Thermography for Predictive Maintenance

Technology Insights
Solar energy is one of the cleanest forms of energy.
With increase in power demand and subsidies available from local governments, countries like India are opting more for solar energy; Availability of sun shine and vast land area is an added advantage. With this the number of challenges in operation and maintenance services has also increased. One such challenge is optimal performance of each Photovoltaic module (and other related equipments) for failure free operation, optimal generation and high returns on investment. To monitor the performance of each PV module at a lower maintenance cost is becoming more difficult. Hence, we need a holistic and evidence-based approach to do predictive maintenance, and one such feasible solution is thermography.
The selection of thermal imaging is purely our choice. We can use a hand held Thermal Imaging camera for better detailing or a UAV/Drone for larger imaging, depending on level of analysis we need to do. All PV module have a glass front surface. Thermal measurement of glass surface is not easy due to its thermal emissivity. Glass has a property of reflecting the temperature of things around it. To avoid measurement error, we need to position the thermal imager between 5˚ to 60˚ (keeping 0˚ as perpendicular) which comes best by practice (initially trial and error will help). The emissivity for any type of Solar PV module can be set to 0.95 and reflected temperature to 20˚C in the device for optimal thermal recording. The distance of measurement must be set manually, one or two meters for a hand-held device is best. For correct and informative thermal recording, shadowing and reflections must be prevented. One more way to get better results from thermography is to perform inspection on rear of the module as well, hence avoiding most of the interfering factors.
 Fig.01 Cell heating abnormally 
avi 1 
Fig.02 Insulated cable abnormality inside an Inverter
After recording the warmer area will be clearly visible; if we observe a certain or entire part of a solar panel to be hotter than its neighbor, it indicates an anomaly. Depending on the shape and location, they indicate various problems such as: 
If an entire module is warmer than usual that might indicate interconnection problem
If individual cells or strings of cells are showing up as a hot spot or a warm patchwork pattern, then the cause can usually be found either in defective bypass diodes, in internal short-circuit.
Shadowing and cracks in cells show up as hot patches in the thermal image.
The temperature rise of a cell or part of a cell indicates a defective cell or shadowing.
Thermal images obtained under load, no-load, and short-circuit conditions should be taken for analytical comparison. A comparison of thermal images of the front and rear faces of the module might also give valuable information. The further analysis is performed on software tool provided along with the data obtained and is compared with the data sheet of the module. The module can be observed on hourly basis for two to three days if needed and then conclusion is drawn.
The thermographic inspection of PV modules usually indicates potential defects at the cell and module level or electrical interconnection issues. The inspections can be carried out when Solar Power plant is in operating condition and do not require a system shut down. Thermal imaging cameras are primarily used for capturing abnormalities in the form of heat and does not detect faults. It records data which we must carry out analysis for further conclusions. Classification and assessment of the abnormality, if any detected, requires a sound understanding of solar technology, knowledge of the inspected system and additional electrical measurements. Documentation is a must and should contain all inspection conditions, additional measurements, and other relevant data. This will help to maintain the solar panel functionality and to extend their lifetime. 



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