Print of the PAPER VC3.48, 16th EPVSEC, Glasgow, May 2000
18 TYPES OF PV MODULES UNDER THE LENS
D. Chianese, N. Cereghetti, S. Rezzonico and G. Travaglini
LEEE-TISO, CH-Testing Centre for PV-modules
University of Applied Sciences of Southern Switzerland (SUPSI)
CP 110, CH - 6952 Canobbio
Phone: +41 91 / 940 47 78, Fax: +41 91 / 942 88 65
Internet: http://leee.dct.supsi.ch, E-mail: leee@dct.supsi.ch
ABSTRACT: At the Testing Centre for photovoltaic components (TISO-LEEE) the modules, chosen among the ones most
frequently used in the PV power grid connected plants in Switzerland, undergo a series of tests in order to verify their
characteristics, reliability, medium and long-term performance and high voltage reliability.
The most important aspect of the tests is to compare energy output of modules exposed in identical real outdoor conditions.
The aim of these tests is to answer the questions which need to be posed when planning a PV plant.
At the moment of purchase 1/3 of the modules are no longer under guarantee, while after a year of exposure under
real environmental conditions and at MPP, 2/3 of the modules have a power which is lower than the limit stated in the
guarantee.
Not only do the amorphous-Silicon modules undergo initial degradation, but also a number of crystalline-Silicon modules
show significant degradation. Comparison between c-Si modules exposed to different high voltage conditions demonstrate
that, in the short term, a correlation between the degradation observed and the voltage applied does not exist
Keywords: Energy Rating - 1: Degradation - 2: Qualification and testing - 3:
1. INTRODUCTION
The nominal electrical parameters supplied by the
manufacturers normally refer to typical parameters
recorded during the manufacture of the modules which use
specific or even calculated measurements. Project planners
and installers are increasingly asked to provide production
and behaviour estimates for the systems they install, in
particular when they involve Solar Grants and Contracting.
They therefore ask the following questions:
1. Is the electrical data supplied by the manufacturers
useful for our purpose?
2. Do the modules degrade over time? In what way?
3. Is there a difference in energy production (Wh/Wp)
between the different types of modules?
4. What is the actual energy output of the different
modules?
5. Does the high voltage between frame and cells of a
module in a plant influence its efficiency?
In order to answer such questions the LEEE-TISO
testing centre for PV components has, since 1991, carried
out systematic tests, under real operating conditions, on the
most important modules currently on the market ([1] to
[5]). At Lugano-Trevano, the modules for each cycle of
tests are exposed on stands tilted at 45° and 7° south of
azimuth and kept at maximum power point (MPP). Each
module is equipped with a Maximum Power Tracker
adapted for its power range.
In more recent times, 18 modules divided into two
cycles with 11 (cycle 5) and 7 (cycle 6) modules
respectively have been test. They were selected from those
most commonly found on the market or which had
interesting innovations. In order to guarantee impartiality
and neutrality regarding measurements, the modules were
purchased anonymously unknown to the manufacturer.
Two examples for each kind of module were acquired.
Figure 1: View of the LEEE-TISO test facility with the
18 modules under the lens.
2. TESTING PROCEDURE
During normal testing procedures [1], the electrical
characteristics of the modules were measured, at regular
intervals, at standard test condition (STC) at the ESTI
laboratory of the Joint Research Centre in Ispra (I) as
described in the following table:
Table I:
Pn
P0
P6
P12
Electrical measurements during the tests.
Registration of data of manufacturers
Before exposure: electrical behaviour @STC
Exposure under real environmental conditions at
MPP during 1 YEAR
After 6 months: electrical behaviour @STC
Exposure under real environmental conditions at
MPP for the next 6 month.
After 12 months: final tests @STC
Print of the PAPER VC3.48, 16th EPVSEC, Glasgow, May 2000
3. ELECTRICAL CHARACTERISTICS OF THE
MODULES
3.1 Initial Power P0
The results of the measurements carried out at LEEETISO show real initial power@STC of the modules (P0)
differ from the nominal power of the manufacturer (Pn)
(see table II,1st column). This is not surprising since the
nominal value Pn is a mean indicative value, while the
value of each single module should fall within the variance
of the production parameters. What is surprising is that,
even before exposure, a 1/3 of the modules were under the
minimum limit of the guarantee (-10%). The technology
used (mono or poly-crystalline or amorphous-silicon) has
no bearing on the failure to respect limits. This means that
either the manufacturers don’t know the variance with
respect to their products or their system of measurement
differs from from the World PV Scale (WPVS) [13],
adopted by JRC at ISPRA where measurements @STC
were carried out.
3.2 Initial degradation and high voltage tests
PV solar modules with c-Si cells also show a
degradation in performance when exposed to light at real
operating conditions ([1] to [5]).
During the first periods of measurements, the modules
were exposed to real environmental conditions at MPP and
with high voltage at 1.2kV applied between the frame and
the active part of the module. In order to separate voltage
effect on initial photodegradation, a test on a sample of 6
identical modules (c-Si) exposed at different voltages (0,
300Vdc, 600Vdc, 900Vdc, 1200Vdc, 1800Vdc) was
carried out. The results demonstrated that no short-term
correlation exists between the voltage applied and
degradation (see figure 2).
101
0V
300V
600V
900V
1200V
1800V
100
99
98
97
Table II: Difference between nominal power (Pn) and
measured power (P0, P6, P12) of module type
tested (average of two modules). Modules no
longer under guarantee, before and after one
year of exposure, on dark background.
∆P (%)
Type of
module
CYCLE 5
CYCLE 6
n
-7.3
-2.2
-3.1
-4.3
-10.7
-7.2
n.a.
-1.1
-5.5
-6.9
n.a.
-0.9
-3.0
0.0
1.6
-6.8
-6.7
-3.4
-1.6
-0.4
-1.9
-2.2
-0.5
-1.0
n.a.
0.3
0.0
-3.2
n.a.
-0.3
-1.8
-1.7
-3.9
-0.2
-1.4
-1.1
95
94
93
92
1/6/98
∆P0-n ∆P6-0 ∆P12-6 ∆P12-0 ∆P12-
-4.7
BP580
1.4
GPV75M
-13.1
KC80
-10.8
MST43MV
4.4
PWX500
-13.6
SDZ34-10
-4.1
SF75
-13.7
SP75
-3.2
Sunslates Q
-18.4
US64 R
n.a.
ASI16-2300
-8.4
H800A
-8.5
I55
-13.1
M30-16GEGK -15.0
PL800
-6.5
RSM100
-6.8
SFM36Bx
-7.9
ASE-50-PWX
96
-8.8
-2.6
-4.9
-6.4
-11.1
-8.1
0.3
-0.8
-5.5
-9.6
n.a.
-1.2
-4.8
-1.7
-2.4
-7.0
-8.0
-4.5
-13.1
-1.3
-17.3
-16.5
-7.2
-20.6
-3.8
-14.4
-8.5
-26.4
-18.6
-10.8
-12.9
-14.5
-17.0
-13.1
-14.3
-12.0
Q average of 6 modules without significant reduction of
power; R value of P12 measured by manufacturer. Bold
type : Significant difference.
Measurement error @STC: absolute ± 2.4% of P0;
repetitivity ± 0.85% of P6 e P12 with respect to P0.
In cycle 6, an old generation ASI16-2300 module, with
17 years of outdoor exposure, was installed for comparison
with new generation modules.
25/9/98
19/1/99
15/5/99
8/9/99
2/1/00
Figure 2: Degradation of Pm, measured @STC and
normalised, for 6 modules exposed under
different voltage conditions.
Degradation of these modules is mainly due to a
reduction of Isc (from –5.5% to –7.5%) whilst Voc and Fill
Factor (FF) didn’t undergo any significant changes (-1.8%
and +0.6% respectively).
In the new measurement procedures, applied to cycles
5 and 6, PV modules are not submitted to high voltage but
only to normal operating conditions at MPP.
Photodegradation in the performance of the modules when
exposed to light (table II, 2nd column) was also observed in
cycles 5 and 6.
As reported also by other authors [9,10,11], during the
first hours of exposure initial degradation occurs; this is
principally linked to a decrease of carrier lifetime in the
bulk material. Part of this degradation also occurs if the
modules are stored in the dark over a time of some weeks,
but final stable module efficiency after photodegradation
is equal even after predegradation in the dark [9].
Storage time and subsequent predegradation of the
modules purchased is not known so the initial P0 value
could correspond to the power of the m-Si modules which
have undergone the degradation described above.
However, apart from the predegradation discussed
above, significant degradation can not only be seen in 11
c-Si modules out of 15 (table II, 3rd column: ∆P6-0 ), but
also over the whole period in 13 c-Si modules out of 15
(table II, 4rd column: ∆P12-0 )
In the standard modules with c-S cells, the average
reduction after one year of exposure was –4.8% peaking at
-9.6%
Print of the PAPER VC3.48, 16th EPVSEC, Glasgow, May 2000
a-Si
120
110
a-Si
cycle 5
cycle 6
P0/Pn (%)
P6/Pn (%)
P12/Pn (%)
1.00
0.909
0.901 0.884
-40%
0.868
0.865
0.862 0.857
0.852
0.849
-35%
0.809
0.80
0.686
100
-24.6%
0.60
PR
Relative difference
90
-25%
-20%
80
0.40
70
0.20
-15%
-11.1%
60
0.00
Figure 3:
-30%
Power ratio of measured value (P0, P6, P12)
vs. nominal power.
0.0%
-0.9%
-2.8%
-10%
-6.7%
-5.8% -6.3%
-4.5% -4.9% -5.2%
-5%
0%
Energy production (cycle 5) with respect to
the power measured (Peff.).
Figure 4:
1.00
After a year of exposure 13 modules out of 17 were
found to be outside the limits of the guarantee (figure 3 and
table II, 5rd column: ∆P12-n ). In the case of PV plants the
modules should be substituted or partially reimbursed by
the manufacturer. In practice, however, it is still difficult
for the consumer to verify the real power of the modules.
0.856
PR
0.807
0.788
0.80
-40.4%
Relative difference
-45%
-40%
0.776
0.752
0.746
0.746
0.740
0.732
-35%
0.677
-30%
0.60
0.510
-20.8%
0.40
-13.5%
-12.1% -12.8% -12.8%
0.20
-5.7%
-7.9%
-14.5%
-25%
-20%
-15%
-9.2%
-10%
-5%
4. ENERGY RATINGS
Two modules of different types but of equal power can
have different levels of energy production even though
exposed in the same way, at the same time and under the
same metereological conditions. The principal parameters
which influence energy production are the temperature
coefficients, behaviour at low irradiation, spectral response
and reflections [10] [11].
0.00
0.0%
Figure 5: Energy production (cycle 5) with respect to the
manufacturer’s given nominal value. (Pn).
1.0
-20%
0.874
0.868
0.853
0.850
0.807
0.8
PR
-7.7%
0.2
-2.4%
0.0%
-10%
-8.4%
-5%
-2.8%
-0.8%
0%
Energy production (cycle 6) with respect to
the power measured.
Figure 6:
1.0
PR
0.8
-15%
Relative difference
0.4
0.0
-17.3%
0.801
0.723
0.6
4.1 Technical comparison
Figures 4 and 6 show energy ratings in the form of
Performance Ratio (PR) calculated on the basis of real
measured power values (Peff.=(P6+P12)/2) for cycles 5
and 6 respectively.
The relative percentage differences in energy
production of the different types of modules are calculated
with respect to the first module on the left.
In this type of comparison, the SDZ34-10, Sunslates
and M30-16GEGK modules are at a disadvantage. They
are designed to be laid on a sloping roof and therefore the
test conditions (open rack) do not respect their real
operating conditions. Moreover, during cycle 5 a Sunslates
tile broke while another lost half its power; for this reason
the relative difference reached 24.6%. Energy comparisons
between cycle 5 modules and cycle 6 modules are not
possible in that they took place in two different periods
under different metereological conditions.
Energy ratings calculated using Peff. refer therefore to
a technical comparison of the different types of modules
tested.
The US64 module has the highest PR (calculated only
with P12 measured by the manufacturer) and bearing in
mind its characteristics at high temperatures, should have
even better values in warm areas of the world;
0%
0.764
0.754
0.750
Relative difference
0.747
0.709
-19.9%
0.702
-20%
-15%
0.612
0.6
-7.2%
0.4
-1.3%
Figure 7:
-10%
-5%
0.2
0.0
-8.1%
0.0%
-1.9%
-2.2%
0%
Energy production (cycle 6) with respect to
the manufacturer’s given power value.
Print of the PAPER VC3.48, 16th EPVSEC, Glasgow, May 2000
unfortunately, however, the manufacturer’s nominal power
is much lower than the actual figures (see chap.4.2).
From a technical point of view, the production
differences for standard modules reach a maximum of
8.4%.
4.2 Consumer viewpoint comparison
Figures 5 and 6 represent energy ratings in the form of
Performance Ratio calculated on the basis of nominal
power (Pn) declared by the manufacturer for the cycle 5
modules and for the period 1998-1999, that is to say for a
one year period after the first four months of expusure
during which initial degradation of power took place
(important for the a-si modules) as well as for cycle 6 (May
1998 - May 1999). This graph shows energy production for
the powers actually purchased by the consumers, which, for
the most part, as seen in chapter 3, are below the limits of
the guarantee.
The relative differences as a percentage of energy
production of the various types of modules are calculated
with respect to the first on the left. Apart from the
Sunslates modules, these differences reach values of up to
20%. This is due to the fact that the nominal powers, which
are explicitly used in PR calculations, are, in fact, not
‘contained’ in the modules, as can be seen in the P0/Pn
ratio in figure 3.
The PWX 500 and ASE-50-PWX-D modules,
produced by the same manufacturer but with a different
label, have a similar PR with respect to Peff.; but, since
Photowatt (PWX500) declares a higher nominal power,
there is a difference in production of 8% between the two
modules. In cycle 5 the BP580 module shows better
performance due to a correct declaration of power. The
good energy rating of the old ASI16-2300 module with
respect to other cycle 6 modules should be noted: after 18
years of exposure, and with nominal power being equal,
this module produces more.
5. CONCLUSIONS
The photovoltaic solar modules, produced using
crystalline-silicon cells, show a degradation in performance
when exposed to light.
The initial degradation of c-Si modules takes place
during the first period of exposure to light. This is often
followed by further degradation during the following
months irrespective of the season of exposure. The causes
of this further degradation are still being studied.
Stabilized power after 12 months of exposure to light is
lower than the guarantee limits for 12 out of 17 modules
purchased. Definition of nominal power values given by
the manufacturers for photovoltaic modules must therefore
be revised. Measurement of electrical characteristics for
every module which leaves the production line would only
partly prevent non-respect of nominal power values since
the modules still have to undergo initial photoinduced
degradation.
Comparison between c-Si modules
exposed at
different high voltage conditions show that no short-tem
correlation exists between initial degradation and voltage
applied.
The choice of a module for a specific application must
not only take into account performance (or power) of a
module but also other factors linked to energy rating under
different metereological and construction conditions.
Measurements show a mean relative difference of PR
(Performance Ratio) in an open –rack system tilted at 45°
and based on real measured power values, of –4.6%, with a
maximum difference of 8.4% (modules which are
integrated in the roof as tiles are not taken into
consideration).
REFERENCES
[1]
M. Camani, N. Cereghetti, D. Chianese and S.
Rezzonico, Comparison and behaviour of PV
modules, 2nd World PVSEC, Vienna, July 1998.
[2] M. Camani et al.: Tests of reliability on crystalline
and amorphous silicon modules, 14th EPVSEC,
Barcelona (S), June 1997.
[3] M. Camani et al.: Vergleich und Beurteilung der PV
Module, 13. PV Symposium, Staffelstein (D), March
1998.
[4] M. Camani et al. : Centrale di prova per componenti
e sistemi per progetti nel campo della tecnica
fotovoltaica, periodo IV: 1994-1996, Final Report.
[5] G. Travaglini, N. Cereghetti, D. Chianese, S.
Rezzonico e M. Camani; Leistungsverhalten von PV
Modulen; Simposio nazionale fotovoltaico, Zürich
(CH), November 1999.
[6] D. Chianese et al.; Degradation of crystalline silicon
modules, Simposio nazionale fotovoltaico 1999,
Zürich (CH), November 1999.
[7] S. Sterk, K.A. Münzer, S.W. Glunz, Investigation of
the Degradation of Crystalline Silicon Solar Cells,
14th EPVSEC, Barcelona (S), July 1997.
[8] J. Schmidt, A.G. Aberle, R. Hezel, Investigation of
Carrier Lifetime Instabilities in CZ-grown Silicon,
26th IEEE PVSC, Anaheim (US), October 1997.
[9] A. Moehlecke et al., Stability Problems in (n)p+pp+
Silicon Solar Cells, 2nd World PVSEC, Wien, 1998.
[10] D.L. King, PV Module and Array Performance
Characterization Methods for All System Operating
Condition, NREL/SNL PV Program Review
Meeting, November 1996.
[11] Bücher et al. , OPTIMOD project contract n°JOR3CT96-0095.
[12] N. Cereghetti et al., Behaviour of Triple Junction
a-Si modules, this conference.
ACKNOWLEDGEMENTS
This project is financially supported by the Swiss Federal
Office of Energy and the AET (Azienda Elettrica Ticinese).
The authors would like to extend particular thanks to the
staff of the ESTI of the Joint Research Centre (JRC) in Ispra
and to Dr Mario Camani.