Pinus uncinata Ramond ex DC.: Conservation and proposal of management at its southwestern limit of distribution (Castillo de Vinuesa, Soria, Spain)

Pinus uncinata forms forests in the centre and southwest of the Alps and in the subalpine Pyrenees (at around 1700 – 2600 m) (Costa Tenorio et al., 1997). The species reaches the southwestern limit of its distribution at the top of Mount Castillo de Vinuesa (Soria, Spain). The small population on this mountain occupies just 66 ha, but is very important from a geobotanical viewpoint since it is just one of two populations (the other being in the Sierra de Gúdar range in Teruel, Spain) isolated from the main area where the species is found in the Iberian Peninsula (The Pyrenees)

Holocene distribution of woody taxa at the westernmost limit of the Circumboreal/Mediterranean boundary: evidence from wood remains

Información paleoecológica relativa a la distribución de táxones arbóreos durante el holoceno, basado en restos fósiles leñosos.

Critical comparative evaluation of design methodologies for the ultimate limit state of fatigue in concrete structures

La aparición de la fatiga ha sido ampliamente investigada en el acero y en otros materiales metálicos, sin embargo no se conoce en tanta profundidad en el hormigón estructural. Esto crea falta de uniformidad y enfoque en el proceso de verificación de estructuras de hormigón para el estado límite último de la fatiga. A medida que se llevan a cabo más investigaciones, la información sobre los parámetros que afectan a la fatiga en el hormigón comienzan a ser difundidos e incluso los que les afectan de forma indirecta. Esto conlleva a que se estén incorporando en las guías de diseño de todo el mundo, a pesar de que la comprobación del estado límite último no se trata por igual entre los distintos órganos de diseño. Este trabajo presentará un conocimiento básico del fenómeno de la fatiga, qué lo causa y qué condiciones de carga o propiedades materiales amplían o reducen la probabilidad de fallo por fatiga. Cuatro distintos códigos de diseño serán expuestos y su proceso de verificación ha sido examinado, comparados y valorados cualitativa y cuantitativamente. Una torre eólica, como ejemplo, fue analizada usando los procedimientos de verificación como se indica en sus respectivos códigos de referencia. The occurrence of fatigue has been extensively researched in steel and other metallic materials it is however, not as broadly understood in concrete. This produces a lack of uniformity in the approach and process in the verification of concrete structures for the ultimate limit state of fatigue. As more research is conducted and more information is known about the parameters which cause, propagate, and indirectly affect fatigue in concrete, they are incorporated in design guides around the world. Nevertheless, this ultimate limit state verification is not addressed equally by various design governing bodies. This report presents a baseline understanding of what the phenomenon of fatigue is, what causes it, and what loading or material conditions amplify or reduce the likelihood of fatigue failure. Four different design codes are exposed and their verification process has been examined, compared and evaluated both qualitatively and quantitatively. Using a wind turbine tower structure as case study, this report presents calculated results following the verification processes as instructed in the respective reference codes.

Architectures for Video-on-Demand content distribution in managed networks

La demanda de contenidos de vídeo ha aumentado rápidamente en los últimos años como resultado del gran despliegue de la TV sobre IP (IPTV) y la variedad de servicios ofrecidos por los operadores de red. Uno de los servicios que se ha vuelto especialmente atractivo para los clientes es el vídeo bajo demanda (VoD) en tiempo real, ya que ofrece una transmisión (streaming) inmediata de gran variedad de contenidos de vídeo. El precio que los operadores tienen que pagar por este servicio es el aumento del tráfico en las redes, que están cada vez más congestionadas debido a la mayor demanda de contenidos de VoD y al aumento de la calidad de los propios contenidos de vídeo. Así, uno de los principales objetivos de esta tesis es encontrar soluciones que reduzcan el tráfico en el núcleo de la red, manteniendo la calidad del servicio en el nivel adecuado y reduciendo el coste del tráfico. La tesis propone un sistema jerárquico de servidores de streaming en el que se ejecuta un algoritmo para la ubicación óptima de los contenidos de acuerdo con el comportamiento de los usuarios y el estado de la red. Debido a que cualquier algoritmo óptimo de distribución de contenidos alcanza un límite en el que no se puede llegar a nuevas mejoras, la inclusión de los propios clientes del servicio (los peers) en el proceso de streaming puede reducir aún más el tráfico de red. Este proceso se logra aprovechando el control que el operador tiene en las redes de gestión privada sobre los equipos receptores (Set-Top Box) ubicados en las instalaciones de los clientes. El operador se reserva cierta capacidad de almacenamiento y streaming de los peers para almacenar los contenidos de vídeo y para transmitirlos a otros clientes con el fin de aliviar a los servidores de streaming. Debido a la incapacidad de los peers para sustituir completamente a los servidores de streaming, la tesis propone un sistema de streaming asistido por peers. Algunas de las cuestiones importantes que se abordan en la tesis son saber cómo los parámetros del sistema y las distintas distribuciones de los contenidos de vídeo en los peers afectan al rendimiento general del sistema. Para dar respuesta a estas preguntas, la tesis propone un modelo estocástico preciso y flexible que tiene en cuenta parámetros como las capacidades de enlace de subida y de almacenamiento de los peers, el número de peers, el tamaño de la biblioteca de contenidos de vídeo, el tamaño de los contenidos y el esquema de distribución de contenidos para estimar los beneficios del streaming asistido por los peers. El trabajo también propone una versión extendida del modelo matemático mediante la inclusión de la probabilidad de fallo de los peers y su tiempo de recuperación en el conjunto de parámetros del modelo. Estos modelos se utilizan como una herramienta para la realización de exhaustivos análisis del sistema de streaming de VoD asistido por los peers para la amplia gama de parámetros definidos en los modelos. Abstract The demand of video contents has rapidly increased in the past years as a result of the wide deployment of IPTV and the variety of services offered by the network operators. One of the services that has especially become attractive to the customers is real-time Video on Demand (VoD) because it offers an immediate streaming of a large variety of video contents. The price that the operators have to pay for this convenience is the increased traffic in the networks, which are becoming more congested due to the higher demand for VoD contents and the increased quality of the videos. Therefore, one of the main objectives of this thesis is finding solutions that would reduce the traffic in the core of the network, keeping the quality of service on satisfactory level and reducing the traffic cost. The thesis proposes a system of hierarchical structure of streaming servers that runs an algorithm for optimal placement of the contents according to the users’ behavior and the state of the network. Since any algorithm for optimal content distribution reaches a limit upon which no further improvements can be made, including service customers themselves (the peers) in the streaming process can further reduce the network traffic. This process is achieved by taking advantage of the control that the operator has in the privately managed networks over the Set-Top Boxes placed at the clients’ premises. The operator reserves certain storage and streaming capacity on the peers to store the video contents and to stream them to the other clients in order to alleviate the streaming servers. Because of the inability of the peers to completely substitute the streaming servers, the thesis proposes a system for peer-assisted streaming. Some of the important questions addressed in the thesis are how the system parameters and the various distributions of the video contents on the peers would impact the overall system performance. In order to give answers to these questions, the thesis proposes a precise and flexible stochastic model that takes into consideration parameters like uplink and storage capacity of the peers, number of peers, size of the video content library, size of contents and content distribution scheme to estimate the benefits of the peer-assisted streaming. The work also proposes an extended version of the mathematical model by including the failure probability of the peers and their recovery time in the set of parameters. These models are used as tools for conducting thorough analyses of the peer-assisted system for VoD streaming for the wide range of defined parameters.

Precipitated iron: a limit on gettering efficacy in multicrystalline silicon

A phosphorus diffusion gettering model is used to examine the efficacy of a standard gettering process on interstitial and precipitated iron in multicrystalline silicon. The model predicts a large concentration of precipitated iron remaining after standard gettering for most as-grown iron distributions. Although changes in the precipitated iron distribution are predicted to be small, the simulated post-processing interstitial iron concentration is predicted to depend strongly on the as-grown distribution of precipitates, indicating that precipitates must be considered as internal sources of contamination during processing. To inform and validate the model, the iron distributions before and after a standard phosphorus diffusion step are studied in samples from the bottom, middle, and top of an intentionally Fe-contaminated laboratory ingot. A census of iron-silicide precipitates taken by synchrotron-based X-ray fluorescence microscopy confirms the presence of a high density of iron-silicide precipitates both before and after phosphorus diffusion. A comparable precipitated iron distribution was measured in a sister wafer after hydrogenation during a firing step. The similar distributions of precipitated iron seen after each step in the solar cell process confirm that the effect of standard gettering on precipitated iron is strongly limited as predicted by simulation. Good agreement between the experimental and simulated data supports the hypothesis that gettering kinetics is governed by not only the total iron concentration but also by the distribution of precipitated iron. Finally, future directions based on the modeling are suggested for the improvement of effective minority carrier lifetime in multicrystalline silicon solar cells.

Factors influencing metal price selection in mining feasibility studies

The mineral price assigned in mining project design is critical to determining the economic feasibility of a project. Nevertheless, although it is not difficult to find literature about market metal prices, it is much more complicated to achieve a specific methodology for calculating the value or which justifications are appropriate to include. This study presents an analysis of various methods for selecting metal prices and investigates the mechanisms and motives underlying price selections. The results describe various attitudes adopted by the designers of mining investment projects, and how the price can be determined not just by means of forecasting but also by consideration of other relevant parameters.

Analysis of the Iberian Power Forward Price Formation

The price formation of the Iberian Energy Derivatives Market-the power futures market-starting in July 2006, is assessed until November 2011, through the evolution of the difference between forward and spot prices in the delivery period (“ex-post forward risk premium”) and the comparison with the forward generation costs from natural gas (“clean spark spread”). The premium tends to be positive in all existing mechanisms (futures, Over-the-Counter and auctions for catering part of the last resort supplies). Since year 2011, the values are smaller due to regulatorily recognized prices for coal power plants. The power futures are strongly correlated with European gas prices. The spreads built with prompt contracts tend also to be positive. The biggest ones are for the month contract, followed by the quarter contract and then by the year contract. Therefore, gas fired generation companies can maximize profits trading with contracts of shorter maturity.

An Empirical Analysis of the Spanish Gas Price Structure

We can say without hesitation that in energy markets a throughout data analysis is crucial when designing sophisticated models that are able to capture most of the critical market drivers. In this study we will attempt to investigate into Spanish natural gas prices structure to improve understanding of the role they play in the determination of electricity prices and decide in the future about price modelling aspects. To further understand the potential for modelling, this study will focus on the nature and characteristics of the different gas price data available. The fact that the existing gas market in Spain does not incorporate enough liquidity of trade makes it even more critical to analyze in detail available gas price data information that in the end will provide relevant information to understand how electricity prices are affected by natural gas markets. In this sense representative Spanish gas prices are typically difficult to explore given the fact that there is not a transparent gas market yet and all the gas imported in the country is negotiated and purchased by private companies at confidential terms.

Electricity price forecasting accounting for renewable energies: optimal combined forecasts

Electricity price forecasting is an interesting problem for all the agents involved in electricity market operation. For instance, every profit maximisation strategy is based on the computation of accurate one-day-ahead forecasts, which is why electricity price forecasting has been a growing field of research in recent years. In addition, the increasing concern about environmental issues has led to a high penetration of renewable energies, particularly wind. In some European countries such as Spain, Germany and Denmark, renewable energy is having a deep impact on the local power markets. In this paper, we propose an optimal model from the perspective of forecasting accuracy, and it consists of a combination of several univariate and multivariate time series methods that account for the amount of energy produced with clean energies, particularly wind and hydro, which are the most relevant renewable energy sources in the Iberian Market. This market is used to illustrate the proposed methodology, as it is one of those markets in which wind power production is more relevant in terms of its percentage of the total demand, but of course our method can be applied to any other liberalised power market. As far as our contribution is concerned, first, the methodology proposed by García-Martos et al(2007 and 2012) is generalised twofold: we allow the incorporation of wind power production and hydro reservoirs, and we do not impose the restriction of using the same model for 24h. A computational experiment and a Design of Experiments (DOE) are performed for this purpose. Then, for those hours in which there are two or more models without statistically significant differences in terms of their forecasting accuracy, a combination of forecasts is proposed by weighting the best models(according to the DOE) and minimising the Mean Absolute Percentage Error (MAPE). The MAPE is the most popular accuracy metric for comparing electricity price forecasting models. We construct the combi nation of forecasts by solving several nonlinear optimisation problems that allow computation of the optimal weights for building the combination of forecasts. The results are obtained by a large computational experiment that entails calculating out-of-sample forecasts for every hour in every day in the period from January 2007 to Decem ber 2009. In addition, to reinforce the value of our methodology, we compare our results with those that appear in recent published works in the field. This comparison shows the superiority of our methodology in terms of forecasting accuracy.