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.