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.

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.

This is the energy detected by infrared cameras. The cameras do not see temperatures, they detect thermal radiation.

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 it could be enough. Is up to you to decide.

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 (RaptorMaps, Above, Scopito, …).

Drone survey route plan

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.

Substrings with defect

If the junction box 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.

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, …).

Short circuit defect on solar panel

If you want to see a particular pattern, e-mail us through info@solarud.eu and we can help you identify what you need.

Conclusion

It is a 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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