The goals of this study are to: (1) parameterize current-voltage (I-V) performance over a wide range of illumination and temperatures: (a) 50-1150 W/m{sup 2} irradiance, 5-65 C; (b) obtain array temperature coefficients; and (c) quantify energy production; (2) investigate high-voltage leakage currents from the CIS modules in a high-voltage array: determine dependence on moisture, temperature, and voltage bias and ascertain corrosion problems if any; and (3) study long-term power and energy production stability

del Cueto, J. A.

Sekulic, B. R.

UNT Digital Library

An Unlikely Combination of Experiments with a Novel High-Voltage CIGS Photovoltaic ArrayJ.A. del Cueto, B.R. Sekulic National Renewable Energy LaboratoryGolden, ColoradoUnited States of America 4th World Conference on Photovoltaic Energy ConversionWaikoloa, HawaiiMay 7-12, 2006NREL/PR-520-39853Disclaimer and Government LicenseThis work has been authored by Midwest Research Institute (MRI) under Contract No. DE-AC36-99GO10337 with the U.S. Department of Energy (the “DOE”).  The United States Government (the “Government”) retains and the publisher, by accepting the work for publication, acknowledges that the Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for Government purposes.Neither MRI, the DOE, the Government, nor any other agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe any privately owned rights.  Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not constitute or imply its endorsement, recommendation, or favoring by the Government or any agency thereof.  The views and opinions of the authors and/or presenters expressed herein do not necessarily state or reflect those of MRI, the DOE, the Government, or any agency thereof. Introduction Long-term (2020) goal of the US Solar Program Multi-Year Plan: ¾ Commercialization of photovoltaic (PV) modules with 30-year lifetimes or more, capable of sustaining less than 0.5% annual performance degradation rate, and at costs consistent with market-rates of electricity. Motivation: quantify the performance, stability and reliability of high efficiency thin-film copper-indium-diselenide (CIS) PV modules in a high-voltage array.Motivation: PV Module Reliability High voltage is a known stress-mechanism that precipitates PV module degradation: ¾ In the 1980's, the Jet Propulsion Laboratory (JPL) investigated the connection between high-voltage leakage currents from modules and their degradation:¾ Found that series resistance increases, brought about by electrochemical corrosion of contacts, is a prime failure mechanism induced by high-voltage stress¾ Established key thresholds of accumulated charge that would result in 50% failure in certain PV modules: For crystalline-silicon (c-Si) ranging 1–10 coulombs per linear centimeter (C/cm) of module perimeter, For amorphous-silicon (a-Si) modules 0.1–1 C/cmMotivation: PV Performance & Stability Actual energy production of modules under real field conditions continues to be an issue:¾ Currently only measurements of power at Standard Reporting Conditions (SRC) are used to rate modules But SRC misses most of actual operating conditions PV arrays commonly employ inverters and measurements reported usually include only optimum power-point voltage, current and power ¾ When performance degrades, measurements of optimum power point voltage & current are usually not enough to discern the failure mode, ¾ Would benefit to have more in-depth I-V characterization.Goals of This Study Parameterize current-voltage (I-V) performance over a wide range of illumination & temperatures: ¾ 50 – 1150 W/m2 irradiance, 5° – 65 °C¾Obtain array temperature coefficients¾Quantify energy production Investigate high-voltage leakage currents from the CIS modules in a high-voltage array:¾Determine dependence on moisture, temperature, and voltage bias¾Ascertain corrosion problems if any Study long-term power & energy production stability.HVST2 CIGS Array 24 Shell Solar thin-film CIGS modules¾ Nominally ±300 VDC open-circuit & 1 kW total array power ¾ Deployed in 2 bipolar strings, 12 modules in each connected in series ¾ Each string: aperture area~4.877 m2, segregated into 2 groups of 6 modules each group for each string¾ Each group of 6 modules perimeter length ~ 1.915 m ¾ Efficiency baseline tested at standard test conditions (STC, ηSTC) prior to deployment: All modules tested between 36 and 44 W power, at STC, Average ηSTC for the positive (+) and negative (-) strings of the array: respectively, 9.64% and 9.53%.¾ Dry hi-pot and wet hi-pot tests : for both tests, the leakage currents ranged 0.1–0.6 microamps. HVST2 CIGS Array Current-Voltage (I-V) control & measurements:¾via programmable e-load¾ I-V traces once every 15min, in 1st quadrant ¾ peak-power other times Temperatures: ¾type ‘T’ TC back-of-module one for each +/- string ¾Air Temp. & humidity (RH) Modules mounted facing south, at ~40° tilt ~latitude ~39.7°, plane-of-array (POA) Irradiance sensed with pyranometers, same POAOutdoor Test Facility (OTF) View Looking East Toward ArrayHVST2 Array Connection 24 CIGS modules in positive (+) and negative (-) strings¾ 12 modules per string, 2 groups of 6 modules each per string¾ Frames not grounded at supports, but instead connected to resistive network and then to ground to facilitate leakage current sensing Electrical Characterization & Control ¾ via dual-channel Programmable Electronic Load (PEL) interfaced to Data Acquisition System (DAS) running Visual BASIC Code¾ PEL is not a power supply/source¾ PEL can control in constant voltage, current or resistance¾ DAS sends varying set-points to PEL to trace I-V curves or track peak powerProgrammable Electronic Load: Control Modes:constant current or voltage, resistance LoadingHigh Voltage Array: 24 CIGS PV modules2 strings + / - ,  with 12 modules per string,2 groups of 6 modules in series each,±300 volts Voc , 2.5 amps Isc from each stringPowerLeadsVsense63308 63308Vsense+_+_ +_+_2.5 A , 150 VDC- +- +- + - +- String + String2.5 A , 150 VDC2.5 A , 150 VDC2.5 A , 150 VDCHVST2 Array Leakage Currents Sensing Frames allowed to float and electrically daisy-chained together on support structure Electrical connection made to resistive network before ground:¾ Leakage currents pass through   52.5 kOhm resistor combination, ¾ Leakage currents appear as voltage across 2.49 kOhm resistor, are sensed by CR23x data-logger differential input channels¾ Arc by-pass to ground provided in case of too much leakage. Each group of 6 modules has own sensing circuit¾ Group 1 always higher offset bias being further from ground+-- String- StringGroup 1- StringGroup 2S/N  05504S/N  06082S/N  17437S/N  27892S/N  34213S/N  01397S/N  34368S/N  21496S/N  34208S/N  70602S/N  27893S/N  06096+ String+ StringGroup 1+ StringGroup 2S/N  34195S/N  05508S/N  16674S/N  05677S/N  34222S/N  27891S/N  17926S/N  04290S/N  06098S/N  34226S/N  06422S/N  06597Earth GndR1   50 kOhmR2   2.49 kOhmNeon Lamp90vSignal GndCurrent HiCurrent LoCampbell CR23xDifferential InputChannel AmplifiersData Analysis1. Integration of peak-power tracking and irradiance data versus time yielding daily energy output, insolation2. Accumulation of all I-V trace records, statistical filtering for predominantly clear-sky conditions, followed by segregation into irradiance bins & regression vs. Temp.3. Integration of the leakage current data through module frames resulting in accumulated daily leakage charge. 4. Efficiency quotients from two distinct angles: i. usual I-V trace data representing power measurements from each string, divided by the incident power on each string, ii. Effective Efficiency (ηEFF) =  ratio of the daily energy output from each string divided by the daily insolation energy)1(][ dtIrrAreadtPDailyStringDailyMAXEFF ⋅⋅⋅= ∫∫ηPeak Power Tracking Data Top: Effective efficiency (ηEFF)derived daily from energy output(time-integrated power) ¾ Not temperature corrected due to integration and convolution of all field conditions occurring daily¾ Varies seasonally ~7%-9½% Mid: Performance ratio (PR)¾ PR = ηEFF / ηSTC energy-based, vary 70%-95% of ηSTC  Bottom: +/- strings sampled average daytime module temperatures¾ ηEFF is function of temperature, ¾ (1/ ηEFF) dηEFF /dTair ~ -0.38%/°C Tair –> average daytime air temp.678910111-Apr-051-May-0531-May-051-Jul-0531-Jul-0531-Aug-0530-Sep-0531-Oct-0530-Nov-0531-Dec-0530-Jan-061-Mar-061-Apr-060102030405060708090100Module Temperatures (°C) or Performance Ratio (%)+ Str PR - Str PR+Str Tmod -Str Tmod+ Str Eff -Str EffEffective Efficiency(Energy Out -Based)Performance From Full I-V Traces FF (top) and Efficiency (bottom) from full I-V traces, corrected to 25°C temperature, for irradiance = 1000±50 W/m2 depicted¾ No statistically significant changes in efficiency or FF with time in 1st year¾ Seasonal variations observed¾ + string: 9.1% ± 0.3% efficiency, 61%-62% FF¾ - 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%7%8%9%10%+Str Eff-Str Eff50%55%60%65%70%50%55%60%65%70%+Str FF-Str FF-Str FF(%)+ Str FF(%)+ Str Eff(%)-Str Eff(%)5/1/05 7/31/05 10/30/05 1/29/06 5/1/06TimeVoc & FF From Full I-V Traces Dependence analyzed across all irradiance every 50 W/m2, corrected to 25°C module temperature (Tmod)¾ Voc shows logarithmic type dependence with irradianceCoefficients (1/Voc) dVoc/dTmodvary ~ -0.37 to -0.30  %/°C¾ FF data, seem to peak at ~   300 W/m2,  then decrease   with increasing irradianceConsistent with series-resistance limited behaviorDrops off at low irradianceCoefficients (1/FF) dFF/dTmodvary ~ -0.05 to +0.20  %/°C going from high to low irradianceIrradiance (W/m2)Voc (volts)FF (%)20021022023024025026027028029030055565758596061626364650 100 200 300 400 500 600 700 800 90010001100+ Str Voc - Str Voc+ Str FF -Str FFLeakage Currents QL = Daily integrated leakage charge: Computed & plotted for each group of 6 modules vs. average daytime temperatures Plotted on Arrhenius Graph¾ Appears thermally activated¾ Scale 0.01-10 milliCoulombs Variations or scatter:¾ Sizes of group-1 QL are  ~2.8 - 2.9 * Group-2 QL, reflects partition of voltages between modules & higher offset bias for group 1¾ Humidity, integration over day convolutes multiple humidity values∫ ⋅=DailyLeakLeak dtIQ101/Tmod (°K-1) *1000Daily Accumulated Charge Leaked per Group ofSix Modules  (milliCoulombs)3.1 3.2 3.3 3.4 3.5 3.6 3.70.010.11-Str Group 1 + Str Group 1- Str Group 2 + Str Group 2Leakage Charge: Activation and Size QL = integrated leakage charge Thermal Activation energies:¾0.6-0.78 eV¾consistent with leakage conduction through soda-lime glass For largest leakage  ~10 mC, Tmod average ~23-30°C¾Not a problem in Colorado due to our cold dry climate¾In hotter climate, say with 200 days/year at these temperatures, In 10 years, would accumulate ~20 Coulombs leakage for each group of 6 modules or about 0.1 C/cm  0.1 C/cm => is right at threshold for 50% failure rate determined by JPL for a-Si thin-film modules101/Tmod (°K-1) *1000Daily Accumulated Charge Leaked per Group ofSix Modules  (milliCoulombs)3.1 3.2 3.3 3.4 3.5 3.6 3.70.010.11-Str Group 1 + Str Group 1- Str Group 2 + Str Group 2Leakage Currents: Response to Array Bias Response of high-voltage leakage currents to bias vs. time of day shown for ± groups 1 on two days ¾ Peak-power tracking at top, & stepped-voltage vs. time profile at bottom,¾ Corresponding ±Vbias shown, read at left, leakage currents read at right¾ For stepped-bias, response appears qualitatively different:Forcing effect in phase with large step increments in biasLeakage exhibit transients and larger value by ~ 2x over peak powerCurrents actually flip polarity for several minutes when decrementing, but not when incrementing Ion motion & relaxation in glassTime of Day (hours)3:00 6:00 9:00 12:00 15:00 18:00 21:00Vbias (±volts)0100200300ILeak ( μAmps)-0.15-0.10-0.050.000.050.10-0.30-0.20-0.100.000.100.200.30ILeak ( μAmps)Vbias (±volts)0100200300V (- string)V (+ string)+Str ILeak1-Str ILeak14/18/063/14/06±Vbias-Str ILeak1+Str ILeak1Conclusions Performance characterized from both power and energy output considerations:¾ Significance of energy-based effective efficiency, rows 2, 4: Average insolation * effective efficiency (at 13°-14°C Average daytime air temperature) = delivered energy output per dayWe present energy-based performance temperature coefficient:   -0.38%/°C vs. average daytime air temperatures Stability:¾ Does not appear to be degrading statistically or otherwiseString Average Air Temp.AverageModule Temp. Irradiance or Insolation Eff (%) Method PR + 14 28 6.12 kW-hrs/m2/day 8.12Energy 84.3%+ Temp. Corrected 25 1001.5  Wm2 9.11Power 94.5%- 13 27 6.18 kW-hrs/m2/day 7.95Energy 83.5%- Temp. Corrected 25 1001.4 Wm2 8.87 Power 93.1% Conclusions Leakage Currents:¾Measured and characterized¾ Appear thermally activated¾May point to potential corrosion problems in hot environments, where average daytime air temperatures are at, or exceed 25-30 °C for 200 days or more per year, after 10 years.Appendix: IV Trace Statistical Filtering for Clear sky vs. Diffuse Illumination Two conditions applied: Isc is linear with Irr & Irr=Irr(cos(AOI)) Irr ~ k2*cos(AOI) ± 2 σ¾ differentiates between predominantly clear vs. diffuse¾ Works well , but at larger AOI the diffuse starts to become dominant over direct beam Isc ~ k1*Irr ± 2 σ¾ mitigates data traced w. rapid changes in Irr and/or obscuration by snow¾ Carried out in piecewise intervals

Unlikely Combination of Experiments with a Novel High-Voltage CIGS Photovoltaic Array

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Unlikely Combination of Experiments with a Novel High-Voltage CIGS Photovoltaic Array

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