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Which inspection method do you prefer? (2/3)

There are various inspection methods that can be done to a solar asset. Depending on the dimension and the purpose, you can choose the type of inspection that suits you the most.

Indeed, it is necessary and important to know the solar photovoltaic system condition, both during the construction as the O&M phase.

So, we’ll talk about 3 different inspection methods for solar parks, their benefits, and limitations, namely: I-V Curves, Thermography and Electroluminescence.

Inspection Method – Thermography

What is thermography and how it works?

All objects emit infrared radiation (IR) as a function of their temperature. Infrared energy is generated by the vibration and rotation of atoms and molecules. As so, with the higher motion, the higher the temperature, the more infrared energy is emitted. Thermal imaging, is a technology that captures and displays temperature variations across objects or surfaces. It operates by detecting and measuring infrared radiation, which is emitted by objects based on their temperature.

Thermographic cameras contain specialized sensors that can detect infrared radiation, which is not visible to the human eye. As you may know, all objects emit heat in the form of infrared radiation, with the amount and wavelength depending on the object’s temperature. Thermographic devices collect this emitted infrared radiation from the object’s surface and convert it into a thermal image.

This thermal image is typically displayed with varying colors or shades (pallets), where usually warmer areas are represented by lighter colors, and cooler areas appear as darker colors. Thermographers use these thermal images as inspection method to identify temperature anomalies or specific patterns, and analyze them for various applications in field like electrical maintenance (cabling and connections, electrical switchboards, and solar panels.

The cameras do not see temperatures, they detect thermal radiation.

Drone Thermal Camera

IR is the technique for producing an image of invisible (to our eyes) infrared light emitted by objects due to their thermal condition.

The thermal cameras look like any other camera, but with the capability of record the spectral band from 7 to 14 microns (7,000nm–14,000nm) in wavelength which is where terrestrial temperature targets emit most of their infrared energy.

The thermal camera price range is from 100€ to 10.000€ depending on the type of camera, their functionalities, and their resolution. The best resolution for industrial/commercial applications regarding price/quality is 640×512.

The other thing that you must be aware is that if the camera is or is not radiometric. What is that mean? If you want to know exactly the temperature of each image pixel, you must have a radiometric camera. They record all the meta information regarding the temperature read by the camera and then, with a proper software, you can know exactly the temperature at a given spot.

On the other hand, if the camera does not record radiometric images, you can only see the differences in the color pallet. Depending on the applications and the inspection method quality, it could be enough. Is up to you to decide.

Non radiometric thermal image (by drone)
Inspection Method: Thermography
Radiometric thermal image

The thermographic image you’ve provided serves as an excellent illustration of the impact of soiling, especially near the frame, on solar panels. Soiling effectively hinders the panels’ ability to absorb solar light (photons), which in turn obstructs the photovoltaic effect. Instead of converting sunlight into electrical energy efficiently, the heat generated by the sun accumulates, leading to the formation of hotspots.

One effective solution to prevent such issues is the use of a solar water drainer like Solarud. Solarud’s drainage function helps keep the panels clean and free from accumulated water, dust, and debris. By maintaining the cleanliness of the panels, Solarud contributes to optimal energy production and reduces the risk of hotspots, ensuring the long-term efficiency and reliability of the photovoltaic system.

Thermography of solar plants

The solar modules have a visible temperature visible through thermography that vary with the sun irradiance and with the condition of the panels. If the solar panel is damaged in any way, the temperature is higher in that point than in their pairs. There are a few defects that could be detected by drone thermography, and it is the most effective way to inspect a solar plant as a whole.

It is possible to define a route and a timestamp for the record images and the drone do the rest while you rest. Then you only have to analyze the images or ask to someone able to do that for you. Is there several companies that are specialized in that (RaptorMapsAboveScopito, Sitemark).

Drone survey route plan using Mission Planner

There is a standard to follow for PV drone inspections with the guidelines of good practices to respect. The standard is “IEC 62446-3 TS: Photovoltaic (PV) systems – Requirements for testing, documentation and maintenance – Part 3: Outdoor infrared thermography of photovoltaic modules and plants”.

Between other things, the main rules to perform are the follow:

– the minimum irradiance level should be 600 W/m2. This could be measure with an irradiance sensor at the time of the survey/inspection;

– The GSD should be up to 3cm/px. GSD (Ground sample distance) is defined as the distance between the centers of two adjacent pixels measured on the ground. The standard states a 3 cm length of edge per pixel equals 5 x 5 pixel on a 6“PV cell. With a normal camera the fly high is around 20~25 m above the solar modules;

– The drone speed should be less than 3 m/s;

– The solar panels should be clean before the drone survey;

– The wind speed should be less than 28 km/h as it could affect the thermal measurements;

– Maximum 2 okta of sky covered by cumulus clouds;

– The temperature of the defects should be relative to 1000 W/m2;

– The angle of view, the angle between the module surface and the IR-camera should be 90°; – The results should be analyzed by a thermographic expert (ITC certification is advisable).

Regarding with the defects, the standard classifies 12 different defects that could be found, classified and characterized by drone thermography:

  1. Modules in open circuit (crystalline Si and thin film);
  2. Module in short circuit (crystalline Si);
  3. Crystalline Si module with broken front glass (crystalline Si);
  4. Substring in short circuit (crystalline Si);
  5. 1x Substring in open circuit, loss of connection within module junction box or cell connector (crystalline Si and thin film);
  6. 2x Substrings in open circuit, loss of connections within module junction box (crystalline Si and thin film);
  7. Single cell with difference in temperature (crystalline Si);
  8. Module with cells shaded by dirt (crystalline Si and thin film);
  9. Thin film module with broken front glass;
  10. Transfer resistance at cross- connections of a thin film module;
  11. Transfer resistance at cell connections of a crystalline Si module;
  12. Heated module junction box (crystalline Si and thin film).

Each one of those defects have a class of abnormality that depending on the specific patterns and measured temperatures have to be compared with the examples of thermographic images and differences in temperature (according to the standard’s Annex C). Three classes of abnormalities and their follow up action are then presented. This is important since there might be imminent danger (electric shock or fire) to personal and property.

Classification of defects according to the drone thermographic standard.

This classification varies with the type of defects and the difference of the temperature measured between the defective pattern and the healthy neighbor solar cells. That is why is important to some applications to have a radiometric camera, as we explained above.

It is also important to refer that this classification consider not only the effect on production but also the safety of persons and property. High temperatures could lead to fire or serious burns (a defective solar cell could reach above the 100°C).

How to “read” the patterns?

Depending on the cells or group of cells that are damage, a pattern is formed with a higher temperature than the good cells/panels.

If a solar module has the defects visible on the thermal images shown above, they are single cells with difference in temperature, caused by soiling that could have been prevent with Solarud.

If a substring is hotter than the others, than the diode is by-passing the current and that substring is not working.

Solar panels are typically composed of 60 or 72 solar cells connected in series. Diodes are used to prevent reverse current flow, ensuring that electricity generated by the cells flows in the intended direction, from the cells to the output terminals. Panels are equipped with bypass diodes, which are especially useful in situations where shading occurs. When a section of the panel is shaded or not functioning correctly, the bypass diode allows the current to bypass the shaded area, preventing it from decreasing the overall panel performance. That is why the half cut cell PV modules have such a good fame now. They are able to “split” the solar panel in 6 different areas only with 3 diodes.

Identifying such temperature anomalies through this inspection method is crucial for pinpointing underperforming or malfunctioning sections of a solar panel. It helps solar technicians and maintenance teams locate and address issues promptly, ensuring that the PV system operates efficiently and at its full capacity. This defect is also detected by IV-Curves but using thermography is more efficient regarding the time needed to inspect a large area.

Substrings with defect

If the junction box (where the diodes are) of one solar panel is hotter than the junction box of the modules at their side, there is a problem in it (Heated module junction box). It is advisable to go further on that because it may lead to electric arc and consequently lead to fire.

The elevated temperature within the module junction box strongly suggests an anomaly within that specific panel. This issue could be related to various components within the junction box, including diodes, connectors, or wiring. What makes this a cause for concern is the risk of electrical problems stemming from this overheating.

Of particular concern is the possibility of electric arcs forming within the junction box. Electric arcs are powerful electrical discharges that can result in damage, overheating, and, in the worst-case scenario, fires. Addressing this situation proactively is crucial to prevent any further complications.

If several cells are hotter than others, in a way apparently random, in the solar module, then the problem may be the module in short circuit (or broken). It is also important, as the thermal survey occur, the record of visible images of the same spots. The visible image could say, per si, the origin of the problem (soiling, broken glass, snail trails, backsheet scratch…).

Short circuit defect on solar panel

some thermal patterns observed in solar panels may necessitate further on-site investigation, often in combination with complementary inspection methods, to arrive at a conclusive understanding of a specific problem. While thermographic imaging can provide valuable initial insights, certain issues may require a more in-depth assessment to determine their root causes and potential solutions.

In some cases, despite diligent field inspections and diagnostics, the most suitable course of action may involve contacting the manufacturer. Manufacturers possess in-depth knowledge of their solar modules, having designed and tested them extensively. They are well-equipped to provide guidance and expertise in diagnosing and addressing issues that may arise with their specific product.

Manufacturer support can be invaluable when dealing with complex or unique problems, as they can offer tailored solutions and recommendations based on their expertise. Furthermore, manufacturers may have access to historical data and engineering insights that can aid in problem resolution.

Ultimately, when faced with challenging or persistent issues related to solar panel performance and thermal patterns, collaboration with the manufacturer can be a strategic step towards achieving a comprehensive understanding of the problem and finding an effective resolution. It highlights the importance of maintaining open lines of communication and seeking expert guidance when needed to ensure the continued efficiency and reliability of solar panel systems.

Conclusion: Inspection Method – Thermography

It is one of the best inspection method to know the overall condition of the solar park. It is the fastest and the cheapest way to detect defects in solar modules and identify in thousands of panels which ones that must be replace or followed to see the evolution of the detected defects.

Currently, a drone inspection could be contracted for 100~250€/MWp, depending on the age, location, and layout of the solar field. The report should comply with the standard IEC DTS 62446-3 TS and a certificate thermographer should do the image analysis.

The drone thermography inspection is a good inspection method to reduce the sample to be inspected by I-V curves and visual on foot inspection.

A regular IR inspection should be done in your solar asset to ensure the good condition of the essential energy source production: the solar modules.

All the best,
The Solarud Team

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