ASK THE EXPERTS: Transformerless Inverters and PV Degradation

Intermediate
Potential-Induced Degradation
Reduction of a Solar-Electric Module’s Efficiency Over Time

I read that potential-induced degradation (PID), the reduction of a solar-electric module’s efficiency over its lifetime, is sped up by the use of transformerless grid-tied inverters, which allow the modules to cycle electrons at the grid’s AC frequency. Module manufacturers are advertising PID-free modules, which supposedly counteract this problem.

Transformer-based inverters don’t cycle and have a constant flow of electrons. So would using one allow my modules to last longer? Does anyone have real experience in this area, or is it all theoretical at this point?

Zachary Strassman • via email

PID is the result of PV modules being held at a high potential (voltage), which causes them to temporarily decrease power output. With high-voltage arrays, ungrounded inverters, and increasing module efficiency with antireflective coatings; PID is showing up in the field—not just the lab.

PID is due to leakage of electrons out of the silicon cells. These migrating electrons accumulate on the surface of the silicon cell, where they affect the ability of the P-N junction in the cell to convert sunlight. This power reduction can occur over days or weeks. It is seen at higher array voltages, higher temperatures, and, especially, under higher humidity conditions. Estimates of power losses due to PID range from 10% to 60%, although the most I’ve seen (on a negatively grounded SunPower array) is about 25%.

The type of encapsulant and backsheet used on the modules are factors that can influence PID. Some compounds are more prone to leaking current than others, and some are more affected by moisture than others.

Another factor is the antireflective coating—some types, while improving efficiency, also make modules more prone to PID. How the silicon’s cells are layered (for example, back-contact cells in which the P and N layers are flipped) also affect the module’s PID propensity.

The third major factor affecting PID is the array’s polarization and voltage. Most negatively grounded arrays are not affected by PID. Historically, however, negatively grounded SunPower arrays could suffer PID. PID is generally only seen on high-voltage (100+ volts) arrays as well. Whether modules are PID-prone is largely dependent on the antireflective coating and encapsulant.

The good news is that, in crystalline silicon modules, PID is mostly reversible. By properly grounding the array, the extra electrons that have leaked to places they shouldn’t are sent back, restoring the module to its normal state. By holding the cells at the proper high voltage for a few days (48 to 96 hours) with an external source of voltage, PID can be almost completely reversed. Properly grounding the array so it gets a few hours of the proper voltage every day will also reverse the effects of PID.

Electrical standards organizations are developing tests and standards to rate whether a particular module is susceptible to PID. Look for these certifications on modules in the future. If you do not have PID-prone modules, the type of inverter you use may not matter. But if you do have PID-prone modules, use an inverter that grounds the modules in the proper direction to avoid PID for those particular modules.

Zeke Yewdall • Mile Hi Solar

Comments (4)

Bob Croteau's picture

I am confused about the anit-reflective coating. The diagram shows it under the silicon cell where reflection back to the cell would seem an advantage. Is it a feature of the encapsulent?
I have SolarWorld 250 Mono modules that I understand have the anti reflective coating, but I thought it is like my eye glasses and telescope primary lens, as a thin film on the surface that appears as a bluish-green tint when viewed an an angle. I understood the film increased transmission through the surface of the glass by reducing reflection at a molecular level. Please clarify for me. Thanks

Michael Welch's picture

Hi Bob. There is an error in that diagram, and we will have it corrected soon. The antireflective coatings are ABOVE the PV material, the point being to get more light into the PV cells.

Marsha Robison's picture

What are the testing procedures and is there a fix for this in an existing system. I have had an untraceable loss in power from my array which happened about 2 years after I installed it. I was told by someone that I bought a battery bank off of to change my 9 panel, 160watt each, 1k pv array configuration from 3 panels 480 each to 2 panels 320 each. No one could figure out how my system went from putting out 960watt to max 720......

Michael Welch's picture

Hi Marsha. Your installer should be able to figure this out for you. I doubt if this is a degradation issue, but is more likely a wiring issue, module failure, or an error in reading the monitoring meter.

There really is not enough info to help you, you don't even say if this is a grid-tied system, or where the readings are coming from that tell you there is a loss in power output.

I doubt if changing the number of modules in the strings would help. You really need a knowledgeable person at your home to begin checking the system out. If your installer is no longer able to help, please send me an email telling me where you are, and maybe I can help find someone to help.

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