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Reliability of photovoltaic modules II. Interconnection and bypass diodes effects
Solar Energy Materials and Solar Cells 31 (1994) 469-480 North-Holland
Solar Ent~j~ Matt~l$
and Solar Cells
Reliability of photovoltaic modules II. Interconnection and bypass diodes effects N a b e e l A. AI-Rawi, M a a n M. AI-Kaisi a n d D h i a J. A s f e r Solid State Electronics Research Laboratory, School of Electrical Engineering, University of Technology, P.O. Box 35010, Baghdad, Iraq Received 9 July 1992
The objective of this study is to evaluate the effect of interconnection and bypass diodes on improving the reliability of a photovoltaic module. The study of module design has been carried out for different cell interconnections from simple series to a more complex series-parallel configuration aiming to do an evaluation and analysis to determine the optimum item configuration which achieves a reliable module of more than 90%. Bypass diodes are often required to limit the potential of a reverse voltage "HOT SPOT". The role of bypass diodes for module reliability improvements has been studied practically. From the results obtained, it is found that the reliability increases as the number of diodes increases. Many experiments to study the effects of interconnections on reliability have been carried out. Results from tests on many modules are presented and discussed.
1. Introduction
The ability of a PV electric generating system to satisfy the intended load is influenced by two phenomena. The first is the output power affected by the variation of the irradiance level, and the second is the component effect resulting from the changes of PV conversion efficiency of the module for one reason or another. Fault tolerant design techniques are used to minimize the complete failure of the PV module and array, but output decreases approximately proportional to cell failure. Because of the fault tolerant design of the array subsystem, individual solar cells and modules can fail but the system remains capable of generating power at a r e d u c e d level. F a i l u r e s i n s u b s y s t e m c o m p o n e n t s t h a t a r e n o t f a u l t t o l e r a n t c a n r e s u l t i n a c o m p l e t e loss o f p o w e r [1]. A c c o r d i n g t o t h e a i m a n d b a s i s o f r e l i a b i l i t y , t h e a b o v e m e n t i o n e d t e c h n i q u e is o n e o f t h e d i f f e r e n t r e l i a b i l i t y a p p r o a c h e s , w h e r e t h e l a t t e r is a c t u a l l y a f u n c t i o n o f c o m p o n e n t f a i l u r e [2]. 0927-0248/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0 9 2 7 - 0 2 4 8 ( 9 3 ) E 0 0 7 2 - L
470
N.A. AI-Rawi et al. / Reliability of photovoltaic modules. H
2. Interconnection and reliability To study the effects of the interconnections of the cell on module power and reliability, I - V characteristics have been measured using sun light simulator facilities with their software, to plot and calculate the various design parameters
[3-5]. For comparison, two types of crystalline silicon of modules (A) and (B) of different manufacturing origin have been tested. Each module consists of 16 cells arranged in a different configuration (m * n) as shown in fig. 1, where m is the number of parallel strings in the module. n is the number of cells in each string. The characteristics are made under the following conditions: (i) Normal operation (all cell illuminated evenly). (ii) One of the cells in the module is completely shaded. (iii) Four successive cells in the module are completely shaded. Fig. 2 shows the relation between maximum power and number of cells (n) in each parallel string in the module. For two types of modules (A) and (B), and when the module operates normally (no shadowing), it is shown that the maximum power increases as n increases, or in other words as the number of parallel strings (m) decreases. Fig. 3 shows the case when one cell in the module is completely shaded. It appears from the figure that the maximum power increases at small number of n and then decreases as n becomes greater than four, showing that the maximum power is obtained at a 4 . 4 arrangement. Fig. 4 shows the case when four cells in the module are completely shaded showing that maximum power occurs at the 4 . 4 arrangement. The effect of the different cell arrangements on cell and module efficiencies have been studied, where the I - V characteristics for the two types (A) and (B)
rn=l , n=16
m:2
, n=3
rn=4, n=4
I m=16
~ n=l
Fig. 1. Different cell arrangements.
471
N.A. Al-Rawi et al. / Reliability of photovoltaic modules. H 2110 B m.n =16
16.0
A
I
x
"-~ 12.0
/
a. g 0 E E X
4-0
NO SHADOWING
I 0.0 0.0 Number
I
I
,
,
4-0
8.0
12.0
1E~0
of c e i l s
in e a c h s t r i n g
Fig. 2. The effect of interconnection on the maximum output power of a module at no shadowing. have b e e n m e a s u r e d for the following modules configurations: 1 * 16 16 cells in series. 2.8 8 cells in series in two parallel strings. 4.4 4 cells in series in four parallel strings. 1 6 . 1 16 cells in parallel. Fig. 5 shows the relation between cell and m o d u l e efficiency with the n u m b e r of parallel strings m in the m o d u l e for type (A) and (B) w h e n all cells are illuminated equally, indicating that cell and m o d u l e efficiency decrease as m increases. T h e lower values represent the efficiency for a pure parallel connection of cells, while the higher values represent the efficiency for a pure series [6,7]. Fig. 6 shows the effect of interconnections on cell and m o d u l e efficiency, where one cell in the m o d u l e is completely shaded. While fig. 7 shows the case when four successive cells are completely shaded. 20.0 16.0
rn~ =!6
ONE CELL SHADED
3:
~, 12.0 o
a.
E
8-0
E x 4.0 O Ig
0.0
0.0
4.0 Number of c e l l s
8.0 in e a c h
12.0 string
16.0
Fig. 3. The effect of interconnection on the maximum output power of a module for one cell shaded.
472
N.A. Al-Rawi et al. / Reliability of photovoltaic modules. H
20.0 16.0
m,n=16
FOUR CELL SHADED
-..:lZ.O ,3 ¢v
¢
~.0 E
._E 4.o X O
0"~. 0
4.0
8,0
Number of cells In each
12.0
1I;.0
strincj
Fig. 4. The effect of interconnection on the output power of a module for four cell shaded.
Figs. 6 and 7 indicate that the maximum cell and module efficiencies is obtained when the arrangement is 4*4. The pure series and pure parallel do not give maximum output power due to the absence of fault tolerance and losses in wires respectively. To study the effect of PV cell interconnection on module reliability, complete shadowing has been made on cell(s) successively in the module (A and B) for different cell interconnection configurations to simulate a fault in the module. The Isc of the whole module after shadowing each cell has been recorded, and the smaller variation in current gives an indication to the reliability of the module. The experiments have been carried out under different conditions (irradiance and temperature) due to the outdoor nature of the test.
20.0
NO SHADOWIN6 16.C
m.n =1 6
1 cell type B 2 cell typeA 3 cell typeB
x ~X ~
"~ 4.0 0.0
0.0
,
,
t,.0 8.0 Number of parallel st.r;ncjs
,
12.0
,
16.0
Fig. 5. The effect of interconnection on module efficiencyat no shadowing.
N.A. AI-Rawi et al. / Reliability of photovoltaic modules• H
473
2043 ONE CELL SHADED re.n=16
0<
1 cell type B 2 cell type A
3 module type B
,2.0
4 m o d u l e type A (3
.c 8.0 ,e,w,-
,,, 4~] 0.0(~.0
4'.o
a:o
i~.o
1~.o
Number of parallel s t r i n g s Fig. 6. The effect of interconnection on module efficiency for one cell shaded.
20.0 FOUR CELL SHADED 16.o
5
I cell t y p e ]3 2cell type A
m.n =16
3 module type ]B 4 module t y p e A
C Ib
~ 8.0 4.O 0.0 0.0
Lo
8'.o
li.o
,6.0
Number of p a r a l l e l s t r i n g s Fig. 7. The effect of interconnection on module efficiency for four cell shaded.
1.0 [ ~ TYPE A TYPE B
00-5 I-4
"t 0.0 0.0
8-0
16.0
Cell number Fig. 8. Variation in module current due to shadowing for the 1 * 16 series connection•
474
N.A. Al-Rawi et al. / Reliability of photovoltaic modules. H t.O
r-]
TYPE
[]
TYPE ]13
A
~ H
0.5 1
0.0 0.0
IN 16.0
8.0 Cell number
Fig. 9. Variation in module current due to shadowing for the 2.8 connection. Fig. 8 shows the variation of module short circuit current and cell number, where each cell in the module (panel) has been numbered from 1 to 16 to examine problems caused by differences in individual cells parameters. Complete shadowing is made successively on one cell of a module, showing a high fluctuation in output current when cells are connected in series. This means that variation in output power indicates a low reliability for this type of connection. Fig. 9 shows the variations in short circuit current when an experiment is made on a module consisting of two parallel strings each with 8 cells in series, and shadowing is done on one cell successively in the module, showing that this kind of interconnection has less variation than with the series type of connection [8]. For a module consisting of four parallel strings each containing four cells in series, and after shadowing is made on one cell, the short circuit variation with respect to cell number is shown in fig. 10. It is shown that variation is minimized,
~
1.0
TY PE A TYPE B
I'll rfl 114 114
o F-q
--. 0.5 E
IN
IH
114
IH
IM IH
11,4 11,4
I1'1 11,4
F-I
11.4 I H 114 114 11,4 11.4 IH
I1"t
IH IH
II1 J,
0.C O0
8.0 Cell number
16.0
Fig. 10. Variation in module current due to shadowingfor the 4*4 connection.
N.A. AI-Rawi et al. / Reliability of photovoltaic modules. H 1.0
475
mm IH IM IH IM IH IM IH IH
O
H
IH IH
i-i
IH ILl IH IH
-.- 0-5 E
In IM IH IH IH IH IN
0.0 0.0
IIA
8.0
IH IU 16.0
Cell number TYPE(A) [] TYPE {la ) [ ]
Fig. 11. Variation in module current due to shadowing for the 16.1 parallel connection. meaning better reliability compared with the other types of configurations shown in figs. 8 and 9. The variations in module current due to shadowing for an arrangement of sixteen parallel cells is shown in fig. 11. It is clear that this arrangement has the smallest variation in output current compared with all kinds of connections, indicating that the most reliable interconnection is the one with sixteen parallel cells.
3. The bypass diode and reliability improvement The motivation of array designers is maximization of array output power over the operational life time of the installation with respect to shadowing and various internal mode failures [9-11]. Bypass diodes are frequently used to permit the current to pass through the circuit even when one or more of the series items fail totally. Bypass diodes are used to decrease the effect of the hot spot problem which arises in the module when the cell becomes back biased and operates in the negative voltage quadrant as a result of short circuit mismatch, cell cracking or shadowing [12,13]. To study the improvement gained by adding bypass diodes when physically and economically feasible, and to what extent it will increase the fault tolerance, which means the reliability improvement of a module of array system, experiments have been made on the following panels: (1) Laboratory panel consisting of 16 cell (type A). (2) Laboratory panel consisting of 16 cell (type B). (3) Commercial panel consisting of 36 cell (type A). (4) Commercial panel consisting of 32 cell (type B).
476
N.A. Al-Rawi et al. / Reliability of photouoltaic modules. II
1.0
1.0 FI TYPE A
0.8 0.6
_NO DIODE
0.6
USED
o.4
0.0
TY PE A TYPE B
_TWO DIODE USED
0.4
o. 0.2 00
[]
0-8
~1 T Y P E B
0"21~,~ .
.
.
.
.
.
8,0 CELL NUMBER
.
.
0.
16.0
8.0 CELL NUMBER
16.0
1.12
FITYPE A E] TYPE B
0.8
mORE DIODE USE[ Q.
0.4 0-2 0.0
0 -0
~
~I.
~ TYPE A ~I TYPE B
=~ 0.(5
_FOUR DIODE USED
IIII1111111111 oI £
~0.
8.0 CELL NUMBER
16.0
0.0
8.0
16.0
CELL NUMBER
Fig. 12. The effect of bypass diodes on output power under shadowing effect.
For the laboratory panels in (1) and (2) above which were made to our specification, and after complete shadowing is made on cell number one in the panel, the related data has been recorded for each panel. The same procedure is repeated successively on the remaining ceils (from 2 to 16) in the panel. These steps were done without bypass diodes and were repeated for one diode across the 16 ceils, two bypass diodes across 8 cells and four bypass diodes across 4 cells in the panel. For the commercial panel (in (3) above) no diodes, one diode, and two diodes are used. And for the panel in (4) above, no diodes and one diode are used due to the terminals available in the panel for diode connection. Fig. 12 shows the relation between the output power and the cell number for laboratory panels type (A) and type (B) respectively, for complete shading of the cell, and for different number of bypass diodes connected across groups of cells in the panel. From (a) and (b) in the above figures, it is shown that there is a noticeable reduction in the output power, since they have the same fluctuation and the same power reduction, indicating that one bypass diode has no effect in case (b), while in (c) and (d) the variation decreases and the output increases (mean reduction decreases) indicating that useful improvement is obtained using two and four bypass diodes.
N.A. Al-Rawi et al. / Reliability of photovoltaic modules. H
477
1.0 C] TYPE A ~l TYPE B NO DIODE USED
O,8 0.6
0.4 0.2 0.0 0.0
8.0 16.0 CELL NUMBER
24.0
32.0
1.0 ['-']TYPE A ~TYPE B ONE DIODE USED
0.8
~06
[ ] TYPE A TWO DIODE USED
, 'ii8
0.6
a. 0.4
• a. 0.4
=y 0.2
o.z
0.0 0.0
8.0
16.0
24.0
32.0
0.0
0.0
ft.0 18.0 27.0 CELL NUMBER
36.0
Fig. 13. The variation of output power with shaded cell number for different numbers of diodes.
Fig. 13 shows the variation of output power with shaded cell number for different numbers of diodes, for commercial panels type (A) and (B) respectively, showing a similar trend as has been obtained in fig. 12.
4. Results and discussion
For a number of groups of different cell arrangements, the pure series connection gave the maximum output power. Whereas the parallel type of connection produces the least generating power, the other types of configurations gave output powers in between as shown in fig. 2. The maximum power from the panel under partial shadowing is obtained when the panel consists of four parallel strings each containing four cells in series as shown in fig. 3 and 4. This is different from the previous case where the maximum power occurs for panels consisting of pure series cells. Fig. 3 and 4 show that the panels have poor output power for the two extreme cases (pure series and pure parallel arrangements), this can be explained as follows: (a) In the series type, the fault (simulated by shadowing in this study) causes a large loss if not a complete loss of output power. That is because this type of connection has no fault tolerance.
478
N.A. Al-Rawi et al. / Reliability of photovoltaic modules. H
This gives the answer for the reason of poor output power in a series type of connection with partial shadowing, although it gives the highest power with no shadowing. (b) In the parallel connection and in spite of showing a high fault tolerance, there are large losses in the output power due to the losses in the wires, whereas the losses are proportional to r/2 (n is number of parallel cells) and the resistance of the wires (I2R). A series of experiments have been made on two types of panels for different cell interconnections in the panels. They show that in the series connection the highest cell and module efficiencies are obtained, while the parallel connection gave the lower values of cell and module efficiencies under normal operation (no shadowing). On the other hand ( 4 . 4 ) gave the optimum configuration under the shadowing or fault condition, as has been shown in figs. 6 and 7. The conversion efficiency of modules is less than that of the cells because of: (a) electrical losses in the interconnection, (b) the presence of an electrically inactive area between cells in the module (panel). The high efficiency obtained for type (B) cells can be attributed to manufacturing techniques and the design of the grid pattern.
Table 1 The average percentage of reduction in the output current due to artificial faults (shadowing). cells arrangement in modules
~ _ ~
_~_~
A v e r a g e p e r c e n t of r e d u c t i o n (%) in o u t p u t curreat due to shadowing f o r module ~ (A)
A v e r a g e p e r c e n t of r e d u c t ion (S) in o u t p u t c u r r e n t due t o shadowing faar modul e t y p e (B)
53.4 %
90.8 %
44.5 %
47.4 %
43.3 %
46.7 %
23.3 %
23.3 %
16.2 %
12.5 %
7.9
%
5.9
%
1.5
%
1.2
%
N.A. Al-Rawi et al. / Reliability of photovoltaic modules. H
479
From the sets of experiments carried to investigate the effect of different types of cell interconnections, two types of modules (A and B) have been tested. The principle used to find the best reliability (high fault tolerance) is by applying shadowing on successive cells on modules and monitoring the variation in output power. The following results have been found: (1) The pure series connection shows the lower reliability. (2) The pure parallel connection shows the highest reliability. (3) The above results are in good agreement with the theoretical results [13]. (4) The average percentage of reduction in the output current due to fault making (shadowing) on cells successively in the module, for the above two types and the other types of connection, are given in table 1. 5. Conclusion The objective of this study was to evaluate the effect of interconnection and bypass diodes on reliability of photovoltaic modules. A series of experiments have been carried out on two types of cells of different origin and different cells interconnection arrangements. It can be concluded from this study that, under normal operation (no shadowing), the losses in module output power is minimum when the cells are interconnected in a pure series connection, while it is maximum when ceils are interconnected in pure parallel schemes. Under abnormal operation (partial shadowing), the losses are high in pure series and parallel connections, while the losses are minimum in the combined (series-parallel) schemes. We found also that the electrical mismatch of cells due to shadowing reduces module efficiency. To study the role of bypass diodes on the reliability, shadowing is used to simulate a fault and the following is concluded: (i) Shunting the module with one bypass diode does not improve the reliability of the module. (ii) Connecting two diodes across the module improves the reliability. (iii) Reliability of the module increases as the number of shunted diodes increases. List of symbols I0 Im Pns Pws
module module module module
current at no shadowing. current at one cell shaded. power at no shadowing. power with shaded cell.
References [1] R.G. Ross Jr. Photovoltaicmodule and array reliability, Proc. 15th IEEE PhotovoltaicSpecialist Conf. (IEEE, Orlando, 1981).
480
N.A. AI-Rawi et al. / Reliability of photouoltaic modules. H
[2] R.M. Turfler, T.J. Lambarske and K.E. Bardwell, Technique for Aggregating Cells in series and parallel, Conf. Proc. 14th IEEE 1980, pp. 518-322. [3] G.R. Mon, D.M. Moore and R.G. Ross Jr., J. Sol. Energy Eng. (November 1984). [4] R.G. Ross Jr., IEEE Trans. Reliability. R-31 (August 1982). [5] L.M. Stember, Sol. Cells 3 (1981) 269. [6] L.B. Page and E. Perry, IEEE Trans. Reliability R-37 (1988). [7] J.W. Bishop, Computer simulation of the effects and electrical mismatches in photovoltaic cell interconnection circuits, EST Project, Commission of the European Communities Joint Research Centre, Ispra, Italy, 1989. [8] B.W. Jenney and D.J. Sherwin, IEEE Trans. Reliability R-35 (December 1986). [9] M.A. Green, E.S Hasyim, S.R. Wenham and M.R. Wilison, Silicon solar cells with integral bypass diodes, Proc. 17th IEEE Photovoltaic Specialist Conf. Ressimmee 1984, pp. 513-517. [10] C.H. Cox, D.J. Oliver Smith and R.W. Mountain, Reduction of photovoltaic cell reverse breakdown by a peripheral bypass diode, Proc. 16th IEEE Photovoltaic Specialist Conf. San Diego 1982, pp. 834-339. [11] N.F. Shepared Jr. and R.S. Sugimure, The integration of bypass diodes with terrestrial photovoltaic modules and arrays, Proc. 17th IEEE Photovoltaic Specialists Conf. Ressimmee 1984, pp. 679-681. [12] R.M. Diamond, Advanced Developments of integral diodes solar cells, Proc. IEEE PV Specialists Conf. 1972, pp. 196-200. [13] M. Giuliano et al., Bypass diodes design application and reliability studies for solar cell arrays, Proc. IEEE PV Specialist Conf. 1981, pp. 997-1000.
Solar Ent~j~ Matt~l$
and Solar Cells
Reliability of photovoltaic modules II. Interconnection and bypass diodes effects N a b e e l A. AI-Rawi, M a a n M. AI-Kaisi a n d D h i a J. A s f e r Solid State Electronics Research Laboratory, School of Electrical Engineering, University of Technology, P.O. Box 35010, Baghdad, Iraq Received 9 July 1992
The objective of this study is to evaluate the effect of interconnection and bypass diodes on improving the reliability of a photovoltaic module. The study of module design has been carried out for different cell interconnections from simple series to a more complex series-parallel configuration aiming to do an evaluation and analysis to determine the optimum item configuration which achieves a reliable module of more than 90%. Bypass diodes are often required to limit the potential of a reverse voltage "HOT SPOT". The role of bypass diodes for module reliability improvements has been studied practically. From the results obtained, it is found that the reliability increases as the number of diodes increases. Many experiments to study the effects of interconnections on reliability have been carried out. Results from tests on many modules are presented and discussed.
1. Introduction
The ability of a PV electric generating system to satisfy the intended load is influenced by two phenomena. The first is the output power affected by the variation of the irradiance level, and the second is the component effect resulting from the changes of PV conversion efficiency of the module for one reason or another. Fault tolerant design techniques are used to minimize the complete failure of the PV module and array, but output decreases approximately proportional to cell failure. Because of the fault tolerant design of the array subsystem, individual solar cells and modules can fail but the system remains capable of generating power at a r e d u c e d level. F a i l u r e s i n s u b s y s t e m c o m p o n e n t s t h a t a r e n o t f a u l t t o l e r a n t c a n r e s u l t i n a c o m p l e t e loss o f p o w e r [1]. A c c o r d i n g t o t h e a i m a n d b a s i s o f r e l i a b i l i t y , t h e a b o v e m e n t i o n e d t e c h n i q u e is o n e o f t h e d i f f e r e n t r e l i a b i l i t y a p p r o a c h e s , w h e r e t h e l a t t e r is a c t u a l l y a f u n c t i o n o f c o m p o n e n t f a i l u r e [2]. 0927-0248/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0 9 2 7 - 0 2 4 8 ( 9 3 ) E 0 0 7 2 - L
470
N.A. AI-Rawi et al. / Reliability of photovoltaic modules. H
2. Interconnection and reliability To study the effects of the interconnections of the cell on module power and reliability, I - V characteristics have been measured using sun light simulator facilities with their software, to plot and calculate the various design parameters
[3-5]. For comparison, two types of crystalline silicon of modules (A) and (B) of different manufacturing origin have been tested. Each module consists of 16 cells arranged in a different configuration (m * n) as shown in fig. 1, where m is the number of parallel strings in the module. n is the number of cells in each string. The characteristics are made under the following conditions: (i) Normal operation (all cell illuminated evenly). (ii) One of the cells in the module is completely shaded. (iii) Four successive cells in the module are completely shaded. Fig. 2 shows the relation between maximum power and number of cells (n) in each parallel string in the module. For two types of modules (A) and (B), and when the module operates normally (no shadowing), it is shown that the maximum power increases as n increases, or in other words as the number of parallel strings (m) decreases. Fig. 3 shows the case when one cell in the module is completely shaded. It appears from the figure that the maximum power increases at small number of n and then decreases as n becomes greater than four, showing that the maximum power is obtained at a 4 . 4 arrangement. Fig. 4 shows the case when four cells in the module are completely shaded showing that maximum power occurs at the 4 . 4 arrangement. The effect of the different cell arrangements on cell and module efficiencies have been studied, where the I - V characteristics for the two types (A) and (B)
rn=l , n=16
m:2
, n=3
rn=4, n=4
I m=16
~ n=l
Fig. 1. Different cell arrangements.
471
N.A. Al-Rawi et al. / Reliability of photovoltaic modules. H 2110 B m.n =16
16.0
A
I
x
"-~ 12.0
/
a. g 0 E E X
4-0
NO SHADOWING
I 0.0 0.0 Number
I
I
,
,
4-0
8.0
12.0
1E~0
of c e i l s
in e a c h s t r i n g
Fig. 2. The effect of interconnection on the maximum output power of a module at no shadowing. have b e e n m e a s u r e d for the following modules configurations: 1 * 16 16 cells in series. 2.8 8 cells in series in two parallel strings. 4.4 4 cells in series in four parallel strings. 1 6 . 1 16 cells in parallel. Fig. 5 shows the relation between cell and m o d u l e efficiency with the n u m b e r of parallel strings m in the m o d u l e for type (A) and (B) w h e n all cells are illuminated equally, indicating that cell and m o d u l e efficiency decrease as m increases. T h e lower values represent the efficiency for a pure parallel connection of cells, while the higher values represent the efficiency for a pure series [6,7]. Fig. 6 shows the effect of interconnections on cell and m o d u l e efficiency, where one cell in the m o d u l e is completely shaded. While fig. 7 shows the case when four successive cells are completely shaded. 20.0 16.0
rn~ =!6
ONE CELL SHADED
3:
~, 12.0 o
a.
E
8-0
E x 4.0 O Ig
0.0
0.0
4.0 Number of c e l l s
8.0 in e a c h
12.0 string
16.0
Fig. 3. The effect of interconnection on the maximum output power of a module for one cell shaded.
472
N.A. Al-Rawi et al. / Reliability of photovoltaic modules. H
20.0 16.0
m,n=16
FOUR CELL SHADED
-..:lZ.O ,3 ¢v
¢
~.0 E
._E 4.o X O
0"~. 0
4.0
8,0
Number of cells In each
12.0
1I;.0
strincj
Fig. 4. The effect of interconnection on the output power of a module for four cell shaded.
Figs. 6 and 7 indicate that the maximum cell and module efficiencies is obtained when the arrangement is 4*4. The pure series and pure parallel do not give maximum output power due to the absence of fault tolerance and losses in wires respectively. To study the effect of PV cell interconnection on module reliability, complete shadowing has been made on cell(s) successively in the module (A and B) for different cell interconnection configurations to simulate a fault in the module. The Isc of the whole module after shadowing each cell has been recorded, and the smaller variation in current gives an indication to the reliability of the module. The experiments have been carried out under different conditions (irradiance and temperature) due to the outdoor nature of the test.
20.0
NO SHADOWIN6 16.C
m.n =1 6
1 cell type B 2 cell typeA 3 cell typeB
x ~X ~
"~ 4.0 0.0
0.0
,
,
t,.0 8.0 Number of parallel st.r;ncjs
,
12.0
,
16.0
Fig. 5. The effect of interconnection on module efficiencyat no shadowing.
N.A. AI-Rawi et al. / Reliability of photovoltaic modules• H
473
2043 ONE CELL SHADED re.n=16
0<
1 cell type B 2 cell type A
3 module type B
,2.0
4 m o d u l e type A (3
.c 8.0 ,e,w,-
,,, 4~] 0.0(~.0
4'.o
a:o
i~.o
1~.o
Number of parallel s t r i n g s Fig. 6. The effect of interconnection on module efficiency for one cell shaded.
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474
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Fig. 9. Variation in module current due to shadowing for the 2.8 connection. Fig. 8 shows the variation of module short circuit current and cell number, where each cell in the module (panel) has been numbered from 1 to 16 to examine problems caused by differences in individual cells parameters. Complete shadowing is made successively on one cell of a module, showing a high fluctuation in output current when cells are connected in series. This means that variation in output power indicates a low reliability for this type of connection. Fig. 9 shows the variations in short circuit current when an experiment is made on a module consisting of two parallel strings each with 8 cells in series, and shadowing is done on one cell successively in the module, showing that this kind of interconnection has less variation than with the series type of connection [8]. For a module consisting of four parallel strings each containing four cells in series, and after shadowing is made on one cell, the short circuit variation with respect to cell number is shown in fig. 10. It is shown that variation is minimized,
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N.A. AI-Rawi et al. / Reliability of photovoltaic modules. H 1.0
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Fig. 11. Variation in module current due to shadowing for the 16.1 parallel connection. meaning better reliability compared with the other types of configurations shown in figs. 8 and 9. The variations in module current due to shadowing for an arrangement of sixteen parallel cells is shown in fig. 11. It is clear that this arrangement has the smallest variation in output current compared with all kinds of connections, indicating that the most reliable interconnection is the one with sixteen parallel cells.
3. The bypass diode and reliability improvement The motivation of array designers is maximization of array output power over the operational life time of the installation with respect to shadowing and various internal mode failures [9-11]. Bypass diodes are frequently used to permit the current to pass through the circuit even when one or more of the series items fail totally. Bypass diodes are used to decrease the effect of the hot spot problem which arises in the module when the cell becomes back biased and operates in the negative voltage quadrant as a result of short circuit mismatch, cell cracking or shadowing [12,13]. To study the improvement gained by adding bypass diodes when physically and economically feasible, and to what extent it will increase the fault tolerance, which means the reliability improvement of a module of array system, experiments have been made on the following panels: (1) Laboratory panel consisting of 16 cell (type A). (2) Laboratory panel consisting of 16 cell (type B). (3) Commercial panel consisting of 36 cell (type A). (4) Commercial panel consisting of 32 cell (type B).
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Fig. 12. The effect of bypass diodes on output power under shadowing effect.
For the laboratory panels in (1) and (2) above which were made to our specification, and after complete shadowing is made on cell number one in the panel, the related data has been recorded for each panel. The same procedure is repeated successively on the remaining ceils (from 2 to 16) in the panel. These steps were done without bypass diodes and were repeated for one diode across the 16 ceils, two bypass diodes across 8 cells and four bypass diodes across 4 cells in the panel. For the commercial panel (in (3) above) no diodes, one diode, and two diodes are used. And for the panel in (4) above, no diodes and one diode are used due to the terminals available in the panel for diode connection. Fig. 12 shows the relation between the output power and the cell number for laboratory panels type (A) and type (B) respectively, for complete shading of the cell, and for different number of bypass diodes connected across groups of cells in the panel. From (a) and (b) in the above figures, it is shown that there is a noticeable reduction in the output power, since they have the same fluctuation and the same power reduction, indicating that one bypass diode has no effect in case (b), while in (c) and (d) the variation decreases and the output increases (mean reduction decreases) indicating that useful improvement is obtained using two and four bypass diodes.
N.A. Al-Rawi et al. / Reliability of photovoltaic modules. H
477
1.0 C] TYPE A ~l TYPE B NO DIODE USED
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Fig. 13. The variation of output power with shaded cell number for different numbers of diodes.
Fig. 13 shows the variation of output power with shaded cell number for different numbers of diodes, for commercial panels type (A) and (B) respectively, showing a similar trend as has been obtained in fig. 12.
4. Results and discussion
For a number of groups of different cell arrangements, the pure series connection gave the maximum output power. Whereas the parallel type of connection produces the least generating power, the other types of configurations gave output powers in between as shown in fig. 2. The maximum power from the panel under partial shadowing is obtained when the panel consists of four parallel strings each containing four cells in series as shown in fig. 3 and 4. This is different from the previous case where the maximum power occurs for panels consisting of pure series cells. Fig. 3 and 4 show that the panels have poor output power for the two extreme cases (pure series and pure parallel arrangements), this can be explained as follows: (a) In the series type, the fault (simulated by shadowing in this study) causes a large loss if not a complete loss of output power. That is because this type of connection has no fault tolerance.
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This gives the answer for the reason of poor output power in a series type of connection with partial shadowing, although it gives the highest power with no shadowing. (b) In the parallel connection and in spite of showing a high fault tolerance, there are large losses in the output power due to the losses in the wires, whereas the losses are proportional to r/2 (n is number of parallel cells) and the resistance of the wires (I2R). A series of experiments have been made on two types of panels for different cell interconnections in the panels. They show that in the series connection the highest cell and module efficiencies are obtained, while the parallel connection gave the lower values of cell and module efficiencies under normal operation (no shadowing). On the other hand ( 4 . 4 ) gave the optimum configuration under the shadowing or fault condition, as has been shown in figs. 6 and 7. The conversion efficiency of modules is less than that of the cells because of: (a) electrical losses in the interconnection, (b) the presence of an electrically inactive area between cells in the module (panel). The high efficiency obtained for type (B) cells can be attributed to manufacturing techniques and the design of the grid pattern.
Table 1 The average percentage of reduction in the output current due to artificial faults (shadowing). cells arrangement in modules
~ _ ~
_~_~
A v e r a g e p e r c e n t of r e d u c t i o n (%) in o u t p u t curreat due to shadowing f o r module ~ (A)
A v e r a g e p e r c e n t of r e d u c t ion (S) in o u t p u t c u r r e n t due t o shadowing faar modul e t y p e (B)
53.4 %
90.8 %
44.5 %
47.4 %
43.3 %
46.7 %
23.3 %
23.3 %
16.2 %
12.5 %
7.9
%
5.9
%
1.5
%
1.2
%
N.A. Al-Rawi et al. / Reliability of photovoltaic modules. H
479
From the sets of experiments carried to investigate the effect of different types of cell interconnections, two types of modules (A and B) have been tested. The principle used to find the best reliability (high fault tolerance) is by applying shadowing on successive cells on modules and monitoring the variation in output power. The following results have been found: (1) The pure series connection shows the lower reliability. (2) The pure parallel connection shows the highest reliability. (3) The above results are in good agreement with the theoretical results [13]. (4) The average percentage of reduction in the output current due to fault making (shadowing) on cells successively in the module, for the above two types and the other types of connection, are given in table 1. 5. Conclusion The objective of this study was to evaluate the effect of interconnection and bypass diodes on reliability of photovoltaic modules. A series of experiments have been carried out on two types of cells of different origin and different cells interconnection arrangements. It can be concluded from this study that, under normal operation (no shadowing), the losses in module output power is minimum when the cells are interconnected in a pure series connection, while it is maximum when ceils are interconnected in pure parallel schemes. Under abnormal operation (partial shadowing), the losses are high in pure series and parallel connections, while the losses are minimum in the combined (series-parallel) schemes. We found also that the electrical mismatch of cells due to shadowing reduces module efficiency. To study the role of bypass diodes on the reliability, shadowing is used to simulate a fault and the following is concluded: (i) Shunting the module with one bypass diode does not improve the reliability of the module. (ii) Connecting two diodes across the module improves the reliability. (iii) Reliability of the module increases as the number of shunted diodes increases. List of symbols I0 Im Pns Pws
module module module module
current at no shadowing. current at one cell shaded. power at no shadowing. power with shaded cell.
References [1] R.G. Ross Jr. Photovoltaicmodule and array reliability, Proc. 15th IEEE PhotovoltaicSpecialist Conf. (IEEE, Orlando, 1981).
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[2] R.M. Turfler, T.J. Lambarske and K.E. Bardwell, Technique for Aggregating Cells in series and parallel, Conf. Proc. 14th IEEE 1980, pp. 518-322. [3] G.R. Mon, D.M. Moore and R.G. Ross Jr., J. Sol. Energy Eng. (November 1984). [4] R.G. Ross Jr., IEEE Trans. Reliability. R-31 (August 1982). [5] L.M. Stember, Sol. Cells 3 (1981) 269. [6] L.B. Page and E. Perry, IEEE Trans. Reliability R-37 (1988). [7] J.W. Bishop, Computer simulation of the effects and electrical mismatches in photovoltaic cell interconnection circuits, EST Project, Commission of the European Communities Joint Research Centre, Ispra, Italy, 1989. [8] B.W. Jenney and D.J. Sherwin, IEEE Trans. Reliability R-35 (December 1986). [9] M.A. Green, E.S Hasyim, S.R. Wenham and M.R. Wilison, Silicon solar cells with integral bypass diodes, Proc. 17th IEEE Photovoltaic Specialist Conf. Ressimmee 1984, pp. 513-517. [10] C.H. Cox, D.J. Oliver Smith and R.W. Mountain, Reduction of photovoltaic cell reverse breakdown by a peripheral bypass diode, Proc. 16th IEEE Photovoltaic Specialist Conf. San Diego 1982, pp. 834-339. [11] N.F. Shepared Jr. and R.S. Sugimure, The integration of bypass diodes with terrestrial photovoltaic modules and arrays, Proc. 17th IEEE Photovoltaic Specialists Conf. Ressimmee 1984, pp. 679-681. [12] R.M. Diamond, Advanced Developments of integral diodes solar cells, Proc. IEEE PV Specialists Conf. 1972, pp. 196-200. [13] M. Giuliano et al., Bypass diodes design application and reliability studies for solar cell arrays, Proc. IEEE PV Specialist Conf. 1981, pp. 997-1000.