Stereo high-speed video of photovoltaic modules undergoing laboratory hail tests was processed using digital image correlation to determine module surface deformation during and immediately following impact. The purpose of this work was to demonstrate a methodology for characterizing module impact response differences as a function of construction and incident hail parameters. Video capture and digital image analysis were able to capture out-of-plane module deformation to a resolution of ±0.1 mm at 11 kHz on an in-plane grid of 10 × 10 mm over the area of a 1 × 2 m commercial photovoltaic module. With lighting and optical adjustments, the technique was adaptable to arbitrary module designs, including size, backsheet color, and cell interconnection. Impacts were observed to produce an initially localized dimple in the glass surface, with peak deflection proportional to the square root of incident energy. Subsequent deformation propagation and dissipation were also captured, along with behavior for instances when the module glass fractured. Natural frequencies of the module were identifiable by analyzing module oscillations postimpact. Limitations of the measurement technique were that the impacting ice ball obscured the data field immediately surrounding the point of contact, and both ice and glass fracture events occurred within 100 μs, which was not resolvable at the chosen frame rate. Increasing the frame rate and visualizing the back surface of the impact could be applied to avoid these issues. Applications for these data include validating computational models for hail impacts, identifying the natural frequencies of a module, and identifying damage initiation mechanisms.
Deflection and stress calculated from an experimentally validated, high-fidelity finite element model (FEM) of a photovoltaic module experiencing mechanical load was compared to results from a simplified FEM treating the module laminate as a homogenized composite using a rule of mixtures approach, and further compared to analytical calculations treating the module as a Kirchoff-Love flat plate. The goal of this study was to determine the error incurred by analyzing module mechanics with varying levels of simplification, since resolving the aspect ratios of a module is computationally expensive. Homogenized FEMs were found to underpredict peak deflection under a 1.0 kPa load by between 13 and 19% for lower and upper bound application of the rule of mixtures. However, module shape was captured, implying that a useful replication of a resolved model could be achieved with a reduced, calibrated material stiffness. Homogenized stress results captured glass layer tensile stress components to within 46 to 52% at a sample location of interest, though agreement was poor through the remainder of the laminate due to the lack of material resolution. For plate theory, deflection was overpredicted by 45 to 67% for upper and lower bound homogenizations, and frame-adjacent module shapes were not adequately replicated. Stress results mirrored FEM trends but magnitudes were not well correlated to resolved model values. These results support the use of homogenized laminate models for module shape derivation, though resolved models remain necessary for stress analyses. The accuracy of plate theory was found to be inadequate for most applications.
Photovoltaic modules are subjected to various mechanical stressors in their deployment environments, ranging from installation handling to wind and snow loads. Damage incurred during these mechanical events has the potential to initiate subsequent degradation mechanisms, reducing useful module lifespan. Thus, characterizing the mechanical state of photovoltaic modules is pertinent to the development of reliable packaging designs. In this work, photovoltaic modules with strain gauges directly incorporated into the module laminate were fabricated and subjected to mechanical loading to characterize internal strains within the module when under load. These experimental measurements were then compared against results obtained by high-fidelity finite-element simulations. The simulation results showed reasonable agreement in the strain values over time; however, there were large discrepancies in the magnitudes of these strains. Both the instrumentation technique and the finite-element simulations have areas where they can improve. These areas of improvement have been documented. Despite the observed discrepancies between the experimental and simulated results, the module instrumentation proved to be a useful gauge in monitoring and characterizing the mechanical state. With some process improvements, this method could potentially be applied to other environments that a photovoltaic module will encounter in its lifetime that are known to cause damage and degrade performance.
New interconnect schemes that replace metallic solders with electrically conductive adhesives (ECA) are appearing in recent embodiments of crystalline silicon photovoltaic (PV) modules. Recently, potential ECA interconnect failure modes were identified and characterized, which included cohesive cracking and debonding of the adhesive joint. In this work, we elucidate on how and to what extent the driving force for ECA degradation develops in shingled cell modules. We have employed a multiscale modeling approach, using the finite-element method, to accurately predict the driving force for both accelerated stress testing conditions and on-sun exposure of PV modules. When we compare our driving force predictions for a generic PV module with the experimentally characterized fracture properties of a candidate ECA, we found that interconnect failure of only poor quality or otherwise damaged joints is likely to occur. Furthermore, we show how a 2-D submodel can efficiently predict limits for the debond driving forces without needing to employ the multiscale modeling approach.
Detailed finite element models of a 60-cell crystalline silicon photovoltaic module undergoing a ±1.0 and ±2.4 kPa pressure load were simulated to compare differences created by a constrained frame boundary condition versus replicating manufacturer recommended rack mounting. Module deflection, interconnect strain, and first principal stresses on cell volumes were used as comparison metrics to assess how internal module damage was affected. Average results across all loads scenarios showed that constraining the frame of the module to its initial unloaded plane reduced peak deflections by approximately 13%, interconnect strains by 11%, and first principal stress by 11% when compared to a module with correctly modeled racking. Analysis of results based on damage metrics indicated that the constrained boundary condition reduced interconnect stress at most locations and increased fatigue life by an average of 34%, and likewise reduced the average probability of cell fracture by 82%, though individual results were highly variable. Nonetheless, location-specific trends were generally consistent across constraint methodologies, indicating that the constraint simplification can be applied successfully if corrected for with increased load, additional test cycles, or an informed interpretation of results. The goal of this work was to exercise a methodology for quantifying differences created by a simplified test constraint setup, since expedient experimental simplifications are often used or considered to reduce the complexity of exploratory mechanical tests not related to standards qualification.
Detailed finite element models of a 60-cell crystalline silicon photovoltaic module undergoing a ±1.0 and ±2.4 kPa pressure load were simulated to compare differences created by a constrained frame boundary condition versus replicating manufacturer recommended rack mounting. Module deflection, interconnect strain, and first principal stresses on cell volumes were used as comparison metrics to assess how internal module damage was affected. Average results across all loads scenarios showed that constraining the frame of the module to its initial unloaded plane reduced peak deflections by approximately 13%, interconnect strains by 11%, and first principal stress by 11% when compared to a module with correctly modeled racking. Analysis of results based on damage metrics indicated that the constrained boundary condition reduced interconnect stress at most locations and increased fatigue life by an average of 34%, and likewise reduced the average probability of cell fracture by 82%, though individual results were highly variable. Nonetheless, location-specific trends were generally consistent across constraint methodologies, indicating that the constraint simplification can be applied successfully if corrected for with increased load, additional test cycles, or an informed interpretation of results. The goal of this work was to exercise a methodology for quantifying differences created by a simplified test constraint setup, since expedient experimental simplifications are often used or considered to reduce the complexity of exploratory mechanical tests not related to standards qualification.
A finite element model of a four-cell photovoltaic mini-module was developed and compared to experimental results from an accelerated stress test protocol in order to validate that computational models can accurately represent their physical counterparts when subjected to mechanical loading and to assess mini-module representativeness against full scale photovoltaic modules. Deflected shapes across the simulated mini-modules were compared to measured mini-module shapes when subjected to various pressure loads. Displaced mini-module shape results constrained to the experimental protocols of 0.4 mm and 1.1 mm of displacement at the mini-module center were compared to experimental results of full-size modules subjected to module qualification test load levels of 1.0 kPa and 2.4 kPa, to assess if the bending of mini-modules was representative of full-sized modules under the load. Temperature cycling was incorporated into the model to simulate the impacts of stress due to thermal expansion of the backsheet and cells. A preliminary uncertainty analysis was performed to show how variations in material properties and geometric parameters change the simulation results.