Alexander Zak Bradley

Weathering and durability of PV backsheets and impact on PV module performance

Polymeric backsheets form the outer protective layer of most crystalline and multi-crystalline silicon cell photovoltaic panels. The mechanical, electrical, optical and chemical properties and durability of these backsheets are critical to the long term reliability, durability and safety of the photovoltaic modules. The stability of these backsheet properties is typically determined based on accelerated testing using individual stresses. However, the impact of multiple stresses applied sequentially or simultaneously can lead to changes in materials properties that are more predictive of performance in the field. An important consideration in the development of accelerated test protocols is the level and duration of the stress, including temperature variation, light intensity and spectral power distribution, humidity, rainfall and powered module current. In this paper, we discuss observations of the aging and degradation of solar panel from the field. Then how these changes correlate to accelerated testing results, and how accelerated tests can be modified to better match observations in the field.

Mapping chemical and mechanical property degradation in photovoltaic modules

An understanding of material interactions and degradation pathways in both fielded modules and modules used for accelerated testing is important to understand how photovoltaic (PV) materials affect reliability. As part of the effort to build this understanding, a suite of destructive and non-destructive testing protocols has been developed to compare material performance and reliability under the stresses of different service environments. The characterization approaches to be discussed in this presentation describe our recent experience mapping the physical and chemical changes observed in degraded PV modules. Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) has been shown to provide a unique combination of sensitivity, spatial resolution and quantitation suitable for the study of ion migration pathways in encapsulants after PID (Potential Induced Degradation). Similarly, the depth resolution and sensitivity of imaging depth profiles determined by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) have expanded our ability to follow chemical changes in cells extracted from modules following PID testing and Damp Heat exposure. As an extension of our ongoing efforts to correlate chemical and physical degradation properties, we have recently added NanoHardness Testing (NHT) of the extracted core to our analysis protocol. NHT has been found to be sufficiently sensitive to detect mechanical property differences in backsheet structures taken from modules after field exposure.

Safety and performance analysis of a commercial photovoltaic installation

Continuing to better understand the performance of PV systems and changes in performance with the system life is vital to the sustainable growth of solar. A systematic understanding of degradation mechanisms that are induced as a result of variables such as the service environment, installation, module/material design, weather, operation and maintenance, and manufacturing is required for reliable operation throughout a system’s lifetime. We wish to report the results from an analysis of a commercial c-Si PV array owned and operated by DuPont. We assessed the electrical performance of the modules by comparing the original manufacturers’ performance data with the measurements obtained using a solar simulator to determine the degradation rate. This evaluation provides valuable PV system field experience and document key issues regarding safety and performance. A review of the nondestructive and destructive analytical methods and characterization strategies we have found useful for system, module, and subsequent material component evaluations are presented. We provide an overview of our inspection protocol and subsequent control process to mitigate risk. The objective is to explore and develop best practice protocols regarding PV asset optimization and provide a rationale to reduce risk based on the analysis of our own commercial installations.

Quantifying PV module defects in the service environment

We report the results from a survey of commercial c-Si photovoltaic (PV) systems. These systems range in size from 1kw to 20MW representative of residential, commercial, and utility scale projects from newly commissioned to >25 years in operation. These results provide the industry with valuable PV system field experience and document PV module defects. Furthermore, a breakdown of module subcomponent defects identified is presented. We provide an overview of our inspection protocol used for documentation. In addition, we hope to raise awareness about visual defects that may be indicative of quality issues requiring further investigation or proactive operation and maintenance (O&M). As a result of our learning, risk mitigation techniques can be developed based on field data to eventually help identify and reduce these defects. The objective is to quantify and disseminate these learnings regarding PV asset management and provide a rationale to reduce risk based on the analysis of PV materials, modules, and systems in the service environment.

Multi-stress durability testing to better predict outdoor performance of PV modules

Photovoltaic modules in the outdoor environment are subjected to a wide range of stresses which can operate simultaneously and sequentially and can vary based on climate and installation. These stresses can include temperature, temperature variation, localized heating, humidity, moisture (rain, snow, humidity, condensation), weathering, mechanical stress, abrasion and internal electric fields. These multiple stress make prediction of service lifetime challenging. Frequently resistance to an extended single stress is improperly used to assess durability. We have used sequential and simultaneous multistress exposure of materials and modules to better predict the synergistic effects of these stresses on module performance. We have also assessed the change in component materials properties to better understand performance changes. Finally, we compare these results to inspection of modules from the field to validate the test methods proposed.

Development of backsheet tests and measurements to improve correlation of accelerated exposures to fielded modules

Matching accelerated test results to field observations is an important objective in the photovoltaic industry. We continue to develop test methods to strengthen correlations. We have previously reported good correlation of FTIR spectra between accelerated tests and field measurements. The availability of portable FTIR spectrometers has made measurement in the field convenient and reliable. Recently, nano-indentation has shown promise to correlate changes in backsheet mechanical properties. A precisely shaped stylus is pressed into a sample, load vs displacement recorded and mechanical properties of interest calculated in a nondestructive test. This test can be done on full size modules, allowing area variations in mechanical properties to be recorded. Finally, we will discuss optical profilometry. In this technique a white light interferogram of a surface is Fourier transformed to produce a three-dimensional image. Height differences from 1 nm to 5 mm can be detected over an area of a few cm. This technique can be used on minimodules, and is useful to determine crack and defect dimensions. Results will be presented correlating accelerated tests with fielded modules covering spectroscopic, mechanical, and morphological changes.

Analysis of the Degradation and Aging of a Commercial Photovoltaic Installation

Simple and accurate methods are needed to monitor and assess PV systems. It is important to characterize and understand the value of the system with regard to safety and performance (including seasonal and geographical variations) as well as operation and maintenance. This documentation is becoming necessary for the secondary or resale value of PV assets. We report the results from an analysis of a commercial c-Si PV array owned and operated by DuPont. Our technical assessment consists of remote monitoring, field inspection with visual examination and thermal imaging to create a pareto chart of degradation modes, and laboratory analysis. A comparison of remote monitoring and site inspection is presented as well as laboratory analysis (nondestructive and destructive test methods) of modules removed from the service environment. Degradation modes and quality issues became evident as electrical, optical, physical or chemical defects developed with system age. This evaluation provided system data, documented quality issues, and quantified the cost of ownership.

Optical properties of PV backsheets: key indicators of module performance and durability

Polymeric backsheets are an important component affecting the performance and durability of photovoltaic modules. The optical properties of the backsheet should be considered in the design and performance of a photovoltaic module and the stability and durability of optical properties have an impact on power, safety and appearance. Changes in optical properties in fielded modules and accelerated durability testing are compared. IR analysis was conducted on various backsheet materials in accelerated durability testing and compared to outdoor performance to better understand the relevant chemical changes and associated degradation mechanisms. The connection between optical properties and chemical changes is discussed.