Characteristic for PV plants is that they will underperform as a result of Potential Induced Degradation or PID. This phenomenon occurs on all types of PV plants, often without the owner of the installation being aware of it.
PID implies that solar modules are subjected to accelerated efficiency losses and therefore don’t achieve the expected return during their lifetime.
The presence of PID can be detected by means of several tests, which are described below.
How to detect PID?
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. For large PV plants, the infrared camera is attached to a drone. The result of an infrared drone inspection of a large PV plant is shown in the picture below.
Fig. 2: infrared drone inspection of a PV plant
A PV module affected by PID has a higher temperature than surrounding modules that are unaffected by PID. This is the case for the modules in the picture below. The yellow color of some solar cells indicates a higher temperature, which is an important indication for PID.
Fig. 3: infrared drone inspection of a PV plant (zoom on PID affected PV modules)
The infrared test is easy to perform because the installation does not need to be turned off.
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 module can also be measured by means of a simple voltmeter. This parameter gives more information about a solar module than the open circuit voltage.
Solar modules affected by PID have a lower operating voltage than modules not affected by PID. This is the reason why solar modules with PID have higher leakage currents and consequently have to deliver the same power than the other solar modules in de string. As a consequence, these solar modules 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 modules 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 module 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 5 both the voltage and the current have dropped. There is clearly a lot of PID on the module.
6. Installing PIDbox and monitor impact
The tests described above are suitable for detecting the presence of Potential Induced Degradation in a PV plant.
As the degradation rate of a PV plant is case specific, waiting too long to diagnose the presence of PID increases the risk of mechanical defects caused by the heat dissipation.
This is why iLumen recommends to install one or more PIDbox solutions in the field and monitor the impact on a string level. Especially, as our customers consider this as the easiest and cheapest way to calculate the impact of the investment today.
Fig. 5: PIDbox of iLumen
The PIDbox is a patented device that is installed between the solar modules and the inverter to regenerate the affected solar cells and to extend their lifetime.
iLumen’s PIDbox is unique in the sense that it is the only PID solution in the market that works independently of the inverter. As a result, inverter warranty is never void. iLumen acquired several patents for this unique series technology.