WHAT IS PID?
POTENTIAL INDUCED DEGRADATION
PID (Potential Induced Degradation) is a phenomenon that negatively affects the cells of PV modules. It is a process which occurs only a few years after installation. PID causes an accelerated degradation in performance which expands exponentially.
PV SYSTEM DEGRADATION
Detecting PID on a PV system by comparing output data, is difficult. After all, the solar irradiation depends on the place and time of measurement. In general, the Performance Ratio (PR) of a PV park will decrease by 2.5% to 30% when PID occurs.
GENERATING ELECTRICITY WITH SOLAR CELLS
To understand PID, it is important to know how electricity is normally generated by solar panels.
A solar panel consists of individual photovoltaic cells. Each of these cells generates electricity from the collected sunlight. Technically spoken, the combination of two semiconductor materials exchange charges, resulting in an internal electric field.
The incident sunlight dislodges electrons. These electrons then flow along the contacts and generate (green) electricity.
PID: DISRUPTED ELECTRICITY GENERATION
When PID occurs, the aforementioned process is disrupted by the presence of electrical charge carriers, mainly the sodium ions of the glass which moved to the solar cell and pn junction.
Active solar panels that have a negative potential with respect to earth, also have this negative potential with respect to the earthed mounting system. Due to this potential difference, charges move through the materials of the PV module. This movement typically occurs from the glass plate through the EVA encapsulation and the ARC (Anti-Reflective Coating) to the cell.
These electrical charges have a strong negative influence on the performance of the PV cell. As a result, a solar cell will generate less electricity when it is affected by PID.
Image: Charges moving from the glass plate through the EVA encapsulation and the ARC (Anti-Reflective Coating) to the cell.
STRENGTH OF THE PID EFFECT
PID effect in individual PV module
In an individual PV module, the PID effect is stronger in cells that are located at the periphery (closer to the aluminium frame). The PID effect is less strong in cells that are located in the centre of the module.
PID effect in string PV modules
In a string of PV modules, the PID effect is stronger in panels that are closer to the negative side of the string. After all, the negative potential with respect to earth is the highest at that side.
CAUSE OF PID
PID is caused by the high potential difference with respect to earth. This occurs within the panels when the PV system is in operation. A typical PV module normally produces about 30 V. A daisy chain of panels can result in a multiple of this.
The inverter converts this corresponding DC voltage into an AC voltage, making it usable for the power distribution network. The distribution of the voltage with respect to earth, as well as the potential on the operating PV modules, is determined by the design of the inverters and the electricity grid.
Distribution potentialby inverter
KEY FACTORS OF PID
It has only recently become apparent that PID is a serious threat to the performance of many PV systems. The main factors that determine whether a system is subject to PID are:
There are indications that many cell types are sensitive to the exposure of a large negative potential. The structure of the cell itself has an influence on the PID process. This is due to the density of the charge carrier of the silicon used, and the chemical composition of the ARC (Anti-Reflective Coating) layer on the cell.
The used materials also play a role. This includes, among other things, the chemical composition of the used glass and the EVA in which the PV cells are encapsulated.
Configuration of PV system
The maximum negative potential to which the PV cells are subject plays an important role. This depends on the number of panels per string, the inverter type and the earthing of the PV field.
For solar panel owners, neither the PID effect, nor its impact on the output of the PV system is immediately visible. Therefore, a PID Check is the best guarantee to timely detect PID.
Amongst the external factors that cause PID are temperature and humidity.
The best way to detect PID on a PV system is through your online monitoring system. The best day to compare the operating voltages (Vmpp or DC Voltage) is a day with good irradiation. Also make sure to compare inverters that have the same string configuration.
The following graphic shows how PID in an advanced stage looks like during the day:
In case of equal string configurations, the operating voltages per inverter are basically on a similar curve. In practise, PID is a random process, with a different impact per cell, panel and string. As a result, PID occurs more on some inverters than on others. A wide range of operating voltages can be detected, resulting in different outputs per inverter.
You can also measure the open-circuit voltage on site, instead of the operating voltage. A big difference between two similar strings is a clear indication that PID is present.
ON CELL, STRING AND PANEL LEVEL
PID detection at cellular level
Detecting PID at cellular level is typically done in laboratory conditions. The PV module is then examined with an electroluminescence (EL) test. This involves sending a current through the panel and observing the emitted (infrared) light. The image below shows the condition before and after the PID effect. In this test, a panel with PID will display a typical chessboard pattern. Mainly the cells on the outside (close to the frame) will be less illuminated because the diode of these cells is impacted.
PID detection at string level
When PID is detected at string level, there is a clear difference between the beginning and end of the string of panels, with a gradient in the EL image over the entire string. A typical random chessboard pattern will be visible for panels that are nearest to the negative side.
PID detection at panel level
If PID occurs at cellular level, this will affect the performance of the PV module. This clearly results in a shift of the IV curve, where the Maximum Power Point (MPP) of the module will be shifting downwards. As a result, the transformation of sunlight into electricity will be less efficient.
HOW TO PREVENT/FIX PID?
Pidbull is the patented solution of Edison Energy.
If you are in doubt, you can use Pidbull to test the extent in which PID is present in your solar park. Pidbull then completely solves the problem and ensures the highest possible output.
If you prefer absolute certainty, you can profit from the preventive effect of Pidbull. In this case, your yield will never decrease because of PID.