1 j

SANDIA REPORT SAND85 - 0656 ' Unlimited Release . UC -63 Printed February 1986

Prototype Flat-Plate Building-Block Array Fields: Two-Year Field Evaluation

K. J. Snyder, J. W. Campbell, H. N. Post, M. G. Thomas

Preparad by Sandi. Nation" laboratorill Albuquerque, New Mexico 81185 ~lInd Livermore, Calilornia 94550 lor the United Stat .. Deparlment 01 Energy under Conlract OE-AC04-76DPOO789

SF2900Q(B-81 )

'-oed by Sandi. National L.boratories, operated for 1M United StaUl Del».lment 01 Ene'l)' by Sandia Co:poration. NOTICE: nul ..,port w .. prepared as an _nt of work aponlOled by an "laney of the United Stille! Government.. Neither the Uoiled States Govern­menl IIOr any agency thereof. nor any of their empioyen. nor any of their c:ontrlc!.On, lubcont.ract.orll. or their employ_. make, Iny .... Ilrtallty. e. ­p._ or implied, or asaumK any lellal liability or I'ftpon.ibility for the accu racy, c:ompJetenese , or ...... fulneu of any informat ion. applI..atU$, prod­uct, or pr~ d;":I.-.:i, Of rep._ nt. thAt i1ll ...... would not infringe privately owned .i, hu.. Reference he ..... in toan)' lpecme commercial product, ProceM, OJ llervice by u.de name. tnldemark , manufactUrer, or otherwi&e, d~ not nflC:ftUI'ily oonatitutcl or imply ita enciorHment, recommendation. G!' ravorilll by the United SUUI Government, any q:ency thereof or any of their conlracton or l uboontractora. The vie ..... IlIld opiniona eaprmaed he"' ­in do not ntoeaarily ,tt.t<I or retlert thoee oIlhe United St.alft Government. any "eney Ihe~r or any 01 their contractors or IUbcontrlcto~.

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SAND85 - 0656 Unlimited Release

Printed February 1986

Distribution Category UC - 63

Prototype Flat-Plate Building-Block Array Fields: Two-Year Field Evaluation

K. J. Snyder Solar Operations Division

EG&G, WASe Albuquerque

J. W. Cam pbell, H. N. Post, M. G. Thomas Photovoltaic System Divisions Sandia National Laboratories

Albuquerque, NM 87185

Abstract An evaluation of two years of operation was conducted on two modular nat-plate array fields at the Photovoltaic Advanced Systems Test Facility at Sandia National Laboratories. The evalua­tion focused on foundations. structures, and the electrical subsystem component.c;. Module characteristics and array performance were also assessed. The two 30-kWp array fields were com,tructed in 1983 using low-cost approaches to minimize the balance-of-system (80S) costs. The results of the evaluation show little 80S degradation in the structures and foundations. Degradation in the electrical subsystem was minimal and included 0.5 % failures in the bypass di ­odes, as well as some preliminary evidence of bonding deterioration in the module junction box. Delamination of some modules was also observed and resulted in several module failures in one of the arrays, a result outside the objective of the 80S evaluation. The results of the evaluation sup· port the modularized low-cost approach for the design and installation of photovollaic system!>.

ACKNOWLEDGMENTS

We would like to express our appreciation to Win Boyer of EG&G

for the collection of performance data and to Dave Menicucci, Division

6221, for his review of the performance data.

4

CONTENTS

INTRODUCTION

MODULAR ARRAY FIELD DESCRIPTIONS

Battelle Design

Hughes Design

SUMMARY OF OPERATION EXPERIENCE

EVALUATION

Methodology and Data

Visual Survey of the Modules

Visual Survey of the BOS

Function Checks

Bi weekly Su rveys

COMPARISON OF INITIAL AND PRESENT CHARACTER ISTI CS

Modules

BOS

BOS Effects on Energy Production

CONCLUSIONS

REFERENCES

Figure

1

2

3

4

5

6

ILLUSTRATI ONS

Battelle Prototype Array Field

Structure and Foundation Design for the Battelle Building Block

Hughes Prototype Array Field

Structure and Foundation Design for the Hughes Building Block

Shattered Front Cover of Solec Module in Battelle Field

Cl oseup of Solec Module Showing Area Where Fault to Module Frame Occurred

5

Page

9

10

10

12

14

14

21

21

26

30

31

31

34

34

36

39

41

11

11

1 3

13

15

15

ILLUSTRATIONS (Continued )

Figure

7 End View of Battelle Array Sho wing Support Structur e

8 Ba cks ides of So le c Modules in Bat telle Field Showing Cable Blocks , Cable Splices. and Bypass Diodes

9 Bypas s Diode o n So lec Module

1 0 Battelle Field Junction Box Showi ng Blocking Diode, Heat Sink, a nd Metal - Oxide Var i stors

11 Back View o f Hughes Array Sho wing Support Struc ture, Array Junction Box , and Solarex Modules

1 2 Cl oseup of Backside o f Hughe s Ar ray S ho wing AMP So l a r-Lo k Conne c tors and Bypass Di ode Housi ng

13 Cl oseup of Hughes Field Junction Box S howing Block ing

14

15

16

1 7

18

1 9

20

21

22

23

24

25

26

27

28

29

30

31

Diodes , Heat Sinks, and Metal - Oxide Varistors

Closeup of Hughes Terminal Bl ock Showing Me tal- Ox id e Va ri stors

Su r vey Form for the Battelle Field

Su r vey Form for the Hughes Field

Example o f Corrosion Obse rved on So l ec Modu l e

Exampl e of Ba ck Splitting on Sole c Module

Example of Back Spl it t ing on So l arex Module

Detached Cable Block o n Sole c Module

Poorly Closed Mastic Pad in Battel l e Fie ld

Split Transfer Beam in Battelle Fi e l d

Bi weekly Su r vey Form

Front View o f S ho rted So l ec Module No . 480

Back View of Shorted So le c Module No . 480

Front Met a l Foo ting of Hughe s Array at Insta l lation

Back Co ncrete Footi ng of Hughes Array at Installation

Sampl e I-V Cu r v e from Hughes Field , March 1984

Sample I - V Curv e from Hughes Fie l d , Novembe r 1984

Samp l e I-V Curve fr om Battelle Field, Ma rch 1 984

Sample I -V Curve fr om Bat t e l le Field. November 1 984

6

Page

16

16

17

17

18

18

1 9

19

22

23

24

24

25

25

27

28

32

33

33

35

35

37

37

38

38

TABLES

Tabl e Page

1 Summary of Visual Survey 26

2 Summary of Condi tion of Mastic Pads 26

3 Summary of Condition of Wo oden Be ams 29

4 Circuit Test Data 39

7-8

PROTOTYPE FLAT-PLATE BUILDING-BLOCK ARRAY FIELDS:

2-YEAR FIELD EVALUATION

INTRODUCTION

For the past 4 yr, Sandia National Laboratories, as managers of

the U. S. Department of Energy Photovoltaic (PV) Systems Development

and Evaluation Projects, has been involved in a comprehensive program

to develop and evaluate modularized array fields that offer the lowest

possible array field balance-of-system (BOS) life-cycle costs. l The

development of these fields was heavily based on the experience gained

from a group of intermediate-sized, full-scale system experiments that

were installed, and originally operated, through the sponsorship of

the U. S. PV Program and that have since been transferred to private

ownership. Experience from three systems in this group contributed

significantly to the design of the modularized array fields. The

three systems are located in Beverly, MA; Lovington, NM; and El Paso,

TX. Array field BOS installation costs (1980) for these systems

ranged from $424/m 2 for Lovington to $543/m2

for the El Paso system.

The lessons learned from these systems identified the following areas 2

in which significant cost reductions could be made:

1. A favorable site that requires little site preparation should

be selected.

2. Standarized designs and installation techniques are required

to reduce costs to acceptable levels.

3. Off-the-shelf materialsl rather than specialty materials,

must be used in the designs.

Under contract to Sandia National Laboratories, Battelle-Columbus

Laboratories and Hughes Aircraft Company independently developed

9

designs for modularized building-block array fields. 3 - 6 Both designs

used commercially available PV modules of the same size (2 by 4 ft)

and output voltage. Each could be readily modified to accommodate

other module sizes. Complete engineering drawing packages and con-

struction specification documents for each design are available.

In 1983, two modular flat-plate array fields were installed at

the PASTF, located in Albuquerque, NM, as a part of this program: one

using the Battelle design, the other using the Hughes design. Each

field has a peak-power rating of approximately 30 kw. This report

details the evaluation of each of the two array fields and identifies

degradation in the BOS that has occurred during the first 2 yr of

operation and the effect of this degradation on the electrical per­

formance of the fields.

MODULAR ARRAY FIELD DESCRIPTIONS

Battelle Design

The Battelle modular array field, shown in Figure I, consists of

three building blocks. The building block corresponds to a 400-Vdc,

10 kW, source circuit equipped with one junction box. Two 5-Vdc

modules, each containing six parallel strings of cells and a bypass

diode, are wired in parallel, and 82 pairs are wired in series to form

the building block. The high degree of paralleling and use of diodes

minimizes the effect of cell failures and allows for a 20-yr operation

with a maximum estimated power degradation of 15%, assuming normal

cell failure rates and no module replacement. All field wiring is

direct-burial cabling. The continuous metal support structure is

attached to a buried bare-capper-wire counterpoise network to provide

a uniform site ground.

The low-cost structure and foundation design is shown in Figure 2.

The support system incorporates galvanized steel structural support

members and galvanized steel foundation stakes (highway-sign posts)

driven approximately 3.5 ft into the ground. Treated wood beams (20-yr

or greater lifetime) permit simple fastening of the structural members

10

Figure 1. Battelle prototype Array Field.

b .. m &lpport

Higllw8Y-'ign ruk.

Figure 2 . Structure and Foundation Design for the Battelle Building Block.

using lag screws and provide cost-reducing flexibility in a lignment

during installation.

11

The Battelle field used PV modules procured from Solec Interna­

tional, Inc. These modules are 2 x 4 ft in size and carry a designa­

tion of Model S-4611. The Gardner-Zemke Company of Albuquerque was

the construction subcontractor on the project and was primarily

responsible for all aspects of the installation, including materials

(except modules) and labor. For the Battelle field, the overall

direct array field BOS cost in 1980 dollars was $121.88/m2 of collec­

tor area.

Hughes Design

The Hughes modular array field, shown in Figure 3, also consists

of three building blocks. The building block is a ±200-Vdc, 10-kW,

bipolar source circuit that consists of two 200-Vdc monopolar subar -

rays positioned in an east-west row. Each subarray has 2 parallel

circuits of 40 series-connected 5-vdc modules, each containing a

bypass diode and

high reliability

6 parallel cell strings. This design a l so exhibits

and tolerance for cell failures. A daisy chain

module-to-module wiring scheme is used in which the circuits run in

horizontal rows, fold back on themselves, and terminate in a common

junction box at one end of the subarray structure . The modules are

connected using quick-disconnect plug-in connectors . Power from eac h

building block is routed from a subarray junction box to a power

collection center (PCC) via direct-burial cabling . The PCC contains a

standard circuit breaker switch and bus panel, fault detection sens­

ing, and a power control module (PCM) for every two building blocks.

Likewise, the PCM contains transient protection devices (metal-oxide

varisters (MOVs)), blocking diodes, and snubbers . Thus, the control

of the array field is sectionalized in 20-kW increments.

The structure and foundation design, illustrated in Figure 4,

uses a galvanized steel-channel support structure and a hybrid founda-

tion. The front foundation is a buried metal foot that is an integral

part of the grounding network; the rear foundation is a co ncrete curb.

The Hughes modular array field uses 2- by 4-ft, polycrystalline­

cell, PV modules procured from Solarex Corporation.

12

Figure 3. Hughes Prototype Array Field.

GALVAHIZED STEEL-----'--_o_ CHANNELS

METAL FOOT

~- 2,,4 FOOT SOLAR CELL MODULE

SUBARRAY JUNCTION BOX

CURB

PVC CABLE RISERS FOR DIRECT BURIAL CABLE

Figure 4. Structure and Foundation Design for the Hughes Building Block.

Abbott Mechanical Contractors, Inc. of Albuquerque was the construc­

tion subcontractor responsible for installing the field. For the

Hughes field the overall direct array field BOS cost in 1980 dollars

was $134 . 27/m2 of collector area.

13

SUMMARY OF OPERATION EXPERIENCE

S ince installation, both array fields have delivered energy to

fixed resistive l oads or to the utility grid. The output of each

array field was measured and recorded over the 2-yr period. During

this period, no energy-related degradation has been observed in the

structural SUbsystems of either array field. Minimal degradation of

the electrical subsystems has been observed but has not appreciably

affected energy production~ however, all failures were repaired or

replaced.

Only one incident was observed in the Hughes field. A circuit

breaker used as a switch to short-circuit the array field failed.

This had no effect on energy production. Several module failures

occurred in the Battelle field during the same period. Electrical

shorts from the circuit to the frame developed in six modules. In

some cases, the rapid heating at the point of the short circuit caused

the front surface of the module to shatter (see Figures 5 and 6) . Two

other modules failed due to open circuits, and the connections between

the module and the output lead had to be resoldered. Also, a total of

five bypass diodes , about 0 . 5% of the total number of diodes, have

fai l ed open in the two fields during the first 2 yr.

EVALUATION

The objective of this evaluation was to determine what changes

have occurred in these two arrays since their installation that could

affect long-term operating cost and array energy production . Mainte-

nance and repair of the BOS, because of degradation, may be major

factors in system cost . The rate at which energy production degrades

is important in estimating the useful life of the system, a critical

parameter in determining the amortized costs of the energy production.

The evaluation of the BOS considered the following elements:

structural integrity, electrical integrity, module bypass diodes, and

electrical protection components. Figures 7 through 14 depict the

14

Figure 5. Shattered Front Cover of Solec Module in Battelle Field.

Figure 6 . Closeup of Solec Module showing Area Where Fault to Module Frame Occurred.

15

Figure 7. End View of Battelle Array Showing Support Structure .

Figure 8. Backsides of Solec Modules in Battelle Field Showi ng Cable Blocks, Cable Splice, and Bypass Diodes.

16

Figure 9. Bypass Diode on solee Module.

Figure 10. Battelle Field Junction Box Showing Blocking Diode, Heat Sink, and Metal-Oxide Varistors.

17

Figure 11.

Figure 12.

Back view of Hughes Array Showing Support Structure, Array Junction Box, and Solarex Modules.

Closeup of Backside of Hughes Array Showing AMP Solar-Lok Connectors and Bypass Diode Housing.

18

.... '"

w w-' ~.

..

Figure 13 . Closeup of Hughes Field Junction Box Showing Blocking Di odes, Heat Sinks, and Metal - Oxide Varistors .

Figure 14 . Closeup of Hughes Terminal Block Showing Metal- Oxide Varistors.

items of interest. While structural integrity does not direct ly con -

tribute to energy production, the loss of structural integrity could

result in damage to the modules and reduction of energy production.

Electrical integrity affects the energy delivered by the system

vis- a - vis the energy produced by the modules. For exampl e, a shorted

module bypass diode can reduce the system energy

bypass diode will

by shunting the

eliminate t he energy module output. An ope n module

produced by a complete source circ uit if the same module also develops

an open circuit. As a specific example , all of the modules in a

Hughes source string are in series, and an ope n circuit in a module

a nd bypass diode would interrupt the power flow from 16% (one source

c ircuit ) of the field. The problem is not as severe in the Battelle

array, where two modules (with their bypass diodes) are connected in

parallel before being series-connected with o ther modu le pairs. In

the Battelle array, four failures (two module fai l ures and two diode

failures) must occur simultaneously in a paralleled module pair to

interrupt the power flow from that complete source circuit .

The evaluation of the electrical performance cons idered the

following elements: module integrity, source- string current-voltage

characteristics , source- string-to-ground leakage currents, and energy

production. Module integrity is impor tant to performance because

module outpu t can be reduced if the environment penetrates the module

and creates leakage paths outside of the source string . Source-string

cur rent-voltage characteristics can be used to determine changes in

performance by modeling the string i n terms of a series resistance,

shunt resistance, diode factor, short-circuit c urrent, and ope n

circuit voltage . Observed changes in the model parameters over a

period of time may lead to determining the rate of performance degra­

dation . Leakage currents to ground diminish the energy delivered by

the array and may be dependent on existing environmental conditions,

especially moisture . Energy production is the primary factor used in

performance evaluation . Comparison of recent energy production with

that just after installation und er similar e nvironmental conditions

may also lead t o estimates of the expec ted useful life o f the array.

20

Methodology and Data

The 2-yr evaluation consisted at an in-depth study and several

biweekly surveys. The in-depth study was done in two parts: a visual

survey and a series ot function and performance tests. Biweekly

surveys, which followed the in-depth survey, covered essentially the

same areas as the latter but were not as thorough. Particular atten-

tion was paid to the worst of the potential problem areas that were

found during the in-depth survey.

For purposes of this evaluation, each array was viewed as con­

sisting of two segments: the PV modules (as manufactured by Solec and

Solarex), and the BOS, which included the site itself and all parts

and structures not integral to the individual modules.

Visual Survey of the Modules

The serial numbers and locations of each module in each array

were recorded on survey forms (Figures 15 and 16). The front surfaces

of the 492 Battelle and 480 Hughes modules were examined to identify

modules showing corrosion of the interconnect busses and what appeared

to be separation of the glass and the encapsulant, which might permit

water seepage. An example of the corrosion is shown in Figure 17.

The back surfaces were examined for splitting or other damage.

Examples of such damage are shown in Figure 18 (Battelle) and in

Figure 19 (Hughes).

Module cable blocks (984 on Battelle and 960 on Hughes ) were

checked by gently tugging each cable. If there was no discernible

motion, an ItOK~ rating was given (indicated by a check mark on the

form). Slight motion of the block and a visible gap was rated "L" for

Loose, and a block with considerable movement or completely detached

was rated "VL" (see Figure 20) or "O~ for Very Loose or Off.

summarizes the results of the visual survey.

21

Table 1

'" '"

BATTELLE SURVEY: ROW Modute No. Pad -Ipanel ConnectOf a.am CondllJon I Metal J TIMO Bot. cond. 1 Po. . I Nea. -lathT""-ooth DlrectJ Comment. ISUoDOrt Siruct.

Figure 15. Survey Fo rm f or the Battelle Field.

Modu .. ·C ....

and Surlec .. -------

• • III .i t- o

« 2 C

u; ~

8-" .i!

III ... 0 A. •

li <.> .. ; 0

~ ~

, .. 0 J< a:

, "8 "

> • III > II a: • ~

c c

'" 0

<.>

'" III X • • <-' z ~ " • M

X .. I-

" N , " 0

" -

"

23

" o '"

>. • > " o

OJ

• " 0

"-'"

Figure 17. Example of Corrosion Observed on Solec Module.

Figure 18 . Example of Back Splitting on Solec Module .

24

~-----~

Figure 19. Example of Back Splitting on Solarex Module.

Figure 20. Detached Cable Block on Salee Module.

25

Table 1

Summary of the Visual Survey

Back Loose Se l2:aration Corrosion S12:1ittins Cable Blks

Battelle 118/492 63/492 10/492 133/984

Hughes 17/480 0/480 8/480 12/960

Visual Survey of the BOS

The electrical insulating mastic pads (Battelle array only) were

~ated on thei~ appa~ent elect~ical insulating abi li ties and on their

appearance. A pad was rated as " solid" if it showed no gaps and had a

neat appearance . Pads with small edge gaps and not quite as neat in

appearance were rated as "acceptable." A "questionable" rating was

given to pads showing large edge gaps and a poor appearance. Co nnec-

tions with exposed crimps or with missing pads were rated nfailed."

Eight connections in Row Five and all connections in Row six were

taped rather than padded, leaving a total of 202 padded connections .

Table 2 summarizes this inspection.

Table 2

Summary of Condition of Mastic Pads

Condition Number of Cases

Solid 119

Acceptable 63

Questionable 16

Failed 4

Total 202

All of the mastic pads that had pulled apart (see Figure 21) were

supplied by one manufacturer. Pads from two different manufacturers

were used, and no problems have been observed with the pads supplied

26

Figure 21. Poorly Closed Mastic Pad in Battelle Field.

by the second manufacturer. Therefore, it appears that one batch of

pads (not the pads themselves) were deficient. At this point, the

deterioration of the pad- seal presents more of a safety concern than

one of performance.

The Solar-Lok connectors (Hughes array only) were examined for

full insertion, and several random connector plugs were checked for

corrosion and crimp strength. No problems were found.

Above-ground metal support structures were examined for corro­

sion, physical damage, and fastener integrity. No corrosion or damage

of any significance was found on either array: only minor rusting on

the cut ends of the frame members was noticed. Four loose bolts were

found on the Battelle array, and 22 were found on the Hughes array.

These appear to have been overlooked on installation.

27

The wooden transfer beams (Battelle only) were examined for flaws

such as checking, splitting, warping, and twisting . A rating scale

was used to rate the beam conditions. A "good" rating indicated a

beam that was in good condition with virtually no flaws. A "fair"

rating indicated a beam that was still structurally sound but had

moderate flaws. A "questionable " rating indicated a beam that was of

debatable st~uctural integrity with a considerable number and/or

degree of flaws. A "failed" rating indicated sufficient problems (see

Figure 22) to allow or possibly cause damage to the modules. The

results are tabulated in Table 3.

Figure 22. Split T~ansfer Beam in Battelle Field.

Although cracks have developed in about 10% of the wood support

members, no concommitant problems have resulted. Splitting of a

member at an end or a complete break in the middle of the member could

cause damage to the module and require replacement. The cracking

28

Table 3

Summary of Condition of Wooden Beams

Condition

Good

Fair

Que stionable

Failed

Number of Cases

121

88

35

8

appea r s to be due to excessive moisture loss . Duri ng post ­

installation inspection, it was determined that several wooden beams

did not meet the specifications. Therefore, because there have been

no observed problems and because some of the wood received was "out ­

o f-spe c ," there is no basis to infer that the design is in any way

deficient in this area.

The southwest s take on the Battelle a rray was chosen for ins pec ­

tion because it is located at the lowest corner of the field wher e

runoff water col lects. There was no sign of corrosion .

On the Hughes array, there was no evidence of spalling or other

deterioration on the concrete rear footing, nor was there any indica­

tion of deterioration on the front footing. The southwest corner was

chosen here also because it was the lowest spot in t he Hughes array

f ield.

All junction . and electrical boxes were examined, and no problems

were found.

The array site was inspected primarily for the effectiveness of

the soil sterilizer, indicated by the amount and l ocat ion of vegeta­

tion growth. There were a few weeds starting to gro w along the north

edge and southeast corner of the Hughes field. No growth was seen in

the Battelle field.

29

Function Checks

The second part of the survey was to test for proper functioning

of the various electrical components and devices and to determine the

performance level of the arrays.

Bypass Diodes -- Both arrays were placed on fixed loads for this

test. The diodes in the Battelle array were checked by placing a

clamp-on ammeter on a lead of each of the two diodes in each panel and

observing the current, first with the panel unshaded, then shaded. No

current when unshaded and divided current when shaded indicated prop-

erly functioning diodes. A shorted diode would have been indicated by

current in an un shaded panel; no shorted diodes were found. No cur-

rent in a diode of a shaded panel indicated an open diode~ four open

diodes were found.

The diode leads on the Hughes modules were not long enough to use

a clamp-on meter. A voltmeter was used to observe the voltage across

the diode, first with the module unshaded, then with the module

shaded. Individual module voltage when shaded and a forward voltage

drop across the diode when shaded indicated a functioning diode.

Module voltage when shaded or a forward voltage drop when unshaded

would have indicated an open or shorted diode, respectively. No

shorted diodes were found; one open diode was found.

Blocking Diodes -- The blocking diodes for both arrays were

tested in two ways. A preliminary check was made using the diode test

setting on a digital multimeter. A further test for reverse leakage

current was made with a Tektronix 576 curve tracer, and the results

were compared to published specifications for each diode type. All

blocking diodes were within specifications.

Fuses and Metal-oxide Varistors -- The fuses and MOVs in both

arrays were visually inspected only. There was no evidence of any

damaged MOVs or any reason to believe that they had functioned since

installation.

30

Biweekly Surveys

The biweekly surveys were an attempt at detecting any s hort-term

degradation or changes. Items that had been flagged as potential

problem spots during the in-depth survey were g iven special attention.

six of these surveys were done on each array, using the same basic

te.chniques that were used for the 2-yr survey . A sample of the for m

used to record observations is shown in Figure 23.

During the period of biweekly surveys, four Battelle modules

developed shorts to their frames, r esu lting in shattered cover glass

and burned backing and encapsulant. Figures 24 and 25 show the

effects of one such short. Of these four modules, only one had shown

any visible evidence of change (corrosion) during the 2-yr survey.

There were no other changes in either the Battelle or the Hughes

modules.

There were no noticeable changes in any of the BOS components in

either array during the biweekly survey period.

No changes in the appearance or functioning of the diodes, fuses,

or MOVs were detected in any of the biweekly surveys.

Efforts were made throughout the biweekly survey period to moni­

tor the ground currents of each array under different conditions.

Attempts to monitor and document the ground currents in the Battelle

field proved futile because of their highly variable and sporadic

nature. Ground currents in the Hughes field were essentially zero

under all conditions.

COMPARISON OF INITIAL AND PRESENT CHARACTERISTICS

The initial surveys, done immediately following assembly of the

arrays, did not include documentation on many of the areas that were

covered in the 2-yr survey .. Further, the data collected in the

initial su r vey were directed toward construction acceptance and were

31

BIWEEKLY CHANGE SURVEY Row Panel Loc. IIem ObHrvatlonl Date

Figure 23. Biweekly Survey Form.

32

Figure 24. Front View of Shorted Solec Module No. 480

Figure 25. Back View of Shorted Solec Module No. 480

33

not documented in a form for comparison with survey data to be col-

lected in the future. Consequently, comparison was limited to the

areas cO.vered in the ini tial survey. Also, since much of the survey

process is visual inspection, much of the evaluation is sUbjective.

Modules

The only suspicious items noted initially were yellowing corners

in four Battelle modules. These modules do not appear to have changed

at all since the initial survey. As mentioned above, six other

modules failed and had to be replaced. No initial problems with the

Hughes modules, front or back (except for minor shipping damage, which

had been repaired prior to the initial survey), were noted.

The initial survey identified 99 loose cable blocks on the

Battelle modules. The 2-yr survey shows 133 loose blocks (Table 1).

No loose blocks were initially found for the Hughes modules.

BOS

The first mastic pad survey was done in April 1983. This report

listed 40 bad pads, but the criteria used for these ratings were not

given by the inspector. The 2-yr survey shows 20 bad pads (rated

"questionable" or "failed") (Table 2).

The initial survey of the wooden transfer beams on the Battelle

array describes six that were in bad condition. Photographs docu-

men ted the condition of a few selected beams. There have been no

measurable or noticeable changes in these beams. Eight bad beams

(rated "failed") were recorded during the 2-yr survey (Table 3), in­

cluding the six recorded initially.

The initial condition of the in-ground structures is shown in the

photographs in Figures 26 and 27. The current condition indicates

that there were no problems initially.

34

Figure 26. Front Metal Footing of Hughes Array at Installation

Figure 27. Back Concrete Footing of Hughes Array at Installation

35

There was no weed growth initially in either array field.

very few weeds grew in the Hughes site after 2 yr.

Only a

No fuses have blown during normal operations.

BOS Effects on Energy Production

The changes in the BOS aspects of the array fields could be

detrimental to energy production if they require time and expense to

correct. Thus far in the discussion, we have not directly addressed

any effects on energy production in the two array fields. Component

degradation in the array fields known to affect energy production

include failed diodes (four in the Battelle and one in the Hughes

field) and eight failed modules observed in the Battelle field. Each

of these components was replaced once the failures were detected.

Other changes in the array fields might also contribute to energy

loss. current-voltage (I-V) curves of source strings of each array

were taken using a Gemini curve tracer. The data were stored on

flexible discs and processed through normal PASTF channels. Samples

of I-V curves from 20-kW source circuits of the Hughes array are shown

in Figures 28 and 29, and similar samples from the Battelle array are

shown in Figures 30 and 31. As shown in Table 4, the time period

between the two sets of data on each array was approximately a months.

The data presented do not indicate any appreciable (i.e., mea-

sureable) degradation over the a-month period. The data further

indicate power ratings of about 29 kW for the Hughes array and about p

28 kW P

for the Battelle array under peak conditions. These values are

very nearly the original ratings, but

verified through field measurements.

the original ratings were not

Maximum power data were also

available during the installation period, and regression analysis did

not indicate any degradation.

In summary, no energy reduction could be observed. Thus, we

conclude that within the accuracy and precision o f the measurements

and analyses, the BOS changes noted in this report did not affect the

performance of the array fields.

36

o

8

o

o ... :;: z w

'" '" ::> 0 U o

~

o

o o

DATE 810302. TIME 103817. REC. NO. SO

~

\ 1\

0. 0 SO.O 100.0 J50.0 200,0 250. 0 300.0 350. 0 400 .0 -4SO . 0 500.0

VOLTAGE

Figure 28. Sample I-V Curve from Hughes Field, Marc h 1984

60.13

h e ·52 .4 FI"'P.t: PIl!!! X • 15860 Watt s 50 . 0 v"p - 353 Vott :s - • "5 FInop.t:

~ ~ -~

4" . 13

~ a Je.i!I > ~ ~ 21:L0

10 . 13

'Voe - 466 Vo 1 t s

100 "'" , .. "'" sao ft(RAY VOLT AGE (VOlTS )

Flgure 29. Sample I-V Curve from Hughes Field I Novembe r 1 9 84

3 7

o

8

o o ~

o o N

o o

DATE 810308. TIME 131903. REC. NO. 125

.......... ~

'\ \

\ 0.0 50.0 100.0 150.0 200 . 0 250 .0 300 .0 350.0 400 .0 450.0 500.0

VOLTAGE

Fig ure 30. Sample I-V Cur ve from Battel le Field, March 1984

613.13

I", .:: '" S3 RIIp'" P", ,, x '" 192113 Watt.., 'Jmp '" 394 lIo lts

- 5121.13 - <16.3 Anps

~ ~ -"

"0.13 z ~ a 313 . 111 > ~ ~ 213.13

113.0

Voe '" SL3 Volts

, .. '00 '00 .00 s •• MRflY VOLT~E (VOLTS)

Figure 31 . Sample I - V Curve from Batte lle Field, November 1 984

38

Table 4

Circuit Test Data

Description Hughes Battelle

Date 3-02-84 11-27-84 3-08-84 11-15-8 4

Time 10:38 10: 4 2 13:49 13: 01

Total normal inso. (W/m2) 965 957 960 966

Ambient air temp. ( de9. C) 13.5 0.6 15.7 13.5

Approx. ce ll temp. (deg . c) 42 28 48 48

Maximum power (kW) 17787 18210 16180 15860

Vol tage at maximum power 369 394 347 353

Current at maximum power 48.1 46.3 46.6 45.0

Efficiency at max. power 7.6% 7.9% 7.4% 7.2%

Fill fac tor .66 .67 .65 .65

CONCLUSIONS

Two 30-kW modularized array fields were installed in the PASTF p

as part of a

techniques.

program

Readily

to develop low-cost BOS designs a nd installation

avai lable materials and standardized designs

fields.

led

to considerable savings for the installation of these two 2 Specifically, previous BOS costs exceeded $400/m for each of three

fielded systems, while the modular array fields were installed for 2

less than $140/m. These cost reductions were obtained despite the

fact that inexperienced contractors were employed to install the

fields.

To verify the applicability of these designs and techniques, we

established a program to characterize the BOS aspects of these fields

over time. This report describes the changes observed in the first

2 yr after the initial in stallation. No significant eve nt s have been

observed, and the energy production from the fields remains unde­

graded. This result appears to verify the specific designs employed

39

and the modular design approach. Nevertheless, longer-term evaluation

is planned, and results from the continuing evaluation will be used to reevaluate this conclusion.

A number of changes have occurred in the fields. Five of nearly

a thousand bypass diodes have failed op?n and have been replaced.

This type of failure rate appears to be somewhat higher than predicted

but has not resulted in any significant loss of energy production due

to replacement. Some of the wooden supports used in the Battelle

field have experienced more cracking than anticipated, but the cracks

have in no way affected the operation of the field. In addition, the

s uppl ier agreed that some of the lumber did not meet specification .

Some loose nuts and bolts have been observed and tightened. Again,

this maintenance was performed when needed and at virtually no cost.

One method of module interconnection utilized mastic pads covering an

infield wire-to-wire connection. Some of these pads are coming apart

and may eventually lead to a safety and /o r corrosion problem .

However, there is evidence that the materials used were "not in spec."

not

The maj or changes seen in the fields relate

really a part of the objectives for this BOS

to modules and are

evaluation . However,

we have observed some delamination and some splitting of the "tedIar"

backings of the modules. For the Solarex modules, these changes

cannot be correlated with any observable degradation in performance.

On the other hand , eigh t So lec modul es failed in the 2-yr evaluation

period. If these modules had not been replaced, s i gnificant e nergy

degradation would have resulted.

In conclusion, the low-cost hardware and insta ll ation procedures

used in the modular approach to fixed, flat-plate design and installa -

tion for two fields at the PASTF appear to be acceptable. Very little

BOS degradati o n has been observed , and little maintenance has been

required. At thi s time, there is no ev idence that the simplicity of

the designs and use of readily available materials has in any way

decreased the expected lifetime or performance of the array fields.

4 0

REFERENCES

1. H. N. Post and E. L. Burgess, "New Designs and Installations of photovoltaic Array Fields with Low Balance-of-System Costs," Proceedin a of the 10th Ener Technolo Conference, Washington, DC, Fe ruary

2. M. Thomas, The Value of PV Systems Experiments: volume I: A Preliminar Assessment of the Lessons Learned from Nine Intermediate-Size Systems, SAND84-0900 I Albuquerque: Sandia National Laboratories, August 1984).

3. D. C. Carmichael et aI, Development of a Standard Modular Design for Low-Cost Flat-Panel Photovoltaic Array Fields, SAND81-7183 (Albuquerque: Sandia National Laboratories, November 1982). WorK performed by Battelle-Columbus Laboratories.

4. G. T. Noel, Installation of a Modular Photovoltaic Array Field with Low Balance-of-System Costs, SAND83-7027 (Albuquerque: Sandia National Laboratories, January 1984). Work performed by Battelle-Columbus Laboratories.

5. G. J. Naff, Photovoltaic Array Field Optimization and Modularity Study, SAND81-7193 (Albuquerque: Sandia National Laboratories, March 1983). Work performed by Hughes Aircraft Company.

6. Modular photovoltaic Array Field, SAND83-7028 (Albuquerque: Sandia National Laboratories, September 1984). work performed by Hughes Aircraft Company.

41

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