Influencia de las restricciones medioambientales en el sector eléctrico

El objetivo de esta tesis es la caracterización de la generación térmica representativa de la existente en la realidad, para posteriormente proceder a su modelización y simulación integrándolas en una red eléctrica tipo y llevar a cabo estudios de optimización multiobjetivo económico medioambiental. Para ello, en primera instancia se analiza el contexto energético y eléctrico actual, y más concretamente el peninsular, en el que habiendo desaparecido las centrales de fuelóleo, sólo quedan ciclos combinados y centrales de carbón de distinto rango. Seguidamente se lleva a cabo un análisis de los principales impactos medioambientales de las centrales eléctricas basadas en combustión, representados sobre todo por sus emisiones de CO2, SO2 y NOx, de las medidas de control y mitigación de las mismas y de la normativa que les aplica. A continuación, a partir de las características de los combustibles y de la información de los consumos específicos, se caracterizan los grupos térmicos frente a las funciones relevantes que definen su comportamiento energético, económico y medioambiental, en términos de funciones de salida horarias dependiendo de la carga. Se tiene en cuenta la posibilidad de desnitrificación y desulfuración. Dado que las funciones objetivo son múltiples, y que están en conflicto unas con otras, se ha optado por usar métodos multiobjetivo que son capaces de identificar el contorno de puntos óptimos o frente de Pareto, en los que tomando una solución no existe otra que lo mejore en alguna de las funciones objetivo sin empeorarlo en otra. Se analizaron varios métodos de optimización multiobjetivo y se seleccionó el de las ε constraint, capaz de encontrar frentes no convexos y cuya optimalidad estricta se puede comprobar. Se integró una representación equilibrada de centrales de antracita, hulla nacional e importada, lignito y ciclos combinados en la red tipo IEEE-57, en la que se puede trabajar con siete centrales sin distorsionar demasiado las potencias nominales reales de los grupos, y se programó en Matlab la resolución de flujos óptimos de carga en alterna con el método multiobjetivo integrado. Se identifican los frentes de Pareto de las combinaciones de coste y cada uno de los tres tipos de emisión, y también el de los cuatro objetivos juntos, obteniendo los resultados de costes óptimos del sistema para todo el rango de emisiones. Se valora cuánto le cuesta al sistema reducir una tonelada adicional de cualquier tipo de emisión a base de desplazarse a combinaciones de generación más limpias. Los puntos encontrados aseguran que bajo unas determinadas emisiones no pueden ser mejorados económicamente, o que atendiendo a ese coste no se puede reducir más allá el sistema en lo relativo a emisiones. También se indica cómo usar los frentes de Pareto para trazar estrategias óptimas de producción ante cambios horarios de carga. ABSTRACT The aim of this thesis is the characterization of electrical generation based on combustion processes representative of the actual power plants, for the latter modelling and simulation of an electrical grid and the development of economic- environmental multiobjective optimization studies. In this line, the first step taken is the analysis of the current energetic and electrical framework, focused on the peninsular one, where the fuel power plants have been shut down, and the only ones remaining are coal units of different types and combined cycle. Then it is carried out an analysis of the main environmental impacts of the thermal power plants, represented basically by the emissions of CO2, SO2 y NOx, their control and reduction measures and the applicable regulations. Next, based on the combustibles properties and the information about the units heat rates, the different power plants are characterized in relation to the outstanding functions that define their energy, economic and environmental behaviour, in terms of hourly output functions depending on their load. Optional denitrification and desulfurization is considered. Given that there are multiple objectives, and that they go in conflictive directions, it has been decided the use of multiobjective techniques, that have the ability of identifying the optimal points set, which is called the Pareto front, where taken a solution there will be no other point that can beat the former in an objective without worsening it in another objective. Several multiobjective optimization methods were analysed and pondered, selecting the ε constraint technique, which is able to find no convex fronts and it is opened to be tested to prove the strict Pareto optimality of the obtained solutions. A balanced representation of the thermal power plants, formed by anthracite, lignite, bituminous national and imported coals and combined cycle, was integrated in the IEEE-57 network case. This system was selected because it deals with a total power that will admit seven units without distorting significantly the actual size of the power plants. Next, an AC optimal power flow with the multiobjective method implemented in the routines was programmed. The Pareto fronts of the combination of operative costs with each of the three emissions functions were found, and also the front of all of them together. The optimal production costs of the system for all the emissions range were obtained. It is also evaluated the cost of reducing an additional emission ton of any of the emissions when the optimal production mix is displaced towards cleaner points. The obtained solutions assure that under a determined level of emissions they cannot be improved economically or, in the other way, at a determined cost it cannot be found points of lesser emissions. The Pareto fronts are also applied for the search of optimal strategic paths to follow the hourly load changes.

Transport and mixing enhancement in fluid-thermal microsystems

En esta tesis se investiga de forma experimental el transporte pasivo de magnitudes físicas en micro-sistemas con carácter de inmediata aplicación industrial, usando métodos innovadores para mejorar la eficiencia de los mismos optimizando parámetros críticos del diseño o encontrar nuevos destinos de posible aplicación. Parte de los resultados obtenidos en estos experimentos han sido publicados en revistas con un índice de impacto tal que pertenecen al primer cuarto del JCR. Primero de todo se ha analizado el efecto que produce en un intercambiador de calor basado en micro-canales el hecho de dejar un espacio entre canales y tapa superior para la interconexión de los mismos. Esto genera efectos tridimensionales que mejoran la exracción de calor del intercambiador y reducen la caída de presión que aparece por el transcurso del fluido a través de los micro-canales, lo que tiene un gran impacto en la potencia que ha de suministrar la bomba de refrigerante. Se ha analizado también la mejora producida en términos de calor disipado de un micro-procesador refrigerado con un ampliamente usado plato de aletas al implementar en éste una cámara de vapor que almacena un fluido bifásico. Se ha desarrollado de forma paralela un modelo numérico para optimizar las nuevas dimensiones del plato de aletas modificado compatibles con una serie de requerimientos de diseño en el que tanto las dimensiones como el peso juegan un papel esencial. Por otro lado, se han estudiado los fenomenos fluido-dinámicos que aparecen aguas abajo de un cuerpo romo en el seno de un fluido fluyendo por un canal con una alta relación de bloqueo. Los resultados de este estudio confirman, de forma experimental, la existencia de un régimen intermedio, caracterizado por el desarrollo de una burbuja de recirculación oscilante entre los regímenes, bien diferenciados, de burbuja de recirculación estacionaria y calle de torbellinos de Karman, como función del número de Reynolds del flujo incidente. Para la obtención, análisis y post-proceso de los datos, se ha contado con la ayuda de un sistema de Velocimetría por Imágenes de Partículas (PIV). Finalmente y como adición a este último punto, se ha estudiado las vibraciones de un cuerpo romo producidas por el desprendimiento de torbellinos en un canal de alta relación de bloqueo con la base obtenida del estudio anterior. El prisma se mueve con un movimiento armónico simple para un intervalo de números de Reynolds y este movimiento se transforma en vibración alrededor de su eje a partir de un ciero número de Reynolds. En relación al fluido, el régimen de desprendimiento de torbellinos se alcanza a menores números de Reynolds que en el caso de tener el cuerpo romo fijo. Uniendo estos dos registros de movimientos y variando la relación de masas entre prisma y fluido se obtiene un mapa con diferentes estados globales del sistema. Esto no solo tiene aplicación como método para promover el mezclado sino también como método para obtener energía a partir del movimiento del cuerpo en el seno del fluido. Abstract In this thesis, experimental research focused on passive scalar transport is performed in micro-systems with marked sense of industrial application, using innovative methods in order to obtain better performances optimizing critical design parameters or finding new utilities. Part of the results obtained in these experiments have been published into high impact factor journals belonged to the first quarter of the Journal Citation Reports (JCR). First of all the effect of tip clearance in a micro-channel based heat sink is analyzed. Leaving a gap between channels and top cover, letting the channels communicate each other causes three-dimensional effects which improve the heat transfer between fluid and heat sink and also reducing the pressure drop caused by the fluid passing through the micro-channels which has a great impact on the total cooling pumping power needed. It is also analyzed the enhancement produced in terms of dissipated heat in a micro-processor cooling system by improving the predominantly used fin plate with a vapour chamber based heat spreader which contains a two-phase fluid inside. It has also been developed at the same time a numerical model to optimize the new fin plate dimensions compatible with a series of design requirements in which both size and wight plays a very restrictive role. On the other hand, fluid-dynamics phenomena that appears downstream of a bluff body in the bosom of a fluid flow with high blockage ratio has been studied. This research experimentally confirms the existence of an intermediate regime characterized by an oscillating closed recirculation bubble intermediate regime between the steady closed recirculation bubble regime and the vortex shedding regime (Karman street like regime) as a function of the incoming flow Reynolds number. A particle image velocimetry technique (PIV) has been used in order to obtain, analyze and post-process the fluid-dynamic data. Finally and as an addition to the last point, a study on the vortexinduced vibrations (VIV) of a bluff body inside a high blockage ratio channel has been carried out taking advantage of the results obtained with the fixed square prism. The prism moves with simple harmonic motion for a Reynolds number interval and this movement becomes vibrational around its axial axis after overcoming at definite Reynolds number. Regarding the fluid, vortex shedding regime is reached at Reynolds numbers lower than the previous critical ones. Merging both movement spectra and varying the square prism to fluid mass ratio, a map with different global states is reached. This is not only applicable as a mixing enhancement technique but as an energy harvesting method.

Thermal behaviour of Sunrise, a balloon-borne solar Telescope

Sunrise is a solar telescope, successfully flown in June 2009 with a long duration balloon from the Swedish Space Corporation Esrange launch site. The design of the thermal control of SUNRISE was quite critical because of the sensitivity to temperature of the optomechanical devices and the electronics. These problems got more complicated due the size and high power dissipation of the system. A detailed thermal mathematical model of SUNRISE was set up to predict temperatures. In this communication the thermal behaviour of SUNRISE during flight is presented. Flight temperatures of some devices are presented and analysed. The measured data have been compared with the predictions given by the thermal mathematical models. The main discrepancies between flight data and the temperatures predicted by the models have been identified. This allows thermal engineers to improve the knowledge of the thermal behaviour of the system for future missions.

Thermal effects in Ni/Au and Mo/Au gate metallization AlGaN/GaN HEMT's reliability

AlGaN/GaN high electron mobility transistors (HEMT) are key devices for the next generation of high-power, high-frequency and high-temperature electronics applications. Although significant progress has been recently achieved [1], stability and reliability are still some of the main issues under investigation, particularly at high temperatures [2-3]. Taking into account that the gate contact metallization is one of the weakest points in AlGaN/GaN HEMTs, the reliability of Ni, Mo, Pt and refractory metal gates is crucial [4-6]. This work has been focused on the thermal stress and reliability assessment of AlGaN/GaN HEMTs. After an unbiased storage at 350 o C for 2000 hours, devices with Ni/Au gates exhibited detrimental IDS-VDS degradation in pulsed mode. In contrast, devices with Mo/Au gates showed no degradation after similar storage conditions. Further capacitance-voltage characterization as a function of temperature and frequency revealed two distinct trap-related effects in both kinds of devices. At low frequency (< 1MHz), increased capacitance near the threshold voltage was present at high temperatures and more pronounced for the Ni/Au gate HEMT and as the frequency is lower. Such an anomalous “bump” has been previously related to H-related surface polar charges [7]. This anomalous behavior in the C-V characteristics was also observed in Mo/Au gate HEMTs after 1000 h at a calculated channel temperatures of around from 250 o C (T2) up to 320 ºC (T4), under a DC bias (VDS= 25 V, IDS= 420 mA/mm) (DC-life test). The devices showed a higher “bump” as the channel temperature is higher (Fig. 1). At 1 MHz, the higher C-V curve slope of the Ni/Au gated HEMTs indicated higher trap density than Mo/Au metallization (Fig. 2). These results highlight that temperature is an acceleration factor in the device degradation, in good agreement with [3]. Interface state density analysis is being performed in order to estimate the trap density and activation energy.

On-Chip Thermal Monitoring: Design, Placement and Interconnection of Temperature Sensors

La temperatura es una preocupación que juega un papel protagonista en el diseño de circuitos integrados modernos. El importante aumento de las densidades de potencia que conllevan las últimas generaciones tecnológicas ha producido la aparición de gradientes térmicos y puntos calientes durante el funcionamiento normal de los chips. La temperatura tiene un impacto negativo en varios parámetros del circuito integrado como el retardo de las puertas, los gastos de disipación de calor, la fiabilidad, el consumo de energía, etc. Con el fin de luchar contra estos efectos nocivos, la técnicas de gestión dinámica de la temperatura (DTM) adaptan el comportamiento del chip en función en la información que proporciona un sistema de monitorización que mide en tiempo de ejecución la información térmica de la superficie del dado. El campo de la monitorización de la temperatura en el chip ha llamado la atención de la comunidad científica en los últimos años y es el objeto de estudio de esta tesis. Esta tesis aborda la temática de control de la temperatura en el chip desde diferentes perspectivas y niveles, ofreciendo soluciones a algunos de los temas más importantes. Los niveles físico y circuital se cubren con el diseño y la caracterización de dos nuevos sensores de temperatura especialmente diseñados para los propósitos de las técnicas DTM. El primer sensor está basado en un mecanismo que obtiene un pulso de anchura variable dependiente de la relación de las corrientes de fuga con la temperatura. De manera resumida, se carga un nodo del circuito y posteriormente se deja flotando de tal manera que se descarga a través de las corrientes de fugas de un transistor; el tiempo de descarga del nodo es la anchura del pulso. Dado que la anchura del pulso muestra una dependencia exponencial con la temperatura, la conversión a una palabra digital se realiza por medio de un contador logarítmico que realiza tanto la conversión tiempo a digital como la linealización de la salida. La estructura resultante de esta combinación de elementos se implementa en una tecnología de 0,35 _m. El sensor ocupa un área muy reducida, 10.250 nm2, y consume muy poca energía, 1.05-65.5nW a 5 muestras/s, estas cifras superaron todos los trabajos previos en el momento en que se publicó por primera vez y en el momento de la publicación de esta tesis, superan a todas las implementaciones anteriores fabricadas en el mismo nodo tecnológico. En cuanto a la precisión, el sensor ofrece una buena linealidad, incluso sin calibrar; se obtiene un error 3_ de 1,97oC, adecuado para tratar con las aplicaciones de DTM. Como se ha explicado, el sensor es completamente compatible con los procesos de fabricación CMOS, este hecho, junto con sus valores reducidos de área y consumo, lo hacen especialmente adecuado para la integración en un sistema de monitorización de DTM con un conjunto de monitores empotrados distribuidos a través del chip. Las crecientes incertidumbres de proceso asociadas a los últimos nodos tecnológicos comprometen las características de linealidad de nuestra primera propuesta de sensor. Con el objetivo de superar estos problemas, proponemos una nueva técnica para obtener la temperatura. La nueva técnica también está basada en las dependencias térmicas de las corrientes de fuga que se utilizan para descargar un nodo flotante. La novedad es que ahora la medida viene dada por el cociente de dos medidas diferentes, en una de las cuales se altera una característica del transistor de descarga |la tensión de puerta. Este cociente resulta ser muy robusto frente a variaciones de proceso y, además, la linealidad obtenida cumple ampliamente los requisitos impuestos por las políticas DTM |error 3_ de 1,17oC considerando variaciones del proceso y calibrando en dos puntos. La implementación de la parte sensora de esta nueva técnica implica varias consideraciones de diseño, tales como la generación de una referencia de tensión independiente de variaciones de proceso, que se analizan en profundidad en la tesis. Para la conversión tiempo-a-digital, se emplea la misma estructura de digitalización que en el primer sensor. Para la implementación física de la parte de digitalización, se ha construido una biblioteca de células estándar completamente nueva orientada a la reducción de área y consumo. El sensor resultante de la unión de todos los bloques se caracteriza por una energía por muestra ultra baja (48-640 pJ) y un área diminuta de 0,0016 mm2, esta cifra mejora todos los trabajos previos. Para probar esta afirmación, se realiza una comparación exhaustiva con más de 40 propuestas de sensores en la literatura científica. Subiendo el nivel de abstracción al sistema, la tercera contribución se centra en el modelado de un sistema de monitorización que consiste de un conjunto de sensores distribuidos por la superficie del chip. Todos los trabajos anteriores de la literatura tienen como objetivo maximizar la precisión del sistema con el mínimo número de monitores. Como novedad, en nuestra propuesta se introducen nuevos parámetros de calidad aparte del número de sensores, también se considera el consumo de energía, la frecuencia de muestreo, los costes de interconexión y la posibilidad de elegir diferentes tipos de monitores. El modelo se introduce en un algoritmo de recocido simulado que recibe la información térmica de un sistema, sus propiedades físicas, limitaciones de área, potencia e interconexión y una colección de tipos de monitor; el algoritmo proporciona el tipo seleccionado de monitor, el número de monitores, su posición y la velocidad de muestreo _optima. Para probar la validez del algoritmo, se presentan varios casos de estudio para el procesador Alpha 21364 considerando distintas restricciones. En comparación con otros trabajos previos en la literatura, el modelo que aquí se presenta es el más completo. Finalmente, la última contribución se dirige al nivel de red, partiendo de un conjunto de monitores de temperatura de posiciones conocidas, nos concentramos en resolver el problema de la conexión de los sensores de una forma eficiente en área y consumo. Nuestra primera propuesta en este campo es la introducción de un nuevo nivel en la jerarquía de interconexión, el nivel de trillado (o threshing en inglés), entre los monitores y los buses tradicionales de periféricos. En este nuevo nivel se aplica selectividad de datos para reducir la cantidad de información que se envía al controlador central. La idea detrás de este nuevo nivel es que en este tipo de redes la mayoría de los datos es inútil, porque desde el punto de vista del controlador sólo una pequeña cantidad de datos |normalmente sólo los valores extremos| es de interés. Para cubrir el nuevo nivel, proponemos una red de monitorización mono-conexión que se basa en un esquema de señalización en el dominio de tiempo. Este esquema reduce significativamente tanto la actividad de conmutación sobre la conexión como el consumo de energía de la red. Otra ventaja de este esquema es que los datos de los monitores llegan directamente ordenados al controlador. Si este tipo de señalización se aplica a sensores que realizan conversión tiempo-a-digital, se puede obtener compartición de recursos de digitalización tanto en tiempo como en espacio, lo que supone un importante ahorro de área y consumo. Finalmente, se presentan dos prototipos de sistemas de monitorización completos que de manera significativa superan la características de trabajos anteriores en términos de área y, especialmente, consumo de energía. Abstract Temperature is a first class design concern in modern integrated circuits. The important increase in power densities associated to recent technology evolutions has lead to the apparition of thermal gradients and hot spots during run time operation. Temperature impacts several circuit parameters such as speed, cooling budgets, reliability, power consumption, etc. In order to fight against these negative effects, dynamic thermal management (DTM) techniques adapt the behavior of the chip relying on the information of a monitoring system that provides run-time thermal information of the die surface. The field of on-chip temperature monitoring has drawn the attention of the scientific community in the recent years and is the object of study of this thesis. This thesis approaches the matter of on-chip temperature monitoring from different perspectives and levels, providing solutions to some of the most important issues. The physical and circuital levels are covered with the design and characterization of two novel temperature sensors specially tailored for DTM purposes. The first sensor is based upon a mechanism that obtains a pulse with a varying width based on the variations of the leakage currents on the temperature. In a nutshell, a circuit node is charged and subsequently left floating so that it discharges away through the subthreshold currents of a transistor; the time the node takes to discharge is the width of the pulse. Since the width of the pulse displays an exponential dependence on the temperature, the conversion into a digital word is realized by means of a logarithmic counter that performs both the timeto- digital conversion and the linearization of the output. The structure resulting from this combination of elements is implemented in a 0.35_m technology and is characterized by very reduced area, 10250 nm2, and power consumption, 1.05-65.5 nW at 5 samples/s, these figures outperformed all previous works by the time it was first published and still, by the time of the publication of this thesis, they outnumber all previous implementations in the same technology node. Concerning the accuracy, the sensor exhibits good linearity, even without calibration it displays a 3_ error of 1.97oC, appropriate to deal with DTM applications. As explained, the sensor is completely compatible with standard CMOS processes, this fact, along with its tiny area and power overhead, makes it specially suitable for the integration in a DTM monitoring system with a collection of on-chip monitors distributed across the chip. The exacerbated process fluctuations carried along with recent technology nodes jeop-ardize the linearity characteristics of the first sensor. In order to overcome these problems, a new temperature inferring technique is proposed. In this case, we also rely on the thermal dependencies of leakage currents that are used to discharge a floating node, but now, the result comes from the ratio of two different measures, in one of which we alter a characteristic of the discharging transistor |the gate voltage. This ratio proves to be very robust against process variations and displays a more than suficient linearity on the temperature |1.17oC 3_ error considering process variations and performing two-point calibration. The implementation of the sensing part based on this new technique implies several issues, such as the generation of process variations independent voltage reference, that are analyzed in depth in the thesis. In order to perform the time-to-digital conversion, we employ the same digitization structure the former sensor used. A completely new standard cell library targeting low area and power overhead is built from scratch to implement the digitization part. Putting all the pieces together, we achieve a complete sensor system that is characterized by ultra low energy per conversion of 48-640pJ and area of 0.0016mm2, this figure outperforms all previous works. To prove this statement, we perform a thorough comparison with over 40 works from the scientific literature. Moving up to the system level, the third contribution is centered on the modeling of a monitoring system consisting of set of thermal sensors distributed across the chip. All previous works from the literature target maximizing the accuracy of the system with the minimum number of monitors. In contrast, we introduce new metrics of quality apart form just the number of sensors; we consider the power consumption, the sampling frequency, the possibility to consider different types of monitors and the interconnection costs. The model is introduced in a simulated annealing algorithm that receives the thermal information of a system, its physical properties, area, power and interconnection constraints and a collection of monitor types; the algorithm yields the selected type of monitor, the number of monitors, their position and the optimum sampling rate. We test the algorithm with the Alpha 21364 processor under several constraint configurations to prove its validity. When compared to other previous works in the literature, the modeling presented here is the most complete. Finally, the last contribution targets the networking level, given an allocated set of temperature monitors, we focused on solving the problem of connecting them in an efficient way from the area and power perspectives. Our first proposal in this area is the introduction of a new interconnection hierarchy level, the threshing level, in between the monitors and the traditional peripheral buses that applies data selectivity to reduce the amount of information that is sent to the central controller. The idea behind this new level is that in this kind of networks most data are useless because from the controller viewpoint just a small amount of data |normally extreme values| is of interest. To cover the new interconnection level, we propose a single-wire monitoring network based on a time-domain signaling scheme that significantly reduces both the switching activity over the wire and the power consumption of the network. This scheme codes the information in the time domain and allows a straightforward obtention of an ordered list of values from the maximum to the minimum. If the scheme is applied to monitors that employ TDC, digitization resource sharing is achieved, producing an important saving in area and power consumption. Two prototypes of complete monitoring systems are presented, they significantly overcome previous works in terms of area and, specially, power consumption.

Concentrating solar power technologies. The DESERTEC megaproject

The European strategies on energy have been searching for years to reduce the dependency of Europe from fossil fuels. Underlying this effort, there exist geopolitical, economic, environmental reasons and the reality that oil reservoirs will dry out some day. Renewable energies have become a milestone of this strategy because their huge potential has emerged after years of uncertainty. One of the better developed renewable sources, which is nearer to commercial maturity is solar-thermal energy. In this paper, the current state of this technology will be described as well as the developments that may be expected in the short and mid terms, including the thermoelectric solar megaproject DESERTEC, a German proposal to ensure energy resources to the mayor areas of the EU-MENA countries. The reader will acquire a picture of the current state of the market, of the technical challenges already achieved and of the remaining ones.

Thermal and mechanical effects of different excitation modes based on low frequency laser modulation in optical hyperthermia

The low frequency modulation of the laser source (menor que30KHz) allows the generation of a pulsed signal that intermittently excites the gold nanorods. The temperature curves obtained for different frequencies and duty cycles of modulation but with equal average power and identical laser parameters, show that the thermal behavior in continuous wave and modulation modes is the same. However, the cell death experiments suggest that the percentage of death is higher in the cases of modulation. This observation allows us to conclude that there are other effects in addition to temperature that contribute to the cellular death. The mechanical effects like sound or pressure waves are expected to be generated from thermal expansion of gold nanorods. In order to study the behavior and magnitude of these processes we have developed a measure device based on ultrasound piezoelectric receivers (25KHz) and a lock-in amplifier that is able to detect the sound waves generated in samples of gold nanorods during laser irradiation providing us a voltage result proportional to the pressure signal. The first results show that the pressure measurements are directly proportional to the concentration of gold nanorods and the laser power, therefore, our present work is focused on determine the real influence of these effects in the cell death process.

Steady-state thermal analysis of an innovative receiver for linear Fresnel reflectors

The study of the performance of an innovative receiver for linear Fresnel reflectors is carried out in this paper, and the results are analyzed with a physics perspective of the process. The receiver consists of a bundle of tubes parallel to the mirror arrays, resulting on a smaller cross section for the same receiver width as the number of tubes increases, due to the diminution of their diameter. This implies higher heat carrier fluid speeds, and thus, a more effective heat transfer process, although it conveys higher pumping power as well. Mass flow is optimized for different tubes diameters, different impinging radiation intensities and different fluid inlet temperatures. It is found that the best receiver design, namely the tubes diameter that maximizes the exergetic efficiency for given working conditions, is similar for the cases studied. There is a range of tubes diameters that imply similar efficiencies, which can drive to capital cost reduction thanks to the flexibility of design. In addition, the length of the receiver is also optimized, and it is observed that the optimal length is similar for the working conditions considered. As a result of this study, it is found that this innovative receiver provides an optimum design for the whole day, even though impinging radiation intensity varies notably. Thermal features of this type of receiver could be the base of a new generation of concentrated solar power plants with a great potential for cost reduction, because of the simplicity of the system and the lower weigh of the components, plus the flexibility of using the receiver tubes for different streams of the heat carrier fluid.

Wind intermittency and CO2 reductions: the case of the spanish power system

Renewable energy sources are believed to reduce drastically greenhouse gas emissions that would otherwise be generated from fossil fuels used to generate electricity. This implies that a unit of renewable energy will replace a unit of fossil-fuel, with its CO2 emissions, on an equivalent basis (with no other effects on the grid). But, the fuel economy and emissions in the existing power systems are not proportional with the electricity production of intermittent sources due to cycling of the fossil fuel plants that make up the balance of the grid (i.e. changing the power output makes thermal units to operate less efficiently). This study focuses in the interactions between wind generation and thermal plants cycling, by establishing the levels of extra fuel use caused by decreased efficiencies of fossil back-up for wind electricity in Spain. We analyze the production of all thermal plants in 2011, studying different scenarios where wind penetration causes major deviations in programming, while we define a procedure for quantifying the carbon reductions by using emission factors and efficiency curves from the existing installations. The objectives are to discuss the real contributions of renewable energies to the environmental targets as well as suggest alternatives that would improve the reliability of future power systems.

Design and analysis of helium Brayton power cycles for HiPER reactor

Helium Brayton cycles have been studied as power cycles for both fission and fusion reactors obtaining high thermal efficiency. This paper studies several technological schemes of helium Brayton cycles applied for the HiPER reactor proposal. Since HiPER integrates technologies available at short term, its working conditions results in a very low maximum temperature of the energy sources, something that limits the thermal performance of the cycle. The aim of this work is to analyze the potential of the helium Brayton cycles as power cycles for HiPER. Several helium Brayton cycle configurations have been investigated with the purpose of raising the cycle thermal efficiency under the working conditions of HiPER. The effects of inter-cooling and reheating have specifically been studied. Sensitivity analyses of the key cycle parameters and component performances on the maximum thermal efficiency have also been carried out. The addition of several inter-cooling stages in a helium Brayton cycle has allowed obtaining a maximum thermal efficiency of over 36%, and the inclusion of a reheating process may also yield an added increase of nearly 1 percentage point to reach 37%. These results confirm that helium Brayton cycles are to be considered among the power cycle candidates for HiPER.

Application and modeling of GaN FET in 1MHz large signal bandwidth power supply for radio frequency power amplifier

In this paper, implementation and testing of non- commercial GaN HEMT in a simple buck converter for envelope amplifier in ET and EER transmission techn iques has been done. Comparing to the prototypes with commercially available EPC1014 and 1015 GaN HEMTs, experimentally demonstrated power supply provided better thermal management and increased the switching frequency up to 25MHz. 64QAM signal with 1MHz of large signal bandw idth and 10.5dB of Peak to Average Power Ratio was gener ated, using the switching frequency of 20MHz. The obtaine defficiency was 38% including the driving circuit an d the total losses breakdown showed that switching power losses in the HEMT are the dominant ones. In addition to this, some basic physical modeling has been done, in order to provide an insight on the correlation between the electrical characteristics of the GaN HEMT and physical design parameters. This is the first step in the optimization of the HEMT design for this particular application.

On the leakage-power modeling for optimal server operation

Leakage power consumption is a com- ponent of the total power consumption in data cen- ters that is not traditionally considered in the set- point temperature of the room. However, the effect of this power component, increased with temperature, can determine the savings associated with the careful management of the cooling system, as well as the re- liability of the system. The work presented in this paper detects the need of addressing leakage power in order to achieve substantial savings in the energy consumption of servers. In particular, our work shows that, by a careful detection and management of two working regions (low and high impact of thermal- dependent leakage), energy consumption of the data- center can be optimized by a reduction of the cooling budget.

Assessing the potential of hybrid fossil–solar thermal plants for energy policy making: Brayton cycles

This paper proposes a first study in-depth of solar-fossil hybridization from a general perspective. It develops a set of useful parameters for analyzing and comparing hybrid plants, it studies the case of hybridizing Brayton cycles with current solar technologies and shows a tentative extrapolation of the results to integrated combined cycle systems (ISCSS). In particular, three points have been analyzed: the technical requirements for solar technologies to be hybridized with Brayton cycles, the temperatures and pressures at which hybridization would produce maximum power per unit of fossil fuel, and their mapping to current solar technologies and Brayton cycles. Major conclusions are that a hybrid plant works in optimum conditions which are not equal to those of the solar or power blocks considered independently, and that hybridizing at the Brayton cycle of a combined cycle could be energetically advantageous.

Exact solution for the conjugate fluid-fluid problem in the thermal entrance region of laminar counterflow heat exchangers

A generalized Lévêque solution is presented for the conjugate fluid–fluid problem that arises in the thermal entrance region of laminar counterflow heat exchangers. The analysis, carried out for constant property fluids, assumes that the Prandtl and Peclet numbers are both large compared to unity, and neglects axial conduction both in the fluids and in the plate, assumed to be thermally thin. Under these conditions, the thermal entrance region admits an asymptotic self-similar description where the temperature varies as a power ϳ of the axial distance, with the particularity that the self-similarity exponent must be determined as an eigenvalue by solving a transcendental equation arising from the requirement of continuity of heat fluxes at the heat conducting wall. Specifically, the analysis reveals that j depends only on the lumped parameter ƙ = (A2/A1)1/3 (α1/α2)1/3(k2/k1), defined in terms of the ratios of the wall velocity gradients, A, thermal diffusivities, α i, and thermal conductivities,k i, of the fluids entering, 1, and exiting, 2, the heat exchanger. Moreover, it is shown that for large (small) values of K solution reduces to the classical first (second) Lévêque solution. Closed-form analytical expressions for the asymptotic temperature distributions and local heat-transfer rate in the thermal entrance region are given and compared with numerical results in the counterflow parallel-plate configuration, showing very good agreement in all cases.

Thermal, stress, and traps effects in AlGaN/GaN HEMTs

GaN y AlN son materiales semiconductores piezoeléctricos del grupo III-V. La heterounión AlGaN/GaN presenta una elevada carga de polarización tanto piezoeléctrica como espontánea en la intercara, lo que genera en su cercanía un 2DEG de grandes concentración y movilidad. Este 2DEG produce una muy alta potencia de salida, que a su vez genera una elevada temperatura de red. Las tensiones de puerta y drenador provocan un stress piezoeléctrico inverso, que puede afectar a la carga de polarización piezoeléctrica y así influir la densidad 2DEG y las características de salida. Por tanto, la física del dispositivo es relevante para todos sus aspectos eléctricos, térmicos y mecánicos. En esta tesis se utiliza el software comercial COMSOL, basado en el método de elementos finitos (FEM), para simular el comportamiento integral electro-térmico, electro-mecánico y electro-térmico-mecánico de los HEMTs de GaN. Las partes de acoplamiento incluyen el modelo de deriva y difusión para el transporte electrónico, la conducción térmica y el efecto piezoeléctrico. Mediante simulaciones y algunas caracterizaciones experimentales de los dispositivos, hemos analizado los efectos térmicos, de deformación y de trampas. Se ha estudiado el impacto de la geometría del dispositivo en su auto-calentamiento mediante simulaciones electro-térmicas y algunas caracterizaciones eléctricas. Entre los resultados más sobresalientes, encontramos que para la misma potencia de salida la distancia entre los contactos de puerta y drenador influye en generación de calor en el canal, y así en su temperatura. El diamante posee une elevada conductividad térmica. Integrando el diamante en el dispositivo se puede dispersar el calor producido y así reducir el auto-calentamiento, al respecto de lo cual se han realizado diversas simulaciones electro-térmicas. Si la integración del diamante es en la parte superior del transistor, los factores determinantes para la capacidad disipadora son el espesor de la capa de diamante, su conductividad térmica y su distancia a la fuente de calor. Este procedimiento de disipación superior también puede reducir el impacto de la barrera térmica de intercara entre la capa adaptadora (buffer) y el substrato. La muy reducida conductividad eléctrica del diamante permite que pueda contactar directamente el metal de puerta (muy cercano a la fuente de calor), lo que resulta muy conveniente para reducir el auto-calentamiento del dispositivo con polarización pulsada. Por otra parte se simuló el dispositivo con diamante depositado en surcos atacados sobre el sustrato como caminos de disipación de calor (disipador posterior). Aquí aparece una competencia de factores que influyen en la capacidad de disipación, a saber, el surco atacado contribuye a aumentar la temperatura del dispositivo debido al pequeño tamaño del disipador, mientras que el diamante disminuiría esa temperatura gracias a su elevada conductividad térmica. Por tanto, se precisan capas de diamante relativamente gruesas para reducer ele efecto de auto-calentamiento. Se comparó la simulación de la deformación local en el borde de la puerta del lado cercano al drenador con estructuras de puerta estándar y con field plate, que podrían ser muy relevantes respecto a fallos mecánicos del dispositivo. Otras simulaciones se enfocaron al efecto de la deformación intrínseca de la capa de diamante en el comportamiento eléctrico del dispositivo. Se han comparado los resultados de las simulaciones de la deformación y las características eléctricas de salida con datos experimentales obtenidos por espectroscopía micro-Raman y medidas eléctricas, respectivamente. Los resultados muestran el stress intrínseco en la capa producido por la distribución no uniforme del 2DEG en el canal y la región de acceso. Además de aumentar la potencia de salida del dispositivo, la deformación intrínseca en la capa de diamante podría mejorar la fiabilidad del dispositivo modulando la deformación local en el borde de la puerta del lado del drenador. Finalmente, también se han simulado en este trabajo los efectos de trampas localizados en la superficie, el buffer y la barrera. Las medidas pulsadas muestran que tanto las puertas largas como las grandes separaciones entre los contactos de puerta y drenador aumentan el cociente entre la corriente pulsada frente a la corriente continua (lag ratio), es decir, disminuir el colapse de corriente (current collapse). Este efecto ha sido explicado mediante las simulaciones de los efectos de trampa de superficie. Por su parte, las referidas a trampas en el buffer se enfocaron en los efectos de atrapamiento dinámico, y su impacto en el auto-calentamiento del dispositivo. Se presenta también un modelo que describe el atrapamiento y liberación de trampas en la barrera: mientras que el atrapamiento se debe a un túnel directo del electrón desde el metal de puerta, el desatrapamiento consiste en la emisión del electrón en la banda de conducción mediante túnel asistido por fonones. El modelo también simula la corriente de puerta, debida a la emisión electrónica dependiente de la temperatura y el campo eléctrico. Además, también se ilustra la corriente de drenador dependiente de la temperatura y el campo eléctrico. ABSTRACT GaN and AlN are group III-V piezoelectric semiconductor materials. The AlGaN/GaN heterojunction presents large piezoelectric and spontaneous polarization charge at the interface, leading to high 2DEG density close to the interface. A high power output would be obtained due to the high 2DEG density and mobility, which leads to elevated lattice temperature. The gate and drain biases induce converse piezoelectric stress that can influence the piezoelectric polarization charge and further influence the 2DEG density and output characteristics. Therefore, the device physics is relevant to all the electrical, thermal, and mechanical aspects. In this dissertation, by using the commercial finite-element-method (FEM) software COMSOL, we achieved the GaN HEMTs simulation with electro-thermal, electro-mechanical, and electro-thermo-mechanical full coupling. The coupling parts include the drift-diffusion model for the electron transport, the thermal conduction, and the piezoelectric effect. By simulations and some experimental characterizations, we have studied the device thermal, stress, and traps effects described in the following. The device geometry impact on the self-heating was studied by electro-thermal simulations and electrical characterizations. Among the obtained interesting results, we found that, for same power output, the distance between the gate and drain contact can influence distribution of the heat generation in the channel and thus influence the channel temperature. Diamond possesses high thermal conductivity. Integrated diamond with the device can spread the generated heat and thus potentially reduce the device self-heating effect. Electro-thermal simulations on this topic were performed. For the diamond integration on top of the device (top-side heat spreading), the determinant factors for the heat spreading ability are the diamond thickness, its thermal conductivity, and its distance to the heat source. The top-side heat spreading can also reduce the impact of thermal boundary resistance between the buffer and the substrate on the device thermal behavior. The very low electrical conductivity of diamond allows that it can directly contact the gate metal (which is very close to the heat source), being quite convenient to reduce the self-heating for the device under pulsed bias. Also, the diamond coated in vias etched in the substrate as heat spreading path (back-side heat spreading) was simulated. A competing mechanism influences the heat spreading ability, i.e., the etched vias would increase the device temperature due to the reduced heat sink while the coated diamond would decrease the device temperature due to its higher thermal conductivity. Therefore, relative thick coated diamond is needed in order to reduce the self-heating effect. The simulated local stress at the gate edge of the drain side for the device with standard and field plate gate structure were compared, which would be relevant to the device mechanical failure. Other stress simulations focused on the intrinsic stress in the diamond capping layer impact on the device electrical behaviors. The simulated stress and electrical output characteristics were compared to experimental data obtained by micro-Raman spectroscopy and electrical characterization, respectively. Results showed that the intrinsic stress in the capping layer caused the non-uniform distribution of 2DEG in the channel and the access region. Besides the enhancement of the device power output, intrinsic stress in the capping layer can potentially improve the device reliability by modulating the local stress at the gate edge of the drain side. Finally, the surface, buffer, and barrier traps effects were simulated in this work. Pulsed measurements showed that long gates and distances between gate and drain contact can increase the gate lag ratio (decrease the current collapse). This was explained by simulations on the surface traps effect. The simulations on buffer traps effects focused on illustrating the dynamic trapping/detrapping in the buffer and the self-heating impact on the device transient drain current. A model was presented to describe the trapping and detrapping in the barrier. The trapping was the electron direct tunneling from the gate metal while the detrapping was the electron emission into the conduction band described by phonon-assisted tunneling. The reverse gate current was simulated based on this model, whose mechanism can be attributed to the temperature and electric field dependent electron emission in the barrier. Furthermore, the mechanism of the device bias via the self-heating and electric field impact on the electron emission and the transient drain current were also illustrated.