Amy A. Lefebvre

Degradation analysis of field-exposed photovoltaic modules with non-fluoropolymer-based backsheets

The selection of polymeric materials utilized in photovoltaic (PV) modules has changed relatively little since the inception of the PV industry, with ethylene-vinyl acetate (EVA), polyethylene terephthalate (PET), and fluoropolymer-based laminates being the most widely adopted primary components of the encapsulant and backsheet materials. The backsheet must serve to electrically insulate the solar cells and protect them from the effects of weathering. Due to continued downward pressure on cost, other polymeric materials are being formulated to withstand outdoor exposure for use in backsheets to replace either the PET film, the fluoropoymer film, or both. Because of their relatively recent deployment, less is known about their reliability and if they are durable enough to fulfill the ≥25 year warranties of current PV modules. This work presents a degradation analysis of field-exposed modules with polyamide- and polyester-based backsheets. Modules were exposed for up to five years in different geographic locations: USA (Maryland, Ohio), China, and Italy. Surface and cross-sectional analysis included visual inspection, colorimetry, glossimetry, and Fourier-transform infrared spectroscopy. Each module experienced different types of degradation depending on the exposure site, even for the same material and module brand. For instance, the polyamide-based backsheet experienced hairline cracking and greater yellowing and chemical changes in China (Changsu, humid subtropical climate), while in Italy (Rome, hot-summer Mediterranean climate) it underwent macroscopic cracking and greater losses in gloss. Spectroscopic studies have permitted identification of degradation products and changes in polymer structure over time. Comparisons are made to fielded modules with fluoropolymer-based backsheets, as well as backsheet materials in accelerated laboratory exposures. Implications for qualification testing and service life prediction of the non-fluoropolymer-based backsheets are discussed.

Cross-correlation of backsheet degradation between real-world exposed modules and accelerated exposures of backsheet materials(Conference Presentation)

heets are a key polymeric component of a PV module and understanding its degradation is necessary to be able to predict the lifetime of PV modules. We are developing a backsheet predictive tests and a model based on point- in-time data from analytical techniques and datastreams that are applicable to both outdoor and indoor PV module backsheet studies and are supplemented with meteorology data, climatic and brand/model, and other accessible information. The predictive tests and models will specify indoor and outdoor exposure and evaluation data acquisition criteria, variable selection, and temporal duration and variation so as to be able to predict backsheet performance in various climatic zones. This backsheet performance prediction is based on defined backsheet failures in the field, and is quantified by tracking backsheet degradation in the field so as to determine degradation rates. The backsheet lifetime performance predictive tests and models, will be developed using a Stressor / Mechanism / Response framework in which all data are categorized as stressor, mechanism and performance (response) variables and are represented as discrete points-in-time datasets. We will develop and validate these accelerated indoor exposures and evaluations and models and cross-correlate the outdoor and accelerated indoor exposures and evaluations. The evaluation techniques include nondestructive spectroscopy and microscopy techniques and destructive techniques and will provide data in predefined variables, which are used in the predictive modeling.

Chapter 6:Commercial Synthesis and Applications of Poly(Vinylidene Fluoride)

Poly(vinylidene fluoride) (PVDF) was first introduced commercially in the 1960s for architectural coatings, followed shortly thereafter by additional grades for industrial applications with continued technology developments to service demand in other application areas. PVDF homopolymers and copolymers are synthesized commercially as stable aqueous latexes, or using suspension processes in which larger particle sizes are produced. Nearly all grades are melt and solution processable and are relatively low-cost fluoropolymers with excellent weatherability and resistance to chemical attack. They find application where excellent physical properties, coupled with ease of processability, are of critical importance. They are particularly useful in formulations for highly weatherable coatings, or processed for high-purity piping, porous membranes, binders for lithium ion batteries and photovoltaics, among others. PVDF, by volume, is the second-highest produced fluoropolymer worldwide, with the majority of demand from the USA, Europe, China and Japan. It continues to experience good market growth, and is projected to exceed a 5% volume increase per year for the foreseeable future.

Durability of photovoltaic backsheet outer weatherable layers

Photovoltaic backsheets are an essential part of photovoltaic modules. It is imperative that the backsheet perform and be shown to be weatherable over the 25 year expected module lifetime. This paper reports on findings from a study focused on understanding the durability of the outer weatherable layers of commercial backsheets exposed in south Florida. Within one year of exposure, Non-Fluoropolymer outer weatherable layers were found to be degrading, with cracking and erosion of the polymer matrix occurring. In contrast, after four years of exposure there is no evidence of degradation in backsheets protected with a Kynar® PVDF fluoropolymer layer.