Photovoltaic (PV) cables facilitate the distribution of electricity collected from modules to the energy grid. Durable cabling enables continuous operation of PV installations, whereas cables with a lifetime less than the modules must be replaced—reducing electricity generation and adding to the operating expense. This study primarily focusses on the aging of the key cable types using the combined-accelerated stress testing (C-AST) protocol. Representative cables for utility, building, and control/auxiliary applications were examined. Cable jacket materials examined include: polyolefin, polyethylene, polyamide, poly(vinyl chloride), chlorinated polyethylene, thermoplastic elastomer, and ethylene propylene diene monomer rubber. Specimen characterizations applied include: optical microscopy, mechanical profilometry, instrumented indentation, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). A variety of performance and durability characteristics were observed, depending on the base material, polymer formulation, and jacket color. The results from C-AST are analyzed and discussed relative to a recent industry survey on electronic balance of system components in addition to a recent study where similar cables were aged using steady state ultraviolet weathering (International Electrotechnical Commission Technical Specification 62788-7-2). Recommendations are made for the screening, industry qualification, and service life prediction of PV cable jackets.

The consequences of failure for balance-of-systems components (such as photovoltaic (PV) cable connectors) include offline module string(s); low system voltage; arc, ground, insulation, and overtemperature faults; triggered fuse(s); system shutdown; and fire. The degradation modes for connectors are studied here through an industry survey and its subsequent examination, which are compared with field-degraded specimens. A total of 117 specimens were obtained from a variety of locations and climates or accelerated tests. A failure analysis for connectors from PV installations was developed (and applied to 54 specimens), including nondestructive examinations (photography, a custom resistance-current scan, and X-ray computed tomography) and destructive examinations (featuring milling of the external plastic, extraction of the internal convolute spring, and potting and polishing in cross section). Surface and through-thickness composition of the metal pins and springs was quantified using scanning electron microscopy with energy-dispersive X-ray spectroscopy. Fourier transform infrared spectroscopy was used to verify the base polymer materials and compare the chemical structure of the connector body, bushing, end nut, and o-ring. Thermogravimetric analysis and differential scanning calorimetry were used to further verify the degradation of the same polymeric components.

The degradation of photovoltaic (PV) balance of systems (BoS) components is not well-studied, but the consequences include offline modules, strings, and inverters; system shutdown; arc faults; and fires. A utility provider experienced a ~30% failure rate in their power transfer chain, originally attributed to branch connectors. Field-failed specimen assemblies were therefore examined, consisting of cable connector, branch connector, and discrete fuse components. In this study, unused field-vintage specimens are examined using combined-accelerated stress testing (C-AST) to clarify the most influential environmental stressors as well as the effect of external mechanical perturbation. A benchtop prototype fixture was used to develop the perturbation capability for the C-AST chamber. The benchtop experiments were also used to develop the in-situ data acquisition of specimen current, voltage, and temperature. A significant increase in operating temperature (~100°C from ~40°C) and a different failure mode were observed promptly once periodic mechanical perturbation was applied. The current at failure was decreased from 35 A (with failure in the fuses) to 15 A (failure at the male/female metal pin connection). After initial examination using X-ray computed tomography, the external plastic was machined away from failed specimens to allow for failure analysis, including the extraction of the internal convolute springs for morphological examination (optical and electron microscopy).

The degradation of photovoltaic (PV) balance of systems (BoS) components is not well studied, but the consequences include offline modules, strings, and inverters; system shutdown; arc faults; and fires. A utility provider experienced a ∼30% failure rate in their power transfer chain, originally attributed to branch connectors. Field-failed specimen assemblies were, therefore, examined, consisting of cable connector, branch connector, and discrete fuse components. In this study, unused field-vintage specimens are examined using a benchtop prototype fixture to identify the most influential environmental stressors on BoS components as well as the effect of external mechanical perturbation. The prototype fixture was used to develop a perturbation capability for future use in the combined-accelerated stress testing chamber. The benchtop experiments were also used to develop the in-situ data acquisition of specimen current, voltage, and temperature. A significant increase in operating temperature (∼100 °C from ∼40 °C) and a different failure mode (arcing at the metal pins rather than overheating of the fuse filament) were observed promptly once periodic mechanical perturbation was applied. The current at failure was decreased from 35 A (measured for static specimens, with failure occurring in the fuses) to 15 A (for tests with mechanical perturbation, with failure at the male/female metal pin connection). After initial examination using X-ray computed tomography, the external plastic was machined away from failed specimens to allow for failure analysis, including the extraction of the internal convolute springs for morphological examination (optical and electron microscopy). Chemical composition analysis included energy-dispersive X-ray spectroscopy, differential scanning calorimetry, and Fourier transform infrared spectroscopy.

Two generations of multiwire modules were studied under dynamic mechanical loading (DML) with in-situ differential conductance (dG) and electroluminescence (EL) imaging. Energy-dispersive x-ray spectroscopy (EDS) was used to identity the solder alloys. The earlier generation module was found to use an In-based solder alloy, and the current generation a Bi-based alloy. The earlier generation module degraded significantly under DML with increasing resistance, while the current generation module did not demonstrate degradation under DML. Atomic force microscopy scratch testing was used to probe the wear resistance of each solder alloy. These results indicate that current multiwire designs may have higher mechanical durability than earlier generations.

Electroluminescence (EL) imaging is a powerful tool used to identify defects in photovoltaic solar cells. Typically, this type of characterization is performed in the dark using a current injection that equals short-circuit current measured at standard test conditions (STC). Restricting imaging to such a temperature range limits the information obtained about the module and cells. However, it is not trivial to develop a tool that would allow for EL imaging to be performed under a wider range of temperatures. Here we demonstrate an in situ EL imaging capability developed within an environmental chamber that allows for control of sample temperatures between -40 and 90 °C. Additionally, we demonstrate EL imaging of 4-cell mini-modules (MiMo) under front-side mechanical loading. A Raspberry Pi-connected camera with short-pass filter removed is used for EL imaging. The camera is low-cost with a small form-factor, appropriate for use in a harsh, enclosed environment. The camera is installed within a thermally isolating housing mounted within the environmental chamber. Three example cases are given for MiMos that exhibit various forms of degradation including solder fatigue and cell cracking. It is shown that by measuring at conditions above and below STC, different behaviors may be identified. In some cases, restricting characterization to STC may lead to a failure to detect damage existing in the sample.

Current-voltage (IV) curve tracing and electroluminescence (EL) imaging have been developed for in-situ performance characterization of photovoltaic (PV) devices in a Xe lamp-based weathering chamber for combined-accelerated stress testing. The capability allows for progressive failure monitoring during accelerated ageing with dynamic control of the characterization environment (e.g. imaging at specific temperatures and mechanical stress levels). Both light and dark IV curve tracing are implemented with techniques to overcome light and temperature instability inherent to the chamber. A Raspberry Pi-connected camera with infra-red filter removed is used for EL imaging, providing a low-cost, small form-factor solution which is desirable for use in a harsh environment. The camera is installed within a thermally-isolated housing mounted within the climate chamber. Measurement and control are achieved via LabVIEW, where characterization is integrated as part of the test protocol and performed automatically.

Natural soiling has reduced the energy output of PV systems since the technology was first used, and viable mitigation strategies have remained elusive ever since. With the ever-increasing deployments around the world, especially in dusty environments, soiling is becoming a billion-dollar problem, worldwide. Furthermore, as plant operators continue to look for ways to increase revenue, the PV operating voltages have increased to between 1000 V and 1500 V when the sun is shining. This has resulted in some unforeseen consequences nominally combined into what is termed “Potential Induced Degradation.” 1 Recent work by Jiang et. al., 2 at NREL using Atomic Force Microscopy has demonstrated that these large potentials also affect soiling by substantially increasing the attraction of dust to the surface, but also by increasing the adhesion force. Jiang et. al., have also shown that these higher soiling attraction and adhesion forces continue long into the night when the PV is no longer producing power. In this paper, we present a set of field results that demonstrate enhanced soiling rates that is due to the strong electric fields induced by these high voltage PV arrays. This includes observation of enhanced soiling rates measured in the field when a module is held at ±1000 V. This is critical information for installation operators because soiling losses may be higher on some panels than what is measured by typical soiling stations, and because the high voltages are not uniform across an array, some modules may have more soiling than others, leading to potential issues with non-uniform soiling problems at the array level. We present this set of compelling electric field induced soiling results in this paper.

Two CdTe and two copper indium gallium (di)selenide (CIGS)-type modules were tested for potential-induced degradation (PID) with positive and negative 1000 V biases applied to the active cell circuit in an 85°C, 85% relative humidity environmental chamber. Various degradation mechanisms could be seen with signatures such as shunting, transparent conductive oxide (TCO) corrosion, charge carrier lifetime reduction, and dead active layer at edges along with resulting cell mismatch. All modules tested exhibited degradation by system voltage stress in chamber, but only one module type has degraded in parallel field tests. I−V curve data indicated that one CdTe-type module sequentially exhibited shunting followed by a recovery and then series resistance losses. This module type showed TCO delamination from the glass in the environmental chamber tests and also exhibited power degradation within 5 weeks in field tests. Relative rates of Coulomb transfer from the voltage-biased active cell circuit to ground are compared for the modules in chamber tests to those placed outdoors under system voltage stress to extrapolate the anticipated time to failure in the field. This analysis correctly indicated which module type failed in the field first.

An acceleration model based on the Peck equation was applied to power performance of crystalline silicon cell modules as a function of time and of temperature and humidity, which are the two main environmental stress factors that promote potential-induced degradation (PID). This model was derived from module power degradation data obtained semicontinuously and statistically by in-situ dark current-voltage measurements in an environmental chamber. The modeling enables prediction of degradation rates and times as functions of temperature and humidity. Power degradation could be modeled linearly as a function of time to the second power; additionally, we found that the quantity of electric charge transferred from the active cell circuit to ground during the stress test is approximately linear with time. Therefore, the power loss could be linearized as a function of coulombs squared. With this result, we observed that when the module face was completely grounded with a condensed phase conductor, leakage current exceeded the anticipated corresponding degradation rate relative to the other tests performed in damp heat.

The Terrestrial Photovoltaic Module Accelerated Test-to-Failure Protocol was applied to seven crystalline silicon module types to test the durability of the various module constructions on a quantitative basis in chamber and to evaluate the protocol itself. The modules under test are subdivided into three accelerated lifetime testing paths: 85°C/85% relative humidity with system voltage bias, thermal cycling between - 40°C and 85°C, and paths that alternate between humidity with bias (one in each polarity) and thermal cycling. Three of the module types were also fielded to ascertain degradation mechanisms occurring in the natural environment for comparison to the mechanisms seen in the accelerated testing. Potential induced-degradation in modules negatively biased and silicon nitride antireflective coating thinning on cells in modules positively biased are among the important mechanisms that are seen both in the modules stressed in the natural environment and in chamber. Junction box failure, cell breakage, and acid-assisted metallization degradation are included in the mechanisms seen in chamber tests, and they vary significantly between module types. Per a goal of the accelerated test protocol, we found examples of modules with components and process methods that showed degradation mechanisms that occurred faster than incumbents that had satisfactory field experience. These were evaluated as opportunities for durability improvement. Conversely, types that showed substantial improvement were also seen, especially with respect to system voltage stress durability.

A new publicly available data set was completed for use in validating models that estimate the performance of flat-plate photovoltaic (PV) modules. The data were collected for one-year periods at three climatically diverse locations (Cocoa, Florida; Eugene, Oregon; and Golden, Colorado) and for PV modules representing all technologies available in 2010 when the work began. The same makes and models of PV modules were tested at all locations and common data acquisition systems were used with calibrations performed at the National Renewable Energy Laboratory. For use in determining model parameters and coefficients, baseline and post-deployment measurements were performed indoors with solar simulators, including per the requirements of IEC 61853 Part 1: Irradiance and Temperature Performance Measurements and Power Ratings. Outdoors, the PV modules were characterized per the requirements of the Sandia array performance model. A user's manual describes the contents of the data set and how to access the data.

IEC 62804 Ed. 1, System voltage durability qualification test for crystalline silicon modules, is being developed. First, two module designs are compared in chamber and in the natural environment of Florida (USA). From these results, a stress level of 60 degrees C, 85% relative humidity, a bias of nameplate system voltage, 96h dwell, and a pass/fail limit of 5% relative power degradation at 25 degrees C and 1000W/m2 irradiance is initially proposed for the draft protocol. This paper next focuses on one of the main controversies within the development of this standardthe use of damp heat in an environmental chamber versus a conductive foil to complete the circuit to ground during the test. Conventional 60-cell multicrystalline silicon modules with (i) a standard aluminum frame, (ii) a modified frame, and (iii) a rear rail design were tested for potential-induced degradation (PID). These three module designs were stressed at the draft protocol conditions stated above and outdoors, applying negative system voltage bias during hours of daylight to simulate array voltage. The damp heat environmental chamber tests run according to the protocol distinguish the relative resistance of five module designs to PID in the field and correctly rank-order the durability in the field to the extent tested (up to 28months). Finally, the degradation rate is determined at 25 degrees C using a foil to ground the module face on a subset of modules susceptible to PID, and the results with respect to measured field performance of the modules are discussed. Copyright (c) 2013 John Wiley & Sons, Ltd.