Thermal imaging cameras: a fast and reliable tool for testing solar panels.
Quality assurance is of fundamental importance for
solar panels. The failure-free operation of the panels is a prerequisite
for efficient power generation, long life, and a high return on the
investment. To ensure this failure free operation a fast, simple and
reliable method to evaluate a solar panel's performance is required,
both during the production process and after the panel has been
The use of thermal imaging cameras for solar panel
evaluation offers several advantages. Anomalies can clearly be seen on a
crisp thermal image and - unlike most other methods - thermal cameras
can be used to scan installed solar panels during normal operation.
Finally, thermal cameras also allow to scan large areas within a short
In the field of research and development (R&D)
thermal imaging cameras are already an established tool for the
evaluation of solar cells and panels. For these sophisticated
measurements, usually high performance cameras with cooled detectors are
used under controlled laboratory conditions.
However, the use of thermal imaging cameras for
solar panel evaluation is not restricted to the field of research.
Uncooled thermal imaging cameras are currently being used more and more
for solar panel quality controls before installation and regular
predictive maintenance checkups after the panel has been installed.
Because these affordable cameras are handheld and lightweight, they
allow a very flexible use in the field.
Thermogram with level and span in automatic mode (above) and in manual mode (below).
With a thermal imaging camera, potential problem
areas can be detected and repaired before actual problems or failures
occur. But not every thermal imaging camera is suited for solar cell
inspection, and there are some rules and guidelines that need to be
followed in order to perform efficient inspections and to ensure that
you draw correct conclusions. The examples in this article are based on
photovoltaic modules with crystalline solar cells; however, the rules
and guidelines are also applicable to the thermographic inspection of
thin-film modules, as the basic concepts of thermography are the same.
Procedures for inspecting solar panels with thermal imaging cameras
During the development and production process, solar
cells are triggered either electrically or by the use of flash lamps.
This ensures that there is sufficient thermal contrast for accurate
thermographic measurements. This method cannot be applied when testing
solar panels in the field, however, so the operator must ensure that
there is a sufficient energy input by the sun.
To achieve sufficient thermal contrast when
inspecting solar cells in the field, a solar irradiance of 500 W/m2 or
higher is needed. For the maximum result a solar irradiance of 700 W/m2
is advisable. The solar irradiance describes the instantaneous power
incident on a surface in units of kW/m2, which can be measured with
either a pyranometer (for global solar irradiance) or a pyrheliometer
(for direct solar irradiance). It strongly depends on location and local
weather. Low outside temperatures may also increase thermal contrast.
What type of camera do you need?
Handheld thermal imaging cameras for predictive
maintenance inspections typically have an uncooled microbolometer
detector sensitive in the 8–14 μm waveband. However, glass is not
transparent in this region. When solar cells are inspected from the
front, a thermal imaging camera sees the heat distribution on the glass
surface but only indirectly the heat distribution in the underlying
cells. Therefore, 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 thermal imaging camera used for these
inspections needs a thermal sensitivity ≤0.08K. To clearly visualize
small temperature differences in the thermal image, the camera should
also allow manual adjustment of the level and span.
Photovoltaic modules are generally mounted on highly
reflective aluminum framework, which shows up as a cold area on the
thermal image, because it reflects the thermal radiation emitted by the
sky. In practice that means that the thermal imaging camera will record
the framework temperature as being well below 0°C. Because the thermal
imaging camera's histogram equalization automatically adapts to the
maximum and minimum measured temperatures, many small thermal anomalies
will not immediately be visible. To achieve a high contrast thermal
image continuous manual correction of the level and span would be
Thermal image without DDE (left) and with DDE (right).
The so-called DDE (Digital Detail Enhancement)
functionality provides the solution. DDE automatically optimizes image
contrast in high dynamic range scenes, and the thermal image no longer
needs to be adjusted manually. A thermal imaging camera that has DDE is
therefore well suited for fast and accurate solar panel inspections.
Another useful feature for a thermal imaging camera
is the tagging of thermal images with GPS data. This helps to localize
faulty modules easily in large areas, e.g., in solar farms, and also to
relate the thermal images to the equipment, e.g., in reports.
The thermal imaging camera should have a built-in
digital camera so that the associated visual image (digital photo) can
be saved with the related thermal image. A so-called fusion mode,
allowing the thermal and visual images to be superimposed, is also
useful. Voice and text comments that can be saved in the camera along
with the thermal image are beneficial for reporting.
Positioning the camera: take into account reflections and emissivity
Even though glass has an emissivity of 0.85–0.90 in
the 8–14 μm waveband, thermal measurements on glass surfaces are not
easy to do. Glass reflections are specular, which means that surrounding
objects with different temperatures can be seen clearly in the thermal
image. In the worst case, this results in misinterpretations (false
"hotspots") and measurement errors.
This thermal image shows large areas with elevated temperatures. Without
more information,it is not obvious whether these are thermal anomalies
or shadowing /reflections.
In order not to draw false conclusions you need to hold the thermal
imaging camera under a correct angle when inspecting solar panels.
Thermal image made using a FLIR P660 camera on a flight over a solar farm. (Thermogram courtesy of Evi Müllers, IMM)
In order to avoid reflection of the thermal imaging
camera and the operator in the glass, it should not be positioned
perpendicularly to the module being inspected. However, emissivity is at
its highest when the camera is perpendicular, and decreases with an
increasing angle. A viewing angle of 5–60° is a good compromise (where
0° is perpendicular).
Viewing angle recommended (green) and to be avoided (red) during thermographic inspections.
Angle dependence of the emissivity of glass.
Long distance observations
It is not always easy to achieve a suitable viewing
angle during the measurement set-up. Using a tripod can provide a
solution in most cases. In more difficult conditions it might be
necessary to use mobile working platforms or even to fly over the solar
cells with a helicopter. In these cases, the longer distance from the
target can be advantageous, since a larger area can be seen in one pass.
To ensure the quality of the thermal image, a thermal imaging camera
with an image resolution of at least 320 × 240 pixels, preferably 640 ×
480 pixels, should be used for these longer distances.
The camera should also have an interchangeable lens,
so the operator can switch to a telephoto lens for long distance
observations, such as from a helicopter. It is advisable, however, to
only use telephoto lenses with thermal imaging cameras that have a high
image resolution. Low resolution thermal imaging cameras will be unable
to pick up the small thermal details that indicate solar panel faults in
long distance measurements using a telephoto lens.
Looking at it from a different perspective
In most cases installed photovoltaic modules can
also be inspected with a thermal imaging camera from the rear of a
module. This method minimizes interfering reflections from the sun and
the clouds. In addition, the temperatures obtained at the back may be
higher, as the cell is being measured directly and not through the glass
Ambient and measurement conditions
When undertaking thermographic inspections, the sky
should be clear since clouds reduce solar irradiance and also produce
interference through reflections. Informative images can, however, be
obtained even with an overcast sky, provided that the thermal imaging
camera used is sufficiently sensitive. Calm conditions are desirable,
since any airflow on the surface of the solar module will cause
convective cooling and thus will reduce the thermal gradient. The cooler
the air temperature, the higher the potential thermal contrast.
Performing thermographic inspections in the early morning is an option.
Thermal image of the back of a solar module taken with a FLIR P660 camera. Its corresponding visual image is shown on the right.
Another way to enhance thermal contrast is to
disconnect the cells from a load, to prevent the flow of current, which
allows heating to occur through solar irradiance alone. A load is then
connected, and the cells are observed in the heating phase.
Under normal circumstances, however, the system
should be inspected under standard operating conditions, namely under
load. Depending on the type of cell and the kind of fault or failure,
measurements under no-load or short-circuit conditions can provide
Measurement errors arise primarily due to poor
camera positioning and suboptimal ambient and measurement conditions.
Typical measurement errors are caused by:
- too shallow viewing angle.
- change in solar irradiance over time (due to changes in sky cover, for example).
- reflections (e.g., sun, clouds, surrounding buildings of greater height, measurement set-ups).
- partial shadowing (e.g., due to surrounding buildings or other structures).
Table 1: List of typical module errors
(Source: ZAE Bayern e.V, “Überprüfung der Qualität von Photovoltaik- Modulen mittels Infrarot-Aufnahmen” ["Quality testing in photovoltaic modules using infrared imaging”], 2007)
What can you see in the thermal image
If parts of the solar panel are hotter than others,
the warm areas will show up clearly in the thermal image. Depending on
the shape and location, these hot spots and areas can indicate several
different faults. If an entire module is warmer than usual that might
indicate interconnection problems. If individual cells or strings of
cells are showing up as a hot spot or a warmer ‘patchwork pattern’, the
cause can usually be found either in defective bypass diodes, in
internal short-circuits, or in a cell mismatch.
Shadowing and cracks in cells show up as hot spots
or polygonal patches in the thermal image. The temperature rise of a
cell or of part of a cell indicates a defective cell or shadowing.
Thermal images obtained under load, no-load, and short-circuit
conditions should be compared. A comparison of thermal images of the
front and rear faces of the module can also give valuable information.
Of course, for correct identification of the failure, modules showing
anomalies must also be tested electrically and inspected visually.
This thermal image shows an example of the so-called ‘patchwork pattern’, which indicates that this panel has a defective bypass diode.
These red spots indicate modules that are consistently hotter than the rest, indicating faulty connections.
This hot spot within one solar cell indicates physical damage within the cell.
The thermographic inspection of photovoltaic systems
allows the fast localization of potential defects at the cell and
module level as well as the detection of possible electrical
interconnection problems. The inspections are carried out under normal
operating conditions and do not require a system shut down.
For correct and informative thermal images, certain conditions and measurement procedures should be observed:
Thermal imaging cameras are primarily used to locate
defects. Classification and assessment of the anomalies detected
require a sound understanding of solar technology, knowledge of the
system inspected, and additional electrical measurements. Proper
documentation is, of course, a must, and should contain all inspection
conditions, additional measurements, and other relevant information.
Inspections with a thermal imaging camera - starting
with the quality control in the installation phase, followed by regular
checkups - facilitate complete and simple system condition monitoring.
This will help to maintain the solar panels' functionality and to extend
their lifetime. Using thermal imaging cameras for solar panel
inspections will therefore drastically improve the operating company’s
return on investment
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