Performance and Degradation Analyses of two
Different PV Modules in Central Anatolia
Talat Ozden
Dept. of Electrical & Electronics Eng.
Gümüşhane University,
Gumushane, Turkey
tozden@gumushane.edu.tr
Bulent G. Akinoglu
Department of Physics
METU,
Ankara, Turkey
bulo@metu.edu.tr
Sarah Kurtz
Dept. of Materials Science and Eng.
University of California,
Merced, CA, USA
skurtz@ucmerced.edu
The Center for Solar Energy Research
and Applications - GÜNAM
METU,
Ankara, Turkey
tozden@metu.edu.tr
The Center for Solar Energy Research
and Applications - GÜNAM
METU,
Ankara, Turkey
UC Solar,
University of California Merced
Merced, CA, USA
Abstract—Ankara (latitude: around 40°N) is in Central
Anatolia and the climate is dry continental. Due to the rapid
transition to renewable energy policies of Turkey it is now a
must to complete truthful feasibility analyses for solar power
systems countrywide. Long term performance and degradation
analyses have been carried out by the authors for three
different arrays and many research results appearing in the
literature are summarized. Present research analyzes the
outdoor testing results of two PV modules monthly data. One
of the modules has multi crystalline silicon (Poly-Si) cells and
the other is thin film silicon (amorphous Si / microcrystalline Si
tandem structure – a-Si/µc-Si) cells. The results showed that in
five years the average yearly efficiency decreases from 14% to
11% for Poly-Si (14.72% is the nameplate Standard Test
Condition (STC) efficiency) and from 8% to 6% for thin film
modules (9.15% is STC efficiency). Our calculations are based
on measured solar global-horizontal irradiance from which we
calculated plane-of-array irradiation using an anisotropic sky
model. We present the details of these analyses and
degradation rates for the two types of modules to be used in
long term feasibility analyses.
Keywords—Long term degradation, Photovoltaic module,
Photovoltaic performance analyses.
I. INTRODUCTION
There are now many photovoltaic power plant (PV PP)
installations all over the world, and also in Turkey. New,
high-efficiency cell types are being utilized to make new
commercial modules for high-performance PV PPs. But their
performances and lifetimes are affected by climatic
conditions. Therefore, it is important to determine the
lifetime and guaranteed power for manufacturers, PV system
owners, investment and insurance firms for the region where
PV PPs are to be installed.
Although there are some standard laboratory qualification
tests in order to estimate the performances and degradations
of PV modules, these tests may miss identifying some failure
mechanisms. A realistic working environment enables
identifying all failure mechanisms. Also, the acceleratedstress tests cannot be used to define realistic lifetimes since
they do not quantify all failure mechanisms [1]. Therefore,
outdoor testing of PV modules is crucial in order to quantify
the lifetime and energy yields of PV modules in the long
term.
The authors acknowledge the support given by the Ministry of
Development for the construction of outdoor testing facility.
978-1-5386-7538-0/18/$31.00 ©2018 IEEE
There are many articles on long and short outdoor test
result for PV cells/modules/systems in the literature (see for
example [2]–[11]). These tests are carried out at different
meteorological and climate conditions in the long and short
term. They calculated the degradation rates and long term
performances. Some degradation rate values from the
literature are tabulated in Table IV with the corresponding
reference numbers.
This study presents the results of performance
calculations for two different PV module tested for long
term. The modules are thin film and poly crystalline. The
objective of the work is to compare the performances and to
determine the degradation rates calculated by integrated
monthly yield values. The calculated degradation rates are
also compared with literature values.
II. MATERIAL AND METHODS
A. Testing site, modules and monitoring systems
The study is carried out at an outdoor test facility of The
Center for Solar Energy Research and Application
(GUNAM). The outdoor test platform is installed on the roof
of the Physics Department of Middle East Technical
University Campus, Ankara, Central Anatolia. It has 16 test
beds. The test beds are suitable for mounting different PV
module types, powers and frame structures (Fig. 1). The
modules are tilted at an angle of 32°. The testing
environment has cold semi-arid climate [12], [13], according
to Köppen’s classification.
Fig. 1. Outdoor test platform of GUNAM
Testing device is a multi-tracer PV analyzer. It continuously
measures PV performance at a pre-specified time interval. In
addition, there is a weather station to measure the
meteorological parameters, and two black and white high
precision Pyranometers, Kipp&Zonen CMP 11. The
pyranometers are connected to the multi tracer to measure
the incoming energy on the tilted and horizontal surfaces.
The specifications of the modules are given in Table I. The
module numbers in the Table I are seen in Fig. 1.
TABLE I.
Module Type
PMAX
VOC
ISC
VMPP
IMPP
ɳ
Area
[W]
[V]
[A]
[V]
[A]
[%]
[m2]
59.80
3.450
45.40
2.820
9.143
1.400
21.70
8.180
17.80
7.300
12.75
1.020
µc-Si / a-Si (1) 128.0
Poly-Si (2)
TESTED PV MODULES SPECIFICATIONS
130.0
The tests of µc-Si / a-Si and Poly-Si modules started in
April and May 2012, respectively. All modules are regularly
cleaned once in every week.
B. Data
In calculating the performance of PV modules, incoming
energy and yields of modules per unit area are used.
Unfortunately, plane-of-array irradiance data are not
available for the first four years. Two high precision
Pyranometers were installed in 2016. Thus, in the analyses,
the incoming energy data measured by Turkish State
Meteorological Service (TSMS) located about 20 km away is
used. The procedure was verified in our earlier work [6]. The
data measured was on horizontal surface, so an estimation
model was used to determine the incident input energy on
tilted modules. It is an anisotropic model called HDKR [14].
Our previous work verified that the data obtained in this way
have acceptable accuracy [6]. For the first four years,
monthly incoming energy on tilted surface calculated by
using this model is tabulated in Table II. Table II also gives
the measured solar irradiation for the last year, column with
light gray ground color.
TABLE II.
ESTIMATED (APRIL 2012 TO MARCH 2016) AND
MEASURED (APRIL 2016 TO MARCH 2017) MONTHLY AVERAGE SOLAR
IRRADIATON DATA [KWH/M2]
2012
January
February
March
April
May
June
July
August
September
October
November
December
176.4
179.5
223.1
226.6
225.3
221.5
157.9
89.21
71.11
2013
65.68
104.2
141.3
181.5
214.8
221.2
226.9
227.5
199.7
168.4
114.8
81.16
2014
62.37
135.2
159.0
182.2
189.3
202.3
233.5
229.4
180.0
142.1
74.25
57.58
2015
75.53
90.63
140.8
177.7
200.2
166.9
238.2
223.1
201.5
138.6
117.5
86.25
2016
71.08
111.0
140.2
201.1
170.0
211.3
235.9
215.9
191.7
153.2
113.6
83.37
2017
70.44
117.8
147.0
The data was collected by PV analyzer in 10-minute time
intervals. The testing device biases the modules between
measurements at the maximum power point. By integrating
the 10-minute measurements for every day and averaging we
obtained the monthly outputs as tabulated in Table III, for the
two PV modules. Some of the data is missing due to
instrumentation failures.
TABLE III.
µc-Si / a-Si
January
February
March
April
May
June
July
August
September
October
November
December
Poly-Si
January
February
March
April
May
June
July
August
September
October
November
December
MONTHLY CUMULATIVE YIELDS OF TWO PV MODULES
[KWH/M2]
2012
17.72
14.68
17.95
17.71
18.72
16.72
12.47
7.07
5.48
2012
-19.27
22.30
22.28
23.62
21.54
17.00
10.40
8.53
2013
5.30
7.57
10.84
---16.61
-15.35
12.73
9.05
6.29
2013
8.29
11.38
16.06
---21.53
-20.35
18.07
13.61
10.24
2014
5.19
9.57
10.74
13.25
13.99
----10.81
-4.46
2014
8.16
14.71
15.87
19.09
19.60
----14.93
-7.06
2015
4.93
6.63
10.00
-14.51
-17.58
16.25
15.50
10.56
10.41
-2015
7.89
10.09
14.99
-19.81
-22.52
20.35
19.63
14.41
15.68
--
2016
---14.61
12.90
15.86
17.12
15.73
14.96
11.81
8.45
6.15
2016
---20.35
17.96
21.09
21.72
19.47
19.02
16.26
12.65
9.34
2017
5.11
8.46
9.58
2017
7.79
13.03
14.02
C. Method
There are essentially two performance indicators for the
PV modules/systems. They are the efficiency and the
performance ratio. In this study, we preferred to use the
monthly efficiencies to determine the module performance
and degradation rates. This monthly efficiency values are
calculated using:
 ( (hourly output ) )
η=
 ( (hourly I ) )
N
day
j
i
i j
N
day
j
i
(1)
t i j
where j and i are the indices that symbolize the day in a
month and hour in a day, respectively, and N is the number
of days in the month. In Equation (1), all the energy values
are calculated in unit of kWh /m2.
Linear regression method was used to calculate
degradation rates of the PV modules [9]. In this method a
line is fitted to the monthly efficiency or performance ratio
time series as:
y = mt + n
(2)
Then the percent degradation rate is calculated using:
 n − y ( N ) 12 
Rd = 
×  ×100
n
N

(3)
where, N is the number of months of outdoor operation. The
efficiency and performance ratios are calculated using the
hourly integrated energy values by summing them up for a
month and averaging.
III. RESULTS AND DISCUSIONS
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In fig. 2, we present the time series of monthly
efficiencies for the two modules for five years. As can be
observed, the two module efficiencies are below the STC
efficiencies as expected. The outdoor monthly average
efficiencies of µc-Si / a-Si and Poly-Si modules over the five
years are 7.56% and 10.68%, respectively. The minimum
efficiency of Poly-Si module was measured as 9.02% in
August 2016 while the minimum of µc-Si / a-Si module was
measured as 6.51% in April 2017. As is commonly known
the ambient temperature affects the performance of
crystalline modules more than the thin film modules. In fig.
2, this situation can be observed especially during the
summer months July and August of 2016.
Fig. 2.
Monthly efficiencies of two modules for five years
The trend lines are also drawn to guide the eyes and the
regression equations are presented in the legend box. The
parameters of trend lines will be used in order to calculate
the degradation rates. The trend lines clearly demonstrate the
degradations of the two module performances.
calculate the degradation rates in the present work is the time
series of monthly average efficiencies. The results are
comparable with those that have appeared in the literature.
Annual degradation rates of Poly-Si and µc-Si/a-Si modules
are calculated to be 1.3%/year and 1.6 %/year, respectively.
Table IV presents the degradation rates calculated in this
work (the first row) using Eq. (3). In addition, some results
from the literature are given. They are taken from the review
article [7] and the results of some recent studies are also
added.
The results can be utilized for long term techno-economic
analysis of PV PP to be installed in Central Anatolia and this
is our future work plan.
TABLE IV.
ANNUAL DEGRADATION RATE [%/YEAR]
This work
[7] (Pre 2000)
[7] (Post 2000)
[3]
[2]
[15]
[4]
Our previous work [16]
µc-Si / a-Si
1.60
~1.80
~1.10
2.28
1.80
2.00
2.34
2.10
Poly-Si
1.30
~0.70
~0.70
-1.20
1.00
---
Our results are consistent with those that have appeared
in the literature especially for the µc-Si / a-Si module.
However, the degradation rate that we calculated for Poly-Si
module is a little higher (1.30%/year) than the average value
of 0.70%/year presented in [7]. But the result 1.2%/year of
reference [2] is quite close to our finding.
ACKNOWLEDGMENT
The authors would like to thank to helps of Dr. R. Turan
from GUNAM and Mr. C. Yıldırım from Proerk for their
works during installation of outdoor test facility. The authors
also acknowledge the support given by the Ministry of
Development for the construction of outdoor testing facility.
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