Detecting defects on solar cells

Thermal imaging allows anomalies to be seen clearly and can be used to scan installed solar panels during normal operation. It is a time-efficient process as a large area can be scanned in minutes.

Ambient and measurement conditions

A few guidelines need to be observed for this process. Fundamentally there must be sufficient energy from the sun to achieve good a thermal contrast for accurate thermographic measurement; a solar irradiance of 500W/m2 or higher is needed and, optimally, 700W/m2.  

Ideally the sky should be clear as clouds reduce solar irradiance and produce interference through reflections.  However, informative images can still be obtained with an overcast sky if the chosen camera has sufficient thermal sensitivity. Calm conditions are desirable as airflow on the module’s surface will cause convective cooling, reducing the thermal gradient. The cooler the air temperature, the higher the potential thermal contrast, so early morning inspection is the best option.

Choosing a camera

Handheld thermal imaging cameras typically have an uncooled microbolometer detector that is sensitive in the 8-14µm waveband.  However, glass is not transparent in this region. So when solar cells are inspected from the front, a thermal imaging camera sees the heat distribution on the glass surface but only indirectly the thermal performance of the underlying cells.

As a result, the temperature differences that can be measured and seen on the solar panel’s glass surface are small. In order for these differences to be visible, the camera chosen needs a thermal sensitivity of <80mK. It should also allow manual adjustment of the level and span function to optimise visual contrast.

As the camera’s histogram equalisation automatically adapts to the maximum and minimum measured temperatures, many small thermal anomalies will not be immediately visual. With manual correction of level and span however, clear contrast can be achieved.

Digital Detail Enhancement (DDE) is also a helpful function as it automatically optimises image contrast in high dynamic range scenes.  A thermal imaging camera with this ability is therefore well suited to fast and accurate solar panel inspection.

Another feature that should be considered is Multi Spectral Dynamic Imaging – MSX technology.  This technology takes detail from the visual image to improve the thermal image. It makes it easier for the operator to see the problem in even greater detail. Solar panel inspection with MSX is therefore quicker and more effective, reducing time and cost.

On site considerations 

The emissivity of a material is the relative ability of its surface to emit energy by radiation.  It is therefore vital that this value is factored in to any thermal measurement and professional thermal imaging cameras will allow this to be pre-programmed.

As with all highly reflective material, the glass on a solar panel requires particular attention as any thermal image of its surface will also pick up the radiated temperature of surrounding objects including the camera and its operator. In the worst case, this results in false hotspots and measurement errors. By adjusting the viewing angle these problems can be minimised or avoided and for this purpose a tripod will prove a useful accessory.

Image analysis

The shape and location of hotspots on the thermal image will indicate a variety of faults.  If an entire module is warmer than usual interconnection problems should be suspected. When individual cells or strings of cells are abnormally hot or shown as a warmer patchwork pattern, the cause can usually be found either in defective bypass diodes, internal short circuits or a cell mismatch.

Shadowing and cracks in cells are evidenced by hotspots or polygonal patches in the thermal image.  And the temperature rise of a cell or of part of a cell may indicate a defective cell or shadowing.  

Thermal images obtained under load, no-load and short circuit condition should be compared.  And if the front and rear faces of the module have both been inspected, these should be associated too, although temperatures obtained from the back may be higher as the cell is not covered by a glass surface.  

It should also be emphasised that classification and assessment of the thermal anomalies require a sound understanding of solar technology, the system under inspection and additional electrical measurements.