Inspection of Photovoltaic Panels with Thermal Imaging Camera

Thermography is a non-invasive inspection technique that can be performed remotely over large areas and provides immediate feedback; because of these characteristics, it has long been used to detect anomalies in photovoltaic panels.

Thermal camera inspections can be conducted under normal plant operating conditions, during testing at scheduled intervals, or during periodic activities, e.g., panel cleaning.

The first important step in obtaining good results is to choose the most appropriate instrument according to the activities to be performed. The choice of the most suitable thermal imaging camera for PV applications is mainly constrained by the distance from which one can work safely while obtaining sufficiently detailed images. The farther one moves away from the surfaces to be inspected, the higher the resolution of the thermal camera must be in terms of the number of Pixels and low IFOV.

What is IFOV? IFOV (Instantaneous Field Of View) indicates the size of the single pixel of the thermal image at a certain distance; the smaller its value, the more detailed the image. IFOV is generally expressed in mrad (milliradians); for example, the projection of one pixel of a thermal camera with IFOV equal to 1 mrad at a distance of one meter covers the area of 1mm on each side.

To simplify calculations of the performance of each thermal imaging camera, the free 'Spot Size Calculator' software can be consulted on the Hikmicro website, which allows the user to calculate the size of the individual pixel at a certain distance or to know at what maximum distance from the panel the minimum conditions required in terms of resolution are met. For photovoltaic panels, at least 5x5 pixels are required on each individual photovoltaic cell, as per the IEC TS 62446-3 technical specification.

For example, I want to know at what maximum distance I can operate a Hikmicro model M20 thermal imaging camera to inspect a photovoltaic panel with 15cm cells on each side. With the Spot Size Calculator software, I simply select the thermal camera model (possibly the optics if the model has more than one lens), the size of the measurement spot (in our case, 5x5 pixels on a 15cm x 15cm cell), the software calculates the maximum distance from which I can work (17.25m) and the field of view (FOV 7.81m x 5.81m) of the entire thermal image at that distance.

Therefore, it is necessary to always work within the maximum distance that ensures that the thermal imaging camera can cover each cell with at least 5x5 pixels.

IEC 62446 defines the procedures for inspecting PV panels with a thermal imaging camera and the weather conditions within which to conduct inspections are also given in addition to the technical characteristics of the thermal imaging cameras (all Hikmicro thermal imaging cameras have the required minimum characteristics).

When conducting inspections on photovoltaic panels, it is always a good idea to consider the following:

• Always avoid reflection of the sun in the frame
• Have a minimum irradiance of 600 W/m^2 during the inspection
• Avoid wind conditions with wind speeds above 28 km/h
• Always clean surfaces (avoid foreign bodies, e.g., guano) that could later be interpreted as false positives
• Always conduct inspections with a camera angle greater than 30° (ideally between 45° and 85°)
• Set the emissivity of the (clean) glass generally between 0.85 and 0.90

Some defects that can be detected on the panels are:

• Open circuit modules
• Short-circuited modules (Crystalline silicon)
• Crystalline silicon modules with broken front glass
• Shorted strings (crystalline silicon and thin film)
• Strings with defective connections (crystalline silicon and thin film)
• Single defective cell (crystalline silicon)
• High resistive connection points.

For more details and information, reference should always be made to IEC TS 62446-3.