Bifacial solar modules are reshaping the solar energy landscape. By capturing sunlight from both sides, they promise higher energy yields, and when paired with a sleek, frameless design, they offer a modern aesthetic that’s hard to beat.
But as with any innovation, new designs can introduce unexpected challenges. For frameless bifacial modules, one of these is a subtle but serious failure mode known as Potential-Induced Shunting (PIS). It’s a cousin to the more familiar Potential-Induced Degradation (PID), but its mechanism is different and potentially more destructive.
Let’s look under the glass to understand what PIS is, why frameless designs are more susceptible, and how to identify it before it impacts your project’s performance.
PID vs. PIS: What’s the Difference?
To grasp Potential-Induced Shunting, it helps to first understand its relative, PID.
Potential-Induced Degradation (PID) is a performance loss in solar modules caused by high voltage differences between the solar cells and the module’s grounded frame. Over time, this voltage stress can cause ions to move, reducing the cell’s efficiency. Think of it as the cells growing tired and underperforming.
Potential-Induced Shunting (PIS), on the other hand, is more direct and destructive. It’s not just about reduced efficiency; it’s about creating a new, unwanted electrical pathway—a “shunt.” Imagine a tiny short circuit forming directly on the cell. This shunt diverts energy away from its intended path, dissipating it as heat. This not only leads to significant power loss but can also create dangerous hot spots.
The key difference is the outcome: PID reduces cell performance, while PIS creates a leakage current path that bypasses the cell entirely.
The Perfect Storm: How PIS Develops in Frameless Modules
Potential-Induced Shunting doesn’t occur in a vacuum. It requires a specific combination of factors—a „perfect storm“ that frameless bifacial designs are particularly prone to creating.
The main culprits are:
- High System Voltage: Common in large-scale solar arrays.
- Sodium Ions (Na+): Present in the soda-lime glass used for most solar modules.
- Moisture: The catalyst that gets everything moving.
Here’s how it unfolds: In a humid environment, moisture accumulating at the exposed glass edge of a frameless module creates a conductive layer on the surface. Driven by the high system voltage, positively charged sodium ions from the glass begin to migrate through the encapsulant (the polymer layer protecting the cells, often EVA).
Their destination? The front side of the solar cell.
When these ions reach the cell, they create a physical, conductive path—the shunt. This new path allows current to leak from the cell’s emitter to its back, effectively short-circuiting part of the cell.
Image Description: Diagram showing the mechanism of Potential-Induced Shunting in a frameless bifacial module, with leakage current paths highlighted at the module edge.
Framed modules have a natural defense. The aluminum frame and its sealant act as a barrier, making it much harder for moisture to create a continuous path to the cell. Frameless designs, while aesthetically pleasing, remove this barrier, leaving the glass edge vulnerable and allowing this damaging cycle to begin.
Seeing the Invisible: How to Test for PIS
As a hidden threat, PIS isn’t visible to the naked eye. A module can look perfectly fine on the outside while suffering from severe internal shunting. That’s why specialized testing in a controlled environment is critical.
Identifying PIS involves simulating worst-case field conditions in a laboratory setting:
- Environmental Simulation: The module is placed inside a damp heat chamber that maintains a high-humidity, high-temperature environment (e.g., 85°C and 85% relative humidity).
- Voltage Application: A high negative voltage (often -1000V or -1500V, depending on the system) is applied to the cells relative to the module surface, mimicking the electrical stress seen in a live solar array.
- Leakage Current Monitoring: Throughout the test, engineers measure the leakage current. A steady increase in this current is a key indicator that shunts are beginning to form.
The definitive proof, however, comes from Electroluminescence (EL) imaging. EL testing works like an X-ray for solar modules, revealing defects that are otherwise invisible. A healthy module will light up uniformly. A module with PIS will show distinct dark, patchy, or completely black areas where the cells are shunted and no longer producing power.
Here is an EL image of a module before the PIS test; it looks healthy and uniform.
Image Description: Electroluminescence (EL) image of a solar module before undergoing a PIS test, showing uniform cell illumination.
And here is the same module after the test. The dark, inactive zones clearly show where PIS has created destructive shunts, causing massive power loss.
Image Description: Electroluminescence (EL) image of the same solar module after the PIS test, revealing dark, shunted areas indicating severe power loss.
This kind of visual evidence is undeniable and essential for making informed decisions about module design and material selection. Performing comprehensive module reliability testing can prevent catastrophic field failures down the line.
Prevention and Mitigation Strategies
While PIS is a serious concern, it isn’t an unavoidable fate for frameless modules. Awareness and smart design choices can significantly reduce the risk.
- Encapsulant Choice: The type of encapsulant used is crucial. Polyolefin elastomers (POE) generally have much lower water vapor transmission rates than traditional Ethylene Vinyl Acetate (EVA), making it harder for moisture to penetrate and activate the shunting mechanism.
- Edge Sealant: Applying a robust, moisture-resistant edge sealant can effectively re-establish the barrier that a frame would normally provide, protecting the vulnerable glass-encapsulant interface.
- Rigorous Prototyping: Before committing a new module design to mass production, testing its resilience is vital. By prototyping new module designs and subjecting them to accelerated PIS testing, manufacturers can validate their material choices and design integrity, ensuring long-term reliability in the field.
Frequently Asked Questions (FAQ)
Is PIS the same thing as standard PID?
No. Standard PID is a degradation of cell efficiency due to ion mobility, often described as surface polarization. PIS is the formation of a physical shunt—a new electrical pathway—that causes current leakage. PIS is generally more severe and often irreversible.
Are all frameless bifacial modules at risk of PIS?
All frameless modules have a higher risk than framed modules due to their exposed edges. However, the actual risk depends heavily on the quality of the materials used (especially the encapsulant and edge sealant) and the climate where they are installed.
Can PIS damage be reversed?
In most cases, no. Unlike some forms of PID that can be partially reversed by applying an opposite voltage, the shunts created by PIS are physical changes to the cell. The power loss is typically permanent.
What is an encapsulant in a solar module?
An encapsulant is a transparent polymer layer (like EVA or POE) that surrounds the solar cells. Its job is to provide adhesion, electrical insulation, and protection from moisture, vibration, and impact.
Why is high voltage a problem for solar modules?
To generate significant power, solar modules are connected in long strings, which builds up high voltage (often 1000V or 1500V). This high potential difference between the live cells and a grounded surface creates an electrical field that can drive unwanted ion migration, leading to degradation like PID and PIS.
Building for the Future
Frameless bifacial modules represent an exciting step forward in solar technology, but their success depends on long-term reliability. Understanding and designing against failure modes like Potential-Induced Shunting is not just about protecting an investment—it’s about building a more resilient and sustainable energy future.
By combining innovative designs with meticulous material selection and rigorous validation testing, we can harness the full potential of bifacial technology without compromise.