A thermal image of a PV installation shows, in a single pass, which panels, cells or strings are warmer than the rest. That is valuable. But a thermal capture taken in the wrong conditions produces an attractive colour image in which nothing can be seen. Or worse: anomalies that are not there.
The difference between the two comes down to three things you can check in advance.
Why a defective panel gets warmer
A solar cell that is delivering current stays relatively cool. A cell that can no longer deliver current, for example because of a crack, a loose connection or a faulty bypass diode, converts the energy it receives into heat. That cell becomes a hotspot. A complete string that is not working produces a pattern of warm panels side by side.
Those temperature differences only appear when the installation is in operation and receiving enough sunlight. No current, no difference. No difference, no finding.
The three conditions
The technical specification IEC TS 62446-3 sets out when a thermal inspection of a PV installation is reliable. The main conditions:
- Irradiance of at least 600 W/m² on the plane of the panels. Below that, the temperature differences become too small and faults disappear into the measurement noise.
- Little wind. Wind cools the panels and masks hotspots. The standard uses an upper limit of about 28 km/h. We do not fly in an average wind above 8 m/s (about 29 km/h), which almost coincides with it.
- A clear sky. A cloud passing in front of the sun changes the irradiance within seconds. The standard allows a maximum of 2/8 cloud cover.
On top of that, the panels must be clean enough. Bird droppings or accumulated dirt produce a heat signature of their own that looks like a defect.
In Belgium that means: from late spring to late summer, around midday, on a clear day with little wind. A capture in November or under changing cloud can technically be flown, but says little.
What the images do and do not show
A thermal drone capture is strong as a screening tool. In one flight you see the complete installation, and every anomaly gets a location on the layout plan.
It does not show the cause. A warm panel can point to a cell crack, a diode fault, a loose connector, shading or dirt. Classification and the decision to repair belong to your installer or PV specialist, who verifies the anomaly on site with a handheld camera or electrical measurements.
That is why we always deliver three things together:
- The thermal image for each anomaly
- The visual image of the same spot, so dirt and shading can be recognised straight away
- The radiometric files, so your specialist can read out temperatures independently
Together with the weather conditions and the irradiance at the time of capture. Without that information, nobody can assess the measurement afterwards.
When it is worth doing
- At handover of a new installation, as a baseline for the warranty period
- With an unexplained loss of yield that the monitoring cannot locate
- After hail or a storm, to record damage per panel for the insurer
- Periodically on large roofs, where a visual check from the ground shows nothing
For an installation of a few panels on a house, the ratio between preparation and information is less favourable. There, the inverter monitoring is often enough.
What we need from you in advance
- The address and a layout plan or string diagram, if one exists
- Confirmation that the installation will be in operation on the day of flying
- A contact person on site for access to the roof or the site
We schedule the flight on a day that meets the measurement conditions. If the day does not meet them, we do not fly and we move the capture. A cancelled flight costs you a day; a capture in poor conditions costs you a wrong conclusion.
Related service: Roof & Façade, thermal observations on request
Sources: IEC TS 62446-3:2017, Photovoltaic (PV) systems, Part 3: Outdoor infrared thermography of photovoltaic modules and plants.