What is PID?
PID stands for potential induced degradation. It is an important mode of performance degradation in crystalline silicon solar panels. The degradation could be high as 30% or even up to 70% in some cases. The degradation can be reversible or irreversible.
It occurs because of minor, unwanted currents between the semiconductor on the one side, and the glass, anti-reflective coating (ARC), the frame, and the mounting on the other side. The degradation in performance is associated with migration of sodium ions from the, glass plate through the encapsulation (EVA) and the Anti-Reflective Coating (ARC), to the cell. This would be caused when due to a particular manner of string earthing, the semiconductor in a cell acquires a negative potential with respect to the encapsulation and the support structure. The presence of these ions causes an effective shunt path across the cell and reduces the output. The effect is somewhat cumulative with time and has a greater extent when the cell is operated at a higher negative potential with respect these parts.
Promoting Factors in the description above
Temperature and humidity are both known to promote it. However, there is not much one can do about these factors once a system is installed in a given location.
Dependence on Location in String
Recall the words “higher negative potential” in the explanation above. If the positive terminal of the string is taken as system ground and the mounting structure connected to the earth potential, the cell closest to the positive terminal has the least negative potential with respect to earth, and hence the least PID effect. The cell closest to the negative terminal will experience a high negative potential relative to the grounded structure and will undergo maximum PID. Thus cells, modules and panels will experience it according to their position in the string.
Avoiding / Mitigating PID
There following considerations are applicable.
• Location- for a new plant, within other limitations, a site with lower temperature and humidity should be selected. Note that a windy site will also keep the system cooler.
• Use PID Resistant Hardware– For a new set up, panels are available which are resistant to PID. However, the cost will be higher due to use3 of more expensive encapsulating materials, anti-reflective coatings, and other materials. A compromise may have to be made for overall profitability by using panels subject to PID and adopting other mitigating techniques.
• Earthing– Use panels where there is no restriction imposed by the manufacturer on connecting the negative end of the string to system ground.
• Charge Equalizers– are built into inverters. When the inverter is inactive at night, they apply an opposite bias to the panel which cancels out the reversible type of PID effect overnight. Reversible type is also called polarization.
Detection of PID
PID presence can be detected by monitoring Vmpp (maximum power point voltage). Difference in VMmpp among panels increasing along the string is an indication of PID being present. Detection is more likely on a day of good irradiation.