A current clamp connected to a digital oscilloscope was used to monitor
the temporal variation of the electrical current into the DC inverter
inputs. Fig. 1 shows the normalised DC current flowing into one of the
inverter inputs during two different switching regimes, reflecting
relative power output from the system. In Fig.1(a) the current trace
reflects a two-step switching process, whereby the operating point of
the system is switched between MPP, at which 100% of the available
power is extracted and OC, at which the DC current into the inverter and
the extracted power are zero. Note that under OC condition the cells
embedded in the modules emit maximum luminescence intensity, whereas at
MPP about 95% of the photogenerated carriers are extracted, reducing
the luminescence intensity to close to zero. In Fig.1(b) the operating
point is periodically switched in three distinct steps, including an
interim step at which 40% of the MPP current
(Impp ) is extracted from the module, resulting in
approximately 40% of the maximum power being extracted. While not the
focus of this paper, the ability to switch between different operating
points enables calculation of image differences that allow
differentiating series resistance variations from recombination defects,
as demonstrated in the context of daylight PL
imaging
by Bhoopathy et al. .
The switching of the power output from 100% to zero takes place in
< 150 ms, but due to a long dwell time of about 1.5 s the
overall cycle time is about 3 s. This comparatively long cycle time was
chosen intentionally in our proof-of-concept studies reported here as a
conservative setting. We anticipate being able to perform switching at
considerably higher frequencies and with shorter switching times in the
near future, noting though that for residential rooftop applications, as
discussed here, the above switching rate is sufficient to enable rapid
high resolution DPL inspection of all modules of a system within a few
minutes.
Individual camera images were acquired with exposure times between 15 ms
and 20 ms. Image registration was applied to improve DPL image quality,
minimizing DPL image imperfections caused by very significant camera
shake. It is noted that camera shake does not notably affect the quality
of individual camera images, given the above short exposure times.
However, it results in lateral offsets between images, producing
artefacts when difference images are calculated. Image registration
prior to image subtraction eliminates this unwanted effect.
Fig.2 shows two PL images acquired using inverter-based switching on the
residential rooftop system at Wylie’s Baths, as described above. From
the fixed camera position the 50mm focal length lens that was used for
these measurements provides close-up DPL images showing half a module to
one full module, depending on the specific module location on the roof
(i.e. distance to the camera). Each DPL image was calculated from 10
image pairs.