A
significant practical benefit of inverter-based modulation is the
ability to take DPL images of all modules that are connected to the same
inverter, either sequentially and with high spatial resolution (close-up
imaging), as shown in Fig.2, or simultaneously in a single overview
image, with reduced spatial resolution. The larger field of view
required for an overview image can be achieved either by moving the
camera further away from the target or by using a camera objective lens
with a shorter focal length. Fig.4 shows a DPL image of the rooftop
system that was acquired with a 12.5mm focal length lens. The
acquisition of the full roof in a single DPL image could not be achieved
in this study, simply due to limitations with respect to the positioning
of the camera using our improvised camera mount (i.e. the camera could
not be moved further away). The image contains a few artefacts,
including the effect of optical reflections of surrounding trees that
were moving in the wind (green arrow in Fig.4). Note that reflections of
stationary objects do not cause such artefacts, since they cancel out in
the image difference calculation. We also observe an artefact associated
with the shadow cast by the (shaking) mounting pole and the camera (blue
arrow). The latter effect causes the top third of one module to appear
darker. The shading effects by the camera can be avoided by more careful
placement of the camera, in particular, when images are acquired using
an RPA (as demonstrated in the following section). Artefacts associated
with objects such as plants, buildings or other architectural features
are generally not a concern for large utility-scale systems and may be
avoidable in applications on residential PV systems with careful
planning and timing of experiments.
Utility-scale System
Utility-scale inverter-based DPL imaging was demonstrated at a
149 MWDC solar farm in Australia in October 2023. The
farm became fully operational in March 2021, utilising 400 W monofacial
half-cell modules on single-axis trackers. The modules feature PERC
solar cells with a nominal open-circuit voltage of 690mV per cell
according to the module specifications. The farm uses 2.75 MW SMA
central inverters (Sunny Central 2750-EV), with approximately 9,000
modules connected to each inverter. For the proof of concept of
large-scale DPL image acquisition using inverter-based modulation
reported here, only one 2.75 MW inverter was switched. As a precaution,
only <60% of the inverter’s rated DC power was switched at
any time (e.g. between 90% and 30% of maximum power) to avoid
potential risks of thermal cycling on the power electronics. DPL image
acquisition was achieved using the same InGaAs camera (1280 x 1024 pixel
resolution) that was used for the data acquired on the rooftop section
discussed above. The camera was mounted on a customised DJI M600
hexacopter RPA, the latter running custom image acquisition software on
an on-board computer. A Gremsy gimbal was used to stabilise the camera,
enabling image acquisition with up to 50 ms exposure time for the
individual image without significant loss of image sharpness due to
camera shake. Images were acquired using a 50 mm focal length objective
lens equipped with the same bandpass filter described above.