This article provides an in-depth explanation of the different thermal cause classifications in our Solar Inspection Pro and Essential data product. If you are looking for an overview of the Solar Inspection Pro and Essential data product, check out the Solar Inspection Pro and Essential data product page.
Thermal Cause Classifications
Hot Spots indicate a defect at the cell level, where one or several cells have a higher temperature than the neighboring ones.
Depending on the temperature difference (temperature delta) between the heated and normal cells, a Hot Spot may indicate a defect of varying levels of severity.
At Sitemark, we distinguish Hot Spots (only one cell appears to be impacted) and Multi Hot Spots (several cells of one panel are unambiguously impacted).
The causes of a Hot Spot can be numerous. Some causes are reversible, while others are not. Thermal images usually aren’t sufficient to identify the causes of Hot Spots, but thanks to the use of normal (RGB) photos, we are capable of identifying the main cause for Hot Spots.
The most common causes come from external elements that would cast a shadow on the panel. As a cell gets covered by shade, it turns into a resistor and will dissipate the energy produced by the other cells in the string. At Sitemark, we identify the cause according to the following categories:
- Soiling of the panels: deposits of dust or dirt, leaves, etc.
- Droppings: Bird droppings, which are frequent at the seaside or in industrial areas that involve food processing plants.
- Vegetation: overgrown grass or plants that grow in front of the first row of panels or between the panels
- Shadowing: external elements casting a shadow on the panels. Classic examples include physical elements bordering sites such as electrical poles, antennas, high tension cables, fences, trees, etc.
- Physical defect: Degraded panels or cells will show Hot Spots, but the underlying reason may not be visible to the naked eye, let alone from drone pictures. These would cover snail trails, micro-cracks, delamination, back-sheet issues, shattered glass, etc.
Depending on the cause, suggested remedial actions will vary:
- Should the Hot Spots be caused by soiling and droppings, cleaning the panels can be sufficient to bring the panel back to their nominal efficiency.
- Vegetation: cutting the grass should be sufficient. However, some defects that are deemed reversible can bring long term damage to the panels if not promptly treated. We estimate that one season will be enough to turn a reversible Hot Spot into a non-reversible one, hence the importance of cleaning and cutting the grass at least once a year.
- Shadowing: as causes of shadowing are usually external elements, no remedial action can be taken in most cases. The impacted panels should be monitored to assess whether the losses are affecting the production of non-impacted panels. The impacted panels may have to be replaced more often if the layout of the plant cannot be adapted to avoid the shadowed areas.
- Physical defects: depending on the severity of the Hot Spot, the age of the plant and the underlying cause, physical defects may justify a warranty claim against the manufacturer. Acceptance criteria for warranty claims differ from one manufacturer to the next, but a commonly used threshold is a temperature delta of at least 20 degrees Celsius between the overheated cells and normally functioning ones.
Overheated cells acting as a resistor, will dissipate the energy generated by the other cells. Such cells will have an impact on the production of the full string of that panel.
We generally assume that the losses will be proportional to the delta temperature of the Hot Spot, relative to the production level of the plant. Note that one Hot Spot on a string is capable of a loss in production of up to 90% of the nominal production of one panel.
Cells in a solar module are organised in substrings, ie. chains of cells connected in series. A typical panel includes three substrings of 20 or 24 cells each (depending on the panel type). To prevent shaded or broken cells from impacting the production of the whole string, these substrings can be bypassed thanks to bypass diodes.
These substrings will then be isolated and won’t produce any energy at all. As a result, they will have a higher temperature than the cells working normally in the same panel. The defect can then be seen on the thermal images with one third, two thirds or even the whole of the panel being warmer than the neighboring ones.
Bypassed substrings are a fail-safe mechanism of the panels. Generally speaking, this is expected behavior when the cause of the shading is visible and the defect disappears when the cause of the shading is fixed.
However, bypassed substrings can be persistent in two main cases:
- The bypass diode is damaged and will always isolate a substring despite the cause having been fixed
- The substring is physically disconnected and can’t let any current through
In most cases, bypassed substrings are reversible defects and will disappear as soon as the cause is fixed. In the event a bypassed substring is present despite no cause being visible, the most relevant fix would be to consider replacing the panel. Depending on the age of the plant and the cause of the issue, manufacturers may cover the replacement of impacted panels under warranty.
As one-third of the panel is disconnected, the loss will be as high one-third of the production of one module.
Panels with several cells at a high temperature can be classified as Multi Hot Spots. However, if all these cells appear to be located on the same substring, one can assume that the cause of the problem lies in the diode of that substring. One will typically see very high-temperature variations between the cells of one-third of the panel, while the rest of the panel has a homogeneous temperature.
This defect will differ from a Bypassed Substring in that all of the affected cells have the same temperature. In the case of a shorted diode, all the cells are hotter, but the thermographic images will show an irregular checkerboard pattern on that substring.
The diode designed to bypass the substring is damaged and will always let current through. Such damage can typically be caused by a very high current going through the panel, due to lightening for example, leading to a permanent malfunction of the diode.
If replacing the diode is not an option, the full panel will have to be changed.
Although complex to accurately calculate, the losses can be estimated to be up to a third of the module’s nominal production, per substring impacted.
Potential Induced Degradation
Panels show a checkerboard pattern on the thermal images. Significant temperature differences are visible between the cells.
This type of defect usually impacts several contiguous panels, and usually at the end of a string, i.e. close to the negative pole of the string.
Potential Induced Defects (PID) are caused by a voltage difference between the cells and the frame of the panels. This difference may cause residual leakage current to flow through the cells and impact the yield significantly.
This effect is thought to be largely impacted by the following parameters:
- Grounding of the panel frames, the poles, etc.
- The choice of the inverter and its working mode, specifically the potential differences between the poles and the ground that the inverter is set at.
- Panel design, including the type of insulation used, the silicon wafer type, mechanical design of the frame, etc.
Not all causes of PID issues can be acted upon, which leaves the operator with a limited choice.
The main remedial actions include:
- Check the grounding of the frames, panel rows and inverters to assess whether the grounding could be improved. A thorough inspection of the impacted strings and rows may lead to some insight into the underlying causes of PID.
- As a short term fix, swap panels from the positive end to the negative end of the string to avoid physical problems developing in the panels over time.
- Consider changing the working mode of the inverters (if possible) to change the voltage difference between ground and inverter poles.
- Consider commercial solutions that inject a very small reverse current at night to counteract the effects of PID.
Leak currents and overheating of cells will dissipate energy that would otherwise be converted by the inverter.
A commonly accepted estimate in the scientific community is that losses per panel impacted may be as high as 30%. PID issues tend to worsen over time.
Suspected Potential Induced Degradation
The definition and threshold for the classification of PID vary throughout the industry. At Sitemark, we not only mark PID but also panels that show an early-stage behavior similar to PID. These show warmer cells close to the edges of the panels, indicating that the cause of the temperature increase can be found in a voltage difference.
Our threshold is that we mark panels that have more than one edge impacted by such an effect.
Similar to PIDs, but with no short term actions recommended. These defects can also indicate the normal wearing of the panels due to the age of the installation. Proper monitoring of the status of these panels is recommended to observe how the situation evolves.
Temperature differences undoubtedly indicate losses, but it is assumed that such panels will cause lower losses than actual mature PID.
One group of panels is warmer than comparable panels by a few degrees, but the temperature is uniform across the impacted group of panels.
The full string is disconnected and thus, gets warmer because the solar irradiance isn’t converted into electricity. This may occur due to several issues such as:
- A blown fuse
- A disconnected cable
- A damaged or burnt welding point
- A physical issue in a connection box or at the inverter
- Any issue that could cause a physical disconnection or prevent current from flowing through the string
Start with a thorough check of the string connections, these remedial actions should be simple.
Since these issues cause extreme losses and are often easy to fix, they should be treated with the highest level of priority.
As a string of panels is disconnected, the losses amount to the production of all of these panels, for as long as the string issue has existed.
Heated Junction Box
It is expected that junction boxes would slightly warm up, because of the high current density it undergoes, but however a high temperature may indicate issues in the connections that will cause losses or even a fire hazard.
Any heat generated in the junction box is thus dissipated through the back-sheet, wafer and glass layers before it can get measured. This information is what we use to try and assess whether a junction box has a problem, or not.
Our assessment of the health of junction box is thus mainly relative, comparing the brightness of junction boxes compared to its neighbouring ones. This is based on the assumption that if a majority of the panels show a warm junction box, they are not likely to be faulty. The increase in temperature can indeed be due to a high irradiance or specific panel design.
Below you can see an example on the left of one junction box being warmer than the surrounding ones indicating a potential problem vs on the right where the temperature is similar across all panels.