As a part of the Flat-Plate Solar Array Project (FSA), a field-test program was developed to obtain solar photovoltaic (PV) module performance and endurance data. These data are used to identify the specific characteristics of module designs under various environmental conditions. The information obtained from field testing is useful to all participants in the National Photovoltaics Program, from the research planner to the life-cycle cost analyst

Weaver, R. W.

English

NASA Technical Reports Server

.9q R84 2 1 9 8 4  _ .
FIELD TEST EXPERIENCE 
R.W. Weaver 
California Institute of Technology 
Pasadena, California 91109 
Jet Pro pu 1 s ion Laboratory 
INTRODUCTION 
As a part of the Flat-Plate Solar Array Project (FSA), a field-test 
program was aeveloped to obtain solar photovoltaic (PV) module performance 
and endurance data. These data are used to identify the specific char- 
acteristics of module designs under various environmental conditions. The 
information obtained from field testing is useful to all participants in the 
National Photovoltaics Program, from the research planner to the life-cycle 
cost analyst. 
TEST SITES AND DATA PROCESSING 
The Field Test Program plan identified four Southern California test 
sites with characteristics ranging from oceanside to desert environments, 
including one with high urban pollution. Test facilities at these sites 
were constructed and modules were deployed in 1977. All of the modules 
deployed were first tested and inspected at the Jet Propulsion Laboratory 
(JPL!. The testing was done using the Large-Area Pulsed Solar Simulator 
(LAPSS) to obtain I - V  (current-voltage) data, at a reference irradiance 
level and module temperature, the results of which were used as baseline 
data whenever the module was returned to JPL for special testing. The pre- 
deployment inspection was a detailed visual examination of the modules, from 
which an original-condition report was generated. Subsequent inspection 
reports were compared with this report to discover and identity physical 
changes in the module. 
IF 1978 the FSA Field Test Activity assumed responsibility for 12 more 
test sites, which had been established originally by the National Aeronautics 
and Space Administration (NASA) Lewis Research Center as part of the NASA 
energy program. These sites were situated as far north as Alaska and as far 
south as the Canal Zone, and covered virtually all climatic conditions. The 
characteristics of these sites and those of the four JPL sites are shown in 
Figure 2. Lewis Research Center also furnished JPL with all of the data 
that had been acquired for the modules at the 12 sites. The resulting site 
network consisted of 15 remote (unattended! sites and ope at JPL. 
?Ai0 data acquisition systems were developed, one for the remote sites 
and one for the JPL site. The data system for the remote sites was a port- 
able battery-operated unit that rampled I - V  data and displayed the key para- 
meters (e.g., st :-circuit current, open-circuit voltage, peak power). 
After acquiring che data the unit stored it on an erasable storage medium, 
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which is readable on the JPL si te data  system. 
ca l ly  from the remote sites using t h i s  uni t .  
Data vere acquired periodi- 
The JPL s i t e  data sys tem was designed t o  sample module performance 
A I 1  of the data were stored on magnetic disks  for processing by 
l%is system could a l s o  
da i ly .  
minutes. 
scheduled programs or by special  programs on demand. 
process data from the remote sites. 
The system a lso  takes weather and i r radiance data  every f ive  
The endurance data were obtained per iodical ly  from a l l  of the s i t e s  by 
means of physical inspections by a JPL quality-assurance team. 
of these inspections were wr i t ten  up as de ta i led  descr ipt ions of the  physical 
s t a t e s  of the mdules .  
inspection reports  t o  ident i fy  changes i n  the d u l e s  occurring during the 
test period. 
The results 
lhese descr ipt ions were then compared with previous 
FAILURE PROCESSING 
The performance and endurance data were used t o  determine i f  a f a i l u r e  
analysis  of a wdu le  were warranted; i f  so, the  module w a s  removed from the  
f i e l d  and returned t o  JPL for fur ther  analysis.  The c r i t e r i o n  for  perform- 
ance f a i lu re  was f a i l u r e  t o  produce 75% or more power than i t  did when it w a s  
o r ig ina l ly  tes ted.  I f  the module's physical state had deter iorated t o  the 
point tha t  it had become hazardous, or when performanc5 f a i l u r e  w a s  imminent, 
the module was t o  considered t o  have fa i led .  Failed modules were returned 
t o  the JPL f a i lu re  analysis  team for  de ta i led  ana lys i s  t o  determine the type 
and cause of the fa i lure .  
Reports and were d is t r ibu ted  t o  a l l  concerned i n  the  PV program, including 
the module manufacturer. 
The r e su l t s  were published as  ProbledFai lure  
RESULTS 
In  the nearly f ive years of f i e l d  t e s t ing  Blocks I, I1 and 111 modules, 
almost 10% fa i led  the performance t e s t .  Many more experienced physical 
degradation tha t  did,  or would eventually,  r e s u l t  i n  an unserviceable module. 
The plot  i n  Figure 5 depicts  performance f a i lu re s  as a function - f  time i n  
the f i e ld .  The curves show tha t  for  the Flocks I and I1 modules f a i l u r e  
r a t e  increased over th2 l a s t  18 t o  24 months. This means tha t  m. modules 
(per module deployed) were f a i l i n g  a f t e r  the f i r s t  three years than before 
tha t  time. Ibis leads t o  the conclusion that  there  is de f in i t e ly  a time- 
versus-ciesign cor re la t ion  for f i e l d  fa i lures .  
The r e s u l t s  of the physical inspections a r e  shown i n  the chart  i n  
Figure 6 for the Blacks I and 11 modules (type r e f e r s  t o  manufacturer). 
defects  a t e  ranked by severi ty  for each si te and type. 
as  being more severe than any other i n  the char t .  However, when the petform- 
ance data a re  a l so  considered, the sites with hot-humid climates c l ea r ly  have 
more severe environments. 
'Ihe 
No s i t e  stands out 
Some r e su l t s  from non-JPL sites are  described in  Figure 7. The causes 
of f a i lu re  are basical ly  the same as  for  the  JPL tests; only the r a t e  of 
60 
failures is d i f f e ren t .  The notable exception is the number of burs t  c e l l s  
observed a t  the Mount Laguna s i te .  The u l t i m a t e  e f f e c t s  of the three  most 
prevalent defects  are  shown in  Figure 8 .  A l l  of these defec ts  would 
eventually require  tha t  the module be replaced. Figures 9 and 10 descr ibe 
several  of the prime reasons for  c e l l  f a i lu re .  
During the test period, changes i n  the c e l l  g r id  and co l l ec to r  mate- 
r i a l s  w r e  observed. The most comM)n w a s  the d isco lor ing ,  usual ly  a brown- 
ing, of the gr id  and co l lec tor  mater ia l .  
sembled and the discolored area w a s  analyzed. 
react ion with some res idua l  mater ia l  from the manufacturing process. 'Ihe 
"blossoming" e f f e c t  found a t  the ends of some g r id  l i n e s  is a t t r i b u t e d  t o  
the migration of the  s i l v e r  used in  the g r id  mater ia l .  This e f f e c t  w a s  seen 
only in  modules tha t  used s i l v e r  i n  the g r id  mater ia l  and tha t  were 
configured so t ha t  the end of a gr id  l i n e  was near another pa r t  of the 
e l e c t r i c a l  c i r c u i t  tha t  was a t  much d i f f e ren t  e l e c t r i c a l  po ten t ia l .  
Several of the modules were disas- 
The probable cause w a s  a 
The t e s t  results a l s o  indicated other  reasons for l o s s  of power or 
module degradation. Some of these are presen:ed i n  Figure 12. The most 
severe,  r e l a t i v e  t o  the lo s s  of power, is the amount of d i r t  t h a t  is  
deposited on the module surface.  Power losses  of as much as 12% were 
observed within a three-month period. 
loss  from so i l i ng  w a s  tha t  with a g l a s s  supers t ra te  on the module. 
The best  design for  preventing power 
2PL f i e l d  test  r e s u i t s  were campared with test and operat ional  r e s u l t s  
from other centers  i n  the PV program t o  determine i f  s imi la r  f a i l u r e s  were 
occurring elsewhere. The consensus as  t o  the pr inc ipa l  causes of e l e c t r i c a l  
f a i l u r e  was: (1) cracked c e l l s ,  (2 )  broken interconnects ,  ( 3 )  various types 
of shorts .  The pr inc ipa l  types of physical degradation were: (1) delamina- 
t ion  of encapsulants,  ( 2 )  discolora t ion  of encapsulants,  ( 3 )  irr ternal cor- 
rosion of interconnects and g r id  connectors, (4: externa l  connector corrosion. 
There bppears t o  be no co r re l a t ioc  between the physical appearance of the 
module, d i r t  deposi ts  excepted, and performance. The most severe environment 
i s  the hot-humid type .  
A representat ive sample of the andules t h a t  were used i n  t h i s  test  pro- 
gram have been relocated a t  the JPL Goldstone site. 
annually t o  determine what e f f e c t  fur ther  t i m e  i n  the f i e l d  may have on the 
modules. 
Data w i l l  be sampled 
61 
Figure 1. Objectives of Field-Test Activity 
To obtain in-field performance data for life-cycle endurance evaluation 
To determine degradation characteristics and failure modes as they 
Provide verification data to qua!ification testing 
Develop improved in-situ diagnostic testing methods and 
relate to module design characteristics 
analytical techniques 
Figure 2. Field-Testing Historb 
1977 Establish four sites in Southern California 
Automatic data acquisition system installed at the JPL site 
(Block I and II modules) 
1978 Acquired 12 more test sites from Lewis Research Center 
(Block I and II modules) 
Developed a portable Module I-V data acquisition system 
Initiated semiannual inspections of remote sites 
Block 111 modules deployed to sites 
Data analysis techniques developed and applied to all 1979 
data available 
1 98 1 Remote sites decommisioned 
Final data analysis for Blocks I, II, and 111 performed 
1982 Started Block IV deployment and testing 
62 
I 
I 
I 
I 
Figure 4. Test and Inspection Procedures 
Testing 
Modvles were "stressed" via fixed resistors 
Baseline I-V data acquired during installation 
Periodic I-V data taken 
Performance evaluated 
Inspection 
Visual inspection prior to shipping to &e 
Visilal inspection during installation 
Periodic inspections 
Physical change description reports 
63 
ORIGYNAL PAGE !g 
OF POOR QUALliY 
Figure 5. Blocks I, II and 111 Results 
Figure 6. Physical 1ns;ection 
PRfVAlfNT PHYSICAL D E F X S  
CRACKfD CELLS 1 GROUND I E ~ I N A L  CORROSION 
EMBEDDED DIRT 
Summary of Remote-Site Modules 
+ . . , .  
I 
J 
I 
" MOOERATf AMOUNT 
' '. SIGNIFICANT AMOUNT 
' I EXlEEMt ColvDll lON 
CRACKED CELLS - 
FRAME SEACISUBSTRITE DEUMlNATlON - 
INTERCONNECT'Ctll SURFACE CORROSION - 
- 
_ _  
I NTE RCOtNCT B~AKTHROJCH 
' SUBSTRATf DrTf RlORATlON 
,W*BEDDL'l DIRT 
0 
TERMINAL S T R l P l S ~ L f R  JOINT DISCOLORATION 
RUBBER OUTPUT CONMCTOR DETfRIORATION - 24 
p IFRAMt S C R W  FASTENfR C O R k S l O N  
XVtRITV SCALE 
1- 
t SOME 
-
[MODULI FRAhlt CORROSION I 
64 
Figure 7. Results From Non-JPL Sites 
Mount Laguna, CA (July 1979 - July 1981 1 
Cracked cells: 1500 (950 "burst") 
Output: Down 55% 
Encapsulant: Delamination 
MIT/LL 
In-fmld. over 30 months: 6.5% failed 
Causes: Cracbd cells, broken interconnects and shorts 
Physical: Delamination, cracked glass 
Figure 8. Failure Effects 
TYPE EFFECTS -
Cracked cell Loss of power 
Hot-spot heating 
Loss of module 
Broken interconnect Loss of power 
Loss of module 
Short circuit Loss of power 
Loss of module 
Hazardous condition 
Figure 9. Causes of Cracked Cells 
Impact type 
Hail storms 
Rocks 
Other 
"Burst type" 
Outgassing of material between cell and substrate 
Moisture entrapment and subsequent heating 
Other causes 
Manufacturing defects 
Hot spotting 
Module twisting 
65 
Figure 10. Observed Changes in Grids and Collectors 
Discoloring 
Brown coloring - probably due to reaction with contaminants 
White streaking (GE) 77 
Separating from cell 
Manufacturing problem 
' 'Blossoming" 
Silver migrating to ends of grid that are at a high potential relative 
to  nearby cell or circuit component 
Figure 1 1. Other Reasons for Loss of Module Output 
Dirt 
2 to 12% loss 
Partially correctable via cleaning 
Glass is best self cleaner 
Discoloring of encapsulant 
Select proper materisl-glass 
Thermal related 
Cycling effects 
Expansion stress 
Match materials or compensate 
NSMD 
San Nicolas Island, Mines Peak, Pt. Vicente 
Hazards of field testing 
Figure 12. Conclusions 
Electrical degradation or failure is not necessarily a function 
Three primary known causes of failures were cracked cells, 
of physical appearance 
broken interconnects, and elec,trical shorts 
Most severe environment is hot and humid 
66 
DISCUSSION 
CAMPBELL: What percentage of Block I, I1 and 111 modules were glass 
superstrate? 
WEAVER: I can't give you a percentage but I think there were only two 
manufacturers that we tested during that period that used glass. 
LAVENDEL: You, in your failure analysis, mentioned discoloration deveral 
times. I s  this really a hazard or is it mostly cosmetic? 
WEAVER: I think it is mostly cosmetic. Like I said, we very seldom, if ever, 
could find an electrical performance degradation related to a 
discoloring of the system. 
LAVENDEL: Have you ever tried to define the composition of this discolored 
film? 
WEAVER: Yes, we did send it to our Failure Analysis group and I think they 
have found what other people have found, if they peel the encapsulant 
away. Brian (Gallagher), do you want to field that? 
GALLAGHER: I am going to give a short presentation this afternoon on metal 
degradation of a very specific encapsalant, and to answer your first 
question, you will see this afternoon that the first property that 
degrades that is visible is transmission at 400 nanometers: it sterts 
to turn yellow. To your question about whether it really degrades the 
modules or not -- if the yellow transmission at 400 nanometers degrades 
down to 10% of its original value, which looks like a lot, you only have 
from 5% to 10% degradation in the electrical properties of the nodule of 
the total integrated area from 400 nanometers to 1.1 micron. You would 
still only have 5% to 10% degradation. 
detail this afternoon. 
We will cover it a little more 
AMICK: You showed that Block 111 modules are much better from the standpoint 
of reliability than I or 11. Do you understand the reasons why the 
Block 111 modules have improved 80 dramatically? 
much the same. 
I and I1 look pretty 
WEAVER: Well, we would like to think it because we told them what was wrong 
Redundant interconnects came on very strongly in Block 
The redundant 
with I and XI. 
111, there were some in 11, but basically in Block 111. 
interconnects; a better understanding of stress relief in 
interconnects. Better encapsulation procedures, we think, c m e  into 
effect there. Glass, more glass. There was a Block 11 contractor that 
used glass that I don't think is still in the business of terrestrial 
PV; I think they are still in the space business, and some of their 
Block I1 modules are actually putting out more now than when we 
originally put them in the field. 
so expeneive there was no point in going on. 
That was a small cell. But they were 
67 
SCWUTTKE: On the subject of electrical migration: was this typical of all 
modules? At what distance from anode to cathode did it occur, and is it 
typical for all metallurgies -- that is, for all modules? 
WEAVER: I will defer that to either Ed (Royal) or Gordon (Man), because they 
understand that better than I do. 
ROYAL: Gordon (Mon) is going to talk about that this aiternoon. 
WEAVER: I can answer that to some extent -- no, we have not seen it in all of 
them. In the ones that we understand have silver, yes, we have seen it. 
PROVANCE: Have you observed any phenomena with this discoloration per 
location -- in other words was it more prominent in one location than in 
another? The reason I ask that is that sulfur tends t o  sulfide in areas 
of high sulfur concentration, so if it is in an industrialized area some 
of the discoloration, I would think, would be from the sulfiding. 
WEAVER: No, I don't think I could correlate that to an area. The site at JPL 
is the worst ur5an environment, pollution-wise, that we found. Mines 
Peak had almost none. Almost no discoloring at all. 
SCHWUTTKE: But you lost all of your modulcs there-- 
WEAVER: On the last inspection. 
PROVANCE: We have seen this quite prominently in other thick-film 
applications, in microelectronic circuits where silver or 
platinum-silver compositions will tend to discolor or sulfide very 
quickly in various areas of high concentration of industry. 
longer periods of time for the same discoloration to occur in very clean 
areas. 
But much 
WEAVER: The worst case I have seen of it was at Cape Canaveral at the Florida 
Solar Energy Center. Very predominant in those modules there. 
68 


Field test experience

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