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Study Concludes Aerial Thermography Most Suitable Method to Identify Underperforming PV Cells

image credit: Gallardo-Saavedra et al.  Technological review of the instrumentation used in aerial thermographic inspection of photovoltaic plants. Renewable and Sustainable Energy Reviews 93 (2018) 566–579.
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In this post we present data from a study that was published in Renewable and Sustainable Energy Reviews, concluding that aerial thermography is the most suitable method to identify underperforming photovoltaic (PV) cells.  Analysis of this data is critical for optimization of solar operations & maintenance. 

 

Detecting under-producing PV modules is at the heart of optimizing energy production for PV plants.  Buerhop et al. reported that roughly 6% of expected output power is lost as a result of undetected faults in PV modules.  Traditionally faulty PV modules or cells have been located by applying electrical tests to the modules such as the I-V curve test and manual thermography, which are both costly and time-consuming techniques.  It has been well established that solar operations and maintenance (O&M) activities prevent energy losses of PV sites, however, O&M is also one of the highest costs for PV plants.  Additionally, the large size of many of the newer PV plants makes it necessary to develop techniques such as aerial thermography to optimize maintenance activities.  In this post we will discuss the data from a technological review published in Renewable and Sustainable Energy Reviews that concludes “The progress that instrumentation involved in aerial thermography has experienced during recent years has led aerial thermographic inspection to be the most suitable technique to identify underperforming cells at a PV site.”

 

PV modules can present different faults at different stages of their life cycle: manufacturing, transportation, installation and operation.  These faults are responsible for energy dissipation in PV modules, reducing electrical output and generating an abnormal temperature distribution and high stress.  PV faults can be put into three buckets: (1) optical degradation (delamination, bubbles, encapsulate discoloration, glass breakage), (2) electrical mismatch and degradation (cell fractures, snail trails, broken interconnection ribbons, poorly soldered shunts, short-circuited cells, shading) and (3) unclassified faults (Potential Induced Degradation). 

 

In a review of 5 studies conducted using unmanned aerial vehicles (UAV) equipped with thermographic cameras to inspection PV modules the authors conclude that aerial thermography is “an effective, powerful procedure for thermographic fault monitoring at photovoltaic sites, and more reliable, faster and cost-effective compared to traditional methods.”

 

Today’s technology allows for aerial thermographic images and RGB visual images of PV plants to be captured at the same time.  Of interest, in two published studies using aerial thermographic and RGB cameras it was found “that the most frequent faults in ascending order are discoloration/browning, snail trails, hot spots, bypass/disconnect, dirty, shading, cracked cells and oxidation/corrosion.”

 

This study states that “the application of UAVs in thermographic inspection of photovoltaic modules is a major advancement in O&M activities of PV plants.  Thermography presents many advantages compared to traditional tests applied to the modules, such as electrical tests.  Electrical tests allow detection of abnormal underperforming situations but do not recognize the cause or location of the faulty module or cell.  Besides not providing complete information about the defect, it is necessary to shut down the plant during the electrical inspection, which means reducing energy production significantly.  For that reason, thermographic inspection is the most common method to detect faults at photovoltaic plants, being fast and simple to implement and obtaining results in real time, with no need to shut down the plant during the inspection.”  However, “manual thermography presents some significant drawbacks.  It is a costly and time-consuming technique and there are some situations in which it is hard to detect the faulty cells.”

 

The authors continue stating that “the rapid growth of photovoltaic power capacity and the tendency to construct bigger PV sites with higher capacity make development of innovative techniques necessary, such as aerial thermography, so that performing thermography or optimizing maintenance activities is possible.  Therefore, application of aerial thermography at photovoltaic plants needs to be understood not just as an innovative technique to save time and money, but as a necessary technique to be able to assess production.” 

 

  1. Gallardo-Saavedra et al.  Technological review of the instrumentation used in aerial thermographic inspection of photovoltaic plants. Renewable and Sustainable Energy Reviews 93 (2018) 566–579.
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Matt Chester's picture
Matt Chester on May 19, 2020

Buerhop et al. reported that roughly 6% of expected output power is lost as a result of undetected faults in PV modules

Thanks for sharing Ryan-- I'm curious if you have any insights on how the above figure varies over time? Surely it's more of an issue as the years go on, right? Or do these undetected faults start pretty quickly in larger PV installations? 

Ryan Bliss's picture
Ryan Bliss on May 20, 2020

That's a great question Matt, and I have yet to see published data that demonstrates that the rate of power output degradation changes based on the age of the solar cells, although logically that would certainly make sense.  Part of the reason for this may be the fact that many different types of PV faults (optical, electrical mismatch, PID) can occur at varying times in the life of the solar cells (manufacturing, transportation, installation, operation).  The literature reports that the decline in solar power output over time typically ranges between 0.5-3.0% year/year during the life of the solar installation. This is why performing solar inspections (at installation & annually at minimum) is so important to optimize solar power output for utility scale solar plants.  

Buerhop et al. inspected 60 PV plants with an operation time between 0.5 - 10 years using a drone with an infrared camera.  Based on the type and volume of PV faults found and their laboratory analysis (measuring IV-curve, electroluminescence (EL) imaging and IR imaging), they estimated the power output loss to be 6% across all 60 PV plants.  Their study reported that the commonly observed defects found with aerial thermography were: disconnected strings, bypassed substrings and cell defects in crystalline PV modules. Typical cell failure mechanisms were cell breakage, deficient solder joints and shunted cells. The impact of these defects on the module performance, mainly the output power, was of major importance as shown by the electrical simulations.  

I hope this information is helpful.

1.  Buerhop et al. Quality Control of PV-Modules in the Field Using a Remote-Controlled Drone with an Infrared Camera. 27th European Photovoltaic Solar Energy Conference and Exhibition.

 

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