a humid environment.
How to check the PID susceptibility of solar modules?
Chamber PID test (IEC 62804-1)
The most common method for PID testing is to bias PV modules with high voltage in high humidity and temperature environment. This can be achieved by placing modules in a climate chamber with controlled humidity and temperature.
As shown in the above figure, the two leads of the module are shortened and connected to the negative terminal of the high voltage power source. The module frame is grounded and connected to the positive terminal of the high voltage power source. The leakage current can be monitored using an additional apparatus. Module performance can be monitored in-situ at the stress temperature.
The test procedure as described in IEC 62804-1 is based on stressing sample modules in a climate chamber for 96 hours at a minimum temperature of 600C, relative humidity of 85% and with an applied voltage equal to module’s maximum rated system voltage. Another combination of temperature, humidity, and voltage - 85oC, 85%RH and (-ve)1000V is also described in IEC 62804-1.
Placing a conductive layer on the front of the glass
This method is included in IEC 62804-1 as an alternative to chamber PID test. Here, a conductive layer made of Al or Cu foil can be placed on the top of a sample module. The Al foil provides a conductive path on the glass surface which is equivalent to the high humidity conditions. To ensure that Al foil stresses uniformly, a rubber mat might be required to be used over top of the Al foil. As we are using Al foil for the test, the controls are required only for temperature, which provides a significant advantage over chamber testing.
What does a “PID free” tag for modules actually mean?
Certifications are available for PID resistance of modules based on IEC 62804-1 which can be obtained by the module manufacturer from accredited laboratories such as UL and TUV. The pass criteria of modules based on IEC 62804-1 is given below:
- Power degradation <5%
- Dry Insulation > 40Mohm-m2
- Wet Insulation > 40Mohm-m2
Solar panels are considered to be ‘PID free’ when they pass the IEC 62804 standard test. However, the passing of this test does not indicate that modules will be completely PID resistant because of the following reason:
- This standard, tests the accelerated PID process over only 96 hours, while the actual lifetime of the panel is 25 years (~91000 hours).
- Solar panels pass the test when the power losses are less than 5%. When the efficiency losses are limited to 3-4%, the panels are still considered to be “PID free”, even when these losses have to potential to magnify over the lifetime of a module
Therefore, ‘highly PID resistant’ would be a better description for these solar panels than ‘PID free’, which implies that developers must pay attention to maintaining system voltages at optimal levels and ensuring proper negative grounding in order to completely mitigate power loss due to PID.
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2: “ Potential-induced degradation in photovoltaic modules: A critical review”, Energy Environ. Sci, 2017, 10, 43-68, Wei Luo, Yong Sheng Khoo et al
3: “ Testing and analysis for multiple PID mechanisms and stresses” Report by Peter Hacke, NREL
4: “ Potential Induced degradation of Solar cells and Panels” S. Pingel et al, 35th IEEE PVSC, 2010
5: “ Potential Induced degradation of Photovoltaic Modules: Influence of Temperature and surface conductivity “ Ebniali, Faraz et al, Arizona State University, April 2012
6: “ Understanding Potential Induced degradation” Application note by Advanced Engineering.