Conference PaperPDF Available

CHARACTERIZATION AND LONG TERM STABILITY ANALYSIS AT PHOTOVOLTAIC MODULES WITH SHINGLED CELL STRINGS

Authors:
  • PI Photovoltaik-Institut Berlin AG

Abstract

In the last recent years the photovoltaic module design becomes more and more in the research focus to increase the module efficiency. Thereby the optimising of cell interconnection plays a major role since it affects the serial resistance and the power output at higher irradiation levels. A novel interconnection approach is the shingling of solar cell. In this work we characterised and studied the long term stability of commercial shingled pv modules. For its we created a special test sequence for shingled panels. Our results are focused on the low-light performance and compared that to outdoor measurements. As well as the mechanical stability by applying different stress tests. This paper ends in a weak-point-analysis of currently available shingled pv panels with different electrical layouts.
37
th
EU PVSEC, 07 - 11 September 2020, Online Conference
CHARACTERIZATION AND LONG TERM STABILITY ANALYSIS AT PHOTOVOLTAIC MODULES
WITH SHINGLED CELL STRINGS
Stefan Wendlandt, Mussi Gebrelul, Sophie Heller, Peppino Dörder and Lars Podlowski*
PI Photovoltaik-Institut Berlin AG, Wrangelstr. 100, D-10997 Berlin, Germany
Phone: +49 (30) 814 52 64-0, e-mail: wendlandt@pi-berlin.com
*Now with: Solyco Technology GmbH, Baseler Strasse 60, D-12205 Berlin, Germany
ABSTRACT:
In the last recent years the photovoltaic module design becomes more and more in the research focus to increase the
module efficiency. Thereby the optimising of cell interconnection plays a major role since it affects the serial
resistance and the power output at higher irradiation levels. A novel interconnection approach is the shingling of solar
cell. In this work we characterised and studied the long term stability of commercial shingled pv modules. For its we
created a special test sequence for shingled panels. Our results are focused on the low-light performance and
compared that to outdoor measurements. As well as the mechanical stability by applying different stress tests. This
paper ends in a weak-point-analysis of currently available shingled pv panels with different electrical layouts.
Keywords: reliability, interconnection, shingled
1 INTRODUCTION
Shingled modules have the potential to reach high cell-
to-module (CTM) values [1]. This kind of interconnection
allows more creativity by engineering the electrical
layouts. On cell level the cut processes should be
understand and developed to reduce the cut edge
recombination [2]. For its two technologies are currently
under development. These are called Thermal Laser
Separation (TLS) and Lasering and Cutting (L&C). Also a
higher hot spot risk was report in the event of shadowing
at some electrical layouts [3]. The quality key at shingled
designs lies in the properties of the Electrically Conductive
Adhesives (ECA). It has to be conductive and mechanical
flexible at the same time. Therefore we did investigations
on that at different commercial modules.
2 EXPERIMENTAL
2.1 Test Samples
The investigations we has been done on five
commercial available full size pv modules with shingled
solar cell strings. The general information about the panels
are shown in Table 1. However, detailed information of
the BOM’s and process parameter are unknown.
Table I: General information of the tested modules.
Brand
“A”
Brand
“B”
Brand
“C”
Brand
“D”
Brand
“E”
P
max
Label 320 W 330 W 340 W 335 W 365 W
Frontend
ARC-
Glass
ARC-
Glass
ARC-
Glass
ARC-
Glass
ARC-
Glass
Enc.
unknown unknown unknown Unknow
n EVA
Cell
Type
mono c-
Si PERC
mono c-
Si PERC
mono c-
Si PERC
mono c-
Si PERC
mono c-
Si PERC
Shingled
cell cut
1/5 1/5 1/5 1/5 1/5
Intercon-
nection
serial-
parallel
serial-
parallel
serial-
parallel
serial-
parallel
serial-
parallel-
serial
Backend
black
Backshe
et
black
Backshe
et
black
Backshe
et
white
Backshe
et
black
Backshe
et
Module
Size
1.0 x 1.7
1.0 x 1.6
1.0 x 1.6
1.0 x 1.7
1.1 x 1.6
The electrical layouts of the different brands are
displayed in Table 2.
Table II: Electrical layouts of the modules.
Electrical Layouts
Brand “A” & “D”
Brand “B” & “C”
Brand “E”
2.1 Test Sequences
The following test sequence (Figure 1) was developed
and applied to characterize and study the long term
stability of the shingled modules.
37
th
EU PVSEC, 07 - 11 September 2020, Online Conference
Simulation of weak
light influrence
a) Berlin / Germany
b) Sevilla / Spain
STC + Weak
Light
EL@ 1.0x Isc
EL@ 0.1x Isc
Bending
Energy Yield
Monitoring
ML (2400 &
5400 Pa)
Figure 1: Applied test sequence.
Four modules per brand were initial characterized
(STC & Weak light behaviour, EL). Afterwards one
module per brand was outdoor installed for performance
comparison and another one mechanical stress tested.
Therefore the module centre was blended for 10 cm and
then a mechanical load stress test according the IEC were
applied. To determine the changes in the quality IV-
Curves and EL images were initial and after each test
taken.
3 RESULTS
The weak light behavior of the different brands is
shown in Figure 2. There the average of four modules per
brand is displayed.
Figure 2: Weak light behavior of shingled modules.
The rel. efficiency losses to 1,000 W/m² were
determined on the base of low light measurements at 700,
400, 200 and 100 W/m². The standard deviations between
the four modules per brand is negligibly small. So that
there is a uniform quality. The weak light behavior can be
divided into two groups:
a) Group 1: Brand “C”
b) Group 2: Other brands
By studying the curve of Brand “C” it can be supposed
that it this type has a higher series resistance which results
in a better weak light behavior compared to the other
brands. At that point it is speculative to say what the
reasons are for the higher Rs. Of course it can shingled
specific properties like the conductibility of the ECA, the
geometry size of the ECA contacting (punctual or linear),
cut edge recombination related or it relates to general
aspects of the solar cell / module. However, a clear trend
that it has do with the length of the strings couldn’t find
since brand “B” has the same electrical design as brand
“C”.
The ranges between that measuring points in Figure 2
were fitted with the following model [4]:
   



 (1)
This fitting were necessary to calculate the European
η
Euro
and California Energy Commission η
CEC
efficiencies.
These are defined as:
!"  #$#% & '( ) #$#* & +,( ) #$% & -,( )
#$# & .,( ) #$/0 & ',( ) #$1 & +,,( (2)
2 2  #$#/ & +,( ) #$#3 & -,( ) #$1 & .,( )
#$1 & ',( ) #$3% & 4'( ) #$#3 & +,,( (3)
The η
Euro
is an averaged operating efficiency over a
yearly power distribution corresponding to middle-Europe
climate, while the η
CEC
is for climates of higher irradiation
intensity corresponding to California climate like US
south-west regions.
Figure 3: STC, European and California Energy
Commission efficiencies.
Based on the different efficiencies definitions, it can
be conclude that all brands except brand “C” are more
suitable for clear sky regions. To see how comparable
these data are to realistic outdoor situations - the modules
were outdoor for energy yield measurements installed. The
environmental conditions of a clear sky day are shown in
Figure 4.
-14
-10
-6
-2
2
0 200 400 600 800 1000
rel. efficiency loss / %
Ee/ (W/m²)
Brand "A" average
Brand "B" average
Brand "C" average
Brand "D" average
Brand "E" average
Tested samples per brand = 4
Group 1: Rs
Group 2: Rs
0%
5%
10%
15%
20%
25%
Module A Module B Module C Module D Module E
Efficiency / %
ηEuro ηCEC ηSTC
37
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EU PVSEC, 07 - 11 September 2020, Online Conference
Figure 4: Global irradiance on module level and
ambient temperature of a clear sky day (16/08/2020) in
Berlin / Germany.
The environmental profiles were takes with an interval
of dt = 5 sec. Figure 4 shows the quasi perfect sky clear
irradiance profile. The average day temperature was
T
amb
= 30°C.
Figure 5: Specific energy yield [kWh/kWp] relativized
to Brand “E” for a clear sky day in Berlin Germany
(16/08/2020). The modules are relativized to Brand “E”
because it reached to highest specific energy yield on
that day.
The yield results of the clear sky day presented in
Figure 5 are comparable to the η
CEC
efficiency ranking in
Figure 3. Brand “E” has the highest yield at sunny days.
However, to study the impact of the different weak lights
on the energy yield for year, the electrical and thermal
module properties were implemented into PVsyst and the
performance was simulated for Berlin / Germany (diffuse
location) and Seville / Spain (clear sky location). The
simulations were done for brand “C” and brand “A”.
Figure 5: Simulated specific energy yield for Berlin /
Germany (diffuse location) and Seville / Spain (clear
sky location).
Figure 5 shows as expected that the specific energy
yields are higher for brand “C” independently of the
diffuse rate. Due to the different thermal module properties
it isn’t clearly that the yield differences are the based on
the weak light behaviour or one other effects. Therefor a
more detailed study were done (Figure 6).
Figure 6: Thermal and weak light related losses for
Berlin / Germany (diffuse location) and Seville / Spain
(clear sky location).
Figure 6 displays that at both locations the thermal
losses are higher than the weak light related ones. In Spain
the thermal losses are even higher than for Germany. Also
it becomes clear that brand “C” has lower weak light losses
at both locations. Finally, the better weak light of brand
“C” gets most visible by comparison the German vs Spain.
Between both locations, there is a factor of 3x in the weak
light related losses.
As in Figure 1 displayed we applied the bending test
before we did the mechanical test according IEC 61215
standard (5x 2400 Pa and 1x 5400 Pa) to one of modules
per manufacturer. The bending test isn’t part of the IEC
classification but it given an information about the
flexibility as well as the conductibility of the
-5
5
15
25
35
0
200
400
600
800
1000
5:31 7:55 10:19 12:43 15:07 17:31 19:55
Ambient temperature / °C
Global tilted irradiance / (W/m²)
global irradiance
ambient temperature
16/08/2020
Berlin / Germany
-0,5%
-1,2%
-1,0%
-0,4%
-3
-2
-1
0
Brand "A" Brand "B" Brand "C" Brand "D"
rel. specific energy yield Brand / Brand "E"
[%]
16/08/2020
Berlin / Germany
1.050
1.804
1.064
1.840
0
500
1000
1500
2000
Berlin, DE Sevilla, ES
Specific Energy Yield / (kWh/kWp)
Brand "A"
Brand "C"
-4,2
-10,0
-3,8
-9,0
-2,8
-1,3
-2,0
-0,6
-12
-10
-8
-6
-4
-2
0
Berlin, DE Sevilla, ES
power losses / %
Brand "A": temperature related losses Brand "C": temperature related losses
Brand "A": weak light related losses Brand "C": weak light related losses
37
th
EU PVSEC, 07 - 11 September 2020, Online Conference
interconnection. During the test condition the module
centre is bended 1 cm while its stands in the flasher and
the IV-curve and the EL image is taken.
Brand “A”
Initial EL
EL @ Bending
EL after Bending
EL after ML
Brand “B”
Initial EL
EL @ Bending
EL after Bending
EL after ML
Brand “C”
Initial EL
EL @ Bending
EL after Bending
EL after ML
Brand “D”
Initial EL
EL @ Bending
EL after Bending EL after ML
Brand “E”
Initial EL
EL @ Bending
37
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EU PVSEC, 07 - 11 September 2020, Online Conference
EL after Bending
EL after ML
Figure 7: EL images (tested at I
sc
) at different test
stages: a) Initial, b) at bending, c) after bending, d)
after ML
The detailed analysis of the images could observed the
following typical patterns at shingled modules.
Figure 8: EL images of a locally interconnection
problem and cell mismatch due to different cell sizes.
At brand “D” we could observed a higher EL signal at
the pseudo rectangular cell compared to the full
rectangular ones. The higher signal is based on a higher
current density at this cells and results in electrical
mismatch of cells at normal operations. As well as into a
lower CTM-value. We also could detect a typical pattern
of a local electrical interconnection problem of shingled
modules. An example of that kind pattern is exemplary
shown in Figure 8 (right). There the part with bad
interconnection has a lower EL signal.
The initial images of Brand “A” shows micro cracks at
the corner cells. This could be a sign for a mechanical
impact at the transport or handling of the modules.
Figure 7: EL images of a module with mechanical
impact (possible a transport or handling related
impact).
The further pictures in Figure 8 show the EL images at
different test stages. At all modules we noticed only a
negligible mechanical impact of the bending test.
However, a major impact of the ML-Test could detect on
the module designs with long cell strings (Brand “A” and
“D”). There the force impact is quite clear due to the
position of the broken cells visible. At the modules with
the short strings (Brand “B” and “C”) there is quasi no
influence visible. Brand “E” has some broken cells in the
middle at the short edge. The not changed EL images at
the modules with the short cell strings can be described
with the force absorption of the ECA and / or the
encapsulation material due to its resilience and the short
edge length of the cells in direction of the force impact.
We want to point out that the limiting stress factor is the
ECA and the encapsulation material tensile strength. Due
to the brittleness of the silicon and the fact of a quasi-
nonexistence buffer zone at the long cell string modules
show that a high number of cracked cells.
Figure 9: Power loss at different test stages: a) Initial,
b) at bending, c) after bending, d) after ML
P
max
is changing quite similar to the number of cell
cracks in the EL images at the modules with long cell
strings (Brand “A” & “D”). The modules with the short
strings “C” & “D” have different power losses. While
module “B” has a power loss up to -1.9% but no major
damages in the EL image visible. Brand “E” has the
highest power loss. However, all modules show the power
loss <5% after the mechanical impact and therefore they
all would pass the IEC criterion.
4 SUMMARY
We have characterized and stressed different
commercial shingled modules. Thereby we were focused
on the weak light and the mechanical stability.
Considering we low light analyse we have to point out: All
modules per brand has a similar weak light behaviour
(same quality). We could spilt the brands into two groups:
Group 1: has a better low-light performance due to a
higher Rs. Group 2: has a worse low-light performance due
to a lower Rs.
Group 1 is optimized for more diffuse light regions
while Group 2 it is for sunny regions. Our outdoor
measurements could confirm that. We haven‘t seen a clear
-3%
-2%
-1%
0%
1%
Brand "A" Brand "B" B rand "C" B rand "D" Brand "E"
Pmax / Pmax,initial
@ Bending After Bending Af ter ML
37
th
EU PVSEC, 07 - 11 September 2020, Online Conference
impact of the cell string length as well as the shingled
module properties on the low light performance.
In contrast to the mechanical tests there we saw a clear
relationship between the stress on short and long shingled
cell strings. While the bend test has a negligible
mechanical impact on the modules, has the ML-Test a
major impact on the module designs with long cell strings.
There the force impact is quite clear due to the position of
the broken cells visible. At the modules with the short
strings there is quasi no influence visible. This can be
described with the force absorption of the ECA and the
encapsulation material due to its resilience and the short
edge length of the cells in direction of the force impact.
Therefor we want to point out that the limiting stress factor
is the ECA and the encapsulation material tensile strength.
SUMMARY
5 ACKNOWLEDGEMENTS
This study was supported by the European Union’s
Horizon 2020 Programme for research, technological
development and demonstration under Grant Agreement
number 857793 (Project name: HighLite).
6 REFERENCES
[1] A. Mondon, N. Klasen, E. Fokuhl, M. Mittag, M.
Heinrich, H. Wirth: Comparison of Layouts for
Shingled Bifacial PV Modules in Terms of Power
Output, Cell-to-Module Ratio and Bifaciality”, 35
th
European Photovoltaic Solar Energy Conference and
Exhibition
[2] S. Wendlandt, B. Litzenburger, L. Podlowski, I.
Gregory, M. Galiazzo: „Shading and hot spot risk
scenarios for shingled cell array modules”, NREL PV
Reliability Workshop 2018, Denver / Colorado / USA
[3] S. Tonini, G. Cellere, M. Bertazzo, A. Fecchio, L.
Cerasti, M. Galiazzo: “Shingling Technology for cell
Interconnection: Technological aspects and process
intergration”, Energy Procedia, Volume 150,
September 2018, Pages 36-43
[4] M.A. Green, Solar Cells - Operating
Principles,Technology and System Application”,
UNSW, Australia, S.97
[5] Grunow et al. Proc. of the 19th EPVSEC (2004),
p.2190
... Accordingly, the cell fracture probability f with the Weibull parameters for full-cells also strongly decreases from 78 % to 3 % at 5400 Pa push load. A similar behavior is observed by Podlowski et al. [Podl20] in Mechanical Load Tests on different PV modules with shingled solar cells. Modules with strings aligned along the short side of the PV module showed much less or no cracks compared to strings aligned along the long side. ...
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Shading and hot spot risk scenarios for shingled cell array modules
  • S Wendlandt
  • B Litzenburger
  • L Podlowski
  • I Gregory
  • M Galiazzo
S. Wendlandt, B. Litzenburger, L. Podlowski, I. Gregory, M. Galiazzo: "Shading and hot spot risk scenarios for shingled cell array modules", NREL PV Reliability Workshop 2018, Denver / Colorado / USA