What Is The Anti-PID Effect Of Solar Panel?
1.PID Effect
The full name of PID is: Potential Induced Degradation, which means potential induced degradation.
The PID effect was first discovered and proposed by the American company SunPower in 2005. It refers to the long-term operation of components at high voltage, the existence of leakage current between the cover glass, packaging materials and frames, and the accumulation of a large amount of charge on the surface of the cell, which deteriorates the passivation effect on the surface of the cell, leading to a decrease in the fill factor, short-circuit current and open-circuit voltage, making the component performance lower than the design standard. The degree of attenuation can reach 50%, but this attenuation is reversible.
2. Mechanism of PID effect
① High voltage effect
The large-scale application of photovoltaic systems has led to higher and higher system voltages. Battery modules often require multiple modules to be connected in series to reach the MPPT working voltage of the inverter, which leads to very high open circuit voltage and working voltage.
Taking a 72-cell battery module of 450W under STC environment as an example, the open circuit voltage of a 20-string battery module is as high as 1000V and the working voltage is as high as 800V. Since photovoltaic power stations need to be equipped with lightning protection and grounding projects, the aluminum alloy frames of general components are required to be grounded, and a DC high voltage of nearly 1000V will be formed between the battery cells and the aluminum frame, causing a voltage bias between the circuit and the metal grounding frame.
② Ion migration
Under the high voltage between the packaging material of the battery module and the materials on its upper and lower surfaces, and between the battery cell and its grounded metal frame, ion migration occurs, resulting in component performance degradation.
When the solar cell is polarized with a high negative voltage, there is a relevant voltage difference between the battery itself and the module frame. This is at zero potential because most of the time it is grounded, so due to the very short distance between the solar cell and the frame and due to possible impurities in the sealing material, current can be generated between the cell and the frame, creating current leakage for the entire photovoltaic module.
3. Causes of PID effect
① Water vapor entering the solar panel
Water vapor has a significant impact on the PID effect in solar panels. As the temperature rises, the water vapor in the air begins to condense and accumulate on the surface of the solar panel. Over time, this condensation can lead to the accumulation of moisture within the solar panel, which can cause problems.
Water vapor that enters the solar panel can create a closed electrical circuit with the solar cells and other components of the solar panel. This results in a flow of electricity that can cause the solar panel to work more poorly than it should.
② EVA Hydrolysis
The second leading cause of the PID effect is the hydrolysis of Ethylene Vinyl Acetate (EVA) encapsulant material. EVA is a widely used encapsulant material in the production of solar panels. When exposed to high humidity and temperature, EVA is prone to generating acetic acid (vinegar).
Acetic acid produced by the hydrolysis of EVA interacts with the metallic components of the solar panel and creates a path for the flow of current. The flow of this current causes a loss of power output.
③ Chemical reactions on the glass surface
The third cause of the PID effect is the chemical reaction between acetic acid and the glass surface of the solar panel. The combination of acetic acid and the glass surface produces sodium acetate. Sodium acetate is an electrolyte solution that can conduct electricity. This flow of electricity leads to a loss in power output.
④ Sodium ions moving in an electric field
The fourth reason for the PID effect is the movement of sodium ions in an electric field. Sodium is the most mobile ion in glass, and when it enters inside the solar panel, it reacts with the solar cells, creating a closed circuit.
When the solar panels are exposed to a high voltage difference, sodium ions can migrate within the solar panel, creating areas of high electrical potential. This flow of electricity leads to a loss of power output.
4. PID Test Method
There is a specific series of standards - IEC 62804 Photovoltaic (PV) modules: Test method for detecting potential induced degradation. The test conditions for detecting potential induced degradation according to IEC 62084 are:
60°C air temperature
85% relative humidity
Voltage bias of +1000V, -1000V, +1500V or -1500V (depending on the PV module characteristics)
Total test time is 96 hours
The pass criteria are mainly related to the power degradation measured at the end of the test. If it does not exceed 5%, the test is passed. Therefore, this test does not ensure that PID will not occur or that the module is PID-free. PV modules with lower power degradation in IEC 62804 certification may be the most resistant to PID effects. Currently some manufacturers extend the duration (up to 600 hours) certification, and this type of test is reliable for products that are resistant to PID effects.
5. Solutions to the PID effect
PID effect of P-type crystalline silicon modules (conventional ASF cells, PERC cells)
In actual operation of power stations, PID attenuation is common in conventional crystalline silicon modules with frames (soda-lime glass, EVA film). The higher the DC system voltage, the greater the humidity, and the higher the temperature, the more serious the PID attenuation. The PID effect of P-type crystalline silicon modules can be reduced by the following methods:
A. Use quartz glass instead of soda-lime glass to remove Na+ and Ca+2 ions;
B. Use double-glass frameless modules to avoid frame grounding;
C. Use composite frames (nylon, polyurethane materials, etc.);
Improve EVA or increase the density of the nitride film on the surface of the cell;
② PID effect of N-type crystalline silicon modules (TOPCon cells)
The PID effect of N-type crystalline silicon modules is no longer caused by migrating ions (Na+, Ca+2), but by the dielectric polarization of the passivation layer caused by the potential difference between the battery and the module frame. Therefore, the PID effect of N-type crystalline silicon modules can be prevented by introducing a passivation layer with higher conductivity and lower dielectric constant.
③ PID effect of HJT battery components
The structure of HJT battery is completely different from PERC and TOPCon. The passivation layer uses transparent oxide conductive film (TCO) instead of SiN4. Under high voltage bias conditions, there is no insulating layer for accumulated charge, so PID phenomenon will not occur. Therefore, HJT battery has the potential to resist PID.