An Unlikely Combination of Ex

with a Novel High-Voltage 

Photovoltaic Array

J.A. del Cueto, B.R. Sekul

National Renewable Energy Lab

Golden, Colorado

United States of America 

4th World Conference on Photovoltaic En

Waikoloa, Hawaii

May 7-12, 2006

NREL/PR-520-39853
Disclaimer and Government License

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specific commercial product, process, or service by trade name, trademark, manufacturer, or othimply its endorsement, recommendation, or favoring by the Government or any agency thereof. authors and/or presenters expressed herein do not necessarily state or reflect those of MRI, the agency thereof. 
Introduction

 Long-term (2020) goal of the US So

Multi-Year Plan: 

3⁄4 Commercialization of photovoltaic (PV

30-year lifetimes or more, capable of sthan 0.5% annual performance degradacosts consistent with market-rates of e

 Motivation: quantify the performanc

reliability of high efficiency thin-film indium-diselenide (CIS) PV modulevoltage array.
Motivation: PV Module R

 High voltage is a known stress-mechani

precipitates PV module degradation: 

3⁄4 In the 1980's, the Jet Propulsion Laboratory 

investigated the connection between high-vcurrents from modules and their degradatio

3⁄4 Found that series resistance increases, brou

electrochemical corrosion of contacts, is a pmechanism induced by high-voltage stress

3⁄4 Established key thresholds of accumulated 

result in 50% failure in certain PV modules

 For crystalline-silicon (c-Si) ranging 1–10 coul

centimeter (C/cm) of module perimeter,

 For amorphous-silicon (a-Si) modules 0.1–1 C/c
Motivation: PV Performance 

 Actual energy production of module

field conditions continues to be an is

3⁄4 Currently only measurements of power at Sta

Conditions (SRC) are used to rate modules

 But SRC misses most of actual operating conditio

 PV arrays commonly employ inverters an

reported usually include only optimum povoltage, current and power 

3⁄4 When performance degrades, measurements 

point voltage & current are usually not enougfailure mode, 

3⁄4 Would benefit to have more in-depth I-V cha
Goals of This Stud

 Parameterize current-voltage (I-V) 

over a wide range of illumination & 

3⁄4 50 – 1150 W/m

2 irradiance, 5° – 65 °C

3⁄4 Obtain array temperature coefficients3⁄4 Quantify energy production

 Investigate high-voltage leakage cu

CIS modules in a high-voltage array

3⁄4 Determine dependence on moisture, t

voltage bias

3⁄4 Ascertain corrosion problems if any

 Study long-term power & energy pr

stability.
HVST2 CIGS Array

 24 Shell Solar thin-film CIGS modules

3⁄4 Nominally ±300 VDC open-circuit & 1 kW 3⁄4 Deployed in 2 bipolar strings, 12 modules i

series 

3⁄4 Each string: aperture area~4.877 m

2, segreg

of 6 modules each group for each string

3⁄4 Each group of 6 modules perimeter length ~ 3⁄4 Efficiency baseline tested at standard test c

ηSTC) prior to deployment: 

 All modules tested between 36 and 44 W power Average 

ηSTC for the positive (+) and negative 

respectively, 9.64% and 9.53%.

3⁄4 Dry hi-pot and wet hi-pot tests : for both te

currents ranged 0.1–0.6 microamps. 
HVST2 CIGS Array

 Current-Vo

& measure

3⁄4via prog3⁄4 I-V trac

15min, i

3⁄4 peak-po

 Temperatu

3⁄4type ‘T’ 

one for e

3⁄4Air Tem

 Modules m

south, at ~4~39.7°, pla

 Irradiance 

pyranomet

Outdoor Test Facility (OTF) 

View Looking East Toward Array
HVST2 Array Connec

 24 CIGS modu

negative (-) str

3⁄4 12 module

6 modules 

3⁄4 Frames no

but instead network anfacilitate le

 Electrical Char

3⁄4 via dual-cha

Electronic LData Acquirunning Vis

3⁄4 PEL is not a 3⁄4 PEL can co

current or re

3⁄4 DAS sends 

to trace I-V 

Programmable Electronic Load: Control Modes:

constant current or voltage, resistance Loading

High Voltage Array: 24 CIGS PV modules

2 strings + / - , with 12 modules per string,

2 groups of 6 modules in series each,

±300 volts Voc , 2.5 amps Isc from each string

Power

Leads

Vsense

63308 63308

V

sense

+_

_

+

_

+

+_

2.5 A , 150 VDC

- +- + - + - +

- String + String

2.5 A , 150 VDC

2.5 A , 150 VDC

2.5 A , 150 VDC
HVST2 Array Leakage Curre

 Frames allowed to float and 

electrically daisy-chained together on support structure

 Electrical connection made to 

resistive network before ground:

3⁄4 Leakage currents pass through 

52.5 kOhm resistor combination, 

3⁄4 Leakage currents appear as voltage 

across 2.49 kOhm resistor, are sensed by CR23x data-logger differential input channels

3⁄4 Arc by-pass to ground provided in 

case of too much leakage.

 Each group of 6 modules has 

own sensing circuit

3⁄4 Group 1 always higher offset bias 

being further from ground

+ String

+ String

Group 1

+ Strin

Group 

S/N 34S/N 05S/N 16S/N 05S/N 34S/N 27

S/N 17926S/N 04290S/N 06098S/N 34226

S/N 06422S/N 06597

Earth Gnd

Neon Lamp90v

Signal Gnd
Data Analysis

1. Integration of peak-power tracking and 

versus time yielding daily energy outpu

2. Accumulation of all I-V trace records, s

for predominantly clear-sky conditions, segregation into irradiance bins & regre

3. Integration of the leakage current data t

frames resulting in accumulated daily le

4. Efficiency quotients from two distinct a

i. usual I-V trace data representing power me

each string, divided by the incident power o

ii. Effective Efficiency (ηEFF) = ratio of the d

from each string divided by the daily insola

[AreadtP dtIrr )1(]

Daily

String

Daily

EFF MAX ⋅= ⋅⋅ ∫ ∫ η
Peak Power Tracking 

 Top: Effectiv

derived daily (time-integra

3⁄4 Not temper

integration field condit

3⁄4 Varies seas

 Mid: Perform

3⁄4 PR = 

ηEFF / 

70%-95% o

 Bottom: +/- 

average dayttemperatures

3⁄4

ηEFF is func

3⁄4 (1/ 

ηEFF) 

dη



Tair –> 

67891011

1-Apr-05

1-May-05

31-May-05

1-Jul-05

31-Jul-05

31-Aug-05

30-Sep-05

31-Oct-05

30-Nov-05

31-Dec-05

30-Jan-06

1-Mar-06

1-Apr-06

0

102030405060708090

100

Module Temperatures (°C) or Performance Ratio (%) + Str PR - Str PR +Str Tmod -Str Tmod + Str Eff -Str Eff

Effective Efficiency

(Energy Out -Based)
Performance From Full I-

 FF (top) and Efficiency (bottom) 

from full I-V traces, corrected to 25°C temperature, for irradiance = 1000±50 W/m

2 depicted

3⁄4 No statistically significant 

changes in efficiency or FF with time in 1st year

3⁄4 Seasonal variations observed3⁄4 + string: 9.1% ± 0.3% 

efficiency, 61%-62% FF

3⁄4 - string: 8.9% ± 0.3%, 

efficiency, ~60% FF

 Actual FF determination allows 

closer scrutiny in event of degradation (e.g., series, shunt resistances).

7%8%9%

10%

50%55%60%65%70%

+Str F-Str F

+ Str FF(%) + Str Eff(%)

5/1/05 7/31/0
Voc & FF From Full I-V 

 Dependence analyzed across all 

irradiance every 50 W/m

2, corrected 

to 25°C module temperature (Tmod)

3⁄4 Voc shows logarithmic type 

dependence with irradiance

 Coefficients (1/Voc) dVoc/dTmod

vary ~ -0.37 to -0.30 %/°C

3⁄4 FF data, seem to peak at ~ 300 

W/m

2, then decrease with 

increasing irradiance

 Consistent with series-resistance 

limited behavior

 Drops off at low irradiance Coefficients (1/FF) dFF/dTmod

vary ~ -0.05 to +0.20 %/°C going from high to low irradiance

Voc (volts)

200 210220230240250260270280290300

0 100 200 300 4
Leakage Currents



Q

L = Daily i

charge:

 Computed & 

group of 6 mdaytime tem

 Plotted on A

3⁄4 Appears the3⁄4 Scale 0.01-

 Variations or 

3⁄4 Sizes of gro

* Group-2 Q

voltages betoffset bias f

3⁄4 Humidity, i

convolutes 

Leak 

Q

10

1/Tmod (°K-1) *1000

Daily Accumulated Charge Leaked per Group of

Six Modules (milliCoulombs)

3.1 3.2 3.3 3.4 3.5 3.6 3.7

0.01

0.1

1

-Str Group 1 + Str Group 1- Str Group 2 + Str Group 2
Leakage Charge: Activatio



Q

L = integra

 Thermal Act

3⁄40.6-0.78 eV3⁄4consistent w

through soda

 For largest le

Tmod averag

3⁄4Not a proble

our cold dry 

3⁄4In hotter clim

days/year at 

 In 10 year

Coulombs of 6 modu

 0.1 C/cm 

for 50% fJPL for a-

10

1/Tmod (°K-1) *1000

Daily Accumulated Charge Leaked per Group of

Six Modules (milliCoulombs)

3.1 3.2 3.3 3.4 3.5 3.6 3.7

0.01

0.1

1

-Str Group 1 + Str Group 1- Str Group 2 + Str Group 2
Leakage Currents: Response to 

 Response of hig

currents to bias vshown for ± grou

3⁄4 Peak-power trac

voltage vs. time 

3⁄4 Corresponding ±

left, leakage cur

3⁄4 For stepped-bias

qualitatively dif

Forcing effec

step increme

Leakage exh

value by ~ 2x 

Currents actu

several minubut not when 

 Ion mo

Time of Day (hours)

3:00 6:00 9:00 12:00 15:00 18:00 21:00

Vbias (±volts)

0

100200300

ILeak (μAmps)

-0.15-0.10-0.050.000.050.10

-0.30-0.20-0.100.000.100.200.30

ILeak (μAmps)

Vbias (± volts)

0

100200300

V (- string)

V (+ string)

+Str ILeak1

-Str ILeak1

4/18/06

3/14/06

±Vbias

-Str ILeak1

+Str ILeak1
Conclusions

 Performance characterized from both 

energy output considerations:

3⁄4 Significance of energy-based effective e

 Average insolation * effective efficiency (at 1

daytime air temperature) = delivered energy 

 We present energy-based performance temp

 -0.38%/°C vs. average daytime air tempe

 Stability:

3⁄4 Does not appear to be degrading statisti

String Average Air Temp. Average Module Temp. Irradiance or Insolation Eff (%) Method 

+ 14 28 6.12 kW-hrs/m2/day 8.12 Energy 8+ Temp. Corrected 25 1001.5 Wm2 9.11 Power 9

- 13 27 6.18 kW-hrs/m2/day 7.95 Energy 8- 

Temp. 

Corrected 25 1001.4 Wm2 8.87 Power 9
Conclusions

 Leakage Currents:

3⁄4 Measured and characterized3⁄4 Appear thermally activated3⁄4 May point to potential corrosion p

hot environments, where averagtemperatures are at, or exceed 2200 days or more per year, after 
Appendix: IV Trace Statis

Filtering for Clear sky vs. Diffuse 

 Two conditions ap

linear with Irr & I

 Irr ~ k

2*cos(AOI) 

3⁄4 differentiates b

predominantly 

3⁄4 Works well , but 

diffuse starts to bover direct beam

 Isc ~ k

1*Irr ± 2 

σ

3⁄4 mitigates data trac

in Irr and/or obscu

3⁄4 Carried out in pie