0413stachew_source

Development of Accelerated Photovoltaic (PV) WireTesting and Evaluation

Elena StachewOlga EliseevaRoger H. French

Solar Durability and Lifetime Extension (SDLE) CenterMaterials Science and Engineering DepartmentCase Western Reserve UniversityCleveland OH 44106

SDLE Center, VUV-Lab, Materials Science & Engineering Department, Roger H. French © 2012 http://sdle.case.edu May 1, 2023, VuGraph 2

What is a PV wire and its function?

• PV wires and connectors transfer current produced from PV modules to the electrical grid.

Single layer Double layerhttp://www.elecdirect.com/product/44e42301-3731-4d5d-b75c-d4388b8cf44a.aspx, http://www.aeesolar.com/catalog/bulk-wire, http://en.wikipedia.org/wiki/File:PV_connectors_01_Pengo.jpg

http://sdle.case.edu/


What is the problem?

• PV modules are expected to maintain a power output within acceptable limits for over 25 years,

• …therefore, PV wires are also expected to perform adequately for over 25 years.

• A typical standard test in 720 hours in wet and dry UV is not indicative of components exposed for 25 years outdoors.

• Long term behavior of wires exposed to UV is unknown • There are consequences,• If the damaged wire comes in contact with support or frame of the solar

panel, the arcing that can occur is strong enough to cause fires and/or cause injury

http://www.coffscoastadvocate.com.au/news/warning-solar-panel-owners/1256986/



The problem with current standard qualification testing

Czanderna & Jorgensen, NREL 1999

• Problem: • Not indicative of:

• true lifetime performance• real-world degradation

modes, pathways, rates and responses

• Solution: Develop an industry-driven PV wire standard test methodology that reflects observed real-world degradation modes and rates

Carissa Plains Power Plant Failure • Test characteristics:• Initial performance, safety,

quality control• All important but lacking



Components

Provided by First Solar and SDLE: • Southwire, single layer crosslinked polyethylene (XLP) insulation

• General Cable, double layer ethylene propylene rubber (EPR) insulation and chlorinated polyethylene (CPE) jacket

• TÜV, thermoset polyolefin (XL-PE) insulation and non-halogenated crosslinked polyethylene (XL-PO) jacket composite

Single layer Double layer



Accelerated testing and characterization At incremental exposure steps, • Q-Lab QUV Spray 1 (24 hours of dry 5x UV at 70C) • Q-Lab QUV Spray 2 (ASTM G154 Cycle 4: 8 h UV at 70 C and 4 h ⁰

condensation at 50 C)⁰

Between each exposure step,

• Infrared spectroscopy (FTIR-ATR) for changes in chemical composition from the presence of functional groups due to aging.

• Fluorescence spectroscopy for changes in electronic structure through excitation and emission energies.

• Both will show changes due to presence of plasticizers, flame retardants, thermal stabilizers, fillers, lubricants, etc. in wire insulation and jacket.

• Scanning electron microscopy (SEM) for surface features, such as microcracks, surface roughening or chalking.



Accelerated testing diagnosis• Stressors: Irradiance (UV) and weathering (heat,

condensation)

• After each exposure dose, wires are removed from accelerated chambers and a small sample is cut from each wire.

• The sample is analyzed under a variety of techniques to characterize chemical, mechanical or thermal changes and to give mechanistic insights to degradation and damage accumulation. Analysis is currently ongoing.



Results

Images taken courtesy of Olga Eliseeva

After 336 hours of accumulated exposure in QUV Spray 2, surface features start to be observed in both General Cable and TÜV wire but not in Southwire wire. There are no noticeable surface features from QUV Spray 1 samples.

2nd dose 2nd dose 2nd dose2nd dose

BaselineBaseline Baseline

BaselineGeneral Cable –

surface rougheningGeneral Cable–

hydrophobic residueTÜV– whitening Southwire–

none



Further accelerated testing diagnosis

• TGA • Weight loss (of plasticizers) and heat flow

• Energy dispersive x-ray spectroscopy (EDS)• Chemical composition

• Oxygen bomb calorimeter• Measure heat of combustion and ash content

On bare wires and linked chains of wires and connectors,

With 8A DC current power supply,

• Insulation resistance or leakage • Contact resistance



Outdoor testing



Test conditions

• Irradiance: Direct sunlight and in the shade

• Geometrical: Fixed position and dangling in the air

• Electrical: Under real situation of current load (about 8Adc) provided by two 4-cell DelSolar mini modules in series

• Weathering: Arid, Northern industrial and tropical climates (Arizona, Ohio, Florida, Taiwan, India)

Continuous data measurement

• Electrical: current and voltage of PV module

• Current-indicator of solar irradiance• Voltage-indicator of resistance change

• Thermal: connector temperatures (one front, one back)

• Visual inspections: Taking images (including IR), measure gap width for unlocking connection

Outdoor test conditions and measurement



Conclusions and future work

1. Problem: Not indicative of true lifetime performance nor real-world degradation modes, pathways, rates and responses

2. Solution: Develop an industry-driven PV wire standard test methodology that reflects observed real-world degradation modes and rates

• The additional stressor of condensation has shown an effect on the surface of the polymer insulation, especially for General Cable and TÜV.

• Characterizing chemical, mechanical or thermal changes will give mechanistic insights to degradation.

• Cross-correlation between accelerated and outdoor exposure conditions will verify that degradation modes occurring in the lab also occur in the field.



Acknowledgements

This work is funded and supported by Underwriters Laboratories (UL).

Thank you to Liang Ji of UL for his support and guidance.

Thank you to the VUV-lab group for their support and guidance.

First Solar for providing materials for this project.

Thank you to Sheila Pedigo and Bethany Pope for coordinating the summer SURES program.


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