TOWARDS AN OPTIMAL INVESTMENT BUDGET FOR GREEN DATA CENTERS

The growing demand for data storage and computational power has increased the global deployment of data centers. Today, data centers are the backbone of modern companies, but also main consumers of energy and among the major producers of greenhouse gas emissions. Therefore, the development and implementation of sustainable and energy efficient Green Data Centers (GDC) has gained relevance from a scientific and practical point of view. Even though technological progress has revealed opportunities for improvements in energy efficiency, little effort has been made regarding the business case of GDC. In this paper, we analyze the coherence of economic and environmental objectives of GDC investments by conceptualizing a decision model using traditional financial metrics and by applying the model on exemplary data. We analyze both costs and realized energy savings associated with the GDC investment. Besides, we examine the influnce of volatile energy prices on the investment decision. By integrating risk and return into one decision calculus, we determine the optimal GDC investment budget which reconciles long-term economic and environmental objectives. Our theoretical findings are supported by an application example of a GDC investment project. We hereby demonstrate the structural under-investment when disregarding volatile energy prices in decision-making

Numerical analysis of laser powder bed fused stents made of 316L stainless steel considering process-related geometric irregularities

Re-narrowing of a coronary vessel after stent implantation, known as in-stent restenosis (ISR), is a predominant problem in the treatment of atherosclerosis. ISR is caused, e.g., by vessel wall injury during stent implantation, malpositioning, over- or undersizing of the stent, and associated adverse alteration of natural blood flow. Advances in metal additive manufacturing, particularly in laser powder bed fusion (L-PBF), are enabling the generation of micro-scale L-PBF lattice structures and thus potentially coronary stents. By enabling new or even patient-specific stent designs, L-PBF stents could improve the conformity of the implanted stent and the vessel wall, thus potentially reducing ISR rates in the future. Research in the field of L-PBF stents is still in its early stages. Previous studies have mainly focused on the analysis of stent design requirements and basic functionality of L-BF stents. Studies regarding the determination of the specific mechanical behavior of L-PBF stents but also regarding their numerical analysis are currently not available. Due to their similar topology, L-PBF stents resemble L-PBF lattice structures with a low structural density. Therefore, it is reasonable to transfer the findings in the field of L-PBF lattice structures to L-PBF stents. L-PBF lattice structures exhibit process-related geometric irregularities that (negatively) affect their morphology and their mechanical behavior. Therefore, for an accurate (numerical) evaluation of L-PBF lattice structures and thus of L-PBF stents, their mechanical behavior must be determined first, and the influence of the process-related geometric irregularities must be analyzed or considered within the numerical models. Furthermore, the mechanical and morphological behavior of filigree L-PBF stents can be altered by post-processing steps (surface, heat treatment). However, studies on L-PBF lattice structures are mainly limited to as-built structures. Therefore, the aim of this doctoral thesis is to determine the effects of L-PBF process-related geometric irregularities and different post-processing conditions on the mechanical behavior of L-PBF 316L stents, as well as to develop a numerical methodology for their numerical evaluation. In a first step, a finite element analysis (FEA) for the prediction of stent deformation during crimping and expansion was developed and validated using extensive experimental data from conventionally manufactured stents. These models accurately predicted the expansion behavior of two different stent designs with different expansion behavior, as well as different positioning of the stent on the balloon catheter. In the second step, the mechanical behavior of L-PBF 316L was determined using uniaxial tensile tests on standard flat tensile specimens with variable specimen thickness and orientation angle. For each specimen configuration, as-built and heat treated specimens were considered. In the as-built condition, besides the anisotropic mechanical properties of L-PBF 316L already known from the literature, a significant increase in strength with increasing specimen thickness was observed, which stagnated at a specimen thickness of t > 1.5 mm, thus reaching a saturation value. Heat treatment resulted in homogenization but no recrystallization of the microstructure. Thus, the melt pool boundaries and substructures were dissolved, and residual stresses were reduced, whereas the elongated and oriented grains and thus the anisotropic microstructure were preserved. Accordingly, the specimen thickness- and direction-dependent mechanical properties of L-PBF 316L were still observed after heat treatment. Thus, for a reliable structural mechanical evaluation of L-PBF parts, their mechanical properties must be determined using test specimens that are comparable in size, orientation angle, and post-treatment condition to the later L-PBF part. In a final step, the mechanical behavior of L-PBF stents was determined and the expansion behavior of L-PBF stents under different post-processing conditions was evaluated by FEAs. The generation of L-PBF miniature tensile specimens of comparable cross section to stent struts and their experimental evaluation is challenging and highly error-prone. Therefore, a combined experimental-numerical approach was developed for the inverse determination of the mechanical behavior of L-PBF 316L stents based on experimental testing and FEA of uniaxial compression of L-PBF stents. The stent models were reconstructed from computed tomography (CT) scans of real L-PBF stents. In this way, process-related geometric irregularities were depicted enabling an accurate prediction of the stent structure-property relationship. Thus, the macroscopic mechanical behavior of L-PBF 316L stents could be determined for the first time and subsequently described numerically by a material model. Morphological analysis of the L-PBF stents further revealed significant discrepancies between the actual L-PBF stents and its computed aided design (CAD) model due to process-related geometric irregularities (surface roughness, strut waviness, enlarged and inhomogeneous strut diameters, internal defects). Numerical expansion analysis of the L-PBF stent models showed that L-PBF stents can exhibit comparable expansion behavior to conventional stents only after surface and heat treatment. However, subsequent analysis of deformation and stress states showed that L-PBF stents, both in the as-built condition and after surface and heat treatment, may exhibit critical local stress/strain concentrations, especially in the areas of pronounced geometric irregularities. Improvements in the L-PBF process, post-processing steps, and stent design are therefore essential to minimize process-related geometric irregularities and thus their strength-reducing effects, ultimately ensuring the structural safety of L-PBF stents. One possible improvement approach is to manufacture the stents on special µ-L-PBF systems that have explicitly been optimized to produce filigree structures. In this way, a higher geometric accuracy and low surface roughness could already be achieved in the as-built condition of L-PBF stents, and the subsequent required surface treatment could be reduced to a minimum. Furthermore, the fatigue strength, the damage behavior, the interaction of the stent with the blood vessel as well as the biocompatibility of L-PBF 316L stents should be investigated. To effectively use numerical models for the development of L-PBF stents, the potential of synthetic L-PBF stent models should also be investigated. The synthetic stent models represent a statistics-based modification of the original stent CAD model (e.g., local variations of strut cross section along strut length). In this way, the effects of L-PBF process-related geometric irregularities could be represented statistically and thus without explicit reconstruction from CT scans. The development of L-PBF stents is a very complex interdisciplinary task in the fields of manufacturing technology, material science, design development and numerical simulation. To establish L-PBF as a reliable alternative to conventional stent fabrication, further research in this area is essential. By providing a method to determine the mechanical properties of L-PBF stents as well as their numerical analysis, this doctoral thesis could contribute to the further development of L-PBF 316L stents, as well as define necessary research aspects for further work

The long-term sustainability of household bio-sand filtration

The introduction of intermittently operated slow sand filters, suitable for use at household level, is gaining momentum in the developing world. An estimated 100,000 bio-sand filters are already in use, providing improved drinking water to more than half a million people. Laboratory and field research has shown that bio-sand filters are capable of impressive reductions of turbidity and pathogen levels. However, long-term sustainability, social acceptance and appropriateness have not been well documented. An evaluation was therefore conducted in rural Kenya to measure the performance of filters introduced 4 years previously. Measuring turbidity and E.coli removal rates, maintenance practices and user perceptions, the study found all but one filter were still in use with over 70% producing water of good quality. Underperformance of some filters pointed at poor maintenance habits. As a ‘point-of-use’ water treatment solution, bio-sand filtration seems to be very appropriate, effective and cheap. Strategies to introduce this promising technology at a large scale need to be seriously investigated

Reconstruction of coronary arteries from X-ray angiography: A review.

Despite continuous progress in X-ray angiography systems, X-ray coronary angiography is fundamentally limited by its 2D representation of moving coronary arterial trees, which can negatively impact assessment of coronary artery disease and guidance of percutaneous coronary intervention. To provide clinicians with 3D/3D+time information of coronary arteries, methods computing reconstructions of coronary arteries from X-ray angiography are required. Because of several aspects (e.g. cardiac and respiratory motion, type of X-ray system), reconstruction from X-ray coronary angiography has led to vast amount of research and it still remains as a challenging and dynamic research area. In this paper, we review the state-of-the-art approaches on reconstruction of high-contrast coronary arteries from X-ray angiography. We mainly focus on the theoretical features in model-based (modelling) and tomographic reconstruction of coronary arteries, and discuss the evaluation strategies. We also discuss the potential role of reconstructions in clinical decision making and interventional guidance, and highlight areas for future research

Ein Ansatz zur Bestimmung kundenindividueller Finanzierungslösungen am Beispiel gekoppelter Absatz- und Finanzierungsgeschäfte

An Approach Towards Determining Customer-specific Financing Solutions Using the Example of Coupled Sales and Financing ContractsCustomized business models are critical for the success of companies that offer coupled sales and financing contracts in highly competitive markets. The aim of this article is to compare two financing solutions based on a quantitative model and to determine the solution that is advantageous for both sides (company and customer). Therefore the conventional annuity loan with a fixed amount of instalments is compared to a simple loan repayment with a totally variable amount of instalments. The customer-specific loan repayment is advantageous for both sides from integrated risk and return aspects: the risk emanating from coupled sales and financing contracts can be reduced, both for companies (in terms of the merchandising risk of their products and the risk of high loan defaults from the financing transactions) and for customers (in terms of default risk). The practical application of the model is exemplarily illustrated by using a real world dataset of a large brewery

Risk Management along the Value Chain

Eine langfristige und nachhaltige Steigerung des Unternehmenswerts als zentrales Unternehmensziel fordert eine konsequente, wertorientierte Ausrichtung aller Unternehmensteile und -aktivitäten. Das Risikomanagement, welches stets im Rahmen einer integrierten Betrachtung von Ertrags- und Risikoseite durchzuführen ist, hat das Ziel, eine wertorientierte Unternehmensführung bestmöglich zu unterstützen. Unternehmen stehen innerhalb ihres Wertschöpfungsnetzwerks jedoch vielen aktuellen und zukünftigen Herausforderungen gegenüber, und sind mit den sich daraus ergebenden Risiken konfrontiert. Vor diesem Hintergrund gibt die vorliegende Arbeit konkrete Handlungsempfehlungen für die Umsetzung eines Risikomanagements in Wertschöpfungsnetzwerken und trägt auf diese Weise zur Unterstützung einer wertorientierten Unternehmensführung bei. Mit dem Ertrags- und Risikomanagement am Beschaffungsmarkt bei Rohstoffen, dem Ertrags- und Risikomanagement am Absatzmarkt bei Kundenbeziehungen und dem Ertrags- und Risikomanagement am Finanzmarkt bei Investmentfonds werden dabei drei ausgewählte Themenbereiche herausgegriffen.A long-term sustainable increase in shareholder value as a key corporate objective calls for a consistent, value-oriented alignment of all business activities. Risk management that is always to be implemented as part of an integrated risk and return management, aims to support a value-oriented corporate governance in the best possible way. Companies are faced within their value chain with many current and future challenges, and are furthermore confronted with the risks arising from them. Against this background, the present thesis gives specific recommendations for the implementation of a sound risk management in the value chain. In this way, it contributes to support a value-oriented corporate governance. With a risk and return management in the procurement market for raw materials, a risk and return management in the sales market for customers and a risk and return management in the financial market for investment funds, three selected subject areas are focused on

Geometric misalignment and calibration in cone-beam tomography

© 2004 American Association of Physicists in MedicineLorenz von Smekal, Marc Kachelrieß, Elizaveta Stepina, and Willi A. Kalende