Solar panel drone inspection: finding hot spots and defects with PV thermography

On a large roof with modules facing different ways and strings of unequal length, a faulty module does not always show up in the production figures. A thermal camera above the panels often makes such faults visible, provided the system is running and the weather cooperates. Below you can read what a thermal drone inspection of solar panels shows, when the measurement is reliable and where it stops.

What does a solar panel drone inspection show?

A thermal camera shows which cells, modules or strings are warmer than their neighbours. In a working module, part of the sunlight becomes electricity; where no current flows out, that energy stays behind as heat.

The IEA PVPS report on infrared and electroluminescence imaging of PV modules (IEA-PVPS T13-10:2018) describes the typical patterns. Hot spots come from the cell itself, such as a manufacturing fault, from glass broken by hail, or from the shade of a bird dropping, a plant, a mast or the row of modules in front. An active bypass diode routes the current around a group of cells, which then produces nothing; the image shows a warmer junction box. With PID (potential induced degradation), it is mainly the modules close to the negative pole of the string that heat up (IEA PVPS).

According to the same report, a cell less than 10 °C warmer is normally not listed separately. Between 10 and 20 °C it should be followed up in later inspections. Above 20 °C the report recommends replacing the module, as it does for a bypass diode that stays permanently active.

When is PV thermography reliable?

In strong sunshine, under a clear sky and with little wind, while the system is producing power as normal. At low irradiance the smallest temperature differences disappear.

The IEA PVPS report recommends irradiance above 600 W/m², no clouds, little wind and a stable ambient temperature. According to the report, a module takes 5 to 15 minutes to adjust thermally to a change in sunshine, temperature or wind, so passing clouds disturb the image. That matters most for disconnected strings, PID and short-circuited bypass diodes: they cause only a small temperature difference, which grows with irradiance. In weak sunshine they are hard or impossible to see (IEA PVPS).

The system has to be under load. In a test included in the report, the PID pattern was visible close to maximum power and disappeared at open circuit. Wind adds its own temperature differences across the array and makes the image harder to read. Glass also reflects clouds, buildings and trees; the usual remedy is to wait or to reposition the camera. According to the report, the emissivity of the glass is about 0.85 when the camera views it at an angle between 60 and 90 degrees.

These inspections have their own technical specification, IEC TS 62446-3. It sets requirements for the equipment, the environmental conditions, the procedure, the report and the qualification of the inspector (IEC). The general conditions for building thermography are covered in what a drone building inspection shows.

What does the ordinary camera show on solar panels?

The ordinary (RGB) camera shows visible damage and dirt on the front of the modules, and helps explain the thermal image.

The IEA PVPS report on failures of PV modules (IEA-PVPS T13-01:2014) calls visual inspection the quickest way to find defects, and effective for, among other things, burn marks, delamination, yellowed encapsulant and junction box failures. The checklist from the IEC module standards, reproduced in the report, includes browning on the front, bent or broken frames and brittle or detached cables. According to the report, broken glass is more than a yield issue: the module's insulation is no longer guaranteed, especially in wet conditions, and the breakage causes hot spots (IEA PVPS).

How much soiling costs in output and how often cleaning makes sense is covered in how often solar panels should be cleaned.

What can a solar panel drone inspection not do?

A drone measures nothing electrical. For current, voltage and the safety of the installation, you need someone who connects measuring equipment to the system.

The I-V curve, the current-voltage characteristic of a module or string, is measured on the installation itself. The IEA report notes that maintenance companies offer imaging inspections in addition to I-V curve measurements and monitoring. The same goes for electrical safety: IEC 62446-1 describes the tests, inspection criteria and documentation used to verify that a system has been installed safely and operates correctly (IEC).

Electroluminescence (EL) is a complementary method. A current is fed through the modules and the light the cells emit is photographed, which according to the IEA report reveals micro-cracks among other defects. Because that light is faint, EL in the field is done at sunset, at night or under a cover. It can be done by drone, but it is a separate measurement.

What do you do with the findings?

Have each finding located on the roof, clean where dirt or shade is the cause, and pass defects on to your installer. The installer decides on repair or replacement.

The IEA report recommends matching every image to the right place on the string layout and following up critical anomalies as soon as possible.

For the warranty, check the terms of your module manufacturer. The IEA report recommends a first thermal inspection at commissioning, because it can later be hard to tell whether damage came from a quality problem or from missing maintenance, such as bird droppings left on the modules for a long time (IEA PVPS).

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