Aerial IR Thermography for Solar PV Power Plants

 

 

Aerial IR Thermography for Solar PV Power Plants

 Due to its high testing convenience and efficiency, aerial infrared (IR) thermography has gradually become a standard inspection requirement for power plant commissioning and O&M. But why has this testing method become so popular? What specific issues can thermal imaging actually reveal? 

What Does IR Inspection Detect?

IR inspection utilizes an infrared camera to capture the infrared radiation emitted by objects, allowing the temperature at each location to be calculated based on radiation intensity. Therefore, IR inspection primarily screens for thermal anomalies, through which various solar PV system defects can be inferred.

Technical specifications for outdoor IR inspection are provided in IEC TS 62446-3, which highlights a key principle: IR diagnostics rely primarily on identifying specific heating patterns to determine fault types, rather than relying solely on absolute temperature values. Consequently, recognizing heating patterns is far more critical than evaluating raw temperatures, and temperature differences should never be used as the sole absolute criteria for determining anomalies.

The following sections explain how specific heating patterns correspond to various defect classifications.

 

1. Open-Circuit Module String

The image below shows a typical thermal image of an open-circuit module string. The defining characteristic is an entire string of modules displaying a uniform, higher temperature compared to surrounding strings. In this thermal image, a distinct color difference is visible for this particular string, indicating a temperature delta relative to adjacent operational modules.

However, it is important to note that a visible color variation in a thermal palette does not necessarily imply a massive temperature difference. An open-circuit string typically runs only about 2℃ warmer than operating modules. This underscores a crucial point mentioned earlier: temperature difference should never be used as an absolute standard for anomaly detection. In a standard module array, a localized 2℃ variation is within normal operating limits; however, when an entire string shows a uniform 2℃ elevation across all its modules, it is a signature of an open-circuit string. Verification is straightforward: cross-referencing the thermal anomaly with the site's string layout map will confirm whether the pattern aligns precisely with a specific string connection.

 

open-circuit module string

 

2. Short-Circuited Module

The image below illustrates a typical thermal image of a short-circuited module. The defining characteristic is the presence of individual cells within the module exhibiting significantly elevated temperatures, with these high-temperature spots distributed unevenly and randomly across the panel.

The temperature difference caused by this type of short circuit may actually be under 10℃, reinforcing once again that raw temperature delta should not be used as the sole criteria for anomaly detection. While a temperature variance of less than 10℃ within a standard module is usually acceptable, a thermal signature exhibiting the specific spatial pattern shown below is a textbook indicator of a short-circuit anomaly.

 

short-circuited module

3. Front Glass Fracture

The image below shows a typical thermal signature of a module with fractured front glass. The defining characteristic is one or two cells within the module exhibiting significantly elevated temperatures, accompanied by uneven temperature distributions across the remaining cells.

This thermal pattern resembles the short-circuited module described above; however, the key difference lies in the lower number of overheated cells. Typically, the cell located directly at the origin of the impact or fracture point suffers the most severe cracking, giving rise to localized high temperatures.

It is important to emphasize that a fractured front glass does not always manifest in this specific heating pattern. Immediately after a fracture occurs, there may be no initial thermal anomaly. Likewise, if the glass damage does not induce severe underlying cell cracking—or conversely, if the glass shatter causes widespread, severe cracking across all cells simultaneously—the thermal pattern will differ from the image shown. Therefore, diagnosing glass fractures via thermography requires engineering judgment and supporting evidence from visual or secondary inspections.

 

module with fractured front glass

 

4. Substring Short Circuit Within a Module

The image below shows a typical thermal pattern of a substring short circuit inside a module. The defining characteristic is a distinct string of cells within the module where certain individual cells exhibit noticeably elevated and uneven temperatures.

Common root causes include a short-circuited bypass diode or an activated bypass diode. Because this thermal pattern involves multiple potential underlying issues, further diagnostic verification is required to determine whether the root cause stems from a diode failure, physical cell damage, or other operational factors.

 

substring short circuit

 

5. Open-Circuit Substring Within a Module

The image below illustrates a typical thermal image of an open-circuit substring within a module. The defining characteristic is an entire single substring of cells showing a uniformly elevated temperature compared to adjacent substrings.

A common root cause is poor soldering quality during manufacturing, which points to a broader reliability issue. For a detailed root-cause analysis, refer to Task 13 report Degradation and Failure Modes in New Photovoltaic Cell and Module Technologies.

 

open-circuit substring

 

6. Single-Cell Overheating

The image below shows a thermal image of single-cell overheating within a module. A primary root cause for this signature is a fractured or cracked solar cell.

In these cases, the temperature differential can reach several tens of degrees Celsius, creating severe localized thermal stress that risks permanently damaging the surrounding encapsulation materials and backsheet.

  

single-cell overheating

 

For example, as shown in the image below, a cell fracture creates a localized inactive area. Over time, the elevated temperature caused by such cell inactive area gradually induces yellowing of the encapsulation material and can even burn through the backsheet (as illustrated below), leading to serious electrical leakage risks.

  

Dark inactive area

  

Inactive area result in browning of EVA and backsheet burnt through

 

7. Hot Spots Caused by Shading

Foreign objects such as bird droppings, fallen leaves, or debris blocking the module surface can also induce hot spot phenomena (as shown in the image below). Cross-referencing visible-light (RGB) photographs with thermal images is necessary to confirm whether localized overheating is caused by surface shading.

Fortunately, hot spots caused by foreign object shading are straightforward to remediate—simply cleaning the module surface restores normal operating conditions. The critical priority is early detection and prompt intervention to minimize yield loss and prevent prolonged overheating from causing permanent thermal damage to the encapsulation materials or junction box.

 

Hot spot caused by shading

 

8. Interconnection Ribbon Overheating

The image below illustrates a thermal signature indicative of interconnection ribbon overheating. The defining characteristic is a localized heat concentration situated precisely along the interconnecting ribbon between adjacent cells.

This condition is frequently caused by ribbon fatigue, mechanical fractures, or poor solder joint connections, and requires targeted follow-up investigation to confirm the exact failure mechanism.

  

Interconnection ribbon overheating

 

9. Junction Box Overheating

The image below shows a thermal signature of junction box overheating. The localized heat accumulation is commonly caused by high contact resistance or loose electrical connections inside the junction box itself.

A failed or degraded bypass diode can also generate excessive heat in this region. Consequently, further diagnostic inspection is required to isolate the precise root cause.

 

Junction box overheating

 

Conclusion

Aerial IR thermography using drones has become an indispensable inspection procedure for solar PV power plants. It efficiently uncovers numerous hidden anomalies that are completely invisible to the naked eye at a highly competitive cost, making it one of the most cost-effective Quality Assurance and O&M diagnostic tools available.

However, IR thermography primarily captures temperature anomalies and comes with inherent diagnostic limitations. It cannot definitively identify every specific defect mechanism on its own, necessitating complementary inspection methodologies to address these blind spots.

 

 

About the Author

Jay Lin, warmly known as Dr. Jay among industry peers:

• 2004: Engaged in solar module R&D at the Industrial Technology Research Institute (ITRI).
• 2005: Partnered with TÜV Rheinland to establish Asia’s first solar testing and certification laboratory in Taiwan.
• 2007: Served as CTO at a2pak Power, a German-backed solar venture.
• 2011: Founded PV Guider, a consulting firm providing professional advisory services and solar power plant quality control solutions.

 

Current Roles:

• Chief Consultant, PV Guider
• Task Force Leader / Chairman, SEMI Standard Committee
• Member, Technical Committee of CNS National Standards
• Technical Expert, IEA PVPS Task 13 (Photovoltaic Power Systems Programme - Solar Reliability Workgroup)
• Technical Committee Member, European Photovoltaic Solar Energy Conference and Exhibition (EU PVSEC)