What is the PID effect in solar panels?

If you experience that your solar panels are not performing as expected and your electrical appliances are not running well.

Then it could be due to the PID effect in the solar panels.
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The PID effect is negligible in the beginning but it advances with time and becomes more noticeable in the later years of your solar panels.

What is PID effect in solar panels?

It is the degradation in the performance of the solar panels due to the flow of leakage current from the solar cell to the other parts of the module like glass, frame, and the mounting structure.

Now, the net useful current to run the electrical appliances is reduced, affecting the power output of the solar panels.
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In the nascent stage, the power reduction could be as low as 5% of Pmax.

But

it increases with time and can reduce the power output of the solar panels by 30%-40% in the later years. 
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Therefore, it is very important to address this problem in the early stage otherwise the damage could be irreversible and will permanently damage your solar cells in the modules.

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main factors causing PID effect in solar panels

The main factors causing PID in the solar panels are:

• Panel Voltage>= 1000 volts
• Heat
• Humidity

The solar panels with the negative potential of 1000 volts or more w.r.t the ground is most affected by the PID effect.

In PID-affected modules, there is a high potential difference between the solar cell and the module frame that is grounded which results in the leakage current to flow from the solar cell to the frame via encapsulating layer and the glass.
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Also, due to the high potential difference, the Na+ from the glass starts migrating towards the solar cell and damages the PN junction of the solar cell.

Moreover, the fog, mist, rainwater, humidity forms a conductive surface between the solar cell, an encapsulant (Ethyl Vinyl Acetate), glass, and the frame that further accelerates this effect.

This effect is more visible with solar cells having high negative potential and are near the periphery of the module.

The solar cells near the periphery are more exposed to humidity and hence are more affected by the PID effect.

An example of PID effect

I have 20 solar panels with Voc of 50 volts (It is generally around 48 volts but I’m taking it as 50 volts for easy math calculations.)
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When I connect these solar panels in series, the voltage of individual solar panels gets to add up and the total voltage of the system becomes 1000 volts.

Now, the last solar panel in this series combination is at -1000 volts.

Understand like this

• The second terminal of the first panel is at -50 volts
• The second terminal of the 2nd panels is at (-50) + (-50) = -100 volts
• Similarly, the seconds terminal of the 19th panel is at -950 volts
• And the 20th (last) solar panel would be at -1000 volts.

The frame is connected to the ground, which means that it is at zero potential and now there exists a potential difference of 1000 volts in between the solar cell of the last solar and the frame.

This high potential difference will start the flow of electrons from the solar cell to the frame via encapsulant and the glass.

Also, the positive Na+ will start flowing towards the solar cell, affecting its PN junction.

If this problem is not addressed early, it could damage the cell and make it non-functional.

You can see in the picture that this effect is more dominant at the last solar panels and that too at the periphery of the solar module.

The growth rate of this effect is more in the early morning and the late evenings when the sunlight is low.

When the sunlight is low, the surrounding temperature is also low and that increases the voltage of the solar modules even beyond 1000 volts further accelerating the PID effect.
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The PID effect also affects the I-V curve due to the drop in the shunt resistance.

The red line shows the I-V curve of PID affected solar cell

and

the black color indicates the I-V curve of PID-free solar cells.

The PID effect reduces the Pmax of the solar cell by reducing its voltage and current.

How to control the PID effect?

1. When using anti-reflecting coating on the cell surface with an additional oxide coating can increase the dielectric strength that helps in reducing the stray current and hence controls the PID effect.
2. Companies introducing encapsulation material and glass with less Na+ composition in their modules by changing from soda-lime to boro-silicate glass or introducing the barrier layer for Na+ can check the PID effect.
3. The negative grounding of the solar panels should be done to reverse this effect or remove voltage potential at night.
4. Drying of the module can reduce the moisture content in the module. Hence helps in checking the PID effect.

*(For the system size beyond 1000 volts where panel voltage is at negative potential w.r.t ground, the PID effect is more dominant).

Standard test to check the PID effect in solar panels

IEC TS 62804-1 is the standard test that evaluates the potential induced degradation in the crystalline silicon-based solar modules.

In this test, the solar modules are subjected to a negative potential of 1000 volts for 96 hours under the ambient temperature of 60 °C (140 °F) and the relative humidity of 85%.

To pass this standard test, the module’s Pmax must not decrease beyond 5%.

Some top PID free solar module manufacturers

There are many companies that manufacture PID-free solar panels.

I’m listing just a few for your reference.

These are:

• Waaree
• Vikram Solar
• SunPower

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Summary

• The PID effect produces stray current within the module that affects the performance of the solar panel.
• This effect leads to power reduction as high 30%-40% by damaging the solar cells in the modules.
• It is found in system sizes having negative potential beyond 1000 volts
• The PID effect is found near the periphery of the solar module.
• It get worse with humidity
• The use of ARC (Anti Reflecting Coating) on the surface of the cell helps in checking the PID effect.
• Introducing barrier layer for Na+ helps in reducing the migration of these ions to the cell.
• The negative grounding of solar modules can reverse this effect.
• IEC 62804 is the test to evaluate the durability of the panel against the PID effect.

If you’re planning to install a system with voltage beyond 1000 volts then do check the IEC 62804 certifications at the back of the solar panels before buying them.