Laura S. Bruckman

Photodegradation in a stress and response framework: poly(methyl methacrylate) for solar mirrors and lens

Abstract. In the development of materials for enhanced photovoltaic (PV) performance, it is critical to have quantitative knowledge of both their initial performance and their performance over the required 25-year warranted lifetime of the PV system. Lifetime and degradation science, based on an environmental stress and response framework, is being developed to link the intensity and net stress to which materials, components, and systems are exposed to the responses observed and their subsequent degradation and damage accumulation over the lifetime. Induced absorbance to dose (IAD), a metric developed for solar radiation durability studies of solar and environmentally exposed materials, is defined as the rate of photodarkening or photobleaching of a material as a function of radiation dose. Quantitative degradation rates like IAD, determined over a wide range of stress intensities and net stresses, have the potential to predict degradation, failure, and power loss rates in photovoltaic systems over time caused by damage accumulation. Two grades of poly(methyl methacrylate) were exposed and evaluated in two cases of high-intensity ultraviolet exposures. A three- to six-fold increase in photodarkening was observed for one acrylic formulation when exposed to UVA-340 light when compared with concentrated xenon-arc exposure. The other, more highly stabilized acrylic formulation, showed up to three times more photodarkening in the same exposure.

Degradation in PV encapsulation transmittance: An interlaboratory study towards a climate-specific test

Reduced optical transmittance of encapsulants resulting from ultraviolet (UV) degradation has frequently been identified as a cause of decreased PV module performance through the life of service in the field. The present module safety and qualification standards, however, apply short UV doses only capable of examining design robustness or “infant mortality” failures. Essential information that might be used to screen encapsulation through product lifetime remains unknown. For example, the relative efficacy of xenon-arc and UVA-340 fluorescent sources or the typical range of activation energy for degradation is not quantified. We have conducted an interlaboratory experiment to provide the understanding that will be used towards developing a climate- and configuration-specific (UV) weathering test. Five representative, known formulations of EVA were studied in addition to one TPU material. Replicate laminated silica/polymer/silica specimens are being examined at 14 institutions using a variety of indoor chambers (including Xenon, UVA-340, and metal-halide light sources) or field aging. The solar-weighted transmittance, yellowness index, and the UV cut-off wavelength, determined from the measured hemispherical transmittance, are examined to provide understanding and guidance for the UV light source (lamp type) and temperature used in accelerated UV aging tests.

Degradation pathway models for photovoltaics module lifetime performance

Previously published accelerated testing data from Underwriter Labs, featuring measurements taken on 18 identical photovoltaic (PV) modules exposed to two stress conditions, were used to develop an analytical methodology. The results provide insight into active degradation mechanisms and pathways present in PV modules under accelerated testing conditions as indicated by statistically significant relationships between variables. Observed experimental results coincide with a domain knowledge based theoretical degradation pathway model informed by literature, and provide a basis for beginning to investigate the degradation modes and pathways truly present in modules and their effects on module performance over lifetime.

Photovoltaic lifetime and degradation science statistical pathway development: acrylic degradation

In order to optimize and extend the life of photovoltaics (PV) modules, scienti c and mechanistic statistical analytics must be performed on a large sample of materials, components and systems. Statistically signi - cant relationships were investigated between di erent mechanistically based variables to develop a statistical pathway diagram for the degradation of acrylic that is important in concentrating photovoltaics. The statisti- cally signi cant relationships were investigated using lifetime and degradation science using a domain knowledge semi-supervised generalized structural equation modeling (semi-gSEM. Predictive analytics and prognostics are informed from the statistical pathway diagram in order to predictively understand the lifetime of PV modules in di erent stress conditions and help with these critical lifetime technologies.

Predictive models of poly(ethylene-terephthalate) film degradation under multi-factor accelerated weathering exposures

Accelerated weathering exposures were performed on poly(ethylene-terephthalate) (PET) films. Longitudinal multi-level predictive models as a function of PET grades and exposure types were developed for the change in yellowness index (YI) and haze (%). Exposures with similar change in YI were modeled using a linear fixed-effects modeling approach. Due to the complex nature of haze formation, measurement uncertainty, and the differences in the samples’ responses, the change in haze (%) depended on individual samples’ responses and a linear mixed-effects modeling approach was used. When compared to fixed-effects models, the addition of random effects in the haze formation models significantly increased the variance explained. For both modeling approaches, diagnostic plots confirmed independence and homogeneity with normally distributed residual errors. Predictive R2 values for true prediction error and predictive power of the models demonstrated that the models were not subject to over-fitting. These models enable prediction under pre-defined exposure conditions for a given exposure time (or photo-dosage in case of UV light exposure). PET degradation under cyclic exposures combining UV light and condensing humidity is caused by photolytic and hydrolytic mechanisms causing yellowing and haze formation. Quantitative knowledge of these degradation pathways enable cross-correlation of these lab-based exposures with real-world conditions for service life prediction.

A Nonrelational Data Warehouse for the Analysis of Field and Laboratory Data From Multiple Heterogeneous Photovoltaic Test Sites

A nonrelational, distributed computing, data warehouse, and analytics environment (Energy-CRADLE) was developed for the analysis of field and laboratory data from multiple heterogeneous photovoltaic (PV) test sites. This data informatics and analytics infrastructure was designed to process diverse formats of PV performance data and climatic telemetry time-series data collected from a PV outdoor test network, i.e., the Solar Durability and Lifetime Extension global SunFarm network, as well as point-in-time laboratory spectral and image measurements of PV material samples. Using Hadoop/HBase for the distributed data warehouse, Energy-CRADLE does not have a predefined data table schema, which enables ingestion of data in diverse and changing formats. For easy data ingestion and data retrieval, Energy-CRADLE utilizes Hadoop streaming to enable Python MapReduce and provides a graphical user interface, i.e., py-CRADLE. By developing the Hadoop distributed computing platform and the HBase NoSQL database schema for solar energy, Energy-CRADLE exemplifies an integrated, scalable, secure, and user-friendly data informatics and analytics system for PV researchers. An example of Energy-CRADLE enabled scalable, data-driven, analytics is presented, where machine learning is used for anomaly detection across 2.2 million real-world current-voltage (I-V) curves of PV modules in three distinct Köppen-Geiger climatic zones.

Degradation of Back-Surface Acrylic Mirrors for Low Concentration and Mirror-Augmented Photovoltaics

Back-surface acrylic mirrors can be used in low concentration and mirror augmented photovoltaics (LCPV, MAPV) to increase the irradiance on a module. Back-surface mirrors can spectrally filter incoming solar radiation reducing the ultraviolet (UV) and infrared (IR) load on the module, while useful radiation is coupled into a module or photovoltaic cell. Degradation of these mirrors can occur from UV induced photodegradative processes and metallization corrosion. Environmental stresses such as humidity, thermal cycling and exposure to corrosive substances can cause an increase in scattering, reducing mirror performance. In order to increase the lifetime and durability of back-surface acrylic mirrors a better understanding of the degradation modes is necessary. In a study of acrylic back-surface mirrors for LCPV and MAPV applications, optical properties and bidirectional scattering distribution functions (BSDF) were investigated and correlated to simulated exposure protocols. Formulations of Poly(methyl methacrylate) (PMMA) with differing concentration of UV absorbers were used for the aluminum backsurface acrylic mirrors. The formulations of aluminum back-surface acrylic mirrors were exposed in a QUV accelerated weathering tester (QLabs) to ASTM G154 Cycle 4. Total and diffuse reflectance spectra were measured for each mirror under exposure using a diffuse reflectance accessory (DRA) from 180-1800 nm on a Varian Cary 6000i at defined dose intervals. The total reflectance losses in the 250-400 nm region were greater and diffuse-only reflectance increased for formulations of acrylic mirrors that contained the least amount of UV stabilizer after each dose of QUV exposure. Acrylic back-surface mirrors were exposed to salt fog corrosion and QUV and were analyzed using BSDF. There was an increase in scattering from roughening of the mirror surface after exposure to the corrosive environment.

Degradation of poly(ethylene-terephthalate) under accelerated weathering exposures

Poly(ethylene-terephthalate) is one of the most critical materials in photovoltaic module backsheets due to its dielectric breakdown strength. Unfortunately, PET is susceptible to environmental stressors and subject to degradation over time. Stabilizers are added to increase its service lifetime; however, even these degrade over time and leave the material vulnerable. To study degradation of polyester films, various accelerated weathering exposures are applied to PET grades with different stabilizers. A lab-based experiment was performed utilizing a randomized longitudinal design where samples were assigned to four exposure types and followed over time with repeated measurements in order to determine the effect of common stresses like UV light and humidity on the main polymer and stabilizer additives. It was found that material yellowing is dominant with UV light exposures while moisture mostly causes hazing of the samples. Effects were more severe when light was coupled with moisture. UV-Vis absorbance, infrared, and fluorescence spectroscopy provided valuable insights into the chemical degradation mechanisms such as formation of degradation byproduct, chain scissions, and change in crystallinity under different exposure conditions.

Design Considerations and Measured Performance of Nontracked Mirror-Augmented Photovoltaics

Costs associated with conventional photovoltaic (PV) installations (module manufacturing, mounts, wiring and installation labor, etc.) scale linearly with system area, and prudent design practices for increasing the light harvesting may significantly enhance the power output per unit area and cost per watt. The use of PV modules that have been augmented by the addition of a solar mirror provides an opportunity to improve light harvesting of a PV module and cost-per-watt considerations if the mirror is less expensive than the relative increase in power. In order to harvest greater insolation, an optimized design configuration between a flat-panel module and mirror is necessary for a fixed (nontracked) mirror-augmented photovoltaic (MAPV) system. A series of irradiance and energy harvest calculations were developed to screen various MAPV design configurations. We employed optical ray-tracing simulations to determine irradiance nonuniformities on the fixed MAPV system. The simulated results are compared with outdoor experimental field trial results. Over a three-month period of study, the fixed MAPV system produced 12% more power than an equivalent nonaugmented panel. An adjustable angle mounting system known as “time machine” was used to estimate yearly power production by adjusting the relative position of the MAPV system to represent varying times of the year. Current--voltage (I-V) curve tracing of test modules was performed during the field trials on augmented and nonaugmented modules for comparison. The experimental time machine result is in agreement with the irradiance simulation, which shows a gullwing curve of annual power output with peak production on the equinoxes and reduced production on the solstices.

Degradation analysis of polyamide-based backsheets in field photovoltaic modules under different climatic conditions (Conference Presentation)

Polyamide (PA)-based backsheet as an emerging product has been developed in recent years to partially substitute fluoropolymer materials under the intensified cost-reduction pressure. However, a number of reports indicate that backsheet cracks have been observed in fielded modules in a few locations after only a few years of exposure. An in-depth analysis on degradation and crack formation and their dependence on climatic conditions is needed. In this work, the field PV modules with PA-based backsheet under five different climatic conditions up to six years were retrieved and analyzed, including humid subtropical climate (Changshu, China), dry-summer subtropical climate (Rome, Italy), marine west coast climate (Bergamo, Italy), desert climate (Arizona, United States) and tropical climate (Thailand). Macroscopic cracks in backsheet were observed for modules aged in Italy and Thailand, while only hairline cracks showed up in backsheet from Changshu, and no cracks could be seen in Arizona. Backsheet in Changshu also experienced much higher yellowing than other sites, while the gloss loss of the backsheet in Italy is the highest. Spectroscopic analyses were also performed to identify various degradation products and to understand the possible changes in degradation mechanism of backsheets under different climates. The intercorrelations between various degradation modes of PA-based backsheet and weathering factors will be further established, providing a valuable information on the material selection and lifetime prediction for the backsheet.