Recent Achievements in Numerical Simulation in Sheet Metal Forming Processes

Purpose of this paper: During the recent 10-15 years, Computer Aided Process Planning and Die Design evolved as one of the most important engineering tools in sheet metal forming, particularly in the automotive industry. This emerging role is strongly emphasized by the rapid development of Finite Element Modelling, as well. The purpose of this paper is to give a general overview about the recent achievements in this very important field of sheet metal forming and to introduce some special results in this development activity. Design/methodology/approach: Concerning the CAE activities in sheet metal forming, there are two main approaches: one of them may be regarded as knowledge based process planning, whilst the other as simulation based process planning. The author attempts to integrate these two separate developments in knowledge and simulation based approach by linking commercial CAD and FEM systems. Findings: Applying the above approach a more powerful and efficient process planning and die design solution can be achieved radically reducing the time and cost of product development cycle and improving product quality. Research limitations: Due to the different modelling approaches in CAD and FEM systems, the biggest challenge is to enhance the robustness of data exchange capabilities between various systems to provide an even more streamlined information flow. Practical implications: The proposed integrated solutions have great practical importance to improve the global competitiveness of sheet metal forming in the very important segment of industry. Originality/value: The concept described in this paper may have specific value both for process planning and die design engineers

NASA Thesaurus supplement: A four part cumulative supplement to the 1988 edition of the NASA Thesaurus (supplement 3)

The four-part cumulative supplement to the 1988 edition of the NASA Thesaurus includes the Hierarchical Listing (Part 1), Access Vocabulary (Part 2), Definitions (Part 3), and Changes (Part 4). The semiannual supplement gives complete hierarchies and accepted upper/lowercase forms for new terms

A Methodology for Data-Informed Process Control in Progressive Die Sheet Metal Forming

This thesis investigates the coupled relationship between the strip transfer and forming operations in progressive die sheet metal forming, including the effects of the strip layout geometry, and its effect on the process speed and accuracy. Servo-actuated strip lifters and feeder are considered to assist in minimizing the dynamic response of the strip during the transfer process. A methodology is proposed for identifying suitable trajectories to prescribe the motion of active strip lifters and feeder to obtain consistent part quality without risk of process failures for a progressive die operation. Multiple iterations of a finite element (FE) model were constructed in LS-DYNA to simulate a progressive die operation. Various FE analysis techniques were used to reduce the computational cost of the simulations to allow for enough data to be generated for machine learning applications. Both explicit and implicit time-integration schemes were considered in iterations of the FE model. Both single and dual carrier strip layouts were considered. The results of the FE simulations suggest that the single carrier strip layouts produce larger predicted dynamic displacements and rotations of the work-piece as compared to the dual carrier strip layouts during strip transfer. Furthermore, the single carrier strip layout is shown to be susceptible to strip misalignment. The final version of the FE model utilized geometry based on a demonstrator tool being deployed at the Technische Universität München. A total of 1000 simulations were generated, 500 each for the ‘I’ and ‘O’ stretch-web types using a single carrier strip layout. Each simulation considered a unique permutation of control inputs sampled from the set of possible strokes rates and trajectories for the lifters and feeder. Cubic splines were used to generate the trajectories for the strip lifter and feeder by varying the position of two knots used to define the shape of the spline. The results from the 1000 simulations indicate that in general the ‘S’ stretch-web produces a larger variance in the predicted dynamic response and ‘work-piece placement as compared to the ‘I’ stretchweb. Furthermore, the stroke rate and lifter trajectory were shown to have a large influence on the overshooting of the work-pieces during strip transfer and the probability of whether tooling collisions occurred. Multiple machine learning models were trained on the data generated by the final FE model. Two types of classifiers were constructed using neural network and XGBoost architectures. The first classifier predicts whether the clearance between the strip and binder are within a specified tolerance (to prevent collision with the tooling) during strip transfer. The second classifier predicts whether the placement accuracy of the work-piece on the forming die after strip transfer is within a specified tolerance. A range of tolerances were considered when labeling the data for both classifiers. Nestedcross fold validation was used to select the hyperparameter tuning and model selection. The machine learning classifiers were used to test all possible control inputs using a ‘minimum feed clearance’ of 10 mm and a maximum ‘work-piece placement error of 0.11 mm. The maximum stroke rate at which a given pair of lifter and feeder trajectories can operate was identified for all permutations. Five permutations that achieved the highest predicted stroke rate were simulated for an additional five strokes. The classifiers showed a reasonable ability to predict the ‘minimum feed clearance’ and ‘workpiece placement in the extended FE simulations for the selected trajectories, but, was unable to account for the strip misalignment which occurred after several strokes in all simulations. This research successfully demonstrates a methodology for using machine learning models trained on FE simulations to predict process outcomes of a progressive die operation with variable feeder and lifter trajectories. The FE simulations used to train the machine learning models were generated by adopting computationally-effective FE modelling techniques in a single press stroke model. The machine learning models were shown to reasonably predict the process outcomes of novel input permutations in a multi-stroke FE simulation. One of the largest constraints in this research is the FE simulation time which limited the model complexity that could be considered in the training set generation. Furthermore, the demonstration of the machine learning predictions for a multi-stroke process was limited due to the susceptibility of the single carrier strip layout to misalign after strip progression. Future work should consider the use of dual carrier strip layouts for the generation of the training data. Alternative approaches may also be considered, such as a machine learning framework for directly predicting the forward dynamics of the progressive die operation or a co-simulation approach in which a robust controller interacts directly with the FE simulation

Three-terminal tandem solar cells enabled by back-contacted bottom cells featuring passivating, carrier-selective polysilicon based junctions

This thesis investigates back-contacted (IBC) bottom solar cells with passivating and carrier-selective POLO contacts with three terminals (3T-POLO-IBC cell). Such cells form the foundation of monolithic three-terminal tandem solar cells. This novel tandem solar cell enables the use of sub-cells with mismatched photocurrents. Thus, this tandem solar cell technology platform offers the flexibility with respect to subcell material selection, the ease of fabrication, and a robustness to spectral variations of incident light over the course of the day and year. Three building blocks of the 3T POLO IBC bottom solar cell, which are based on each other, are examined: First, the passivating and carrier-selective POLO contact. Second, the integration of POLO contacts on the rear side of a solar cell. Third, the principle of operation of a bottom cell with three terminals. In the first part, the process of charge carrier extraction at selective contacts to the photoabsorber is theoretically explored. The selectivity of a contact is defined on the basis of (reaction) kinetic considerations at the contact in terms of the rate ratio of desired processes to undesired processes. The extraction efficiency of charge carriers at the contact is derived as the ratio of the external voltage versus the internal voltage from a thermodynamic point of view. To emphasize the unifying nature of the definitions in this thesis, the existing literature definitions are calculated from the definitions in this thesis. The extraction efficiency is related to the selectivity coefficient of the contact and the limiting efficiency of a silicon solar cell with given contact selectivity is calculated accordingly. After the detailed theoretical investigation on selectivity, the properties of n+ and p+ POLO contacts are examined. Low saturation current densities between 2 fA/cm² and 18 fA/cm² and contact resistivities between 0.4mOhmcm² and 10mOhmcm² are found at the same time. It is shown that the efficient carrier transport of majority carriers is ensured by pinholes in the interfacial oxide. The resulting logarithmic selectivity coefficient of POLO contacts is determined to be above 15, which is one of the highest values measured. This makes POLO contacts predestined for solar cells with the highest efficiencies. POLO contacts are integrated on the rear side of a back-contact cell with POLO contacts for both polarities. Thereby, the p+ and n+ doped poly-Si on the backside of the solar cell form a parasitic graded p+n+ junction within the defect-rich poly-Si with a carrier lifetime of a few picoseconds. The arising recombination limits the achievable efficiency of the POLO-IBC cell to about 18%. For this reason, the parasitic junction is removed during the cell fabrication process by wet-chemically introducing a trench between the n+- and p+-doped poly-Si regions. The POLO-IBC cell with isolated n+- and p+ poly-Si regions achieves a certified efficiency of 24.25%. For the last part, a third POLO contact is added to the POLO-IBC cell and the 3T-IBC bottom cell is studied in detail using current-voltage measurements. First, the different realization options for a 3T tandem solar are sorted and the corresponding nomenclature is presented. Two different 3T IBC bottom cell architectures are identified. The first one – the unijunction bottom solar cell – contains a single minority carrier contact and two majority carrier contacts. The second one – the bipolar junction bottom solar cell – on the other hand, has two minority carrier contacts and a single majority carrier contact. Both 3T bottom cell architectures are fabricated based on a modified POLO-IBC fabrication process. The principles of operation and loss mechanisms are elucidated using J-V measurements on illuminated devices and by means of analytical modeling. The experiments show that the third contact of a 3T unijunction and bipolar junction bottom cell allows the collection or injection of additional minority or majority carriers from or into the bottom cell. Ideally, the power output of such a 3T bottom cell is nearly independent of the current density applied by the top cell. Therefore, no current matching of both subcells is required. However, the transport of majority carriers or minority carriers through the unijunction or bipolar junction bottom cell causes a loss, which, however, can be made negligible by a specific design of the bottom cell. The design rules are explained in detail. After the detailed investigations, a 3T unijunction bottom cell with a textured n+-POLO front contact with an efficiency of 20.3% and a simplified screen-printed PERC-like 3T bipolar junction bottom cell with 14.4% are developed. The latter is an attractive approach to utilize the dominant PERC technology in a low-cost tandem solar cell with maximum energy yield. Finally, the first 3T GaInP//POLO-IBC tandem cell demonstrator is fabricated with an efficiency of 27.3% and a net efficiency gain of 0.9% is demonstrated compared to the 2T operation of the 3T tandem cell.Die vorliegende Arbeit untersucht Rückkontakt-Bottomsolarzellen mit passivierenden und ladungsträger-selektiven POLO-Kontakten mit drei Anschlüssen (3T-POLO-IBC-Bottomzelle). Sie bilden das Fundament monolithischer Tandemsolarzellen mit drei Anschlüssen. Diese neuartigen Tandemsolarzelle erlaubt die Verwendung von Subzellen, dessen Fotoströme fehlangepasst sind. Damit bietet diese Tandemsolarzellen-Technologie Flexibilität bei der Materialauswahl der Subzellen, einfache Herstellbarkeit und Robustheit gegenüber spektraler Änderung des einfallenden Lichts im Tages- und Jahresverlauf. Es werden drei aufeinander aufbauende Bausteine der 3T-POLO-IBC-Bottomsolarzelle untersucht: Erstens, der passivierende und ladungsträger-selektive POLO-Kontakt. Zweitens, die Integration von POLO-Kontakten auf der Rückseite der Solarzelle. Drittens, die Funktionsweise einer Bottomzelle mit drei Anschlüssen. Im ersten Teil wird der Prozess der Ladungsträgerextraktion an selektiven Kontakten zum Fotoabsorber theoretisch ergründet. Die Selektivität eines Kontaktes wird auf der Grundlage von (reaktions-) kinetischen Betrachtungen am Kontakt als das Ratenverhältnis gewollter Prozesse zu ungewollten Prozessen definiert. Die Extraktionseffizienz von Ladungsträgern am Kontakt wird als das Verhältnis der externen Spannung gegenüber der internen Spannung aus thermodynamischen Gesichtspunkten abgeleitet. Um den vereinheitlichenden Charakter der Definitionen in dieser Arbeit hervorzuheben, werden die bestehenden Literatur-Definitionen aus den Definitionen in dieser Arbeit berechnet. Die Selektivität und Extraktionseffizienz werden miteinander korreliert und daraus der Wirkungsgrad einer Solarzelle mit vorgegebener Kontaktselektivität errechnet. Nach der detaillierten theoretischen Untersuchung der Selektivität werden die Eigenschaften von n+ und p+ POLO-Kontakten untersucht. Es werden niedrige Sättigungsstromdichten zwischen 2 fA/cm² und 18 fA/cm² und gleichzeitig Kontaktwiderstände zwischen 0,4 mOhmcm² und 10 mOhmcm² ermittelt. Es wird gezeigt, dass der effiziente Ladungsträgertransport der Majoritäten durch Pinholes im Grenzflächenoxid sichergestellt wird. Der resultierende logarithmische Selektivitäts-Koeffizient von POLO-Kontakten wird auf über 15 bestimmt. Damit gehören POLO-Kontakte zu den Kontakten mit der höchsten Selektivität und sind für Solarzellen mit höchsten Effizienzen prädestiniert. Die POLO-Kontakte werden auf der Rückseite einer Rückkontaktzelle mit POLO-Kontakten für beide Polaritäten integriert. Dabei formt das p+ und n+ dotierte Poly-Si auf der Rückseite der Solarzelle einen parasitären, gradierten p+n+-Übergang im defektreichen Poly-Si mit einer Ladungsträgerlebensdauer von wenigen Pikosekunden. Die resultierende Rekombination limitiert die erreichbare Effizienz der POLO-IBC-Zelle auf etwa 18%. Aus diesem Grund wird der parasitäre Übergang während des Zellherstellungsprozesses entfernt, indem ein Graben zwischen die n+- und p+-dotierten Poly-Si-Regionen nasschemisch eingebracht wird. Die POLO-IBC-Zelle mit isolierten n+- und p+-Poly-Si-Gebieten erzielt einen zertifizierten Wirkungsgrad von 24,25%. Für den letzten Baustein wird die POLO-IBC-Zelle um einen dritten POLO-Kontakt ergänzt und die 3T-IBC-Bottomzelle mittels Strom-Spannungsmessungen im Detail untersucht. Zuerst werden die unterschiedlichen Realisierungsmöglichkeiten für eine 3T-Tandemsolar einsortiert und die dazugehörige Nomenklatur vorgestellt. Dabei werden zwei verschiedene 3T-IBC-Bottomzellen-Architekturen unterschieden. Eine Unijunction-Bottomsolarzelle enthält einen einzigen Minoritätsladungsträgerkontakt und zwei Majoritätsträgerkontakte. Eine Bipolar-Junction-Bottomsolarzelle hingegen hat zwei Minoritätsladungsträgerkontakte und einen einzigen Majoritätsladungsträgerkontakt. Beide 3T-Bottomzell-Architekturen werden auf Basis eines modifizierten Herstellungsprozesses für POLO-IBC-Solarzellen realisiert. Das Funktionsprinzip und die Verlustmechanismen werden mit Hilfe von J-V -Messungen an beleuchteten Bauelementen und mit Hilfe analytischer Modellierung untersucht. Die Experimente zeigen, dass der dritte Kontakt einer 3T-Unijunction- und Bipolar-Junction-Bottomzelle das Sammeln oder Injizieren von zusätzlichen Minoritäts- oder Majoritätsladungsträgern aus der oder in die Bottomzelle ermöglicht. Im Idealfall ist die Leistungsabgabe einer solchen 3T-Bottomzelle nahezu unabhängig von der Stromdichte, die von der Topzelle angelegt wird. Daher ist keine Stromanpassung beider Subzellen erforderlich. Allerdings verursacht der Transport von Majoritätsladungsträgern bzw. Minoritätsladungsträgern durch die Unijunction- bzw. Bipolar-Junction-Bottomzelle hindurch einen Verlust, welcher jedoch durch eine gezielte Auslegung der Bottomzelle vernachlässigbar klein ausfallen kann. Die Auslegung wird im Detail erläutert. Schließlich wird eine 3T-Unijunction-Bottomzelle mit einem texturierten n+-POLO-Frontkontakt mit einem Wirkungsgrad von 20,3% und eine vereinfachte siebgedruckte PERC-ähnliche 3T-Bipolar-Junction-Bottomzelle mit 14,4% entwickelt. Letztere ist ein attraktiver Ansatz, um die dominierende PERC-Technologie in einer kostengünstigen Tandemsolarzelle mit maximaler Energieausbeute zu nutzen. Abschließend wird der erste 3T-GaInP//POLO-IBC-Tandemzellen-Demonstrator mit einem Wirkungsgrad von 27,3% hergestellt und ein Netto-Wirkungsgradgewinn von 0,9% im Vergleich zum 2T-Betrieb der 3T-Tandemzelle demonstriert

A Two Stage Stand Alone Solar PV System

Stand-alone systems based on solar photovoltaic (PV) have developed as a sustainable alternative pertaining to issue of electrification in areas particularly where the grid is not present. Main challenges in scheming this kind of systems are 1) Maximum power extraction from PV array 2) Dc to ac conversion 3) Protecting battery from over discharging and overcharging and 4) Facilitating satisfactory stepping up of voltage. As many objectives are needed to be met, schemes hitherto for standalone systems need a minimum of three converter stages, resulting in significant decline in efficiency and reliability of system. To tackle this problem, a two stage stand-alone topology comprising a novel transformer coupled dual input converter (TCDIC) is connected to a normal full bridge inverter is discussed in this thesis. Discussed TCDIC can facilitate charge control of battery while tracking maximum power point and keeping proper voltage level at load terminal. Mathematical model of TCDIC through small signal based approach is also included. A suitable control strategy for TCDIC is also presented to generate pulses. The operational efficiency of this topology is verified by performing simulation studies in detail

Development of a Novel Hybrid Multi-Junction Architecture for Silicon Solar Cells

Although existing technology can produce highly efficient solar cells, they remain commercially cost-prohibitive. A low-cost alternative was investigated in this research by developing a novel hybrid multi-junction silicon (HMJ-Si) solar cell architecture through modeling, fabrication, and testing. The architecture consists of stacked silicon solar cells with an air gap between them and was designed with metal grating contacts that exploit interference patterns for light management. The interference patterns were examined in MATLAB and verified using Lumerical FDTD Solutions. Development focused on wafer configuration; diffusion profile; front contact design; optical, electrical, and thermal loss reduction; and efficiency. The architecture was optimized using an unpolished-front, p-type top cell with 128nm of Si3N4, a butterfly front contact, and 400 m grating spaced 900 m apart; a polished-front, n-type bottom cell with 200 m grating spaced 1100 m apart; and both cells having an enhanced back surface field diffusion profile with 500nm silver contacts. Efficiency peaked at 8.42% using a silver-coated wafer in lieu of the bottom cell. The results indicate that the architecture is a viable solar cell design requiring additional research for optimization