An unexplained decrease in power output
Many owners and operators of PV systems are faced with unexpected power loss from PV modules. This seemingly inexplicable drop in power output is usually related to the spread of PID. However, it is often attributed to other factors.
Most of them are irreversible, unlike PID, whose effects can be reversed provided that PID is detected and solved in time. Such regeneration is swift and permanent. It spans the entire remaining life of the PV system.
Data provided by the German Fraunhofer Institute shows that 46% of the 95 PV modules tested in 2012 did not pass a quick check for the occurrence of PID.
Test results here
PID (Potential Induced Degradation) is a degradation of the power output of photovoltaic cells caused by the difference in voltage potential with respect to ground.
PID causes an inappropriate polarization of PV cells resulting in the module's inability to produce electric current. This phenomenon occurs mainly in PV modules closest to the negative pole of the string. Here the potential (voltage to ground) of PV cells typically ranges between ?200 V to ?450 V based on the string length and inverter type. The frame of a PV module, on the other hand, has a potential of 0 V, as it has to be grounded for safety reasons. It is due to this electric voltage between PV cells and their frames that electrons separate from the materials the PV module is made of and flow out via the grounded frame. The result is an increasing charge (polarization) of the PV cell, which adversely changes its characteristic curve.
Under normal conditions, PID may decrease the power output of individual modules by 70% and the output of the whole PV system by 15%.
An example of PID: power loss of 9%
This electroluminescence image shows an early stage of PID degradation of PV cells. In an electroluminescent image, PID is manifested by a darkening of PV cells, predominantly around the frames of PV modules. An early stage of PID causes a measured drop in the PV module's power output by approximately 9%.
An example of PID: power loss of 36%
This electroluminescence image shows an advanced stage of PID degradation of PV cells. PV cells are affected by PID nearly all across their surface. PID in some PV cells is so advanced that these no longer produce any power. These are the black cells. PID has a considerable impact on the drop in the PV module's power output by more than 36%.
An example of PID: power loss of 67%
This electroluminescence image shows a very advanced stage of PID degradation of PV cells. Nearly all PV cells are affected by PID. PID is so advanced that the power loss reaches almost 67%.
For more information about PID, study the following article: https://en.wikipedia.org/wiki/Potential_induced_degradation