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, ' * * 9 7 y 9 B JPL Publication 87-23 DOEIJPL-1012-126

Distribution Category UC-63b

Concentrator Hot-Spot Testing \ Phase I Final Report

i

I

C .C. Gonzalez R.S. Sugimura

May 15, 1987

Prepared for Sandia National Laboratories U.S. Department of Energy Through an Agreement with National Aeronautics and Space Administration

by Jet Propulsion Laboratory California Institute of Technology Pasadena, California

(NASA-CE- 181322) C C B C E N T R A I C G HCI-SPOT N87-24C28 I E S T I L G , E B A S E 1 E i o a l Bekort ( J e t E x c p a l s i c n LaL,) 28 F A v a i l : hlIS HC AC3/H€ A C 1 CSCL 10B Unclas

G3/44 0097498

JPL Publication 87-23 DOEIJPL-1012-126 Distribution Category UC-63b

Concentrator Hot-Spot Testing Phase I Final Report

C . C . Gonzalez R.S. Sugimura

May 15, 1987

Prepared for Sandia National Laboratories U.S. Department of Energy Through an Agreement with National Aeronautics and Space Administration

Jet Propulsion Laboratory California Institute of Technology Pasadena, California

by

Prepared by the Jet Propulsion Laboratory, California Institute of Technology, for the U.S. Department of Energy through an agreement with the National Aeronautics and Space Administration.

This work reported here was sponsored by Sandia National Laboratories of the U.S. Department of Energy and is part of the Photovoltaic Energy Systems Program.

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, com- pleteness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.

Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

This publication reports on work performed under NASA Task RE-152, Amend- ment 420, SNLA Nos. 04-4848 and 02-2047.

ABSTRACT

Results of a study to determine the hot-spot susceptibility of concentrator cells, to provide a hot-spot qualification test for concentrator modules, and to provide guidelines for reducing hot-spot susceptibility are presented. voltaic module when the short-circuit current of a cell is lower than the string operating current forcing the cell into reverse bias with a concurrent power dissipation.

Hot-spot heating occurs in a photo-

Although the basis for the concentrator-module hot-spot qualification test is the test developed for flat-plate modules, issues, such as providing cell illumination, introduce additional complexities into the testing procedure.

The same general guide1 ines apply for protecting concentrator modules from hot-spot stressing as apply to flat-plate modules. Therefere, recemmendat’c!ns are made e!? the numtser e f h v n x r -J r --- dindes required per given number of series cells per module or source circuit. In addition, a new method for determining the cell temperature in the laboratory or in the field is discussed.

i i i

ACKNOWLEDGMENT

The authors thank Elizabeth Richards, Rob Barlow, Alex Maish, and Charles S t i l lwe l l of Sandia National Laboratories f o r providing infor - mation and module components used in the work reported herein, and f o r making valuable suggestions on the content of t h i s report . thank Ron Ross of the Je t Propulsion Laboratory (JPL) f o r many useful discussions and suggestions during the performance of the research reported upon, and f o r valuable suggestions on the content of t h i s report . Finally, we thank Elizabeth J e t t e r of JPL f o r s e t t i n g up equipment, and performing much of the t e s t i n g and data reduction, and Scot t Leland and Otto Whitte of JPL f o r ass i s tance in making and modi- fying tes t equipment and preparation of t e s t samples.

We a l so

iv

GLOSSARY

Back-bias voltage Voltage applied across a ce l l in the negative direct ion.

Bypass diode Diode t h a t i s connected in paral le l across a group of ser ies c e l l s (or paral le l groups o f ser ies c e l l s ) so t h a t i t conducts only when the voltage across the c e l l s i s in the reverse direct ion.

Cel l /heat sink assembly

Cell attached t o a heat sink (as in a module) removed from the module s t ruc ture and without concentrating opt ics .

Cell junction Temperature of ce l l a t a par t icu lar temperature location in the junction.

Dark (reverse voltage) The term "dark" re fers t o an I-V curve taken 1 - V curve in room-ambient l i gh t .

IR camera Camera sensit ive t o infrared radation and producing electr ical signals t h a t can be converted t o a black-and white video image with a grey scale proportional t o the temperature of an emitting surface.

NOCT Nominal operating cel l temperature.

Reverse q u a d r a n t Refers t o t h e current-voltage charac te r i s t ics o f a solar cell when i t i s operating with current in the posi t ive direct ion and with negative voltage across the c e l l .

Reverse-breakdown Reverse voltage where ce l l passes an unlimited voltage amount o f current.

Reverse voltage Cell voltage i n the negative direct ion.

Type A ce l l

Type B ce l l

Cell whose reverse voltage i s limited by the number of ce l l s in se r ies with the c e l l .

Cell whose reverse voltage i s limited by the breakdown voltage of the c e l l .

V

CONTENTS

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

A . OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . 1

B . HOT-SPOT HEATING FUNDAMENTALS . . . . . . . . . . . . . . 1

I1 . TEST IMPLEMENTATION AND RESULTS . . . . . . . . . . . . . . . 5

A . CHARACTERIZATION OF INTERSOL CELLS . . . . . . . . . . . 5

B . HOT-SPOT TESTING . . . . . . . . . . . . . . . . . . . . 6

I11 . HOT-SPOT QUALIFICATION TEST DEVELOPMENT . . . . . . . . . . . 15 IV . CONCLUSIONS AND RECOMMENDATIONS . . . . . . . . . . . . . . . 17 II n r r r n rwr rr V . nLr LnLiiLLa . . . . . . . . . . . . . . . . . . . . . . . . . 13

APPENDIX: TEST PROCEDURE FOR CONCENTRATOR-MODULE HOT-SPOT TEST . A-1

F i q I! r e E

1 . Visualization of Hot-Spot Cell Heating for High-Shunt- Resistance Cell . . . . . . . . . . . . . . . . . . . . . . . 3

2 . Visualization of Hot-Spot Cell Heating for Low-Shunt- Resistance Cell . . . . . . . . . . . . . . . . . . . . . . . 3

3 . Intersol Module Receiver Assembly . . . . . . . . . . . . . . 5

4 . Typical Concentrator Cell Reverse-Quadrant I-V Curves for Various Illumination Levels . . . . . . . . . . . . . . . . . 6

5 . Modified Spectrolab XT-10 Steady-State Solar Simulator . . . 7

6 . Fresnel Lens and Receiver Assembly . . . . . . . . . . . . . 8 7 . Cell Temperature Versus Junction Voltage and Current . . . . 9 8 . Test Setup for Hot-Spot Testing of Receiver Assembly . . . . 11 9 . Test Setup for Hot-Spot Testing of Concentrator Modules . . . 12 10 . IR Camera Video Image o f Cell Hot Spot . . . . . . . . . . . 13 11 . Cell Temperature Versus Power Dissipated for Various Test

Configurations . . . . . . . . . . . . . . . . . . . . . . . 14

12 . Cell Hot-Spot Temperature Increase Above Operating Temperature Versus Number o f Cells per Bypass Diode . . . . . 14

vi i

SECTION I

INTRODUCTION

A. OBJECTIVES

The objective of this study was t o develop an understanding of the hot-spot susceptibility levels of concentrator modules, to define a qualification test for quantifying the hot-spot endurance of concentra- tor modules, and to provide guidelines for improving their endurance.

The study centered on the exploratory testing o f an Intersol second-generation, point-focus module (Serial Number 1069). This module has 14 cells (0.89-inch-square) in series, each attached to an aluminum heat sink, and has a geometric concentration of 84, achieved with planar Fresnel lenses. A single bypass diode is externally mounted across the module’s terminals.

Meeting the above objectives involved conducting a variety o f research activities as described in this report. These included:

(1) Characterizing the reverse-quadrant I-V curves of the Intersol cell s .

(2) Quantifying the hot-spot heating characteristics of the Intersol cells and module.

(3 ) Generating a draft hot-spot test procedure for concentrator modules.

(4) Developing illumination sources to allow operating the modules in the laboratory environment under simulated steady- state field conditions.

(5) Developing techniques for accurately measuring cell temper- atures including localized hot spots.

B. HOT-SPOT HEATING FUNDAMENTALS

Hot-spot heating is caused when the short-circuit current level o f an individual cell or group of cells in an array circuit is reduced below the module operating current level. The reduced short-circuit current fault condition can be the result of a variety of causes in- cluding nonuniform illumination (local shadowing or rnistracking), or individual cell degradation due to cracking. Under this condition the cell carrying the excess current dissipates power equal to the product of the current and the reversed voltage that develops across the cell. It is desirable to ensure that the resulting hot-spot heating due to

1

the reverse biasing does n o t cause propagation of the f a u l t or elec- t r i c a l safety hazards. parallel array when the ce l l carrying the excess current open c i r c u i t s , causes the loss of one of the paral le l s t r i ngs of a s e r i e s block, and subjects the remaining s t r ings of the se r i e s block t o fu l l source- c i r cu i t current.

Fault propagation can occur in a s e r i e s -

The degree of hot-spot heating within an affected ce l l i s depen- dent on a variety of conditions, including the number of s e r i e s c e l l s per bypass diode, the amount of illumination, the amount of over- current in the affected c e l l , and the reverse-voltage I-V character is- t i c s o f t h e affected c e l l . Because the reverse-voltage I-V character- i s t i c s vary considerably from ce l l t o ce l l within a given module, i t i s necessary t o determine the d a r k reverse-voltage I-V curve for a repre- sentative sample of c e l l s .

Based on the shape of t h e i r reverse-quadrant I-V curves, c e l l s are divided into two c lass i f ica t ions . One c l a s s , Type A, i s composed of those ce l l s whose reverse-breakdown voltage ( the reverse-voltage where the cell passes an umlimited amount of current) i s higher t h a n the max- imum available reverse voltage ( V L ) under f i e l d hot-spot conditions. The maximum f i e l d voltage ( V L ) i s determined by e i the r the maximum system voltage ( i f no bypass diodes are used) or by the number of s e r i e s c e l l s per bypass diode ( i f bypass diodes are used). As shown in Figure 1, Type A c e l l s suffer the greatest amount of back-bias power dissipation under conditions of par t ia l illumination; t h i s i s because the back-bias current increases with increasing illumination up t o a maximum a t some par t icular par t ia l illumination leve l . In t h i s case, as the illumination level increases, so does the power dissipated in the cell u p t o tha t illumination level which produces maximum current.

The second c lass of c e l l s , Type B, i s composed of those whose breakdown voltage i s l e s s t h a n the reverse voltage tha t may be applied by the f i e l d array. fu l l str ing current and a t a reverse voltage determined by the voltage d r o p across the c e l l . These c e l l s suffer the grea tes t amount of power dissipation under fu l l shadow; under these conditions they have the greatest back-bias voltage ( V B ) across them (see Figure 2 ) . c e l l s , as illumination increases, Curve B r i s e s , causing the back-bias v o l t a g e ( V Q ) t o decrease, resul t ing in a decrease in the amount of power dissipated in the c e l l .

As shown in Figure 2 , these c e l l s operate a t the

For Type B

2

+ /- CRACKED OR SHADOWED CELL

C

A

k-VL-

I I-

V

Figure 1. Visualization of Hot-Spot Cell Heating for High-Shunt- Resistance Cell

CRACKED OR SHADOWEO CELL f

B

t + / POWER I

I 1 D I S S I I ~ I O N , \

C /

Figure 2. Visualization of Hot-Spot Cell Heating for Low-Shunt- Resi stance Cell

QRTG!" PAGE 1s OF POOR QUALm

SECTION I1

TEST IMPLEMENTATION AND RESULTS

A. CHARACTERIZATION OF INTERSOL CELLS

Initial characterization of the Intersol cells was performed on cell assemblies consisting of only the heat sinks and the attached cells (see Figure 3 ) . Several units were isolated electrically with separate electrical leads to allow the individual reverse-quadrant I-V curves to be measured. Figure 4 displays the typical reverse-quadrant I-V response of these cells, both in the dark and at working irradiance 1 eve1 s.

The measured cells exhibit a high shunt resistance with reverse voltage breakdown at 10-12 volts. Because this breakdown voltage exceeds the moduie voltage, the cell i s voltage limited by tne moduie bypass diode under back-bias conditions; i .e., the cells are classified as Type A cells. When thermal runaway (the specific voltage breakdown where rapidly increasing current leads to increased power dissipation and rapid heating of the cell) was allowed to occur, the cells general- ly broke down and exhibited significant reductions in shunt resistance (partial shunting); this behavior is also noted in Figure 4 , Curves 1A and 1B.

Figure 3 . Intersol Module Receiver Assembly

5

121 I I I I I I I

VOLTAGE, V KEY: -

1A 1B

DARK - BEFORE THERMAL RUNAWAY DARK - AFTER THERMAL RUNAWAY

2 INDOOR ILLUMINATION 3 NATURAL SUNLIGHT

Figure 4 . Typical Concentrator Cell Reverse-Quadrant I-V Curves fo r Various Illumination Levels

B. HOT-SPOT TESTING

T h i s phase of the ac t iv i ty was addressed t o quantifying the h o t - spo t heating leve ls of the Intersol modules. The laboratory t e s t s , developed a t the J e t Propulsion Laboratory (JPL) t o determine the h o t - spo t suscept ib i l i ty of f l a t - p l a t e modules (References 1 and 2 ) , served as a s ta r t ing point fo r the development of the t e s t procedures used here for concentrator modules.

Modul e h o t - s p o t heating 1 eve1 s and endurance a re normal l y quoted fo r representative worse-case f i e l d thermal conditions and peak i r r a d i - ance levels . Building on the t e s t procedures fo r f l a t - p l a t e modul s ,

direct-normal irradiance, 4OoC ambient a i r temperature, and s t i l l a i r conditions (no wind).

the conditions fo r concentrator modules were chosen t o be 80 mW/cm $

In i t i a l attempts a t t e s t ing modules in the outdoor environment led t o extreme data s c a t t e r because of t rans ien t wind cooling and variable irradiance levels . I t quickly became c l ea r t h a t accurate character iza- t ion would require the s t a b i l i t y of a laboratory t e s t setup. Accurate- l y measuring ce l l temperature was a l so a s ign i f icant problem.

The ideal laboratory t e s t setup involves f i r s t using an external radiant heater t o r a i s e the module heat sink and ce l l assembly t o a background (unilluminated) temperature simulating the 4OoC ambient condition plus the influence of adjacent illuminated c e l l s . Next, the

6

cell is electrically reverse-biased to achieve the worst-case hot-spot electrical operating point. For high-shunt-resistance (Type A) cells such as these, the cells must also be partially illuminated to achieve worst-case current levels. Lastly, the maximum hot-spot temperature must be measured.

Achieving illumination of the cells and measuring cell tempera- tures both required development of new techniques and experimental apparatus.

1. Sol ar Simul ator Development

Hot-spot testing of high-shunt-resistance cells typically requires illuminating cells to levels approaching 80% of the outdoor peak illumination level, i .e., to levels near 60 mW/cm2. under steady-state conditions and must also simulate the natural heating of the cell/heat sink assembly that would normally occur from the concentrated sun1 ight.

This must be done

Particular concerns include:

(1) Excessive infrared content in the indoor illumination

( 2 ) Optical incompatibility with the concentrator leading t o

source leading to excessive heating.

insufficient illumination of the cell and/or excessive illumination and heating of the region surrounding the cell.

To meet the above concerns a Spectrolab Model XT-10 steady-state Xenon solar simulator was modified extensively to provide an optical beam simulating the sun’s disk at infinity (see Figure 5 ) . The size of

FRESNEL LENS 7

CONCENTRATOR MODULE

APERTURE 7

SPECTROLAB

SOLAR SIMULATOR MODEL XT-10

Figure 5 . Modified Spectrolab XT-10 Steady-State Solar Simulator

7

\S

the sun’s disk was established via a carefu l lv selected aperture loca t - ed internal t o the simulator a t the focal point of the lamp’s condens- ing optics. The aperture i s imaged t o i n f i n i t y by an object ive lens on the ex i t of the simulator. When directed on the Intersol module ( in - cluding i t s Fresnel lens) the simulator achieves the desired i r radiance l eve l s and causes minimal illumination outside of the c e l l i t s e l f .

2 . Cell Temperature Instrumentation

Determining maximum ce l l temperature during h o t - s p o t heating i s d i f f i c u l t w i t h f l a t - p l a t e c e l l s and especial ly challenging with concen- t r a t o r c e l l s . With f l a t - p l a t e c e l l s the usual technique involves using an IR video camera t o image the h o t spot and quantify the d i f f e ren t i a l temperature between the maximum heating point and a reference thermo- couple attached d i r ec t ly t o the rear surface of the c e l l .

With a concentrator, i t i s d i f f i c u l t t o view the c e l l because of i t s position behind the concentrating opt ics , and i t s IR s ignature i s pa r t i a l ly masked by re f lec ted IR from the so l a r simulator. very d i f f i c u l t t o accurately instrument the ce l l with thermocouples.

I t i s a l so

To achieve a bes t -e f for t temperature determination, the Intersol module was cut away t o allow video IR imaging of the ce l l a t an angle (see Figure 6 ) . In addition, a thermocouple was attached d i r e c t l y t o

Figure 6. Fresnel Lens and Receiver Assembly

8

the cell periphery, outside of the illuminated region and shielded from stray light; a second thermocouple was attached to the heat sink assem- bly. For a closeup of the thermocouples, see Figure 3 .

In addition to the thermocouples and IR imaging, another technique of sensing cell temperature was developed and demonstrated based on the known relationship between the voltage drop across a diode p-n junction (at a particular current level) and the junction temperature. With this technique, the cell’s temperature is determined by measuring the voltage drop across the cell’s p-n junction in the dark using a fixed, reverse-current level. disconnecting the back-bias current that is inducing the hot spot, and removing any illumination from the cell. work, the measurement should be completed in approximately 0.5 second. The method builds on the technique previously developed for measuring the junction temperature o f bypass diodes in situ (Reference 3 ) .

The measuring current is supplied rapidly after

As determined by previous

Several of the concentrator cells were calibrated t o provide cell

The cells were placed in a constant-temperature oven where temperature versus the voitage drop across the ceii for given measuring currents. the junction temperature equilibrated at the measured oven temperature. Figure 7 shows a sample of the vol tage-versus-temperature plots.

VOLTAGE, V

Figure 7 . Cell Temperature Versus Junction Voltage and Current

9

This junction-temperature measurement technique was used to obtain the cell temperature in a variety of trial operating situations. In one, the cell was illuminated while mounted in one of the partial- module assemblies, and while radiant heating was provided to the heat sink. The cell temperature and heat-sink temperature were measured with thermocouples, and the highest and lowest cell temperatures were measured with the IR camera. ally fell in the range of temperatures between the thermocouple cell and the heat-sink measurements.

The measured values of temperature gener-

The preliminary tests indicate that the new procedure is a promis- ing technique for measuring cell temperature, but that the readings are sensitive to the choice o f measuring current when the cell has substan- tial temperature gradients. An analysis of the technique predicts that the readings at the lowest measuring currents should be indicative o f the hottest localized regions of the cell, whereas readings at higher measuring currents should be indicative of larger cooler areas o f the cell. temperature measurement technique will have to await further study.

A definitive assessment of the feasibility of using the junction

3 . Overall Test Set-up

Based on the issues described above, two test setups were devel- oped: one for initial hot-spot characterization in the dark (see Figure 8), and one for complete module hot-spot testing with partial illumination (see Figure 9).

The equipment comprising the test setups included: (1) a power supply for providing the back-bias voltage and current to the test cell, (2) meters and an x-y plotter for measuring the test parameters, ( 3 ) temperature recording devices including thermocouples and the IR camera, ( 4 ) radiant heating source to bring the cell to the desired temperature prior to back-biasing (heat source maintained during entire test), and (5) the light source described above.

Additional equipment for measuring the junction temperature included: (1) a constant-current power supply for providing the mea- suring current, (2) an ammeter for measuring the current and a sample- and-hold voltmeter for measuring the resultant voltage drop across the cell, ( 3 ) a switching circuit for switching between the back-bias power supply and the one providing the measuring current, and ( 4 ) a solenoid- activated shutter for quenching the illumination on the cell during the cell junction temperature measurement.

4 . Test Results

The results of cell back-bias testing can be presented in several ways. power dissipated. The cell temperatures were measured both with ther- mocouples and with the IR camera.

The key parameters that are measured are cell temperature and

The thermocouple readings are

10

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average cell Val ues unless the thermocouple is fortuitously pl aced on or near a hot spot. Figure 10 shows the video image of a hot spot on the cell as produced by the IR camera. visible in the lower corner of the cell.

The hot spot is the bright spot

One method of data presentation, shown in Figure 11, is to plot the cell hot-spot temperature as a function of power dissipated. Shown for comparison is a typical response taken outdoors using natural sun- light. The plot of mean temperature as a function of power dissipation was smoothed to eliminate the extreme data scatter actually observed. (Additionally, the curve was not normalized to worst-case ambient temperature o f 4OoC and still -wind conditions.) Although this method gives an unambiguous indication of the cell's susceptibility to hot- spot heating in terms of power dissipated, it does not provide the module and array designer with a useful tool for optimizing the cir- cuitry to minimize hot-spot problems. For the tested concentrator cells the controllable parameter is voltage, which can be limited by the use of bypass diodes. Therefore, Figure 12 gives the cell temper- ature as a function o f number o f ceiis per bypass ciiocie; the iatter parameter directly determines 'the back-bias voltage. Figure 12 was determined from actual hot-spot tests using the solar simulator with a slight upward current adjustment to be consistent with module rated current at peak power. The module designer must, of course, choose a maximum a1 1 owabl e temperature based on the construction materi a1 s and endurance of his cell assembly. Some of the concerns are solder melt- ing, increase of thermal resistance between the cell and heat sink due to cell delamination, degradation of the cell front surface coating, and cell shorting. Generally a maximum temperature between 100°C and 120°C is appropriate.

Figure 10. IR Camera Video Image of Cell Hot Spot

13

POWER, W

Figure 11. Cell Temperature Versus Power Dissipated for Various Test Configurations

Figure 12. Cell Hot-Spot Temperature Increase Above Operating Temperature Versus Number of Cells per Bypass Diode

14

SECTION I11

HOT-SPOT QUALIFICATION TEST DEVELOPMENT

As the hot-spot t e s t procedure was refined during the above des- cribed invest igat ion, i t was used as a model t o specify a hot-spot endurance qual i f i ca t i on t e s t for concentrator modul es. Thi s d r a f t qual i f i c a t i on t e s t procedure i s i ncl uded as Appendix A. The procedure draws heavily upon the comparable procedure fo r f l a t - p l a t e arrays and uses the same 4OoC, s t i l l -wind ambient conditions as t h a t procedure (Referen es 1 and 2 ) . (1) r5ference t o

( 2 ) the requirement for a specialized illumination source, and ( 3 ) reference t o a fixed 5OoC background module temperature (as opposed t o NOCT fo r f l a t - p l a t e modules). This l a s t 5OoC f igure was selected based on the absence of a design-specific reference temperature s imilar t o NOCT, and the qreat var iab i l i ty in heat t ransfer between adjacent c e l l s in d i f f e ren t types of concentrators.

Specific differences include: 80 mW/cm 5 d i rec t normal irradiance (as opposed t o 100 mW/cm global ) ,

I n concept the 5OoC f igure i s intended t o represent the operating temperature of a single fu l ly shadowed concentrator ce l l (with no back- bias heating) when the remainder of the module i s fu l ly illuminated with 80 mW/cm2 d i r ec t irradiance, and the ambient a i r i s 4OoC and s t i l l . Under these conditions i t i s l ikely tha t thermally isolated c e l l s in point-focus concentrators (such as the In te rso l ) will run a t temperatures somewhat c loser t o the 4OoC ambient t h a n those in thermal- l y integrated receivers (such as the Entech). The 5OoC f igure was f e l t t o be a good compromise.

15

SECTION IV

CONCLUSIONS AND RECOMMENDATIONS

Since the ce l l s in concentrator modules a re already designed f o r maximum cooling using la rge heat sinks, the only prac t ica l means f o r improving hot-spot heating endurance o f marginal modules i s l imi t ing the maximum reverse voltage v ia bypass diodes. The Intersol modules t e s t ed in t h i s a c t i v i t y demonstrated excellent endurance against f u l l - shadow f a u l t conditions, but exhibited very high heating levels under the p a r t i a l illumination conditions that a r e more probable in f i e l d ap- p l ica t ions . Foreseeable causes o f the highly stressful, pa r t i a l illum- inat ion conditions include spot ty so i l ing , local ized shadowing from narrow objec ts , and pa r t i a l c e l l area lo s s from ce l l cracking.

Figure 12 provides the data on the frequency of bypass diodes re- quired t o l i m i t (based on worst-case data) hot-spot heating of the Intersol module t o sa fe temperature levels . Similar da ta can be devel- oped f o r other module designs using the procedures defined herein. I t i s recommended t h a t additional hot-spot t e s t i n g be car r ied out t o fur- t h e r r e f ine these t r i a l procedures and provide additional manufacturers with s p e c i f i c recommendations f o r t he i r designs.

17

REFERENCES

Arnett, .C., and Gonzalez, C.C., "Fllotovoltaic Module Hot-Spot Durability Design and Test Methods," Proceedinqs of the 15th Photovoltaic becialists Conference, Orlando, Florida, May 12-15,

Block V Solar Cell Module Desiqn and Test SDecifications for Intermediate-Load Applications, 1981, JPL Internal Document No. 5501-161, Jet Propulsion Laboratory, Pasadena, California, February 20, 1981.

Otth, D.H., Sugimura, R.S., Ross, R.G. Jr., "Development of Design Criteria and Qualification Tests for Bypass Diodes in Photovoltaic Applications," 1985 Proceedinqs of the 31st Annual Technical Meetinq of the Institute of Environmental Sciences, Las Vegas, Nevada, April 30-May 2, 1985, pp. 242-248.

1981, pp. 1099-1105.

19

APPENDIX A

TEST PROCEDURE FOR CONCENTRATOR-MODULE HOT-SPOT TEST

1. Purpose

The purpose of this test procedure is to evaluate the ability of a module to endure the long-term effects of periodic hot-spot heating associated with common fault conditions such as severely cracked or mismatched cells, single-point open-circuit failures, or nonuniform illumination (partial shadowing).

2. Commentary

Field experience indicates that periodic circuit faults, such as partial shadowing and cracking of cells, must be expected to occur even i t : I A - r : -

able to ensure that possible hot-spot heating due to reverse biasing does not cause propagation of the fault or electrical safety hazards.

I 3 uc311- :c. h:-hl., u-l:-hl- ll-A-- +h--.. S - . . l + . . . . rA :+ : r r r 1 1 1 II i y i i i y I t: I i a u i t : ai I ay3. UIIUCI L I I C ~ ~ I au I L LUIIU I L iu113

Hot-spot heating is caused when module operating current levels exceed the reduced short-circuit current level of an individual cell or group of cells in an array circuit. fault condition can be the result of a variety of causes including non- uniform illumination (local shadowing or mistracking), or individual cell degradation due to cracking. Under this condition the cell(s) carrying the excess current will dissipate power equal to the product of the current and the reversed voltage that develops across the cell ( s ) , heating the cell ( s ) to elevated temperatures.

The reduced short-circuit current

3 . Test Procedure

The preferred procedure for conducting this test includes a series of steps, first to select and instrument appropriate cells for testing, then to determine the hot-spot test levels, and last to conduct the hot-spot endurance test.

a. Cell Selection and Instrumentations. The degree o f hot-spot heating within an affected cell is dependent on a variety of condi- tions, including the number of series cells per bypass-diode, the amount of illumination, the amount of over-current in the affected cell, and the reverse-voltage I - V characteristics of the affected cell(s). erably from cell to cell within a given module, it is necessary first to determine the dark reverse-voltage I - V curve for a representative sample of cells (at least 10). This may require a specially built module. Each cell’s dark characteristics should be determined for reverse voltages from 0 to V L or currents from 0 to IL, whichever limit

Because the reverse-voltage I - V characteristics vary consid-

A- 1

i s reached first, where:

IL = Isc of an average cell at 80 mW/cm2, at standard test

VL = N x Vmp of an average cell at 80 mW/cm2, STC

N = Number of series cells per bypass diode or number of series

conditions (STC)

cell s per modul e, whichever i s 1 ess.

When the family of reverse-voltage I-V curves is plotted for the representative sample of cells, a graph similar to Figure A-1 should be obtained. The individual curves may be either all voltage-limited (Type A ) , current limited (Type B), or a combination of both (as shown). Whether a cell is a Type A or Type B cell is a function of the number of series cells, N, as defined above. A cell that is Type A in one circuit configuration may switch to a Type B cell as N is increased. In general, the cells associated with the highest hot-spot heating levels are those with the highest shunt resistance, although low shunt resistance may be associated with highly localized heating.

For testing, select three individual cells (non-adjacent if not thermally isolated) within the test module from the measured sample - one representative of the highest shunt resistance obtained, one repre- sentative of the average shunt resistance, and one representative of the lowest shunt resistance. negative electrical leads to allow them to be connected individually to separate power supplies. Any parallel current paths around the selected test cells must be eliminated. The lead attachment should minimize disruption of the cell’s heat-transfer characteristics or the hot-spot endurance of the encapsulation/interconnect system.

Provide the test cells with positive and

b. Selection of Hot-SDot Test Level. The objective of this por- tion of the test procedure is to select the level of heating corre- sponding to test conditions that will stress the module in a manner similar to a severe field hot-spot condition. The severity of the field condition will depend on the array circuit configuration, the array I-V operating point, the ambient thermal conditions, the overall irradiance level, and the previously described shunt-resistance char- acteristics of the affected cells. Type A or Type B is important.

In particular, whether cells are

For T w e A Cells. The maximum cell reverse voltage (VL), the ambient thermal environment, and the cell illumination level are the key parameters. circuit without bypass diodes, the maximum reverse voltage imposed on an individual cell can approach the maximum array operating voltage. In this procedure it is assumed that the array designer has used bypass diodes or other means to limit the reverse voltage. The test voltage (Vtest) i s thus set to the maximum reverse voltage than can develop across an affected cell as determined by the number of series cells per bypass diode.

When a module is incorporated into an array source

For Type A cells, Vtest shall be set equal to N times

A- 2

the average Vmp of an individual cell, where N is the number of series cells per bypass diode or the number o f series cells per module, which- ever is less.

The second test condition is the cell background thermal environ- ment, which is selected to simulate a high (4OoC) ambient air tempera- ture with minimal wind, together with heating from adjacent illuminated cell assemblies and heat sinks. For the test, the background thermal environment shall be implemented by uniformly irradiating the heat-sink assembly with a steady-state infrared flux sufficient to achieve an equilibrium cell temperature of 5OoC & 2OC prior to the application of hot-spot power and cell illumination. The IR flux shall remain at this level during the test.

The third key test condition for Type A cells is the illumination level; it directly controls the hot-spot current level and, therefore, the power dissipation level. As shown in Figure A - 2 , there is a unique illumination level that corresponds to worst-case power dissipation for any particuiar Type A soiar ceii . in the test, the irradiance ievei on the test cell shall be adjusted to achieve this worst-case condition with te set equal to the average cell maximum power current at 80 mW/cmJ, sk. The incremental cell heating resulting from the illumina- t i m scurce should cause an incremental cell temperature increase approximately equal to that associated with an equivalent exposure to natural direct normal sunlight.

TYPE 6

I TYPE A

VOLTAGE vL

CURRENT

‘1

Figure A - 1 . Typical Reverse-Voltage I-V Plot for a Sample of Cells.

A-3

HIGH IRRADIANCE

/ WORST-CASE IRRADIANCE ’ M A X I M U M POWER DISSIPATION

\

LOW POWER DIKIPATION

\ \ i

Figure A-2 . Effect of Test Cell Illumination Level on Hot-Spot Power Di ssipation.

T w e B Cells. Type B cells have a cell shunt resistance so low that the maximum reverse voltage is set by the IR drop across the cell associated with the available current level. In these cells, worst- case heating occurs when the cell is totally shadowed, and the current level is at a maximum. Test conditions for Type B cells shal , there- allow f o r room lighting) and a current test level ( I es ) equal to the short-circuit current o f an average cell at 80 mW/cm3, &TC.

fore, maintain a cell illumination level of less than 5 mW/cm 4 (to

As with Type A cells, the background thermal environment shall be implemented using infrared heaters to achieve an equilibrium cell tem- perature of 5OoC & 2OC prior to the application of the hot-spot test current. The air surrounding the heat-sink assembly shall be still.

c. Test Execution. Detailed steps for execution of the test involves subjecting the three selected test cells to cyclic hot-spot heating, at the levels determined above, for a period of 100 h total on-time, as follows:

(1) Apply an IR source to the cell heat-sink assembly and adjust the heating level to achieve a uniform cell temperature equal to 5OoC & 2OC. The ambient air shall be still.

A - 4

( 2 ) Connect a separate dual-limiting, constant-current, constant- voltage power supply t o each t e s t ce l l with polar i ty arranged t o dr ive the c e l l s with reverse voltaue. A d j u s t the voltaue and current 1 imits t o Vtest and Itest-values-determined in- s t ep b above.

( 3 ) For Type A c e l l s only, arrange a so l a r simulator t o illum- inate each t e s t ce l l t o the unique level determined in s tep b of Figure A - 2 . IR source a re turned on and adjusted. t ion level t o achieve, simultaneously, both current and voltage l imit ing a t I t e t and Vtest a f t e r equilibrium test conditions s t a b i l i z e . ?he illumination source should not contain excessive infrared o r u l t r av io l e t irradiance t h a t would lead t o abnormal heating o r accelerated UV aging. The source should also avoid illuminating portions of the concen- t r a t o r i n t e r i o r o r ce l l assembly t h a t a re not normally illum- inated (heated) d u r i n g operation i n natural sunlight.

This must be done a f t e r the power supply and Adjust the illumina-

(4) Connect the power supplies, IR source, and l i g h t sources t o an appropriate timer t o obtain a cycl ic on-off operation with an on-time equal t o 1 h , and an off-time s u f f i c i e n t t o allow the t e s t c e l l s t o cool t o w i t h i n 10°C of the ambient a i r tem- perature.

(5) Conduct the t e s t un t i l a t o t a l of 100 h o f on-time has been accumul ated.

(6) Visually inspect the module and t e s t ce l l a t approximately 24-h intervals during the t e s t , and upon completion of the 100-h sequence. Identify any evidence of degradation, including ce l l cracking and delamination, outgassing o r b l i s te r ing of encapsulants, solder melting, or other defects resul t ing from the t e s t . performance of the module f o r comparison with baseline elec- t r i c a l performance. Perform a pos t - tes t e l e c t r i c a l isolat ion t e s t .

Measure the pos t - tes t e l e c t r i c a l

A - 5

1. Report NO. 87-23

Concentrator Hot-Spot Tes t ing - Phase I , F i n a l Report

2. Government Accession No. 3. Recipient's Catalog No.

Mav 15. 1987 6. Performing Organization Code

4. Title and Subtitle 5. Report Date

J E T PROPULSION LABORATORY C a l i f o r n i a I n s t i t u t e of Technology 4800 Oak Grove Drive Pasadena, C a l i f o r n i a 91109

7. Author(s)

9 . Performing organization Nane and Address

C. C . Gonzalez

13. Type of Report and Period Covered

8. Performing Organization Report N o .

10. Work Unit No.

12. Sponsoring Agency Name and Address JPL P u b l i c a t i o n

9 . Security Clcasif. (of this report)

Unc lass i f i ed

14. Sponsoring k e n c y Code NATIONAL AERONAUTICS AND SPACE ADMINISTRATION Washington, D . C . 20546

15* SuPP1ementary Notes Sponsored by t h e U.S. Department of Energy through SNLA Order Nos. 04-4848 and 02-2047. Also i d e n t i f i e d as DOE/JPL 1012-126 (RTOP o r Customer Code 776-52-00).

20. Security Clolsif. (of this page) 21. No. of Pages 22. Price

Unclas s i f i ed 34

16. Abstract

R e s u l t s of a s tudy t o determine t h e hot-spot s u s c e p t i b i l i t y of c o n c e n t r a t o r c e l l s , t o provide a hot-spot q u a l i f i c a t i o n test f o r concen t r a to r modules, and t o provide g u i d e l i n e s f o r reducing hot-spot s u s c e p t i b i l i t y are presented . Hot-spot hea t ing occur s i n a pho tovo l t a i c module when t h e s h o r t - c i r c u i t c u r r e n t of a c e l l i s lower than t h e s t r i n g ope ra t ing c u r r e n t f o r c i n g t h e c e l l i n t o r e v e r s e b i a s w i t h a concurren t power d i s s i p a t i o n .

t h e t e s t developed f o r f l a t - p l a t e modules, i s s u e s , such as provid ing c e l l i l l umina t ion , in t roduce a d d i t i o n a l complexi t ies i n t o t h e t e s t i n g procedure.

hot-spot s t r e s s i n g as apply t o f l a t - p l a t e modules. Therefore , recommendations are made on t h e number of bypass diodes r equ i r ed per g iven number of series ce l l s per module o r source c i r c u i t . I n a d d i t i o n , a new method f o r determining the c e l l temperature i n t h e l abora to ry o r i n t h e f i e l d i s d i scussed .

Although t h e b a s i s f o r t h e concentrator-module hot-spot q u a l i f i c a t i o n test is

The s a m e g e n e r a l g u i d e l i n e s apply f o r p r o t e c t i n g concen t r a to r modules from

7. Key Word (Selected by Author(s))

Energy Product ion Conversion Techniques S t r u c t u r a l Engineering

18. Distribution Statement

Unc l a s s i f i ed -Unl i m i t ed