this work was performed by the jet propulsion laboratory, · 2016. 7. 17. · solar array (lssa)...

This work was performed by the Jet Propulsion Laboratory , California Institute of Technology , under NASA Contract NAS7- 100 for the U. S . Energy Research and Devel opment Administration, Division of Solar Energy.

The JPL Low- Cost Silicon Solar Array Projec t is f und ed by ERDA and fo rms part of the ERDA Photovoltaic Conversion Program to initiate a maj or effor t toward the devel opment of low- cos t solar arr ays .

5101-27

SUMMARY RESULTS OF BLOCK I (46 kW) MODULE TESTING

May 2, 1977

J. S. Griffith S. G. Sollock

Approved by:

JET PROPULSION LABORATORY CALIFORNIA INSTITUTE OF TECHNOLOGY

PASADENA, CALIFORNIA

5101-27

DISTRIBUTION

Alexander, P. --------------------------------- FHB-201 Anhalt, K. ------------------------------------- 72-112 Arnett, J. ------------------------------------ 157-507 Bickler, D. ----------------------------------- FHB-201 Bishop, W. ------------------------------------- 72-112 Boyd, D. -------------------------------------- FHB-201 Callaghan, w. --------------------------------- 169-422 Carroll, W. ----------------------------------- 157-507 Chamberlain, R. ------------------------------- 156-223 Coulbert, C. ----------------------------------- 67-201 Cumming, G. ------------------------------------ T-1073 Doane, J. -------------------------------------- 79-200 Downing, R. ------------------------------------ 198-B9 Dumas, L. ------------------------------------- 157-102 Forney, R. ------------------------------------ 169-422 Gallagher, B. --------------------------------- FHB-201 Goldsmith, J. --------------------------------- 169-422 Greenwood, R. ---------------------------------- 198-B9 Griffith, J. -------------------------------------- 150 Grippi, R. ------------------------------------ 158-224 Hasbach, w. ----------------------------------- 125-147 Headrick, E. ---------------------------------- 248-101 Jaffe, P. ---------------------------------------- 79-6 Josephs, R. ----------------------------------- FHB-201 Lawson, Leipold, Lutwock, Maxwell,

A. ------------------------------------ FHB-201 M. ----------------------------------- 157-507 R. ----------------------------------- 125-147 H. ----------------------------------- 157-315

McDonald, R. ---------------------------------- 169-422 D.·------------------------------------- 157-410 Moore,

Mueller, Ross, R. Runkle, L. D. Schneider, E. Shumka, A. Sidwell, L. Simoneau, F. Sollock, S. Stirn, R. Stultz, J. Tsou, P. Wang, J.

R. ------------------------------------ 198-B9

iii

157-507 157-102 180-504 158-205 125-147 190-247 158-205 198-201 157-102 156-203 144-218

5101-27

CONTENTS

I. INTRODUCTION---------------------------------------- 1-1

II. MODULE DESCRIPTION---------------------------------- 2-1

III. QUALIFICATION TESTING------------------------------- 3-1

A. PROCEDURES------------------------------------ 3-1

B. EQUIPMENT ANp FACILITIES---------------------- 3-1

1. Large-Area Pulsed Solar Simulator------------- 3-1

2. Inspection Facilities------------------------- 3-1

3. Module Handling and Storage------------------- 3-6

4. Test Chambers--------------------------------- 3-6

C. RESULTS AND DISCUSSION------------------------ 3-6

IV. EXPLORATORY TESTING--------------------------------- 4-1

A. HUMIDITY-FREEZING----------------------------- 4-1

1. Procedures------------------------------------ 4-1

2. Equipment and Facilities---------------------- 4-1

3. Results and Discussion------------------------ 4-2

B. SALT FOG-------------------------------------- 4-2

1. Procedures------------------------------------ 4-2



C. HARD RAIN------------------------------------- 4-2

1. Procedures------------------------------------ 4-2



iv

5101-27

D. HEAT-RAIN------------------------------------- 4-6

1. Procedure------------------------------------- 4-6



E. HUMIDITY-HEAT--------------------------------- 4-7

1. Procedure------------------------------------- 4-7



F. FUNGUS---------------------------------------- 4-8

1 • Procedure 4-8



G. WIND LOADING---------------------------------- 4-10

H. ELECTRICAL ISOLATION TESTING------------------ 4-10

1. Purpose--------------------------------------- 4-10

2. Procedures------------------------------------ 4-11


4. Results--------------------------------------- 4-11

5. Recommendation for Electrical· Isolation Testing----------------------------- 4-12

I. THERMAL RESPONSE------------------------------ 4-12

1. Procedure------------------------------------- 4-12



V. PROBLEM/FAILURE ANALYSIS---------------------------- 5-1

A. GENERAL 5-1

V

5101-27

B. DISCUSSION BY MODULE TYPE--------------------- 5-2

VI. CONCLUSIONS AND RECOMMENDATIONS--------------------- 6-1

REFERENCES------------------------------------------------ R-1

Figures

1. Physical Appearance of the Five Module Types -- 2-2

2. LAPSS Control Console, I/0 Teletype, and XY Plotter--------------------------------------- 3-2

3. A Zeiss Farview Stereo Microscope Used to Inspect a Solar Module------------------------ 3-2

4. Use of a Plastic Foam Lined Cardboard Carton for Transporting and Storing Solar Modules

5. Solar Module Installation in a .9m x .9m x .9m

3-2

(3' x 3' x 3') Temperature Humidity Chamber --- 3-2

6. Hard Rain Test-------------------------------- 4-5

7. Test Setup for Heat-Rain Test----------------- 4-5

8. Lamp Bank Used for Humidity-Heat and Heat-Rain Testing--------------------------------------- 4-5

9. Typical Cell Temperature Measurements--------- 4-14

10. Module Design Thermal Effectiveness----------- 4-15

11. Interconnect Spike Impinging the Metal Substrate------------------------------------- 5-4

12. Encapsulant Split at Edge of Cell------------- 5-5

13. Arrows Indicate Split Encapsulant and Crack Caused By Entrapped Air----------------------- 5-5

14. Delamination Caused by Entrapped Air Underneath the Cells------------------------------------- 5-6

15. Delamination of Cell and Resulting Cracked Cell Caused by Entrapped Air----------------------- 5-6

vi

Tables

1 •

2.

3.

4.

5.

6.

1.

8.

9.

10.

11.

12.

13.

5101-27

Description of Solar Modules------------------ 2-3

Summary of Data from the Qualification Tests 3-4

Comparison of Early and Late Production Modules--------------------------------------- 3-6

Results of Humidity-Freezing Tests------------ 4-3

Results of Salt Fog Tests--------------------- 4-4

Results of Hard Rain Tests-------------------- 4-6

Conditions of Heat-Rain Tests----------------- 4-7

Results of Heat-Rain Tests-------------------- 4-8

Results of Humidity-Heat Tests---------------- 4-9

Thermophysical Properties of Solar Module Materials------------------------------------- 4-16

Optical Properties of Solar Module Materials------------------------------------- 4-16

P/FR Status and Cause Summary----------------- 5-1

Sumnary of Identification Problems and Number of Occurrences----------------------------------- 5-2

ACKNOWLEDGMENT

Appreciation is expressed to J. Arnett for his contribution in the area of electrical isolation testing, and to J. Stultz and L. C. Wen for their report on temperature characterization.

vii

5101-27

SECTION I

INTRODUCTION

On July 18, 1975, a Request for Proposals was issued for the procurement of terrestrial solar cell modules by the Low-Cost Silicon Solar Array (LSSA) Project. Five contractors were selected, and 58 kW of modules were ultimately delivered for use in ERDA Test and Applications Projects. Most of these modules were delivered during the summer of 1976, and were intended to be representative of commercial products available at the time.

Sample modules of this procurement, designated ''Block l" or 11 46 kw", were subjected to contractually required qualification tests by each manufacturer and by JPL. Additional exploratory tests were carried out by JPL to develop test methods for future procurements and to characterize module performance under various environmental conditions. This report summarizes the content and results of the JPL test program.

Selection of a particular manufacturer's module for a given application depends on a number of factors in addition to those addressed by this test program. No endorsement of a particular manufacturer's product should therefore be inferred from these results; rather, the intent was to develop test methodology and determine which types of tests best revealed design and workmanship defects. In general, correlation between the tests described herein and actual .field service has yet to be established.

1-1

5101-27

SECTION II

MODULE DESCRIPTION

The principal features of the five module types are given in Table 1 as determined for a representative sampling of late production modules. Figure 1 illustrates their physical appearance.

2-1

w

tc \\\\\\\\\\.-/

y

F· i gu re l · Ph

X

·•81.,., .. . ............ YSica1 A

PPearanc e of th

V

e Five Module T

YPes

VJ .... 0 .... I z f\) ~

5101-27

Table 1. Description of Solar Modules

Vendor Code V w X y z

Substrate Finned Alum. Acrylic Epoxy Epoxy Alum. Tee Glass Glass

Encapsulant RTV RTV none Silgard Silgard 615 615 184 184

Encapsulant Cover none Glass none Dow QR-4-3117

Surface Hardness 46 .6 55.0 59.4 (Shore 0-2), Avg.

Dimensions, cm

Length 57 66 41 51 61

Width 17 12 33 26 37

Thickness, Max. 2.4 6.8 4.4 0.6 3.3

Module Area,. cm2 942 822 1351 1321 2272

Cell Area, cm2 521 420 594 860 1376

Number of Cells 25 20 24 18 22

Weight, Avg. , g 1262 1566 1563 1142 2589

Power Rating, 100 mw/cm2

28° c,

Nominal Watts 5 5 5 9.3 14.47

Nominal Volts 9.2(Min) 9.2-10.0 9.2-10.0 7 9.2(Min)

Average Measured Pmax• W 5.76 5.24 4.95 9.39 14.99

Efficiency, Percent

Based on Cell Area 11.0 12.5 11.0 10.5

Based on Module Area 6. 1 6.4 7. 1 6.6

Watts per kg 4.56 3.35 3. 17 8.22 5.79

2-3

5101-27

SECTION III

QUALIFICATION TESTING

Qualification tests included temperature cycling and humidity, as described below. The vendor and JPL tested initial production modules; also, the vendors were required to test one module for each 1 kW production in these environments.

A. PROCEDURES

The procedures used for the qualification testing are given in Reference 1. The temperature cycling was between +90°C and -40°C at a temperature rate of 100°C per hour for 50 cycles. The humidity test duration was 168 hours at 70°C and 90% relative humidity.

B. EQUIPMENT AND FACILITIES

1. Large-Area Pulsed Solar Simulator

Electrical performance measurements for the solar modules utilized a large-area pulsed solar simulator (LAPSS) in a specially darkened room. The LAPSS was procured from Spectrolab, Inc. (Ref. 2). The light pulse was powered by a capacitor bank power supply that discharged in about 3 milliseconds through dual xenon arc lamps at one end of a darkened room. The light pulse traveled to the other end of the room through three baffles (to reduce reflection) to a target area about 13 meters away. There, the individual modules were mounted on a vertical panel, adjacent to a standard cell. These units were connected to a control console and I/0 equipment (Fig. 2).

After the proper parameters were fed into the computer in the console and the run was started, the operation was automatic. The capacitors were discharged through the lamps when a predetermined voltage was reached. The module output was swept with a variable electronic load at a time interval when the arc was fairly steady. The standard solar cell monitored the irradiance at the target plane. The cell provided simultaneous correction factors for a series of data points taken as the load varied. Data were stored in the computer temporarily and then printed on the teletype and drawn on the X-Y plotter simultaneously. Data were generally corrected for temperature (to 28°C or 60°C) and to 100 mW/cm2 irradiance. Standard cells were calibrated and supplied to JPL by the NASA Lewis Research Center in accordance with the procedures outlined in Reference 3.

2. Inspection Facilities

Physical inspection was performed on all modules upon receipt at JPL and again after completion of an environmental test. Besides the standard inspection equipment (calipers, scales, surface hardness testers), several

~1

I.A)

I N

Figure 2. LAPSS Control Console, I/0 Te l e t y p e , a nd XY Plot ter

Figure 4. Us e of a Plastic Foam Lined Carboard Carton for Transporting and Storing Solar Modules

Figure 3. A Zeiss Farview Stereo Microscope Used to Ins pect a Solar Module

Figure 5 . Sola r Module Installation in a .9m x .9m x . 9m (3 ' x 3 ' x 3 ' ) Temperature Humidity Chamber

\J1

0

I N -..J

5101-27

Zeiss Farview Stereo microscopes were used by the inspectors (Fig. 3). A Polaroid MP-4 multipurpose camera and a full set of lenses and accessories were ·available for photographic records ranging from full module views to 35X macrophotographs of defects.

3. Module Handling and Storage

Handling and storage arrangements are described in Reference 1. Modules were generally kept in special cardboard cartons lined with plastic foam (Fig. 4). In one case, the vendor's polyurethane shipping container was used. Modules in cartons were stored in standard metal and wooden cabinets when not in use.

4. Test Chambers

Most of the environmental testing at JPL was done in three temperature/humidity chambers (Fig. 5). The modules were installed in special stainless steel racks with Teflon slots to hold the modules. All were basically of 0.9 m x 0.9 m x 0.9 m (3' x 3' x 3') internal dimensions, with mechanical refrigeration. All could readily provide the test conditions required in Paragraphs 11 and 12 of Reference 1. Especially cut cams were used to control chamber temperature and humidity. To handle overflow test work, an outside testing laboratory was used, which provided a test chamber utilizing gaseous CO2 refrigeration.

C. RESULTS AND DISCUSSION

Table 2 presents a summary of the principal data from the 46 kW environmental tests. The modules tested are designated by phases and lots, where the phase indicates the time of production and the lot identifies design differences:

Phase

Phase

Phase

Lot

w Lot

w Lot

X Lot

y Lot

y Lot

1

2

3

1

2

3

2

2

3

Early production modules

Late production modules

15 kW add-on (Z type)

First modules received

Modules tested by vendor; no tests at JPL

Improved interconnects and terminal connectors

Loop at bus bar corners, shorter bus bars, improved bus bar cementing

Humidity tested and selected individual cells

Modules tested by vendor; no tests at JPL

3-3

V

Phase 1 Phase 2

Mean o/oSD Mean o/oSD

Electrical Performance, Modules as Received

PM' Power, Max, W/% 5. 77 1. 5 5, 76 4.0

1sc• Short Circuit Current, A/% o. 550 o. 8 o. 554 2. 6

voe• open circuit voltage, V/% 14. 14 o. 6 13. 98 o. 5

FF, fill factor o. 74 0. 74

Efficiency, cell area basis, % - 11

E!f ici ency, module area basis, % - 6. 1

PM Electrical Degradation >5% >10% >5% >10%

After temperature cycling, pct. of sample 0 0 0 0

After humidity test, pct, of sample 25 0 0 0

Physical Degradation, Degree: l to 4 (worst) Pct. Deg Pct. Deg

After temperature cycling Delamina ti on, % of sample/ degree 50 4 0 Cracked cells,. % of sample/degree 25 1 0

After Humidity

Additional delam, %/degree 50 4 0

· Additional cracked cells, %/degree 25 l

Discoloration of cells ]00 1

Broken intcon strands or cracked intcon

Cracked terminals or bus bars

PFR I s., percent of sample 50 50

Percent of sample, serious degradation 50 0

Sample size 4 4 4 4

NOTES:

(I) Tests were run for 350 total hours of humidity also: 75% were > 5% elect. degrad. 25% >10%

(2) 33% surface cracks, degree 3 (3) 100% box top loose or off (4) 17% surface cracks, degree 2 (5) Different modules tested in temperature cycling, humidity (6) One temperature sample (7) Humidity Fi tst

Phase ·1 Lot 1

Mean °loSD

5.40 1. 9

o. 653 3, 7

11. 64 I. 2

0, 7 l

--

>5% >IO%

25% Open

100 100

100 i

25 I

0

0

0

100 3

100 1

175

75

4 4

w X Phase 1

Phase 2 Phase I Phase 1 Lot 3 Lot l Lot 2

Mean %SD Mean o/oSD Mean o/oSD Mean o/oSD

5. 33 1. 1 · 5. 24 3. 0 5.25 2. 8 5, 31 2. 6

0.643 2.4 0.638 3. 3 0.566 1. 2 o. 562 1. 3

11. 66 o. 5 11. 57 o. 8 13.86 o. 4 13. 79 o. 7

o. 71 o. 71 0,67 o. 6 8

- 12. 5 - -- 6.4 - ->5% >10% >5% >10% >5% >10% >5% >10%

.0 0 0 0 100% Opens 25% Open

50 25 2 5 ( 1) 0 (1) - - No change

75 2 100 1 100 l 0 0 0 0 0 67

- 0

- 0

25 1

0 0 - -0 100 I 100 4 0

150 0 100 100

25

4 4 4 4 4 4 3/4 3/4

y

Phase 2 Phase 1 Phase 1 Phase 1 Phase 2 Lot l Lot 2 Lot 4 Mean o/oSD Mean %SD Mean o/oSD Mean o/oSD Mean %SD

4,95 o. 9 10. 18 2. 6 9.67 3. 3 9. 25 3. 8 9. 39 3, 8

0.524 3. 5 1. 476 2. I 1. 461 4. 0 1. 423 2.4 1. 53 4. 8

13. 76 0, 3 10. 52 0.7 10, 38 o. 6 10. 38 1. 7 lo. 3.0 1. 0

o. 69 o. 66 0.64 o. 63 o. 60

- - - - 1 1

- - - - 7, 1

>5% >10% >5 >10 >5 >10 >5 >10 >5 >IO

50 50 0 25/ 0 0 0

(25% Open) 0

- - 100 75 /7 5 67 0 0

0 0 100/ 4 0 0

75 4 0 25/ 2 0 0

- 0 0 0 0

- 0 0 0 0

0 0 0 25

0 0 0 0

75 0/7 5 100/58 0 25

0/75 100/67 0 0

4 4T 4T

4 4H(5) 12H(5) 4 4

5101-27

Table 2. Sunmary of Data from the Qualification Tests

z Phase I Phase 1 Phase ]

Phase 2 Phase 3 Lot 1 Lot 2 Lot 3 (Add-On)

Mean o/oSD Mean o/oSD Mean %SD Mean %SD Mean o/oSD

15. 50 1. 6 14.60 3. 0 14. 79 3. 3 14. 99 3. 2 14,85 4. 9

1, 707 2. 2 I. 616 2. 2 1. 628 2. 0 1. 624 2. 0 1,614 3. 9

13. 08 0,6 12. 7 1 4. 6 12. 91 o.6 12.94 o. 9 12. 89 o. 7

o. 69 o. 71 o. 70 o. 71 o. 71

- - - 10. 5 -- - - 6. 6 ->5 >10 >5 >10 >5 >10 >5 >10 >5 >IO

17% Open J6) 25 0 25 25 50 25

No change 0 0 0 No change No change

67 1 0(6) 0(7) 100 3 0

0 0(6) 0(7) 100 4 0

20 1 (3) 40(3,6) Slight (3) 50

(2) (2) (4) (4) 0(6) 25 1

0 0 0 0 0

33 4 0 20 0 0

82 20 20 17 5 50

33 0 20 100 25

6 5 5 4 4

3-~

Y Lot 4

Z Lot 2

Z Lot 3

5101-27

Palladium added to titanium-silver contacts

Improved processing of top coating on encapsulant

No top coating

Table 2 is largely self-explanatory. The top portion of the table lists the average electrical characteristics of the modules, together with the percentage standard deviation for these data. The lower portion of the table lists electrical and physical degradation resulting from the tests. The two qualification test environments, temperature cycling and humidity, caused considerable degradation of early production modules, but the design modifications previously noted solved the worst of these problems. Degradation noted on late production modules was generally traceable to workmanship defects. See Section V for further details. Table 3 provides a comparison of problems in early modules and late production modules.

3-5

5101-27

Table 3. Comparison of Early and Late Production Modules

Vendor Phase 1 Phase 2 Phase 3

V Serious delamination. Satisfactory except for Electrical degradation was sti 11. minor cell discoloration a problem after temperature cycling. and occasional cell cracks. Minor delaminotion still present.

w Serious problems with Some improvement was interconnector strand made for Phase 1, lot 2, breakage, delamina- but problems still existed tion, and electrical during Phase 2, espec i a II y degradation dueto delamination and electri-humidity cal degradation from

humidity.

X lot 1 developed Little improvement. Testing opens at bus bar was discontinued after joints. lot 2 devel- temperature cycling. oped an open at the eel I/interconnect contact, cracked cells both in the cen-ter of eel Is and at i nterconnect-cel I contacts.

y Serious electrical de- Satisfactory. Only minor gradation in humidity discoloration observed. of Lots 1 and 2. No problems after pal la-dium was added in contact process for Lot 4.

z The top hard surface A new problem, delamina-of the encapsulant tion and eel I cracking, showed severe crack- occurred. ing in temperature cycling and some de-lamination. Occo-sional interconnect breaks. The junction box covers came off and sometimes the box sides loosened.

3-6

.,

5101-27

SECTION IV

EXPLORATORY TESTING

A number of supplemental tests were run on sample modules to characterize performance and evaluate techniques of environmental testing. Tests in these environments were not a requirement under the contract. These environments included the following:

( 1 ) Humidity-freezing

(2) Salt fog

(3) Hard rain

(4) Heat-rain

(5) Humidity-heat

(6) Fungus

(7) Wind loading

(8) High voltage

(9) Thermal response

The facilities used for these tests were similar to those described in Section III, with exceptions as noted in the following detailed discussion.

A. HUMIDITY-FREEZING

1. Procedures

This test simulated high humidity followed by freezing. The procedure was based on MIL-STD-202E, Method 106D, except that no vibration test was included. The temperature in the chamber was cycled from ambient to 65~C and 95% relative humidity twice; then the temperature was lowered to -1o~c for three hours. The test was repeated for a total of 10 cycles. Modules were installed almost horizontally in the chamber. Droplets of condensed moisture were generally frozen onto the surface of the modules.

2. Equipment and Facilities

The standard 0.9 m x 0.9 m x 0.9 m (3' x 3' x 3') environmental chambers used for qualification testing were suitable for humidityfreezing. The other equipment described in Section III was used for this test.

4-1

5101-27

3. Results and Discussion

The results showed that humidity-freezing was a comparatively severe environment with respect to physical degradation, although all these late-production modules showed less than 5% electrical degradation. Vtype modules uniformly discolored, and one module delaminated somewhat. W modules showed considerable physical degradation, resulting in P/FRs for all modules. The colored spots in the Y modules were found to be unimportant. These modules had essentially no degradation during these tests. The Z modules delaminated and developed junction box problems. The Z modules had previously undergone salt fog tests; the other three types were previously untested. Further information is set forth in Table 4.

B. SALT FOG

1. Procedures

The salt fog test procedure used was MIL-STD-810C, Test Method 509.1. A~er suspending the modules vertically in a test chamber, the temperature was raised to 35°C and the humidity to 95%. A concentrated salt solution was sprayed from an atomizing nozzle into the chamber continuously for two days. An electrical performance test was performed within one hour of module removal from the chamber, and the test was repeated two days later after dryout.


The salt fog chamber was a large test chamber lined with a noncorrosive plastic-fiberglass composite. An external tank contained concentrated sodium chloride solution, which was drawn from the tank by a pump and ejected continuously into the chamber through an atomizer nozzle. The solution was not recirculated.


Salt fog testing for two days appeared to have a negligible effect on these modules. Some minor corrosion of modules with metal frames was observed. Table 5 summarizes the results.

C. HARD RAIN

1. Procedures

The hard rain test simulated a 40-mph wind-driven rain with an average droplet size of 2 mm. No wind was used; water velocity was provided by discharging water under pressure through nozzles. Individual modules were mounted on a motor-driven geared-down shaft parallel to their long axes (Figure 6). Three nozzles mounted at various angles caused water impingement on the modules from the side and ends. Shaft rotation provided exposure of the module to the rain from 360°--front, back, and

4-2

S/N

V781

V809

V812

V813

W05901

W05909

W05913

W05945

Y03232

Y03238

Y03243

Y03254

Z7670720

Z7670756

Z7670759C

Z7670761C

5101-27

Table 4. Results of Humidity-Freezing Test

Electrical Degradation,

Percent

-0. 7

-1.4

-2.8

-0.9

+O .9

-1. 1

-3.3

-2.8

+0.5

+1.8

+0 .1

+O .6

-0.4

-1.6

-4.8

-0.3

Physical Changes, Comments

Cell discoloration, delamination

Cell discoloration

Cell discoloration

Cell discoloration

Encapsulant delamination to glass cover

Cell discoloration, delamination and stains

Cracked insulator, delamination and stains

Cracked insulators, delamination and stains

Red-yellow spots in encapsulant




Delamination at 10 cells, name plate, junction box cover loose

Delamination

Electrical degradation

Junction box filler cracked, some delamination

4-3

S/N

V781 V809 V812-V813

W05901 W05909 W05913 W05945 Y02340 Y03255 Y03263 Y03270

Z7670720 Z7670756 Z7670759C Z7670761C

5101-27

Table 5. Results of Salt Fog Tests

Electrical Degradation,

Percent Moist Dry

-1.2 -0.7 +0.4 o.o

-0.3 -0.2 -0.2 0.0

+3.2· +2 .1 +1.1 -1.0 +0.6 0.0 -0 .5 -0.4 +1.7 -0.3 +1.4 +0.2 -1. 9 0.0 -0.1 -1. 7

Physical Changes, Comments

None None None None None None None None None None None None None None None Encapsulant delaminated at positive terminal

edges. The water was deionized and provided at a rate of about 20 liters/minute (5 gpm), a much higher rate than terrestrial rainfall. Fifteen minutes of rain exposure were provided. Electrical performance testing was performed in less than one hour after exposure.


The hard rain equipment is shown in Figures 6 and 7. The four deionizer tanks shown in Figure 7 provided about 8,000 liters (2,000 gallons) before they had to be exchanged for fresh tanks. This portable water supply stand was used for both hard rain and heat-rain tests (Paragraph IV-D). Tap water supply was used and regulated.


Electrical degradation of modules after this test was negligible. The increase in output of the Z modules is not understood. This is currently under investigation.

4-4

5101-27

Figure 6 . Hard Rain Test

Figure 7 . Test Setup for Heat Rain Test

4- 5

Figure 8. Lamp Bank Used for Humidity- Heat and Heat- Rain Testing

5101-27

Table 6. Results of Hard Rain Tests

Electrical S/N Degradation, Physical Changes, Comments

Percent

V781 +1 .8 None V809 +0.9 None V812 +1. 1 None V813 +1.4 None

W05901 -0. 4 None W05909 0.0 None wo5913 +0.2 None w05945 +0.6 None Y03232 +O .8 None Y03238 -3.8 None Y03243 -1. 7 None Y03254 -1.8 None

Z7670720 +5.0 Anomalous increase in power output Z7670756 +5.5 Anomalous increase in power output Z7670759C +6.3 Anomalous increase in power output Z7670761C +4.8 Anomalous increase in power output

Test Conditions:

Minutes of exposure 15 Shaft Rotation Rate, rpm 5 Droplet size, avg., mm 2 Drop velocity, avg., m/s 17

D. HEAT-RAIN

The test simulated the effect of a sudden hard rain falling on modules previously heated.by a clear-day sun on a warm day.

1. Procedure

The initial heating of the modules could be done outdoors in the sun or indoors under a lamp bank, although all tests reported here were done outside.

When heated outdoors (Figure 7), the test was limited to clear, warm days with low wind. The modules were mounted on a rack which could be tilted and rotated manually for approximately normal incidence to the sun. Thermocouples on the back of each module were connected to a recorder. The modules were allowed to warm in the sun to a stable temperature. The rain was then turned on. This device sprayed the modules with deionized water

4-6

5101-27

at a rate of over 2.5 cm (1 inch) per hour. After the modules reached a stable temperature, the water was turned off. The cycle was repeated a total of five times. Details are shown in Table 7.

Table 7. Conditions of Heat-Rain Tests

Supplier V w y z

Date of test 2/17/77 2/18, 22 2/17/77 2/18, 22

Time of day 1000 to 0945 to 1000 to 0945 to 1400 1330 1400 1330

Number of cycles 4 5 4 5

Maximum module temps., oc 43 to 48 38 to 44 52 to 58 52 to 66

Minimum module temps., oc 23 18 24 24

water temp. , oc 16 17 16 17

Ambient air temp., oc 27 to 29 16 to 27 27 to 29 16 to 27

Time to.heat to a stable 20 to 25 28 to 36 20 to 25 28 to 36 temp., minutes

Time to cool to a stable 8 8 8 8 temp., minutes


Outdoor equipment used for this test is shown in Figure 7 and described in Paragraph C-2. The alternative indoor heater is described in Paragraph E-2.


Weather conditions were favorable in February, 1977, for running this test. Ambient air temperatures w~re over 27°C during the test of the first eight modules. The plastic substrate module (Y-type) reached 65.5°C. The ambient temperature dropped to 16°C during the test on the second day with the Wand Z modules, but 65.5°C was still reached on the plastic Z module.

4-7

5101-27

The test results (Table 8) showed very little change in module electrical output or physical appearance, except the Z modules, which showed electrical output decreases averaging 8%. This problem is still under investigation.

Table 8. Results of Heat-Rain Tests

Electrical S/N Degradation,

Percent

V-781 -1.3 V-809 -1.5 V-812 -1.2 V-813 -1.9

W-05901 -1.3 W-05909 -1. 7 W-05913 -1.5 W-05945 -1.5 Y-03232 -2.4 Y-03238 -0.8 Y-03243 -0 .3 Y-03254 0.0 Z-7670720 -8.7

I Z-7670756 -9.4 Z-7670759 -8.3 Z-7670761 -7.5

E. HUMIDITY-HEAT

Physical Changes & Remarks

None

None Electrical degradation on this test was preceded by an increase on previous test

This test was designed to simulate the effect of a clear, bright sun upon a module following a period of high humidity and/or rain.

1. Procedure

The modules were subjected overnight to high humidity in a chamber at 40.5°C. Chamber temperature was reduced to ambient, and the modules were then quickly put on a rack under an overhead lamp array. The lamps were turned on. Lamp irradiance level was predetermined to achieve maximum module temperature typical in a field installation at full sun on a warm day. Modules were allowed to reach a stable temperature on this rack.

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The humidity chamber was a standard one shown on the left in Figure 8. The lamp bank (right side of Figure 8) was a 2.1 m x 2.1 m (7' x 7') array equipped with tungsten-iodine-quartz heat lamps. Power was provided to the lamps from a 480-V, 3-phase supply controlled by a ganged variable transformer.


Four modules each of the four suppliers were exposed to humidity/heat for 10 cycles. Only one module showed some peeling. Changes in electrical output were minor (Table 9).

Table 9. Results of Humidity-Heat Tests

Electrical S/N Degradation, Physical Changes, Comments

Percent

V-781 +O. 7 Encapsulant peeled for 1/2 11 off the top of one cell (P/FR 1202)

V-809 +1.0 None V-812 +1.3 None V-813 +O .5 None

W-05901 -1.0 None W-05909 -0. 9 None W-05913 -1.2 None W-05945 -1.1 None Y-03232 +0.3 None Y-03238 +0.8 None Y-03243 +1.4 None Y-03254 +0.8 None

Z-7670720 +0.6 None Z-7670758 -1.0 None Z-7670761 +0.4 None Z-76101116 -2.0 None

F. FUNGUS

Equipment installed in humid warm climates generally is attacked by fungi if it contains nutrients for these organisms. The fungus test was a screening test to determine if Block I modules were susceptible to attack.

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1. Procedure

The procedure used was MIL-STD-810B, Test Method 508.1. Modules were sprayed with a salt and fungal culture and put into a chamber maintained at 30°C and 951 relative humidity. A test sample containing known nutrients was examined after 14 days to confirm the growth of fungi. After 28 days the modules were removed and inspected. ·

This test was run without the usual electrical test and inspection after the environmental exposure. Only one module from each supplier was used instead of the customary four. A registered mycologist performed the test off-Lab and reported the results to JPL. This variation from the usual practice was necessary because facilities were not available at JPL to handle modules that might have become heavily contaminated by fungi.


Equipment for this test was a standard humidity chamber that had· been reserved for fungus testing only. In addition, special equipment for fungal culture and counting, salt mixing equipment, and spraying equipment of the mycologist were required (see MIL-STD-810B, Test Method 508.1).

3; Results and Discussion

No fungal attack occurred on any of the four modules tested. Reference 4 reports the susceptibility of silicone compound RTV 615 to attack. However, even the module containing .that compound showed no problems in this test.

G. WIND LOADING

The test and apparatus developed for the simulation of wind loading, together with the test results, are described in detail in Reference 5.

H. ELECTRICAL ISOLATION TESTING

1. Purpose

Determination of the electrical isolation provided by the encapsulation system between the module internal circuitry and the substrate and/or mounting structure is a measure of such parameters as encapsulant integrity, consistency of interconnect spacing and insulation thicknesses, and moisture absorption resistance. Also, subsequent procurements will probably specify isolation from ground for safety reasons. Characterization of insulation capabilities of module designs will assist in establishing electrical isolation design guidelines, suitable test procedures, and appropriate requirement values for future procurements. Consequently, a number of characterization measurements of insulation integrity were made on representative samples of the 46 kW modules.

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Measurements were made of the insulation resistance (IR) and of the ability of the modules to withstand application of high voltage.

2. Procedures

a. Sample Connections: A variety of termination techniques were present in the four designs tested. Therefore, each module was provided with short jumper cables as necessary to short circuit the module output terminations. In the case of the module provided with a pigtail, the cable was stripped to expose the conductors, which were then connected directly. The ground termination for modules with plastic substrates was provided by inserting the standard mounting hardware in the mounting hole closest to the output terminals. The negative output (or ground connection) of the test equipment was then connected to the fastener by means of an alligator clip. Modules with aluminum substrates used clips attached directly to the metal structure.

b. Insulation Resistance: With connections made as described above, the instrument test voltage was raised in 100 VDC increments and held at each level for one minute. The insulation resistance at each step was recorded and the voltage increased until a maximum insulation resistance at 1000 VDC could be determined. Modules which indicated very low insulation resistance (i.e. less than 1 x 108) were investigated to evaluate probable causes of low readings.

c. Voltage Withstanding: The test voltage was applied in 100 VDC incremental steps, each held for 1 minute, until a maximum of 1500 VDC was achieved or until an indication of leakage currents in excess of 15 microamps was noted. This limiting value was selected to minimize the possibility of physical damage to modules which were exhibiting insulation breakdown. The maximum voltage achieved without exceeding the current limit or causing a voltage breakdown was recorded.


a. Insulation Resistance Test: Hammerland Model 5T01, Non-destructive insulation tester.

b. Voltage Withstanding Test: Hipotronics Model HD 115, AC/DC high voltage tester.

4. Results

a. Plastic Substrate Modules: Initial measurements were obtained on the module types which utilize an epoxy-glass laminate substrate material and the framed plexiglass substrate type. These module types demonstrated very high electrical isolation. Insulation resistance values

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ranged from 1 x 1011 to 1 x 1014 ohms. When tested for voltage withstanding capability, all of these modules accepted over 1500 VDC without evidence of current leakage or breakdown.

b. Aluminum Substrate Modules: When subjected to electrical isolation tests both of these module types exhibited a variety of electrical isolation defects ranging from low insulation resistance readings to actual voltage breakdown. In all cases of failure, subsequent analysis indicated that the probable cause was workmanship errors which placed the internal circuitry in close proximity to the metal substrate. In particular, sharp edges and burrs present on mesh interconnects or solder projections on back surface contacts were the source of voltage stress concentrations leading to encapsulation breakdown. Additional discussion of the failure analyses performed on modules typifying these electrical isolation failure modes ~ppears in Section V. Aluminum substrate modules without workmanship defects were shown to be capable of providing insulation resistance values of the order of 1010 to 1012 ohms, and could withstand 1500 VDC.

5. Recommendation for Electrical Isolation Testing

It was shown that the use of IR and voltage withstanding tests were meaningful evaluation tools in assessment of the electrical isolation capabilities of solar cell modules. The implementation of insulation resistance and high voltage testing (usually referred to as a "high-pot '' test) is recommended as a production inspection requirement fol' future module procurements.

I. THERMAL RESPONSE

1. Procedure

Testing was conducted in natural sunlight on all module types. One or more modules were mounted in a 4' x 4' frame tilted 34° (local latitude). Space surrounding the modules was filled with black aluminum plates. Temperature measurements on the cell, substrate, and reference plate were monitored at 10-minute intervals. Local air temperature, total solar irradiation, wind speed, and wind direction were monitored in the vicinity of the five arrays, which were in a single east-to-west row with the active side facing south.


The total solar irradiance on the active side of the module was measured continuously with an Eppley pyranometer. Wind measurements were made continuously with a Meteorology Research, Inc., Ground-Based Meteorological Measurement System. Temperature measurements were made with 28-gage chromel-constantan thermocouples. The wind and irradiance were read out on strip charts. The temperatures were read out at

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15- and/or 10-minute intervals on a Kaye Instruments, Inc., 8000 Data System. The tests were performed in the parking lot on the south side of JPL Building 248 between May 12 and June 30, 1976.


Physical characteristics of these modules are shown in Table 1. Thermal physical properties of the various materials used in the module construction are shown in Table 10. Optical properties measured by TRW are presented in Table 11.

Two of the more interesting test results are presented in Figures 9 and 10. Figure 9 illustrates the variation in cell temperature above the local air temperature as a function of total irradiance.

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50

• PLEXIGLAS WITH AIR GAP

0 ALUMINUM FINNED SUBSTRATE

u 40 0

... -a::: <(

1--

..J

..J 30 w

u 1--- • w u z • w a::: •• w u. • • u. • ci 20 • w • MODULE W a::: ::J 0

1--<( a::: • w 0..

~ w • 1-- 10

0

0

00 «to 0

0 0 0

0

00

0 0 10 20 30 40 50 60 70 80 90 100

SOLAR IRRADIANCE (mW/cm2)

Figure 9. Typical Cell Temperature Measurements

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23.75 co

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., 10 - + - MODULE DESIGN (Y) ., ., .. ., ., -O- MODULE DESIGN (V) ., ., .,

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0 10 20 30 40 50 60 70 80 90 975 TOTAL IRRADIANCE mW/cm 2

Figure 10. Module Design Thermal Effectiveness

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Table 10. Thermophysical Properties of Solar Module Materials

Nominal k, Thermal Conductivity

Material Specific Gravity Specific Heat

Tempered Glass 2.72 0.200

Sylgard 184 1. 05 0.340

Aluminum 2.70 0.208

Paper 0.49 0.510

Silicon Cell 2.33 0.200

GlO Glass 1. 70 0.350 Epoxy

Plexiglas 1. 18 0.350

Air -3 1.09x 10 0.240

Table 11. Optical Properties of Solar Module Materials

Module Design I.D. w V y z X

a N (Encapsulant/Cell) 0.88 0.85 0.85 0.85 0.826

a N (Encapsulant/lnter-Cell) 0.662 0.44 0.55 0.66 -0.02

'T N (Encapsulant/1 nter-Cell) / / 0.32 0.21 0.679

p (Encapsulant/lnter-Cell) 0.34 0.56 0.45 0.34 0.32

£ {Front) 0.864 0.91 0.88 0.88 0.912

£ (Back) 0.762 0.854 0.88 0.88 0.912

E°(Cell) / / / / 0.523

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SECTION V

PROBLEM/FAILURE ANALYSIS

A. GENERAL

The problem/failure reporting and analysis program covers those Low-cost Silicon Solar Array activities which are:

(1) related to environmental, electrical and mechanical test.

(2) associated with problems/failures experienced in the JPL field test operations.

(3) related to problems/failures experienced by the Application Centers for applications under their control and which are attributable to a solar module.

The overall status of the problem/failure reporting system as of April 12, 1977 is given in Table 12, which shows the number of P/FR's written, the number closed and where the problem/failure first occurred.

Table 12. P/FR Status and Cause Summary

No. No. Temp Volt Hum. Heat Vendor PFR's Closed Cycle Humidity !sol. Freeze Rain

V 9 7 2 3 1 2

w 28 6 8 15 1 4

X 11 11 11

y 22 22 4 13 4

z 35 29 27 2 2

TOTALS 105 75 52 33 2 10 2

The 30 open P/FR's resulted from electrical degradation to various degrees. Analysis of the degradation mechanism is continuing.

The JPL analysis of problems/failures is summarized in Table 13 by vendor and by problem. The problems/failures encountered can be categorized as 32J electrical, 39% material, and 29% mechanical. 47% were workmanship-related, and 53$ were design-related.

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Table 13. Sumnary of Identification Problems and Number of Occurrences

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w 14 10 1 6 5 1 1

X 7 2 3 2

y 9 4 3 2 8

z 10 17 l 8 5 4 8

B. DISCUSSION BY MODULE TYPE

During Phase I testing (early production modules) vendor "V" type modules showed three serious defects: (1) Delamination of encapsulant from the substrate. This was attributed to improper preparation of the metal surface and application of the primer used to facilitate the bond between the metal and the silicone rubber. (2) Fractured terminations at the interconnect and the output terminals resulting from thermal stress between the terminal and the interconnect bus. This defect was corrected by adding a stress relief (stranded wire) between the interconnect and the terminal. 3) Voltage breakdown between the cell string and the metal substrate. The insulation provided between the cell assembly and the substrate was found to vary from 2 to 10 mils on some modules, causing a breakdown to occur as low as 50 V because of uneven soldering or sharp points in the interconnect mesh. The vendor changed his design to provide a 10 mil minimum of silicone rubber with improved inspection to avoid sharp appendages in the assembly.

Vendor 11 V" phase II (late production modules) problem/failure analysis consisted of one major effort to determine the cause of cell and interconnect discolorations. The conclusions of this analysis showed the interconnect discoloration was caused by chlorine corrosion which was present in the solder flux used to make solder joints. The brown discoloration on the cell surface was shown to result from hydrolysis of the primer, which also reduced bond strength between the silicone and the cells. The bond strength between the encapsulant and the aluminum substrate was found to be within the normal range specified by the manufacturer.

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Vendor "W" phase I problem/failure analysis was conducted for five major problems resulting from environmental test: (1) Broken mesh strands in the cell interconnect system. Thermal stressing of the module caused fractures in the interconnects because of inadequate mechanical stress relief. The vendor introduced some stress relief in the connections and provided a wire pigtail at each terminal to eliminate the problem. (2) Cracked terminal insulators, which could have been caused by overtorquing and thermal stress combinations. The cracked terminals did not appear to degrade the module electrically. (3) Delamination of the encapsulant from the glass at the ends of the module and around the terminals, which was caused by differential thermal expansion of the different materials used in the module. (4) The insulation breakdown point between the cell string and the substrate was found to be as low as 100 Von some modules. Analysis showed the breakdown points to be at metal ''spikes" from the expanded metal interconnects to the substrate (Figure 11). Such breakdowns were a random occurrence depending on the presence and position of the metal spike relative to the substrate. The vendor has taken steps to correct the problem by flattening the interconnects at assembly and adding in-process inspection points. (5) Electrical degradation was observed after exposure to humidity tests. This degradation is believed to be caused by the interaction of moisture with cell contacts, although the mechanism has not been well-characterized.

Vendor 11 W" module performance during the Phase II (late production testing) showed some improvement with respect to the phase I problems with the exception of electrical degradation when subjected to humidity test. This problem is manifested by an increase in cell-string series resistance and will probably require a design change to either improve the encapsulation system or the cell metallization.

Vendor "X" modules in the phase I and II testing exhibited severe degradation related to the thermal coefficients of the materials used. The module construction used plexiglass as a substrate and cover with an aluminum frame assembly to complete the assembly. Applying thermal stress to the assembly caused warpage and bowing of the plexiglass substrate and cover, which in turn caused the solar cells, interconnects and module buss to fracture, creating open circuits. Production of the modules was limited to prototypes on this procurement; no field applications were made.

Vendor "Y" modules employed a fiberglass substrate with cells fastened to the substrate with an adhesive and a silicone rubber encapsulant. During phase I testing some modules exhibited cracking of encapsulant around the edges of the cells. The delamination and splitting of the encapsulant was caused by entrapped air beneath the solar cells. The vendor made changes in his production processing to outgas the air from the encapsulant and to reduce air bubbles entrapped under cells. (Figures 12 and 13). The most significant problem in the early production, however, was severe electrical degradation of the modules after humidity' exposure. Analysis of the electrical degradation indicated that the cell string· had excessive series resistance in the cells after such exposure. The silicone rubber encapsulant failed to provide adequate

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-... .-.---::----

Figure 11. Interconnect Spike Impinging the Metal Substrate

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Figure 12. Encapsulant Split at Edge of Cell

.... ----

Figure 13. Arrows Indicate Split Encapsulant and Crack Caused by Entrapped Air

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Figure 15 .

Delamination of Cell and Resulting Cracked Cell Caused by Entrapped Air

Figure 14 .

De l amination Caused by Entrapped Air Undernea th the Cells

,,• .. ~ . . .· . , . . . .. :.... ... ..,.

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protection to the evaporated metal contacts, causing a permanent increase in series resistance from the interaction of humidity and the evaporated contact material. The vendor added palladium to the contact metallization system; no further degradation was experienced on production modules.

During Phase II testing these modules still experienced some delamination near the edges of the substrate which could be attributed to improper surface preparation and handling prior to encapsulation.

Vendor "Z" phase I modules employed a fiberglass substrate, cell adhesive and silicone rubber encapsulant with and without a hard top surface coating. Three major problems were evidenced after environmental thermal stress: (1) Delamination of the encapsulant from the substrate, cells and terminations. This problem was caused by air trapped beneath the cells or air bubbles in the encapsulant which separated the encapsulant from the substrate at the bonding plane and caused cracking of cells when strong adhesion between the bonding plane resisted separation. (Figures 14 and 15). (2) Cracking of the top surface plastic used to provide a smooth, hard coating over the silicone rubber encapsulant. This was caused by differences in coefficents of thermal expansion between the silicone and the top coating and by inhomogeneities in the coating, which caused the top coating to crack from simple mechanical fatigue. This problem was solved by controlled mixing, application and curing of the top coating. (3) Junction box covers and sides loosened or came apart. This problem was caused by material voids in the potting process, and differences in coefficients of expansion. The problem did not impair the functional operation of the module.

Phase II testing of manufacturer "Z" showed some delamination and surface coating splitting, but not as severely as the early production modules. The cause of delamination and cell cracking was a combination of entrapped air beneath the cells and warpage of module.

Phase III testing of manufacturer "Z" modules showed an apparent decrease in power output after temperature cycling. Analysis of the problem indicated that lead wires were corroded from sulphur contained in the natural rubber insulation on the conductors. This caused the resistance of the terminations to increase because of inability to make good contact to all the strands of wire in the conductor at both ends. The substandard lead wires were only used on a fraction of the modules manufactured.

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SECTION VI

CONCLUSIONS AND RECOMMENDATIONS

Considering the severity and exploratory nature of this test program, these essentially off-the-shelf modules performed surprisingly well. The trend during the procurement was away from the design problems of Phase I to residual workmanship problems during Phase II. Electrical performance degradation was much less common during the latter tests. although some physical degradation persisted. It should be noted that many of the tests described were under conditions for which the modules were not designed.

Based on the results of this test program, some preliminary conclusions and recommendations seem warranted:

(1) The frequency of interconnect failures indicates that additional attention is needed both in design and workmanship to avoid thermal stress damage.

(2) Silicone rubber encapsulant delamination has been experienced to some extent on all designs using this material. The fact that such conditions are not universal indicates that improved surface preparation and encapsulation procedures can control this problem.

(3) Insulation breakdown resulting from the application of high voltages occurred on some·modules of both design types utilizing metallic substrates. Such failures were controlled but not eliminated by modified fabrication and inspection procedures. Basic design changes will be required for future high voltage applications.

(4) Air entrapment during encapsulation led to numerous problems involving delamination and cell damage from air pocket expansion and contraction during temperature cycling. Positive measures to preclude such entrapment must be employed during module fabrication.

(5) Silicone rubber encapsulants will not protect humiditysensitive cell contacts in a hot and humid environment. To tolerate such an environment, contacts must be moisture resistant or hermetic encapsulation techniques must be employed.

(6) The random examples of cell and interconnect discoloration related to residual processing materials could have been avoided by proper cleaning procedures.

(7) Of those environments to which modules were exposed: temperature cycling, temperature humidity, and humidityfreezing caused the most module damage. Correlation with the effects of actual field service remains to be established.

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1.

2.

3.

4.

5.

6.

7.

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REFERENCES

LSPT-01, Rev. A, JPL Document 5101-6, Solar Cell Module Performance, Environmental Test, Handling, Storage and Inspection Procedure.

Spectrosun LAPSS, various volumes by Spectrolab, Inc.; Technical Manual for the Noya 1200, by Data General Corp.; LAPSS Operating Procedure for Simulated Sunlight Testing of Solar Cell Modules, dated February 2, 1977, by JPL Section 341 personnel.

NASA TM X-71771, Interim Solar Cell Testing Procedures for Terrestrial Applications, July 1975.

DeBell and Richardson, Monthly Technical Letter, December 12, 1976, to January 12, 1977, No. 8, JPL Contract 954527.

LSSA Project Task Report 5101-19, Cyclic Pressure-Load Developmental Testing of Solar Panels, February 1977.

LSSA Project Interoffice Memo, ''Block I Solar Module Problem/Failure Summary," dated April 14, 1977.

LSSA Project Interoffice Memo, "Solar Module Data Reports Update," April 14, 1977.

R-1 NASA-JPl-coml., L.A .. Calif.

this work was performed by the jet propulsion laboratory, · 2016. 7. 17. · solar array (lssa)...

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