PID stands for potential induced degradation. It is an important issue 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 occurs in solar energy systems and can be reversible or irreversible.
Potential-Induced Degradation (PID) is a common phenomenon causing PV panels to lose power generation by up to 80%. Power reduction may occur over time or can happen within days or weeks after installation.
The PID process in the PV module may grow very rapidly and in the shortest period will affect the performance of an entire PV system. Consequently, this results in damaging effects on PV system project financing, operations and economics at all installation levels: residential, commercial and utility-base. It is essential to understand and address the PID problem in its early stages, to ensure PV module performance over the entire system life – PID can be prevented and recovered on system level altogether.
Causes of Potential Induced Degradation (PID)
PID 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 (commonly: 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.
PID promote factors
Temperature and humidity are both known to promote PID. 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 PID according to their position in the string.
Avoiding / Mitigating PID
The 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, there are modules available which are resistant to PID. However, the cost will be higher due to use 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 modules 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 PID is also called polarization.
Any other way to reduce PID Impact :-
Sungrow and TÜV Rheinland have jointly issued PID Zero – an anti-PID solution whitepaper for residential PV systems. Equipped with patented mirror boost topology and an intelligent control algorithm, the innovative PID Zero solution provides 24-hour anti-PID protection, enabling more effective PID suppression during the day and PID recovery at night, significantly reducing power generation losses due to module performance degradation.