Multistress Accelerated Testing and Correlation to Field Performance

D u P o n t P h o t o v o l t a i c S o l u t i o n s Multistress Accelerated Testing and Correlation to Field Performance SNEC 12th (2018) International Photovoltaic Power Generation and Smart Energy Exhibition & Conference William Gambogi, Technical Fellow 2018 DowDuPont. All rights reserved.

Outline Multiple stress factors in the PV Environment Overview of Sequential Testing Protocols Module Accelerated Sequential Testing (MAST) Third Party Test Results Faster MAST Test Sequence Alternative Test Sequences Sequential Testing with Dynamic Mechanical Load Future Directions Conclusions 2018 DowDuPont. All rights reserved. May 29, 2018 D u P o n t P h o t o v o l t a i c S o l u t i o n s 2

Stresses in the PV Field Photovoltaic modules are exposed to a wide range of stress conditions Stresses operate on the module simultaneously and sequentially, synergistic effects are observed Combine Key Stresses in Sequences using Commercial Test Chambers D u P o n t P h o t o v o l t a i c S o l u t i o n s 3

DuPont Sequential Tests Test 1 UV / DH / 3x (TC / UC) Test 2 DH/TC Test 3 Xenon Weatherometer: ASTM G155 protocols Test 4 UV / DML / TC / HF Rationale Our #1 Test. Combines the most important stress factors in the field. Appropriate for component, minimodule, or full-size module testing. Combines two important stress factors in a shorter test not requiring expensive UV equipment. Appropriate for full-size module testing. Combines UV and rainfall simulation, common weathering test conditions in commercially available weatherometer. Combines UV and Temperature cycling with the dynamic stresses of Dynamic Mechanical Load (DML)

Original Module Accelerated Sequential Test Most predictive test for backsheet field performance. Appropriate for component, minimodule, or full size module testing. May 29, 2018 D u P o n t P h o t o v o l t a i c S o l u t i o n s 5

Third Party MAST on Commercial Modules Thermal Cycling Thermal Cycling Thermal Cycling 1000 hours 485 hours 200X 200X 200X 485 hours 485 hours 485 hours Damp Heat UVA UVA UVA UVA 1000 Hours in a Humidity Chamber Amounts to > 25+ years worth of stress 600 Thermal Stress Cycles Mimics thermal stresses seen in the field 1940 Hours in a UVA Chamber Amounts to 24 years worth of UV stress Higher intensity UV source shortens exposure time with same total UV dosage Tests conducted on full-size modules by independent 3 rd party testing lab DNV-GL, USA 2018 DowDuPont. All rights reserved. May 29, 2018 D u P o n t P h o t o v o l t a i c S o l u t i o n s 6

Loss of Backsheet Mechanical Properties - Third Party MAST test at DNV-GL, USA Visible cracks on PVDF-based backsheet over all busbar ribbons after MAST. All cracks are in the long (MD) direction due to weak TD elongation strength. No cracks in Tedlar PVF-based backsheets. PVDF backsheet cracking in minimodules in MAST PVDF backsheet cracking in the field in 5 years, North America Cracking in PVDF-based backsheet after MAST in full-sized modules, minimodules, and in the field D u P o n t P h o t o v o l t a i c S o l u t i o n s 7

Yellowing of PET Backsheets - Third Party MAST test at DNV-GL, USA 3.5 3 2.5 2 1.5 1 0.5 0-0.5-1 b* vs Test Cycle Number Tedlar PVDF PET 0 1UV-TC 2UV-TC 4 3 2 1 0 3UV-TC -1 b* vs Test Cycle Number Tedlar PVDF PET 0 1UV- TC Minimodules after MAST at DuPont Full Size Modules after MAST at DNV-GL Yellowing is a sign of polymer degradation and leads to cracking Yellowed PET backsheet after 15 years in Japan D u P o n t P h o t o v o l t a i c S o l u t i o n s 8

Module Accelerated Sequential Test (FAST MAST) 1676 Hours in a UVX Chamber Amounts to ~20 years worth of UV stress Shortened MAST by using higher intensity UV xenon exposure Shorter time at higher temperature (90C BPT) Result are equivalent to original MAST May 29, 2018 D u P o n t P h o t o v o l t a i c S o l u t i o n s 9

Comparison of MAST and Field Observations May 29, 2018 D u P o n t P h o t o v o l t a i c S o l u t i o n s 10

MAST Alternative Testing Sequences Damp Heat & Thermal Cycling Combines two important stress factors in a shorter test not requiring full size, expensive UV equipment. Test Conditions DH 85C/85%RH Thermal Cycling 85 o C <=> -40 o C UV & Water Spray Combines UV and rainfall simulation, common weathering test conditions using commercially available weatherometer. Test Conditions: ASTM G155 protocols in a weatherometer for backsheet coupon or minimodule. BPT = 90 o C D u P o n t P h o t o v o l t a i c S o l u t i o n s 11

Inner Layer Cracking of PET Backsheets Same PET Backsheet Accelerated Test: Front side exposure in xenon weatherometer * Fielded module with 6 years service in North America with 30% power loss in 5 years of service, 6% linearized power loss per year Inner Layer Cracking can result in Loss of adhesion Moisture ingress Insulation failure and power loss * Exposure conditions: 90 C BPT, 1.1 W/m 2 nm UV, cycle: 102 min UV followed by 18 min UV + water front spray, 3500h (~ 5y outdoor equivalent) 2018 DowDuPont. All rights reserved. May 29 2018 D u P o n t P h o t o v o l t a i c S o l u t i o n s 12

Cracked PET Backsheet Inner Layer in the Field PET-based backsheet inner layer cracked all over module in spaces between cells and around edges Cracks do not extend behind silicon cells Backsheet damage likely caused by a combination of UV exposure and temperature Defect is known to cause inverter tripping and ground faults in the field 2018 DowDuPont. All rights reserved. May 29 2018 D u P o n t P h o t o v o l t a i c S o l u t i o n s 13

Comparison of Stress Tests to Field Results for Backsheet Degradation Stress PPE KPE Polyamide TPT/TPE Comment Field Yellowing Mech Prop Loss Cracking Cracking Front Side Yellowing Yellowing Mech Prop Loss Cracking Low Defects Effects of simultaneous and sequential stresses Damp Heat (1000 hrs) Slight Yellowing No Change Mech Prop Loss No Change Misses UV Degradation UV (4000hrs) Yellowing Mech Prop Loss No Change Mech Prop Loss No Change Misses hydrolysis and moisture DH/UV/TC (MAST Sequential Test) Yellowing Mech Prop Loss Cracking Cracking Front Side Yellowing Yellowing Mech Prop Loss Cracking No Change Combines key stresses, gives best correlation Sequential Tests correlate better with degradation seen in the field Combine most important stress factors Use Stress levels / dosages that match field exposures Accelerate with highest temperature but Do NOT produce degradation not found in the field D u P o n t P h o t o v o l t a i c S o l u t i o n s 14

New Accelerated Sequential Dynamic Mechanical Load Test Protocol UV exposure: 65kWh/m 2 on the front DML 1: 1000 cycles of ±1500 Pa of loading @ 1/6 Hz DML 2: 1000 cycles of ±1500 Pa of loading @ 1 Hz TC200 = Thermal Cycling, -40 C <-> 85 C, ramp and hold per IEC62782, 200 cycles HF30 = Humidity Freeze, 30 cycles Optional DML 3: 1000 cycles of ±1000 Pa of loading @ 4 Hz Designed to better simulate the Field by combining Sequential Testing and with Dynamic Mechanical Loading in field environments mechanical loads are dynamic Current IEC 61215 only uses Static Mechanical Load Tests conducted on full-size modules by independent 3 rd party testing lab DNV-GL, USA D u P o n t P h o t o v o l t a i c S o l u t i o n s 15

Summary of Results DML Sequential Test Fielded Glass Module: 21 yr (JRC) Glass-Glass modules show delamination induced by mechanical load combined with UV exposure, thermal cycling and humidity Due to rigidity and non-breathability allowing trapping of degradation products D u P o n t P h o t o v o l t a i c S o l u t i o n s 16

Future Directions Higher irradiance New equipment with higher intensity and full size module capability Higher temperature Extend work with UV exposures at higher temperature (from 90C to 100C BPT) New stress conditions Static mechanical load, dynamic mechanical load, humidity freeze (-40C, 85C/85%RH), electrical bias, weathering (UV + water spray) Front side exposure Add UV/visible exposure from front of module to assess power loss and inner layer and front side degradation D u P o n t P h o t o v o l t a i c S o l u t i o n s 17

Conclusions Multistress testing better simulates and predicts outdoor performance of PV modules Module accelerated stress testing (MAST) shown to be a good predictor of backsheet defects observed in the field Timing reduced from 9 months to 6 months for MAST test protocol using xenon exposure and higher temperature Inner layer cracking predicted by frontside UV exposure of minimodules, correlated to field Glass/glass module shown to be susceptible to delamination using sequential test including DML New sequential test protocols offer an opportunity to assess new module structures and materials Improved durability test methods are the most effective way to prevent large scale field failures D u P o n t P h o t o v o l t a i c S o l u t i o n s 18

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