Potential Induced Degradation (PID) is the degradation of module power output caused by high potential relative to the ground & other environmental factors.
PID would decrease the Module Power Output by 20% if precautionary measures were not taken then the system yields were at Risk.
|Reasons for PID||Controllable/ Uncontrollable|
| 1. PV System Level
i. High Potential Relative to Ground
| 2. Environmental Factors
Solar enthusiasts would clearly understand from the above Table what were the reasons, which can be controllable & uncontrollable.
- PV System Level
(i) High Potential Relative to Ground
Depending on the Solar Cell Design whether it is Positive Bias or Negative Bias, PID affects the module performance. PID influence on P Type was explained as follows:
Transformer Less Inverters would make the Negative Side Modules of Strings under the influence of Negative Bias & Positive Side Modules of Strings under the influence of Positive Bias.
This new electric field results in degradation of the front passivation and of the power output caused by accelerated surface recombination.
As the charge accumulates, positive ions penetrate through the SiNx (ARC) layer and gradually increase the power degradation caused by recombination and shunting.
So better understanding the basics of PID in order to prevent & reduce the influence on modules would benefit the solar enthusiasts.
System-Level Improvements to Prevent PID:
a. Reducing System Voltages:
By reducing the maximum voltage, and thus the maximum negative potential, system designers create an environment less likely to foster potential induced degradation.
b. Using Inverters with Transformers and Grounding the Negative Pole Grounding:
The negative pole of the array ensures that the solar cells do not have a negative voltage relative to their surroundings. This is only possible if an inverter with transformer is used.
c. Improved O&M Practices:
Module-level monitoring enables plant owners to promptly detect modules affected by various forms of degradation earlier and submit warranty claims or perform maintenance accordingly. Because this phenomenon tends to develop over time, the incremental losses could accumulate into large yield losses and a reduction on the return of investment for system owners.
d. Conducting Accelerated Recovery:
PID can be mostly reversed in moderately affected models through storing modules in high temperatures in the laboratory.
2. Environmental Factors
High temperature and high humidity will make the Positive Ions Na+ move faster.
As Environmental Factors were uncontrollable, penetration of Positive Ions to ARC Layer can be prevented at Cell Level & Module Level. So, Solar PV Module Manufacturer’s coming up with better manufacturing process by improving at Cell Level & Module Level.
Cell-Level Improvements to prevent PID:
Improvements to the cell manufacturing process mitigate the degradation of the cell shunt resistance and passivation associated with PID.
Reference: Trina Solar Manufacturing Process
Improved process: PECVD
Make sure the Anti-PID cells have same color and appearance with normal cells.
Improved process: Cell sorting
Use Anti-PID cells block the sodium ions moving at cell level.
Module Level Improvements to prevent PID:
New process: O3 Passivation
To generate a dense SiO2 layer (Compactness is better than SiNx), this layer help to block NA+ move into the surface of cell.
Improved process: Lay-out
Use EVA with higher volume resistivity to reduce the current leakage, this can block sodium ions at module level. But need to balance the optic performance and the price.
The choice of glass, encapsulation, and diffusion barriers has all been shown to have an impact on PID. For the front glass, several studies have shown sodium to exhibit a causative factor. According to one study, “an ingredient contained in soda-lime glass but not in Quartz glass is required for the effect to occur.
It was suggested that this species might be sodium. While sodium is the prime suspect due to its availability and high mobility, “aluminium, magnesium, and calcium are present in smaller concentration in soda lime glass but not in Quartz glass and might contribute to the difference.”