Owning a PV system is becoming increasingly popular, with benefits including greater independence from the energy market, energy savings and climate protection. Large-scale producers, such as businesses, trades, and agricultural enterprises with a power output of 30 kWp or more, benefit particularly from good returns that make up for the high costs of installation.

The German government is seeking to significantly speed up the expansion of solar power. Its PV Strategy aims at raising the proportion of PV in Germany’s power mix to over 30 per cent by 2035.1 To reach this goal, PV systems must make full use of their maximum efficiency. However, this is not always the case at present. According to estimates by the German Insurance Association (GDV), around 400,000 of the 2 million PV systems in Germany in 2020 had been installed incorrectly, revealing not only technical defects, but also economic deficits.

Possible causes alongside production faults or damage in transit also include errors in installation and planning. In addition, age-related wear, accumulation of dirt on the panels or weather-related damage can also result in impaired efficiency. When PV modules are connected in a string, one defective cell is all it takes to cause a significant reduction in output.
Faults may reduce the system’s efficiency, the service life and, in a worst-case scenario, even cause a fire. Many of these defects can be easily remedied by, say, replacing defective modules or cleaning panel surfaces. Steps to prevent shading of the solar modules by roof structures should already be taken in the planning stage. During PV system operation, vegetation may have to be cut back regularly.

Click Image to Enlarge

Testing and inspection – before damage occurs

Early identification of deficiencies may eliminate high secondary costs, and even generate additional yield. Along with economic advantages, testing and inspection also serve to identify safety-relevant defects. For this reason, law and technical standards require periodic electrical safety tests of PV systems to be performed. In particular, the accident prevention regulation DGUV V3 and the standards EN 62446 (VDE 0126-23), IEC 62548 and DIN VDE 0105-100/A1 do apply in Germany. Depending on the age of the system and other operating conditions, PV systems may have to be tested and inspected every one to four years.
Experts frequently identify simple defects by means of visual testing performed to evaluate a system’s actual state of repair. Target-performance comparison can be carried out with the help of simulation software; it offers indications of defects that may also impact on the output of the PV systems.

Click Image to Enlarge

By applying the voltage-current characteristic, the software measures the actual performance of the system and compares it to the manufacturer’s specifications. In case of deviations, imaging processes are introduced to provide more detailed information. Defects increase electrical resistance, and thus build up more heat. The resulting hotspots are captured by thermal imaging cameras. Inactive modules, disconnected substrings and performance degradation caused by ageing, i. e. potential-induced degradation (PID), are further anomalies that can be identified using this method, provided an adequate level of current is produced by solar radiation.

Highest precision eliminates loopholes

By contrast, inverse thermography, also known as reverse-current thermography, is weather- independent. It detects even the smallest defects at an early stage. In this “reversed” method, current is fed into the PV systems and the difference in temperature is measured when current flows through the cells. Drones can also be used to capture images. The only other method offering even greater detail is electroluminescence measurement (EL measurement). This method likewise involves feeding external current into the modules. Using special cameras at night, the experts then record the electromagnetic radiation at wavelengths of approximately 1,150 nm. EL measurement thus enables the experts to look inside a solar cell and identify defective bypass diodes, failed cells, micro-cracks and even performance degradation caused by light and elevated temperature induced degradation (LETID). 

Conclusion

Modern test methods such as thermography enable testing and inspection to be performed during operation or outside the system’s regular service hours. EL measurement, for example, is performed at night. In addition, modern test methods do not require modules to be dismantled. The use of drones reduces the need to set up cranes or lifting platforms.
The longer a defect goes undetected, the higher the secondary costs it causes. In this case, planners and installation and maintenance companies benefit from the support provided by recognised testing, inspection, and certification (TIC) companies like TÜV SÜD. Drawing on their technical expertise and using ultramodern technical equipment, they track down safety- and efficiency-relevant defects and identify opportunities for improvement. Combining safety inspections with efficiency checks also pays off for smaller systems, particularly when they are still within the manufacturer’s or installation company’s warranty period.

References
[1] PV Strategy, Federal Ministry for Economic Affairs and Climate Action: https://www.bmwk.de/Redaktion/DE/Publikationen/Energie/photovoltaik-stategie-
2023.pdf?__blob=publicationFile&v=4

Text: MBA, B. Eng. Stefan Veit, Head of Product and Quality Management Electrical Engineering, Team Lead Electrical/Building Technology, TÜV SÜD Industrie Service GmbH 

Images: TÜV SÜD