Thermo-visco-elastic modelling of photovoltaic laminates: Advanced shear-lag theory and model order reduction techniques

During lamination, residual thermo-mechanical stresses are induced in the encapsulated solar cells composing photovoltaic (PV) modules. Depending on the material and geometrical configuration of the layers of the laminate, this residual stress field can be beneficial since it may lead to a compressive stress state in Silicon and therefore crack closure effects in the presence of cracks, with a recovery of electrical conductivity in cracked solar cells. It is therefore important to investigate the distribution of thermo-mechanical stresses within the PV laminate with a view to optimizing the coupling between the electrical response and elastic deformation in the operation of PV modules. A promising approach proposed in the present thesis regards the prediction of residual stresses in composite laminates by using a shear-lag theory to model the epoxy-vinil-acetate polymeric layers, accounting for their thermo-visco-elastic response. Moreover, it will be shown that thermomechanical formulations for stress analysis of a PV laminate lead to a system of higher order ordinary differential equations or partial differential equations in which the exact solutions may be impossible to be determined in closed form and hence numerical schemes become desirable. However, the computational cost associated with the implementation of the numerical scheme may be significantly expensive. Therefore, a method to reduce the computational complexity is expected to be very important. To this aim, Model Order Reduction (MOR) techniques are applied hierarchically, first to the thermal system of a PV module in service, and then extended to coupled thermo-mechanical problems. A combination of proper orthogonal decomposition (POD) and discrete empirical interpolation method (DEIM) with a modified formulation is proposed for the first-order thermal equations of photovoltaic system during service and a new coupled second-order Krylov based formulation is developed for model order reduction of the coupled thermo-mechanical model of the photovoltaic module. The results of these reduction schemes show a huge computational gain in the reduced system solutions and a high accuracy of the reduced system outputs

Experimental Analysis for Lubricant and Punch Selection in Shear Extrusion of Aa-6063

Shear extrusion is a forming process which is based on combined backward cup-forward rod extrusion. This extrusion process is attractive due its potential to achieve severe plastic deformation thus enabling texture and microstructural control of materials. Furthermore, the economic potential of shear extrusion for mass production and production of complex shapes provides for numerous applications in automotive, transportation, aero-space and other industries. However, a trending challenge in the use of this method for complex shapes is the design and selection of tools to achieve a high quality product. This paper focuses on deep study of shear extrusion of AA-6063. The process was studied experimentally using variables which affect the forming load as well as the quality of the product. It is concluded from the load-displacement and stress plots that a punch with large diameter and small punch land is desirable for easy forming of the material during shear extrusion. Analysis of the effect of lubricants on deformation load and stress shows that palm oil lubricant remains the best lubricant of the four lubricants examined since its gives the minimum load obtained during shear extrusion

Effect of Punch Diameters on Shear Extrusion of 6063 Aluminium Alloy

This paper reports the effect of punch diameters on the shear extrusion of 6063 Aluminium alloy. During the shear extrusion process, Aluminium billets of considerable diameter 30 mm and height 25 mm were inserted in a die hole and different punches of diameter 12 mm, 14 mm, 16 mm and 18 mm respectively were allowed to come in contact to perform the shear operation. The setup took place under a hydraulic press with maximum capacity of 600 kN. This work is aimed at studying the selection of the optimum punch diameter for shear extrusion using local groundnut oil as the lubricant. Different extrusion pressures were measured and the punch with a diameter of 18 mm gives the highest load of 77.7 kN while the punch with a diameter of 12 mm gives the lowest load of 51.2 kN. An indication shows that, an increase in the punch diameters led to an increase in the height of the extrudates and this in turn reduces the stress induce

Early experience with video-assisted thoracoscopic surgery in Nigeria

Background: Video-assisted thoracoscopic surgery (VATS) is a minimal access surgery that can be used for various diagnostic and therapeutic procedures. However, this tool is underused in our setting because of various reasons, ranging from equipment availability to expertise. Objective: This study aimed to review our early experience with VATS, highlighting the clinical attributes, outcomes, and challenges in our setting. Methods: This was a retrospective study of patients who underwent VATS between November 2015 and May 2019. Patients’ demographics, clinical presentation, diagnosis, procedural success, complications, and length of hospital stay were analyzed. Results: The study included 25 patients (mean age, 41.26±12.78 years). The most common preoperative diagnosis was right catamenial pleural effusion. The conversion rate was 20%, and the average length of hospital stay was 3.4 days. Conclusion: The scope of VATS is very narrow in our setting. Only one center in Nigeria has reported good success rate and minimal complications albeit longer hospital stay. The identified limitations to use of VATS include lack of investment in procurement of appropriate equipment and expertise, which need to be overcome

Abrasive water jet drilling of advanced sustainable bio-fibre-reinforced polymer/hybrid composites : a comprehensive analysis of machining-induced damage responses

This paper aims at investigating the effects of variable traverse speeds on machining-induced damage of fibre-reinforced composites, using the abrasive water jet (AWJ) drilling. Three different types of epoxy-based composites laminates fabricated by vacuum bagging technique containing unidirectional (UD) flax, hybrid carbon-flax and carbon fibre-reinforced composite were used. The drilling parameters used were traverse speeds of 20, 40, 60 and 80 mm/min, constant water jet pressure of 300 MPa and a hole diameter of 10 mm. The results obtained depict that the traverse speed had a significant effect with respect to both surface roughness and delamination drilling-induced damage responses. Evidently, an increase in water jet traverse speed caused an increase in both damage responses of the three samples. Significantly, the CFRP composite sample recorded the lowest surface roughness damage response, followed by C-FFRP, while FFRP exhibited the highest. However, samples of FFRP and hybrid C-FFRP recorded lowest and highest delamination damage responses, respectively. The discrepancy in both damage responses, as further validated with micrographs of colour video microscopy (CVM), scanning electron microscopy (SEM) and X-ray micro-computed tomography (X-ray μCT), is attributed to the different mechanical properties of the reinforced fibres, fibre orientation/ply stacking and hybridisation of the samples.Peer reviewe

A thermo-visco-elastic shear-lag model for the prediction of residual stresses in photovoltaic modules after lamination

Abstract The distribution of residual thermo-elastic stresses in encapsulated solar cells arising from lamination is relevant for the characterization of the long term performance of photovoltaic (PV) modules during service. Accurate modelling of the structural response of the laminate in the transient regime during cooling after lamination is a challenging task from the computational point of view. In this work we propose a semi-analytic model based on the Kirchhoff plate theory and the shear-lag approach for the treatment of the polymeric encapsulant layers and accounting for their time and temperature dependency according to a rheological model derived from fractional calculus considerations. Spatially uniform and non-uniform temperature distributions are compared to accurately assess the amount of the residual compressive stresses raised in the Silicon cells after lamination. The use of more realistic non-uniform temperature distributions leads to lower residual compressive stresses in Silicon as compared to the uniform case

A 3D coupled thermo-visco-elastic shear-lag formulation for the prediction of residual stresses in photovoltaic modules after lamination

Evaluation of the residual stress distribution arising from lamination of photovoltaic (PV) modules is important to address thermomechanically induced failure of PV modules during service. In view of the fact that PV modules contain several Silicon cells, modelling the thermo-mechanical response of PV laminates during cooling after lamination is computationally challenging. Due to the coupling between the thermal and the mechanical fields, the stress state experienced by each silicon cell in a module varies from one position to another. Here, a novel 3D coupled thermo-visco-elastic shear-lag model is proposed to determine the stress distribution in a PV module after lamination. To enhance the prediction of stress distribution in the laminate, viscoelastic properties of the EVA encapsulant are taken into account by using an asymptotic model which is stable for small and large time steps of strain increments. The results for a simulated mini-module show that residual stresses vary significantly from point to point inside the PV module

A nonlocal adaptive discrete empirical interpolation method combined with modified hp-refinement for order reduction of molecular dynamics systems

Model order reduction is an emerging technique to tackle the computational complexities of molecular dynamics (MD) simulations. Different strategies are required to adequately obtain the reduced solutions of different classes of molecular dynamics systems. In this work, a proper orthogonal decomposition (POD) is combined with the discrete empirical interpolation method (DEIM) to study atomic systems. Due to the limitations of the DEIM in capturing the nonlocal response of the nonlinear force field of MD systems, a nonlocal adaptive discrete empirical interpolation method (ADEIM) is proposed. Furthermore, a modified hp-refinement algorithm is introduced to extend the application of the PODDEIM approach to order reduction of multi-dimensional MD systems. In the DEIM, the distance between atoms and hence the reduced internal force vector is estimated based on a local interpolation of the state variables. The internal forces of a multi-dimensional MD system depend on the distance between the atoms, represented in space by more than one coordinate. Therefore, the ADEIM approach seeks to obtain a nonlocal interpolation of the state variables to accurately predict the distance between the interpolated atoms and hence the reduced force vector. Simulation of MD systems with frequently changing neighbour atoms leads to change in the system dynamics, which further leads to change of properties of the snapshots. Therefore, the temporal domain is adaptively subdivided into smaller sub-domains using the adopted hp-refinement procedure. The reduced system parameters are effectively derived over the sub-domains. Considering the computational cost, a modified hp-refinement algorithm is developed in this study, which is further coupled with the POD-ADEIM approach to obtain the reduced-order solution of the MD systems. The results of the proposed approach demonstrate the efficiency and accuracy of the reduced solutions

Model order reduction applied to heat conduction in photovoltaic modules

Modeling of physical systems may be a challenging task when it requires solving large sets of numerical equations. This is the case of photovoltaic (PV) systems which contain many PV modules, each module containing several silicon cells. The determination of the temperature field in the modules leads to large scale systems, which may be computationally expensive to solve. In this context, Model Order Reduction (MOR) techniques can be used to approximate the full system dynamics with a compact model, that is much faster to solve. Among the several available MOR approaches, in this work we consider the Discrete Empirical Interpolation Method (DEIM), which we apply with a suitably modified formulation that is specifically designed for handling the nonlinear terms that are present in the equations governing the thermal behavior of PV modules. The results show that the proposed DEIM technique is able to reduce significantly the system size, by retaining a full control on the accuracy of the solution