Potential Induced Degradation (PID) is the main reason for efficiency losses of solar panels. Therefore it is important to timely detect this phenomenon.
The presence of PID can be detected by means of several tests.
1. Electroluminescence test
An electroluminescence test is done at night with a charge-coupled device camera, while the module is energized by a power source and without sunlight. This test can only take place at night.
A module without PID has a picture of which the cells all have the same brightness. On figure 1 a module that has been affected by PID is shown. The cells that are no longer or not completely illuminated are affected by PID.
Fig. 1: Electroluminescence test
The electroluminescence test can also be performed at a system level, as is shown in this video.
2. Infrared image-forming
This test is done on a sunny day with an infrared camera. A module affected by PID has a higher temperature than surrounding modules that are unaffected by PID. The cells that are hotter (red) than the others (yellow) are potentially infected by PID.
The infrared test is easy to perform because the installation does not need to be turned off.
Fig. 2: infrared test of solar panel
3. Open circuit voltage
A first indication to detect PID is by measuring the open circuit voltage with a voltmeter. If the modules are affected by PID, the open circuit voltage is lower than the reference.
4. Operating voltage
The working voltage of the panel can also be measured by means of a simple voltmeter. This parameter gives more information about a solar panel than the open circuit voltage.
Solar panels affected by PID have a lower operating voltage than panels not affected by PID. This is the reason why solar panels with PID have higher leakage currents and consequently have to deliver the same power than the other solar panels in de string. As a consequence, these solar panels work at a lower operating voltage.
This can easily defined by next formula
P = U * I
If the power (P) decreases due to PID, and the current (I) remains the same by the other panels in series, the voltage (U) is to fall. Moreover the fall of the operating voltage is proportional to the PID-losses.
To check whether a solar panel is PID free, take an IV-characteristic that can be measured by means of an electric tracer. This makes it possible to observe deviations from the IV curve. The measurements should be made on a sunny day in order to obtain sufficient accuracy.
In the example of figure 3 both the voltage and the current have dropped. There is clearly a lot of PID on the panel.