Background and Motivation

There is a growing consensus that the exponential growth of the cumulatively installed photovoltaic (PV) capacity will continue unabated over coming decades. A large group of experts have forecast that sustained 25% compounding annual growth will result in 70-80 TWDC of installed PV capacity by 2050, to be supported by 3-4 TWDC of annual module manufacturing and module installations . The global annual electricity consumption in 2022 was estimated by the International Energy Agency (IEA) to be 28,777 TWh. For comparison, in the above scenario the electricity generated from PV alone will reach an estimated 130,000 TWh per annum, thus exceeding today’s total worldwide annual electricity generation and consumption by more than four times. Consequently, PV will rapidly become the main source of global primary energy over coming decades.
PV modules based on single-junction crystalline silicon solar cells dominate industrial manufacturing and commercial PV deployment today and will remain the dominant technology for the foreseeable future, possibly over coming decades . Power production yield from PV power plants can be affected by module quality and integrity in multiple ways. These include manufacturing issues, damage occurring during transport and installation, degradation during operation under harsh environmental conditions and catastrophic weather events such as hailstorms, among others. With industrial silicon modules now rapidly approaching their theoretical efficiency limit, longevity and module quality are more important than ever since re-powering of solar systems and early retirement of PV modules are no longer economically viable options . For these reasons, systematic in-field testing and monitoring are of increasing importance.
Electroluminescence (EL) and photoluminescence (PL) imaging  emerged around 2005 as powerful diagnostic tools for device and process optimisation, process monitoring and quality testing. PL imaging is now used routinely for PV research and development purposes, whereas EL imaging has become an integral part of quality testing in high-volume cell- and module manufacturing. A wealth of information about material and device defects is contained in luminescence image data, making this measurement principle an ideal candidate for field inspection of modules in operating solar systems. Moreover, since specific information about the presence of defects or degradation that is contained in luminescence image data cannot be gleaned in any other fashion, we are of the firm view that luminescence imaging must become an integral, potentially dominant part of routine field testing, complemented by other inspection methods such as thermal imaging , visual inspection and performance data analysis .
Monochromatic or narrow-band light sources such as lasers or light emitting diodes are commonly employed for laboratory PL imaging applications. Alternatively, and only for outdoor applications during daytime, the sun itself can be used as the excitation source in so-called daylight PL imaging (DPL) applications. A significant technical challenge for DPL imaging, however, is the separation of the luminescence signal emitted by crystalline silicon solar cells from the (typically orders of magnitude more intense) reflected sun light.
An elegant technical solution to this problem is based on PL image acquisition within an ultra-narrow atmospheric spectral absorption band in which incident sunlight is almost completely blocked by water vapor absorption and which coincides spectrally with the room temperature emission from crystalline silicon . Other solutions are based on a lock-in type approach, whereby the emitted luminescence is modulated by manipulating the electrical operating point of modules, for example between open circuit (OC) and the maximum power point (MPP). The PL image is obtained as the difference between two (or more) images, which is the approach we also take in this study.
Different methods to achieve the required variation of a module’s electrical operating point have been demonstrated, including the electrical connection of specialised switching electronics to the module terminals, non-contact module-level optical modulation and non-contact string-level optical modulation. A topical review of these methods was presented in .
Using the PV inverter as a means of rapidly switching between different operating points offers an additional solution. Vukovic et al. demonstrated DPL image acquisition during IV curve sweeps, which some residential inverters commonly perform in certain intervals to determine the global MPP . However, the operating point of a PV string or array can also be deliberately changed via the PV inverter, which allows the acquisition of daylight PL images in a more controlled way . Preliminary results from a demonstration of controlled variation of the operating point, achieved with a customised inverter, and performed on a small test set-up, were recently presented by Koester et al. .
Here, we demonstrate DPL image acquisition using controlled inverter switching on operational PV systems and on a much larger scale, whereby the operating points of all modules connected to an individual inverter are actively manipulated. The method is demonstrated (i) on a 12 KWDC rooftop system located in Sydney, and (ii) on a 2.75 MWDC central inverter in a 149 MWDCAustralian utility-scale solar farm. Both sets of experiments were conducted using SMA inverters, without modifications to the hardware or firmware. Actively controlling the inverter requires knowledge about specific inverter settings and capabilities. Modulation settings and associated safe operation limits as well as details of the required communication protocols and access permissions were discussed with the inverter manufacturer (SMA) prior to all experiments presented here. This is a requirement for future experiments of this nature. Photoluminescence image acquisition in the utility-scale solar farm was achieved from a remote piloted aircraft (RPA), which we consider to be the most practical and economical solution for rapid DPL inspection of solar assets.