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Potential Induced Degradation PID in photovoltaic modules

Photovoltaic PID is a potential degradation phenomenon of photovoltaic modules caused by voltage difference. Discover the causes and how to prevent it

Photovoltaic Potential Induced Degradation (PID) is an electrical phenomenon that results in a power loss in photovoltaic modules. This phenomenon occurs over several months or even years: in fact, it is often overlooked at the beginning of the plant’s life, while it becomes more burdensome in the subsequent operating phases, especially because it is not always easy to trace the causes.

The effects of PID can be reversible or irreversible. These performance degradation effects on the module and system can lead to significant changes in the system’s business plan. In particular, cases of PID have been reported that have led to a decrease in power output of up to 70% in less than a year.

It is therefore essential to constantly monitor the photovoltaic system in order to identify the first signs of performance degradation in the system. In this regard, to obtain financial analyses on the actual profitability of your system, you can use a photovoltaic software.

Let’s analyze in detail what causes photovoltaic PID, its effects, and strategies to identify and prevent it.

photovoltaic system efficiency - Solarius PV software

photovoltaic system efficiency – Solarius PV software

What is Photovoltaic PID

PID stands for potential induced degradation and it consists of the degradation of the photovoltaic effect and therefore the power reduction of the photovoltaic cells.

The potential-induced degradation is due to a high potential difference between the semiconductor material (cell) and other parts of the module (glass, support, or aluminum frame). This potential difference creates a current loss that leads to the migration of negative and positive ions.

Negative ions exit through the aluminum frame, while positive ions (sodium ions) migrate to the surface of the cell. These contaminate the cell, reducing its photovoltaic effect, causing power losses.

This phenomenon affects both modules with crystalline cells and thin-film ones and is strongly dependent on the construction and materials used in the panel, also due to temperature and high humidity, essential for a faster development of the defect. It also depends on time and can start to occur already in the first months of a panel’s life.

PID mainly leads to two types of degradation:

  • reversible (polarization): characterized by the presence of leakage currents through the encapsulating material, frame, and front glass. It is particularly evident in modules at the bottom of the strings (near the negative pole). The power loss in these modules can even reach 70% in a few years;
  • irreversible (TCO layer electrocorrosion): characterized by the interaction between the frame and humidity. The dispersed current generated by PID causes an electrochemical reaction between the previously mentioned elements, generating a corrosion phenomenon. Logically, it is a problem because by decreasing the conductive capacities, the module’s efficiency is reduced. To avoid this problem, either the frame materials are changed or a galvanically isolated inverter is used.

What are the causes of the PID effect

The causes leading to photovoltaic PID effect include various factors, such as:

  • cell quality: the quality of photovoltaic cells can influence the onset of the PID effect, as low-quality cells may be more susceptible to deterioration;
  • module type: the choice of encapsulant and glass has a strong impact on the ease (or difficulty) of ion movement within the module;
  • dielectric properties of glass: the dielectric properties of the glass used in photovoltaic modules can contribute to the onset of the PID effect, as they influence the encapsulant resistivity and leakage currents;
  • potential to which the module is subjected: the potential to which the photovoltaic module is subjected can be a determining factor in the onset of the PID effect, as the negative voltage between the module cells and the frame connected to the ground can cause performance deterioration;
  • environmental factors: high humidity and temperature values can favor the onset of the PID effect, as they can influence the encapsulant resistivity and increase leakage currents.

Methods for detecting the PID effect

The methods for detecting the PID effect are mainly two:

  • measurement of the I-V curves of the photovoltaic modules;
  • electroluminescence (EL) analysis of photovoltaic modules suspected of power loss due to PID.

Electroluminescence Analysis

Outdoor electroluminescence analysis for detecting photovoltaic PID consists of viewing an entire string of photovoltaic modules simultaneously using an infrared thermographic camera. This method allows observing the different levels of solar cell emissions and identifying any anomalies or problems, such as the presence of the PID effect.

In practice, electroluminescence is a technique that allows analyzing the brightness emitted by photovoltaic modules when subjected to an electric voltage, identifying damaged or PID-affected areas.

Measurement of I-V Curves of Photovoltaic Modules

The measurement of the I-V curves of photovoltaic modules consists of measuring the I-V characteristic of the modules, i.e., the relationship between the electric current and the voltage generated by the modules. This analysis helps identify any anomalies or problems, such as the presence of the PID effect.

The measurement of the I-V curve helps determine the efficiency and isolation of the modules, providing detailed information on the health and performance of solar panels. In this way, experts can identify energy losses and diagnose problems related to the PID effect, allowing timely interventions to maintain optimal solar system performance.

How to prevent photovoltaic PID effect

To prevent the Photovoltaic PID effect, it is important to take some preventive measures:

  • use of high-quality materials: using high-quality materials for the production of photovoltaic modules, such as solar cells with optimal dielectric properties, can help reduce the likelihood of the PID effect;
  • careful system design: carefully designing the photovoltaic system, taking into account environmental conditions and specific site requirements, can help reduce the likelihood of the effect;
  • constant performance monitoring: constantly monitoring the performance of the photovoltaic system can help identify any problems related to the PID effect and intervene promptly;
  • use of specific inverters: using specific inverters designed to mitigate the PID effect can help prevent this problem;
  • panel cleaning: regularly cleaning solar panels can help reduce the likelihood of the PID effect;
  • verification of electrical connections: regularly checking the electrical connections of the photovoltaic system can help identify any problems related to the PID effect and intervene promptly.
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