A paper on Potential Induced Degradation (PID) & its impact on Modules Performance

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

Controllable
 2.    Environmental Factors

ii.     Temperature

iii.    Humidity

Uncontrollable

Solar enthusiasts would clearly understand from the above Table what were the reasons, which can be controllable & uncontrollable.

  1. 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.

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The Negative Bias Modules with respect to ground would attract the Positive Ions from the Glass & atmosphere. These ions will penetrate through the EVA sheet & get accumulated at the surface of the Anti-Reflective Coating Layer to create a new electric field in opposite direction of the existing PN Junction Electric Field in the front passivation layer.

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.

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This PID influence on the modules is confined to the regions climate beset by both High Temperature & Humidity i.e., typical Climate of most parts of India.

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

(i) Temperature

(ii)Humidity

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.

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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.”

Kiran Kumar Alla

Kiran Kumar Alla

A Solar Professional passionate about Solar PV System Design & Engineering with an Industry Experience of 5 Years working in Fortum India Pvt. Ltd. as Lead-Technical, Solar. Well versed with AC-DC PV System Design Engineering concepts & eager to learn new Technical Advancements, implement them to optimize the Engineering Practices without compromising Human-Equipment safety, reliability & quality. I love to discuss with my colleagues, friends regarding various aspects of Solar Power Plants starting from Tendering, Design, Project Execution & Operation-Maintenance because I believe in Knowledge shared is Knowledge Gained. The reason for choosing Design profile in Solar Industry because Solar was the answer to electrify all remote parts in India using Off Grid and Grid Connected without polluting the future generations, Energy Security & India has a great Solar potential.