1,720,966 research outputs found
Silicon photovoltaics: experimental testing and modelling of fracture across scales
Full text linkThe study of the properties of materials can be addressed through a multi-scale approach, in order to have the possibility to grasp at each of the levels of analysis the peculiar aspects. Tracing a path inside the state-of-the-art in the available bibliography, historically in the field of mechanics s are found in which the material is studied through nonlocal theories based on continuous or discrete local approaches. More recently, with the advent of great computatio- nal power computers, analytical methodologies based on theories also very complex deriving from the field of chemistry and physics have been developed, capable to discretize at the ato- mic scale the material and study its behavior by applying energy approaches. Starting from the analysis of some of these theories at the nano- and micro-scales, it is possible to investi- gate the separation mechanisms at the molecular level, which may be considered as cracking processes within the material according to the adopted scale of analysis. The application of theories of this kind to large portions of material, in which there are up to some millions of particles involved is reasonably not an applicable solution, since it would require a huge effort in terms of computation time. To work around this problem and find a method suitable for the study of cracking mechanisms, a mixed method (MDFEM) was byconjugating pure molecu- lar dynamics (MD) and the finite element method (FEM), in which the material is discretized by means of one-dimensional elements whose mechanical characteristics are derived from MD. This approach is based on the application of a nonlocal theory in which the contribution of a portion of material placed within a certain distance from the point of fracture is taken into account by means of a parameter of non-locality. Moreover, the study of the evolution of cracking is addressed at the meso-scale by the application of a cohesive non-linear model. Towards the analysis of the macroscale, the theories put forward so far have been ap- plied to the study of phenomena of breakage inside Silicon cells embedded into rigid or semi-flexible photovoltaic modules. By performing various laboratory tests, useful for the characterization of the material and for understating the evolution of cracking process due to multiple causes, a study on the main issues that may compromise the durability and mainte- nance of the expected service levels of photovoltaic panels has been conducted. Experimen- tally results have been interpreted by using appropriate macro-scopic continuum models. The research carried out had the purpose to provide an introduction to a correct design of these systems of energy production in order to increase their durability and resistance to crackin
Recent progresses on modelling of cracking in polycrystalline Silicon for photovoltaic applications
No full textAnalysis of the stress field in photovoltaic modules due to impact loadings
No full textIn the present work the stress field due to hail impacts on semi-flexible photovoltaic (PV) modules is analysed. Experimental tests and finite element simulations have been performed by considering different arrangements for the stack of layers of the panels and different kind of substrates in order to identify the type of support that minimizes the extension of the solar cell region where high tensile stresses take place under impacts. Practical hints for production and installation of the modules are derived from the obtained results
Experimental and numerical investigation on the stress field induced by impact loadings in semi-flexible photovoltaic modules
No full textIn the present work, the effects of impacts on silicon (Si) cells embedded in semi-flexible photovoltaic (PV) modules are investigated from both the experimental and the numerical point of views. Semi-flexible PV modules, frequently installed on curved surfaces, such as on sailing boat decks, mountain refuge roofs, motorhomes and innovative electrical cars, are in fact particularly sensitive to the impact of hailstones, due to the fact that they are entirely made of soft polymeric layers. In order to simulate the hailstone impact, a polyamide sphere with a radius of 20 mm has been shot with a compressed-air apparatus against the PV module at different velocities, up to a maximum value of 10m/s. The effects of the impact load on the Si cells, invisible by the naked eye, have been analyzed through pictures taken with the electroluminescence (EL) technique [1]. Different typologies of substrates on which the panels are laid on have been considered, in order to identify the configuration that minimizes the extension of the zone around the impact point characterized by a high stress level. In the case of a rigid substrate, the damage is localized in a narrow circular area, where the Si cell is completely destroyed, whereas a completely different damage pattern is occurring in the case of a soft substrate, showing several concentric cracks developed around the point of impact. As regards the numerical modelling, an axisymmetric finite element model of the laminate has been proposed using the finite element analysis program FEAP. Linear elastic constitutive laws have been adopted for the materials composing the layers, except for the epoxy material encapsulating the Si cells for which a neo-Hookean constitutive behavior has been considered. An implicit integration scheme has been used to solve the contact problem, using the node-to-segment contact strategy [2] and the penalty method to model the contacting interfaces. Different values of the penalty parameter have been used along the contact interface between the PV module and the substrate, to simulate different substrate stiffnesses. Numerical predictions regarding the extension of the damaged areas are in good agreement with our own experimental results obtained in the laborator
Simulated hail impacts on flexible photovoltaic laminates: testing and modelling
No full textThe problem of simulated low-velocity hail impacts on flexible photovoltaic (PV) modules resting on a substrate with variable stiffness is investigated. For this type of PV module it is shown that the prescriptions of the IEC 61215 International Standard for quality control used for rigid (glass-covered) PV modules should be augmented by taking into account the real mounting condition and the stiffness of the substrate in the simulated hail impact tests. Moreover, electroluminescence inspection of the crack pattern should be made in addition to electric power output measurements. An implicit finite element simulation of the contact problem in dynamics is also proposed, with two different degrees of accuracy, to interpret the experimentally observed extension of cracking. Results pinpoint the important role of stress wave propagation and reflection in the case of soft substrates
Image analysis of polycrystalline solar cells and modeling of intergranular and transgranular cracking
No full textAn innovative image analysis technique is proposed to process real solar cell pictures, identify grains and grain boundaries in polycrystalline silicon, and finally generate finite element meshes. Using a modified intrinsic cohesive zone model approach to avoid mesh dependency, nonlinear finite element simulations show how grain boundaries and silicon bulk properties influence the crack pattern. Numerical results demonstrate a prevalence of transgranular over intergranular cracking for similar interface fracture properties of grains and grain boundaries, in general agreement with the experimental observatio
Fatigue degradation and electric recovery in Silicon solar cells embedded in photovoltaic modules
Full text linkCracking in Silicon solar cells is an important factor for the electrical power-loss of photovoltaic modules. Simple geometrical criteria identifying the amount of inactive cell areas depending on the position of cracks with respect to the main electric conductors have been proposed in the literature to predict worst case scenarios. Here we present an experimental study based on the electroluminescence (EL) technique showing that crack propagation in monocrystalline Silicon cells embedded in photovoltaic (PV) modules is a much more complex phenomenon. In spite of the very brittle nature of Silicon, due to the action of the encapsulating polymer and residual thermo-elastic stresses, cracked regions can recover the electric conductivity during mechanical unloading due to crack closure. During cyclic bending, fatigue degradation is reported. This pinpoints the importance of reducing cyclic stresses caused by vibrations due to transportation and use, in order to limit the effect of cracking in Silicon cells
Electrical recovery and fatigue degradation phenomena in cracked silicon cells
No full textAn experimental study based on the electroluminescence technique is herein proposed to demonstrate the existence of coupling between mechanical deformations and the intensity of the electric field due to cracks in monocrystalline Silicon cells embedded in photovoltaic modules. In spite of the very brittle nature of Silicon, due to the action of the encapsulating polymer and residual compressive stresses resulting from the lamination stage, cracks experience crack closure and contact during mechanical unloading, partially recovering their original electric response. Crack propagation in case of cyclic loading, as, e.g., in case of vibrations due to transportation and use, have also been reported for the very first time. The research results pinpoint the need of improving electric predictions based on the estimation of inactive cell areas, since worst case scenarios not accounting for electro-mechanical coupling are too conservative
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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