What is PID ?
Potential-Induced Degradation (PID) is a common phenomenon causing PV panels to lose power generation by up to 80%. Power reduction can occur within days or weeks after installation. The PID process in the PV panel may grow very rapidly and in the shortest period will affect the performance of an entire PV system and results in damaging effect 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 panels performance over the entire system life - PID can be prevented and recovered on system level altogether.
Why PID ?
The PID occurs when electrons of photovoltaic module are leaking from the semiconductor material into other elements of the panel (frame, glass, mount etc.) and creates current leakage and electricity production degradation.
How can PID be detected ?
PID detection is the main issue in the treatment of PID.
There are different methods, physical and theoretical, providing reliable information on PID affect.
The typical test to know if a module is free of PID is to measure its IV (current–voltage) characteristic with an electronic tracer.
The shape of the IV_curve can show the presence of anomalies in the characteristic of the module, as can be seen the below IV_Curve test.
In the below IV_Curve test, a 20 modules string was measured. The modules on the “+” side of the string (modules 1 & 2) are with the expected curve and characteristic, yet the modules on the “-“ side of the string are showing clear degradation caused by PID.
This method using a voltmeter to measures the open circuit voltage of the PV modules. If a panel is affected by PID its VOC will be likely lower than the expected one. This is since the reduction of shunt resistance can reduce the open circuit voltage. This method works well when PID is high. Otherwise voltage reduction may be not distinguished.
At the graph below VOC of 20 solar modules was measured at the same time and date both at 2013 and 2016. It is clear that at 2013 the voltage on all 20 panels was almost the same, while in 2016 part of the modules produced much lower voltage. That points to the existence of PID.
Other option to use VOC measurements is to compare the VOC of the modules at the “+” side of the string to the VOC of the modules at the “-“ side of the string.
If there is a minimum difference between VOC results of both sides than a PID existence is very likely.
This method uses the fact that PID-affected cells have higher temperatures than the neighbor cells free of PID. If the panel does not affected by PID, all its cells are almost at the same temperature. The imaging with infrared camera (IR camera) allows detecting “ill” cells, as is shown on the picture below. The darker zones have higher temperature and obviously are affected by PID
Electroluminescence (EL) is an optical and electrical phenomenon in which a material emits light in response to an electric current. The higher the current - the brighter the light. EL imaging is performed by special camera at the near infra-red zone. The pictures below show PID affect dynamic. Dark colored cells indicates PID effect, light color indicates “healthy” cells.
Another approach of PID detection is analysis of measurement data provided by on-line management system. Comparison between operating voltages, powers or specific yields per inverter/string/panel in systems with monitoring per module, gives possibility to detect PID existence.
The following graphic shows example of PID detection using Comparison within analyzed object during specific period method.
The monthly power generated by solar modules of a specific inverter during two years is compared. In July 2013, immediately after system installation almost all modules provided approximately the same monthly power (green zone on the below graph), while in July 2015 a large deviation between modules productivity is observed showing a high level of PID effect.
How to prevent & restore PID affect ?
The installation of the Vigdu-P Anti PID device will prevent the evolution of PID and will restore an existing PID in affected panels.
The next two images are taken before (left) and some days after (right) Vigdu-P device installation. The number of “healthy” cells, especially in the left column, was essentially increased.
The following graph shows the growth of the rate between analyzed system with Vigdu-P and reference system with stable functionality. The estimated growth is approximately 35% during 40 days.
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