With advancements in drone technology allowing for more and more efficient data capture and thermal analysis platforms becoming more sophisticated, thermography has become a very popular method for inspection solar sites.
This article will cover some of the basics of how Thermography works on solar sites, some of the major benefits of using this inspection method as well as some of the considerations to take into account when viewing the results.
How does Thermography work?
Using specialised equipment we can capture the infrared energy that is emitted by objects. Then using the specific criteria of the object you are trying to measure these measurements will be converted into the surface temperature of the object.
In the example of solar panels, the thermal camera is used to measure the infrared energy that is being emitted from the panels. In a normal working panel the heat signature across all of the cells will be uniform. However, due to the nature of certain malfunctions in the panels they begin to produce excess heat. Depending on the pattern and intensity of these malfunctions these can be categorised into different anomaly types (Hotspots, Bypassed substrings, PID etc).
You can learn more about the different anomalies and their heat signatures in this support article.
Benefits of Thermography
There are many different methods to inspecting solar panels, each with their own benefits and considerations to make. There are a few highlighted below:
Non invasive inspection method, meaning there is no need to impact the wiring or production of the panels on site. There are obvious financial and safety benefits to this.
Efficient method of inspecting large areas, with advances in drone technology and AI inspecting huge scale solar sites becomes easier and faster.
Shows the site as is with no bias to layout assumptions, whether that highlights incorrectly configured strings or malfunctioning trackers.
Considerations for Thermography
As mentioned above there are also important considerations to keep in mind when looking at thermographic data:
A thermal image will provide a snapshot for what is happening on site at that moment in time. Therefore if there is shading across a panel at that moment in time you will see the issues related to that, but as the sun changes position these issues tend to also pass.
Solar panels need to be connected and functioning for thermography to work. This can also include string issues. For example if there are issues with the individual panels (Hotspots etc) because of the string issue they will not produce a heat signature and will not appear in the thermal imagery.
It can be difficult to compare results from thermographic inspections as environmental conditions change between inspections. You can learn more about normalised weather conditions and how we can make it easier to compare results in the Irradiance section below.
Radiometric vs. Non-radiometric
An important consideration to make is the difference between Radiometric data and non-radiometric data. While the radiometric camera models are typically more expensive, they are capturing an actual temperature measurement for every pixel in each photo. Where as a non-radiometric camera will capture a visual comparison of the temperatures in each photo. This means you will not be able to make any accurate temperature measurements or comparisons between photos using non-radiometric cameras. For this reason Sitemark only works with Radiometric data.
What is irradiance and why is it important for thermography?
Solar irradiance is the amount of energy received in a certain area, the unit that we use for these measurements is Watts per square meter. In solar, the higher the irradiance the more energy that the panels will produce.
There are a number of factors that can effect this from day to day, such as cloud coverage, panel orientation, time of day and more. On top of this there will also be seasonal changes throughout the year as the Earth moves closer and further away from the sun. This will drastically reduce the amount of energy produced by the panels each day.
Why is this important for Thermographic inspections? As we discussed earlier thermal cameras are detecting the infrared radiation emitted by the panels. This energy is provided by the sun, the greater the irradiance the more energy either produced by the panels or in the case of issues emitted as heat.
This can make comparing anomalies between inspections a little tricky. If there are different conditions when capturing the data, the anomalies may appear more or less intensely. If there is higher irradiance during one inspection the hotspots will likely have a higher delta temperature and the issue might appear more severe than when previously inspected. For this reason, the platform can normalise the anomaly information to 1000W/m² to compare apples with apples.
You can find more information about normalised weather conditions in this support article.
Our typical standard for flying thermographic inspections is 600W/m² which is also the requirement for the IEC standard. This level of irradiance provides a good balance between providing enough energy to easily see anomalies and operational flexibility when it comes to capturing the data. This is also why we typically have a solar season where it is much more viable to collect thermal data effectively.
Temporary vs. Permanent Anomalies
As we discussed earlier in the article, it's possible that when conducting multiple inspections, an anomaly may appear in one inspection but not another. Most of these are caused by changes in inspection conditions or are easily explained (time of day, time of year etc). Some examples include shading or vegetation that has been cut back.
However there are some anomalies that may appear in one inspection but not another or even a field inspection where the cause of this is not as obvious. Some typical examples are a bypassed substring, at the moment of inspection it could be correctly bypassing an issue but when inspecting in the field it has returned to functioning correctly.
There are some more interesting cases that we see that typically show up as hotspots or multi-hotspots where there is a physical internal issue that will appear on one inspection but not another. It's worth noting that this can also be observed for issues such as string issues, combiner box issues, inverter issues, open modules, PID, junction box issues, bypassed substrings, diodes etc. Where we typically see these temporary anomalies is with low delta temperature anomalies. For example a hotpot or multi-hotspot with a delta temperature of less than 2-3℃. It's important to monitor these anomalies in the next inspection to see if they persist but it's not alarming if they are not apparent at all.
These types of issues can happen with changing irradiance levels, voltage mismatch between parallel connected strings or any potential clipping or curtailment issues to name a few. Our recommendation is to monitor the issue to see if it appears in the next inspection. You can find more about our remedial action recommendations, you can check out this support article.