1,584 research outputs found

    On pricing kernels, information and risk

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    We discuss the finding that cross-sectional characteristic based models have yielded portfolios with higher excess monthly returns but lower risk than their arbitrage pricing theory counterparts in an analysis of equity returns of stocks listed on the JSE. Under the assumption of general no-arbitrage conditions, we argue that evidence in favour of characteristic based pricing implies that information is more likely assimilated by means of nonlinear pricing kernels for the markets considered.Comment: 20 pages, 3 figures, 1 tabl

    What impact do thin debris layer have on ablation for debris covered glaciers? A combined fieldmodeling study on Zmuttgletscher

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    Debris covered glaciers are commonly found in alpine environments. As a result of a protective debris layer, debris covered glaciers respond differently to changes in climate. Current numerical ablation models simulating debris covered glaciers, however, do not account for enhanced melt for thin debris layers as is proven by empirical data. Debris layers thinner than a specific critical debris thickness as well as partially covered surfaces are often found to have increased melt rates compared to clean ice. As current numerical models attribute insulation to debris layers of any thickness, it is of great importance to include enhanced melt for thin layers to analyse how surface mass balance is affected when enhanced melt is incorporated. In a first part, this study focuses on reproducing the Østrem curve with data collected during a field campaign on Zmuttgletscher. Partially covered surfaces were found to enhance melt by up to 20 % the clean ice melt rate, whereas insulating of thicker debris layers reduced surface melt by as much as 61 %, depicting an accurate Østrem curve. Melt rates were shown to be reduced on fully covered surfaces of any thickness, with melt rates decreasing as debris thickness increased. In a second part, simulations of glaciers with and without enhanced melt for thin debris layers were compared and analysed. Step change experiments as well as sinus simulations revealed that, for a glacier building up with a simplified bed geometry, inclusion of enhanced melt for thin debris layers has no significant impact on surface mass balance. Differences increase with a higher selected enhancement factor but do not affect surface mass balance significantly. With an enhancement 1.6 times the clean ice melt rate for debris layers ranging from <0 to 0.03 m thickness, differences stay insignificantly minimal and do not necessitate the need to implement enhanced melt in numeric melt models

    Optimization and multivariable control of refrigeration systems

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    Los ciclos de refrigeración por compresión de vapor constituyen el método más extendido a nivel mundial para la generación de frío. Estos sistemas se utilizan en áreas tan diversas como regulación de la temperatura en estancias habitadas, almacenamiento y transporte de alimentos y múltiples procesos industriales. Dado el considerable impacto causado por el consumo energético de estos sistemas en los balances económicos y medioambientales de los países desarrollados y en vías de desarrollo, y teniendo en cuenta la escasez creciente de fuentes de energía fósiles y el desarrollo todavía lento de las diferentes tecnologías de energía renovable, la operación óptima en términos de eficiencia energética de los sistemas de refrigeración por compresión de vapor existentes se presenta como un problema clave que abordar. Esta Tesis aborda la operación óptima de los ciclos de refrigeración desde el punto de vista de la eficiencia energética. Aunque el trabajo se centra principalmente en sistemas de una etapa de compresión y un recinto a refrigerar, se analizan también otras configuraciones con varias etapas y varios recintos. Existen varios factores clave para alcanzar la operación óptima de un sistema de refrigeración en el campo del Control Automático: el modelado, la optimización y el control propiamente dicho. En primer lugar, se estudia ampliamente el modelado estático y dinámico de los sistemas de refrigeración. En cuanto al segundo, se desarrolla un modelo dinámico simplificado y orientado al control de un ciclo de una etapa de compresión y un recinto a refrigerar. El objetivo es que pueda ser incorporado en estrategias de control basado en modelo, donde se requieren tanto una baja carga computacional como una descripción suficientemente precisa de la dinámica dominante del sistema, de acuerdo con los objetivos de control. En segundo lugar, se analiza la operación óptima en régimen permanente de un ciclo de una etapa de compresión y un recinto a refrigerar. Dada una cierta demanda de frío, el objetivo de la fase de optimización es calcular el ciclo en régimen permanente que alcanza la máxima eficiencia energética posible asegurando la satisfacción de la demanda de frío y a la vez respetando las restricciones de operación. Una vez calculado, se pretende que este ciclo óptimo constituya la referencia a seguir por parte del controlador. Finalmente, se estudia asimismo el problema de control. En la literatura sobre sistemas de refrigeración se encuentran principalmente dos esquemas: el control convencional y el control centrado en la eficiencia energética. En el primer esquema, además de la referencia impuesta por la demanda de frío, se impone un valor bajo pero constante como referencia para el grado de sobrecalentamiento del refrigerante a la salida del evaporador, to achieve the cycle defined by the optimization stage by manipulating the available control actions. Therefore, the controllability of the one-stage, one-load-demand cycle is analysed using linear theory and a nonlinear pointwise analysis based on the phase portrait method. Given the conclusions of the controllability analysis, a suboptimal hierarchical control strategy is proposed to achieve the highest possible efficiency while satisfying the cooling load. Most contributions of this Thesis are of theoretical nature. Notwithstanding, the application of the proposed control strategy to a multi-compression-stage, multi-loaddemand experimental plant is intended. Then, steady-state identification of the plant is performed from experimental data, whereas validation of the models considering different plant configurations is also carried out.Premio Extraordinario de Doctorado U

    Piecewise log-normal approximation of size distributions for aerosol modelling

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    Application of techno-economic modelling in the platinum mining industry of Southern Africa

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    A research report submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, in fulfilment of the requirements for the degree of Masters of Science in Mining Engineering 2016/10/5Management does not have an efficient mechanism to test strategic and operational alternatives and to assess the impact of these on the value and underlying trade-off variables of the business. Techno-economic models can be applied for this purpose as they provide a framework for undertaking advanced process simulation and business valuation. The purpose of the research report is to identify key components, principles and best practice as applied in techno-economic models, to improve techno-economic modelling for the purpose of decision-making and business optimization. The integrated techno-economic model requires a mining model with production planning and scheduling abilities. The half-level system method can be applied to create production profiles for different mining options and only after optimisation the best option is taken forward for graphical design and detailed scheduling. A metallurgical model incorporates the logic and efficiencies of the treatment process into the techno-economic model from which the refined products are determined for revenue and costing purposes. The financial model integrates with the mining and metallurgical elements and uses detailed costing models and sound financial principles for operating and capital cost estimates. An accurate techno-economic model includes key cash flow components and applies rigorous valuation practice for investment analysis. Techno-economic models are extensively applied in business planning, major project valuations and stay–in-business project valuations. Learnings from the review of these case studies suggest best practice, which allows the models to be applied to different types of business entities and contributes to the accuracy, consistency and efficiency of techno-economic modelling. Integrated techno-economic modelling is also applicable in strategic planning and mine design optimization as it provides a powerful instrument for decision-making and business optimization. The future of the mining business depends on it as an invaluable direction steering tool.MT201

    Advances in simulated moving bed : new operating modes : new design methodologies and product (FlexSMB-LSRE) development

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    Tese de doutoramento. Engenharia Química e Biológica. Faculdade de Engenharia. Universidade do Porto. 200

    Mathematical modelling of shoreline evolution under climate change

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    This study focuses on the impact of potential changes in the wind-wave climate on shoreline change. The `one-line' model for medium to long-term prediction of coastline evolution is employed. New analytical and numerical solutions of this important model are described. Specifically: 1) original semi-analytical solutions are derived that relax the unrealistic assumption of existing analytical work that a constant wave condition drives shoreline change and, 2) a more general form of the one-line model is solved with a novel application of the `Method of Lines'. Model input consists of 30-year nearshore wave climate scenarios, corresponding to the `present' (1961-1990) and the future (2071-2100). Winds from a high resolution, (12km x 12km), regional climate model, obtained offshore of the south-central coast of England at a dense temporal resolution of 3 hours, are used to develop the aforementioned wave climate scenarios, through hindcast and inshore wave transformation. A hypothetical shoreline segment is adopted as a `benchmark' case for comparisons. Monthly and seasonal statistics of output shoreline positions are generated and assessedfo r relative changeso f `significance' between `present' and future. Different degrees of evidence that such changes do exist are found. This study is the first application of such high resolution climate model output to investigate climate change impact on shoreline response. Major findings include: 1) shoreline changes of `significance' are strongly linked to `significant' changes in future wave direction, 2) future changes appear smaller for entire seasons than for individual months, 3) shoreline position variability is often smaller in the future, 4) different climate model experiments produce diverging results; however, general trends are largely similar. The present study, at a fundamental level, offers analytical solutions of the 'oneline' model that are closer to reality and a numerical solution that is of increased effciency.. At a practical level, it contributes to better understanding of the patterns of shoreline response to changing offshore wave climate through: 1) the use of fast and straightforward methods that can accommodate numerous climate scenarios without need for data reduction, and 2) the development of a methodology for using climate model output for coastal climate change impact assessment studies

    The TRIZ-CBR synergy: A knowledge based innovation process

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    Today innovation is recognised as the main driving force in the market. This complex process involves several intangible dimensions, such as creativity, knowledge and social interactions among others. Creativity is the starting point of the process, and knowledge is the force that transforms and materialises creativity in new products, services and processes. In this paper a synergy that aims to assists the innovation process is presented. The synergy combines several concepts and tools of the theory of inventive problem solving (TRIZ) and the case-based reasoning (CBR) process. The main objective of this synergy is to support creative engineering design and problem solving. This synergy is based on the strong link between knowledge and action. In this link, TRIZ offers several concepts and tools to facilitate concept creation and to solve problems, and the CBR process offers a framework capable of storing and reusing knowledge with the aim of accelerating the innovation process

    Simulation of the Eemian Greenland ice sheet

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    This thesis focuses on the simulation of the Greenland ice sheet (GrIS) during the Eemian interglacial period (~125,000 years ago). The warm Eemian summers on Greenland are used as a past analogue for future warmer conditions. The aim of this work is a contribution to the improvement of future sea level rise predictions and to better understand how model uncertainties propagate through the chain of models necessary to simulate ice sheet evolution in past climates. Firstly, the influence of surface mass balance (SMB) models and climate model resolution on the simulation of the Eemian SMB is investigated. The corresponding study shows that both, the selection of the SMB model as well as the climate model resolution are essential for simulating the Eemian SMB, and either of these two factors can have a dominating effect on the results. However, which factor dominates the results depends on the climate state (cold or warm) and particularly the prevailing insolation regime. It is shown that an inclusion of insolation in the selected SMB model is essential for the simulated warm early Eemian conditions. Secondly, the influence of SMB forcing on millennial time scale ice sheet modeling is tested. The simulations with two different SMB forcings reveal a large difference in the evolution of the ice sheet, while ice flow sensitivity tests with changed basal friction and changed ice flow approximation show small differences. Thirdly, regional climate simulations with a full surface energy balance model are analyzed focusing on Greenland surface melt. This analysis shows that all Greenland ice core locations, also GRIP near the summit of Greenland, are affected by surface melt during the Eemian interglacial period. Elevated levels of Eemian surface melt indicate that ice cores might be affected more strongly than previously considered. Therefore, caution needs to be applied when interpreting Greenland ice core records from warm periods such as the Eemian interglacial period. This thesis shows that forcing from a single climate model can lead to a wide range of SMBs and ice sheets. To quantify this large uncertainty, a systematic approach of model intercomparison, similar to what is used to constrain future climate projections, is advised. Climate and SMB model biases and uncertainties need to be explored and outliers rejected, to be able to provide a most likely range for the Eemian GrIS topography and its contribution to sea level
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