Light trapping properties of cylindrical well diffraction gratings in solar cells: Computational calculations

Light trapping using diffraction gratings is a promising approach to increasing absorption in solar cells. In this paper, the computationally calculated absorption enhancement expected from a diffraction grating consisting of a triangular array of cylindrical wells is presented. Angle-extended polychromatic illumination is considered, and special attention is paid to absorption of sub-bandgap photons in an intermediate band solar cell. Results are compared to the absorption enhancement expected from an ideal Lambertian (randomizing) scatterer, which is considered as a baseline. It is found that for cells which absorb very weakly, the diffraction grating provides absorption enhancement above that of the ideal Lambertian scatterer over a wide wavelength range. For cells which absorb more strongly, the grating underperforms the ideal Lambertian scatterer over almost all wavelengths. Finally, the grating period, well height and well radius are optimised. Keywords: Light Trapping, Diffraction Grating, Intermediate Band Solar Cel

Power Systems Monitoring and Control using Telecom Network Management Standards

Historically, different solutions have been developed for power systems control and telecommunications network management environments. The former was characterized by proprietary solutions, while the latter has been involved for years in a strong standardization process guided by criteria of openness. Today, power systems control standardization is in progress, but it is at an early stage compared to the telecommunications management area, especially in terms of information modeling. Today, control equipment tends to exhibit more computational power, and communication lines have increased their performance. These trends hint at some conceptual convergence between power systems and telecommunications networks from a management perspective. This convergence leads us to suggest the application of well-established telecommunications management standards for power systems control. This paper shows that this is a real medium-to-long term possibility

Dust effects on PV array performance: in-field observations with non-uniform patterns

This paper presents the impact of non-homogeneous deposits of dust on the performance of a PV array. The observations have been made in a 2-MW PV park in the southeast region of Spain. The results are that inhomogeneous dust leads to more significant consequences than the mere short-circuit current reduction resulting from transmittance losses. In particular, when the affected PV modules are part of a string together with other cleaned (or less dusty) ones, operation voltage losses arise. These voltage losses can be several times larger than the short-circuit ones, leading to power losses that can be much larger than what measurements suggest when the PV modules are considered separately. Significant hot-spot phenomena can also arise leading to cells exhibiting temperature differences of more than 20 degrees and thus representing a threat to the PV modules' lifetime

Influence of Partial Soil Wetting on Water Relation Parameters of the Olive Tree

A drip versus pond irrigation experiment was carried out with 30-year-old ‘Manzanilla’ olive trees planted at 7 m 5 m in an orchard in Southwest Spain. At the end of the dry season of 1998, we chose two dry-land trees, D1 and D2, and two drip-irrigated trees, I1 and I2. During the experiments, the D1 and I1 trees were pond-irrigated, increasing the soil water content to around field capacity in the whole rootzone. The D2 and I2 trees were drip-irrigated, remaining part of the rootzone in drying soil. The results showed that the ratio between the transpiration of the pond-irrigated D1 tree and that of the drip-irrigated D2 tree (D1/D2 Ep) increased from an average of 0.88 before irrigation to 1.22 fourteen days after the first water supply. For the I trees, I1/I2 Ep varied from 0.76 to 1.02 nine days after the I1 tree was pond-irrigated for the first time. Transpiration, therefore, was restricted when using a drip irrigation system which, despite supplying enough water to cover the calculated crop demand, affected a part of the rootzone only. During the drip versus pond irrigation experiment, the recovery of leaf water potential, stomatal conductance and photosynthesis rate was greater and quicker in the pond-irrigated than in the drip-irrigated trees.– Influence de l’irrigation partielle du sol sur les paramètres des relations hydriques de l’olivier. Une comparaison de l’irrigation goutte à goutte avec l’irrigation en cuvette a été conduite sur oliviers ‘Manzanilla’ âgés de 30 ans plantés à 7 m 5 m dans un verger du sud-ouest de l’Espagne. À la fin de la saison sèche de 1998, nous avons choisi deux arbres sur sol sec, D1 et D2, et deux arbres sur sol irrigué au goutte à goutte, I1 et I2. Durant les expériences, les arbres D1 et I1 ont été irrigués en cuvette, en augmentant la teneur en eau du sol jusqu’à la capacité au champ dans toute la zone racinaire. Les arbres D2 et I2 ont été irrigués au goutte à goutte, laissant une partie de la zone racinaire dans un sol se desséchant. Les résultats ont montré que le quotient entre la transpiration de l’arbre D1 irrigué en cuvette et celle de l’arbre D2 irrigué au goutte à goutte (D1/D2 Ep) a augmenté à partir d’une moyenne de 0,88 avant irrigation jusqu’à 1,22 quatorze jours après le premier apport d’eau. Pour les arbres I, I1/I2 Ep a varié entre 0,76 et 1,02 neuf jours après que l’arbre I1 ait été irrigué en cuvette pour la première fois. Par conséquent, la transpiration était réduite quand on utilisait un système d’irrigation au goutte à goutte qui, malgré l’apport d’eau suffisant pour couvrir les besoins potentiels des cultures, a affecté seulement une partie de la zone racinaire. Durant cette comparaison de l’irrigation goutte à goutte et de l’irrigation en cuvette, la récupération de la teneur en eau des feuilles, de la conductance stomatique et du taux de photosynthèse était supérieure et plus rapide pour les arbres irrigués en cuvette que pour ceux irrigués au goutte à goutte

Control de seguimiento solar de alta precisión con autocalibración

Abstract This work describes the development of high accuracy sun tracking control equipment, to be integrated in high concentration photovoltaic systems. In order to achieve the sub degree accuracies required by these photovoltaic concentrators, a new control approach is presented that primarily based in the computation of analytic sun ephemeris equations, adds in its software a calibration model, that characterizes the concentrator geometry through a set of parameters, in order to precisely convert the sun coordinates supplied by the ephemeris into tracking axes rotation angles. Fitting the calibration model parameters is done through an automatic process that runs during the in‐field installation of the concentration system, in which a set of precise sun position measurements with respect to the tracking axes are taken throughout a day from dawn to dusk. The full development cycle of this product is described going from the theory of its operation, to its validation and testing, its physical implementation as an electronic system, and finally to the steps given towards the volume production of the resultant technology. The work is divided in five chapters that follow an introduction in which an overview is given of the present context of Concentration PhotoVoltaics (CPV), to later focus in presenting the CPV tracker, and finally reviewing the history and state of the art of sun tracking control for high accuracy applications. The tracking control developed in this work is based in an autocalibration process, very common in instrumentation equipment and applied also to the relatively similar problem of the tracking of orbital targets, such as required in big astronomical telescopes. In fact, the novelty of this work resides in adapting these astronomy sun tracking techniques to some sort of “telescopes” that only “see” the electrical power output of a PV generator. These particular telescopes or sun trackers will usually be a two axis pointable structure, where in the approach presented in this work, sun tracking will be firstly based in obtaining the sun’s coordinates with respect to the tracker location through the computation of sun ephemeris equations encoded in a microprocessor. However converting these so called topocentric coordinates into the tracker axes rotations pointing it to the sun, will require another set of equations, i.e. the calibration model or transform as is termed in this work, that usually based in rotation transforms and spherical geometry (or alternatively obtained through quaternions) will parametrically model the orientation of the tracking axes and its rotation origins, and also the orientation with respect to these axes, of the supported CPV array pointing vector, i.e. the one that when perfectly parallel to the local sun vector produces maximum power output. Chapter 1 covers the development of this Calibration Transform (C‐Transform), its application and possible variations to consider the most common tracker axes configurations, and finally the consideration of flexure in the tracking structure and propose how it can be characterized and integrated in an extended calibration transform. Chapter 2 describes the algorithms used for the fitting of the calibration model once a set of accurate sun position measurements is collected by the tracker. Least squares (LS) fitting through the Levenberg Marquardt method is developed for the C‐Transform of Chapter 1. Firstly the correct operation of our encoding of the Levenberg Marquardt (LM) method is tested, as is usual for numerical optimization algorithms, with the Moré set of test functions. Then the numerical accuracy of the LM method when fitting the C‐Transform and having its code compiled to run in a low cost 8 bit microprocessor, is obtained when compared with that of the same code running in a conventional 32 bit desktop PC. Along with these main topics some other are considered in this chapter such as the analysis of the proper definition of the LS merit function when operating on the C‐Transform, checking the right definition of the C‐Transform that doesn't give rise to discontinuities that may slow its fitting, and also exploring the accuracy of an alternative implementation of the LM algorithm in which the derivatives in the Hessian and gradient are calculated through finite differences instead of using their closed analytic forms. Chapter 3 first gets to prove the physical grounds of the calibration model, by using a laboratory tracker that mounts several different samples of CPV modules, and where some of the parameters of the calibration model can be previously adjusted, tweaking the position of both the tracker and the test modules, so after collecting a set of sun positions and then fitting the C‐Transform best fit parameter values should be equal to those preset during the experiment. Once the calibration model is validated, next goal is the precise measurement of the tracking accuracy being achieved by the proposed hybrid calibrated tracking control. This will first require the development of a Tracking Accuracy Sensor (TAS) able to provide real time measurements of the sun tracking error. This sensor essentially consists of a collimating pipe mounted on top of an imaging electronic sensor, with the necessary signal conditioning electronics, developed along with the conversion equations necessary to compute a tracking error angle from its internal electric variables, and also with the calibration procedure to accurately obtain the internal construction parameters of the sensor that intervene in these conversion equations. This sensor will end up featuring the rather impressive resolution of 1/10.000th degree within its 1° acceptance angle. Finally a monitoring system is built around the TAS mounted in the referred laboratory tracker, and by calibrating the controller against this TAS, a best case estimate of the tracking accuracy statistics, can be obtained for several variations of the calibrated routine here developed. In these experiments average daily accuracies of 0.05° are proven possible with accuracies better than 0.1°, 97% of the time. All tests in Chapter 3 to prove the correct operation of the calibrated sun tracking control, had to previously collect a set of in between fifty and a hundred sun position measurements, throughout a day from dawn to dusk, in order to feed the fitting of the calibration model. This was to be manually done, by aiming the laboratory tracker with its CPV modules to the sun till, according to some criterion, best pointing accuracy was obtained, and then recording axes position along with a measurement time stamp. This was a boring and error prone task, that had to be fully automated in order to be feasibly integrated in a commercial tracking controller unit, so it would run with the least amount manual operation and maintenance. In Chapter 4 the necessary hardware and algorithms to implement the automatic collection of sun position measurements are explained. But prior to the full description of the automatic measurement acquisition process, a review of the existent sun ephemerides and their accuracy is presented, including the set finally chosen for the tracking controller described in this work. Regarding sun position measurements these can first start by a preliminary search of the sun within the concentrator’s tracking range, till this gets within the acceptance angle, a problem which as will be described can be attacked mathematically by resorting to the toolbox of Search Theory, a branch of Operations Research developed during WWII for antisubmarine warfare. In particular when helping the search with a coarse sun sensor such as a low power flat PV module, the optimum search path can be derived by making the assumptions of the so called “Flaming Datum Problem” firstly developed for the search of an evading submarine after it reveals its position after attacking a friendly ship. Once the sun is within acceptance angle, to precisely point to the sun we would ideally maximize the power output of the CPV array, but as will be discussed some practical problems appear when choosing power output as the variable to maximize, and this due to the difficulties of maximum power point (MPP) tracking stages in inverters to follow the tracker scanning movements. Therefore other alternative electrical variables under different biasing than MPP are here proposed. Short circuit current is finally the variable chosen for maximization, and the additional hardware developed to automatically short the CPV array and measure its current is described. Searching for the short circuit current maximum is a problem of maximization of a two dimensional function, that in the sun tracking application can be approached more straightforwardly by avoiding computation of derivatives and instead by determining the minimum number of linear maximizations, which will be translated into tracker sweeps, that lead to the overall maximum of the function. In this occasion the mathematical corpus behind corresponds to the Powell‐Brent derivative‐free conjugate direction set method. Following the description of the mechanism for automatically taking a certain position measurement, investigation of the right number of measurements to obtain the most accurate calibrated ephemeris and their distribution or scheduling is presented. The chapter ends with a discussion about the possibility to introduce a statistical filter to detect outliers during sun position measurement acquisition, understood as those faulty measurements affected by whatever errors, or alternatively considers the possibility to introduce maximum likelihood estimators (MLEs) other than the Least Squares, for the fitting of non linear models such as the CTransform. In this respect the so called Cauchy‐Lorentz MLE appears as the most feasible option. Chapter 5, the final one, describes how all the methods developed for the autocalibrated sun tracking control, explained up to this point, have been physically implemented in an electronic unit, commercially known as the SunDog STCU (standing for Sun Tracking Controller Unit). Firstly a detailed physical description from its electronic boards to its peripherals is presented, followed by an account of its features and operational modes, to end with an account of its reliability tests and certification. The chapter follows with a description of the process of the testing and operation of the tracking control unit through the most significant CPV R&D development projects and related prototypes. Here on the one hand several tracker development projects carried out by Inspira for several CPV developing companies, such as Isofoton, Concentrix, Solfocus, Boeing, or Renovalia are presented, focusing on the application and results of the auto calibrated tracking control in them. On the other hand this chapter presents a selection of European R&D projects in the field of CPV, in which Inspira participated being in charge of the development of the CPV tracker, and which have been useful to explore or further develop particular features of the SunDog controller operation. The chapter ends with a description of the first steps taken towards series production of the auto calibrated controller, and their first volume deployment in the ISFOC and Abertura PV plants. Putting an end to this work, the Conclusions section gathers the most relevant results obtained, and outlines some areas whose advancement can further improve the sun tracking controller developed. Resumen En este trabajo se describe el desarrollo de un equipo de control de seguimiento solar de alta precisión para sistemas de alta concentración fotovoltaica. Con objeto de lograr las precisiones de apuntamiento por debajo del grado requeridas por estos concentradores fotovoltaicos, se presenta una nueva aproximación al control de seguimiento que primeramente basado en el computo de efemérides solares analíticas, incorpora a continuación un modelo de calibración, que caracteriza geométricamente el concentrador por medio de un conjunto de parámetros, con objeto de realizar una conversión precisa de las coordenadas solares suministradas por las efemérides en ángulos de giro de los ejes del seguidor. El ajuste de los parámetros del modelo de calibración se realiza por medio de un proceso automatizado que tiene lugar durante la instalación en campo del sistema de concentración, en el que se realizan un conjunto de medidas precisas de la posición del sol en relación a los ejes del seguidor, tomadas a lo largo de un día desde el orto al ocaso. En este trabajo se describe el ciclo completo de desarrollo de este producto yendo desde las bases teóricas de su operación, su validación y pruebas, su realización física como un equipo electrónico, y finalmente los pasos dados de cara a la producción industrial de esta tecnología. Este trabajo se compone de cinco capítulos, que se suceden a continuación de una introducción en la que se explica el presente contexto de la Concentración Fotovoltaica (CFV o en inglés CPV de Concentration PhotoVoltaics), para después pasar a describir las características de los seguidores solares para concentración fotovoltaica, y finalmente ofrecer una revisión de la historia y el estado del arte de los sistemas de control de seguimiento solar en aplicaciones que requieren altas precisiones. El control de seguimiento desarrollado está basado en un proceso de autocalibración, generalmente común en el ámbito del equipamiento de instrumentación o por ejemplo aplicado también en el problema relativamente similar del seguimiento de objetos orbitales, que se da en los grandes telescopios astronómicos. De hecho la novedad de este trabajo reside en adaptar estas técnicas de seguimiento utilizadas en astronomía a un tipo de "telescopios" que solo "ven" la potencia eléctrica producida por un generador fotovoltaico. Estos telescopios o seguidores solares consistirán habitualmente en una estructura orientable en torno a dos ejes, de tal modo que según la aproximación que se presenta en este trabajo, el seguimiento solar estará primeramente basado en obtener las coordenadas del sol con respecto al lugar de instalación del seguidor, a través del cómputo de efemérides solares codificadas en un microprocesador. Posteriormente convertir estas coordenadas topocéntricas en los ángulos de giro a ejercer en los ejes de seguimiento para apuntar al sol, requerirá de otro conjunto de ecuaciones, i.e. según se denomina en este trabajo el llamado modelo de calibración, que basado en transformaciones de tipo rotación y geometría esférica (o alternativamente deducibles usando cuaternios) que parametriza la orientación de los ejes de seguimiento y sus orígenes de rotación, así como la orientación con respecto a estos ejes, del vector de apuntamiento del sistema de módulos de concentración soportados, esto es aquel vector ligado a la superficie orientable del seguidor que cuando queda orientado de manera perfectamente paralela al vector local del sol produce la máxima potencia de salida. El Capitulo 1 está dedicado al desarrollo de esta Transformada de Calibración (por abreviar Transformada C), sus aplicaciones y posibles variaciones orientadas a cubrir las más comunes configuraciones de ejes de seguimiento, y finalmente se detiene a considerar los efectos de la flexión en la estructura de seguimiento y proponer como pueden ser caracterizados e integrados en una transformada de calibración extendida. El Capitulo 2 se centra en los algoritmos utilizados para el ajuste del modelo de calibración una vez que el seguidor a tomado una serie de medidas precisas de la posición del sol. Se desarrolla el ajuste por mínimos cuadrados (MC o en inglés LS de Least Squares) de la Transformada‐C del Capítulo 1 por medio del algoritmo de Levenberg‐Marquardt. Primeramente se probará el correcto funcionamiento de la codificación de este método Levenberg‐Marquardt (LM) utilizando el conjunto de funciones de Moré, el estándar habitualmente utilizado con los algoritmos numéricos de optimización como es LM. A continuación la precisión numérica del método LM en el ajuste de la Transformada‐C, con su código compilado para correr en microprocesador de 8 bits de bajo coste, se obtiene comparándolo con la obtenida por el mismo código corriendo en un ordenador personal convencional de 32 bits. Junto con estos temas esenciales, se consideran otros en este capítulo, como por ejemplo el análisis de la correcta definición de la figura de merito de los MC cuando se opera sobre la Transformada C, también la correcta definición de la Transformada‐C de tal modo que se eviten discontinuidades que puedan ralentizar su ajuste, o la exploración de la precisión de una implementación alternativa del método LM donde las derivadas del Hessiano y el gradiente sean calculadas mediante diferencias finitas en lugar de a través de sus expresiones analíticas. El Capítulo 3 comienza por probar la coherencia física del modelo de calibración, mediante la utilización de un seguidor de laboratorio en el que se instalan diversas muestras de módulos CPV, y donde algunos de los parámetros del modelo de calibración pueden ser previamente fijados, regulando la posición tanto del seguidor como de los módulos de prueba, de tal modo que tras obtener un conjunto de medidas de posición del sol y ajustar la Transformada C, los valores resultantes para los parámetros deberían coincidir con aquellos que fueron manualmente prefijados. Una vez que el modelo de calibración queda validado, el siguiente objetivo es medir la precisión de seguimiento alcanzable por el sistema control de seguimiento calibrado desarrollado. Esto requirió en primer lugar el desarrollo de un Sensor de Precisión de Seguimiento (SPS, en inglés TAS de Tracking Accuracy Sensor) capaz de proporcionar medidas en tiempo real del error de apuntamiento. Este sensor esencialmente consiste en un tubo colimador montado sobre un sensor electrónico de imagen, y que cuenta con la electrónica necesaria de acondicionamiento de señal, todo desarrollado junto con las ecuaciones de conversión necesarias para obtener el error angular de apuntamiento a partir de variables eléctricas internas, y también con un procedimiento de calibración para obtener con precisión los parámetros internos de construcción del sensor que intervienen en las ecuaciones de conversión. Este sensor terminó alcanzando una resolución en la determinación del error de apuntamiento de una diezmilésima de grado dentro de su apertura angular de un grado. Finalmente un sistema de monitorización se ha construido en torno al TAS instalado en el referido seguidor de laboratorio, y mediante la calibración de su control de seguimiento contra este TAS, se puede obtener una estimación para la estadística del error de apuntamiento para una serie de variaciones de la estrategia de calibración aquí descritas. En estos experimentos se ha demostrado que precisiones promedio diarias de 0.05°, donde la precisión es mejor que 0.1°, el 97% del tiempo, son posibles. Todas las pruebas realizadas en el Capítulo 3 orientadas a comprobar la correcta operación del control de seguimiento calibrado, requirieron la recolección previa de un conjunto de entre cincuenta y cien medidas de la posición del sol, a lo largo de un día de orto a ocaso, con que luego realizar el ajuste del modelo de calibración. Esto fue realizado de manera manual, apuntando el seguidor de laboratorio al sol, hasta que de acuerdo con un cierto criterio se conseguía su apuntamiento optimo, momento en el cuales e registraban las posiciones de los ejes de seguimiento junto con la hora exacta en la que se realizaba la medida. Se trataba en cualquier caso de una tarea aburrida y susceptible de errores, que era necesario automatizar completamente, de tal modo que se realizará con los menores requerimientos de intervención manual y mantenimiento. En el Capítulo 4 se describe el hardware y los algoritmos necesarios para implementar la colección automática de medidas de posición del sol. Pero antes de entrar con la descripción completa del proceso de adquisición de estas medidas, se presenta una revisión de las efemérides solares existentes y sus respectivas precisiones, incluidas las finalmente elegidas para su integración en el equipo de control de seguimiento objeto de este trabajo. En cuanto a las medidas de posición del sol, estas comienzan con una búsqueda preliminar del sol en el interior del rango se seguimiento del concentrador, hasta que este entra dentro de su apertura angular, problema este que será abordado recurriendo a las herramientas de la Teoría de Búsqueda, una rama de la Investigación Operativa desarrollada durante la Segunda Guerra Mundial en el marco de la guerra antisubmarina. En concreto, cuando dicha búsqueda se instrumenta mediante un sensor grueso de la posición del sol tal como un pequeño panel plano FV, el camino optimo de búsqueda puede obtenerse tomando las suposiciones del llamado "Problema del Punto Llameante" (en inglés "Flaming Datum Problem") que surge inicialmente en la búsqueda de un submarino enemigo que escapa tras haber revelado su posición al atacar a un barco amigo. Una vez el sol es detectado al entrar en la apertura angular del concentrador, un apuntamiento preciso pasaría por maximizar su potencia de salida, pero según veremos problemas de índole practico aparecen cuando se elige la potencia de salida como la variable a maximizar, y esto es debido a las dificultades de las etapas de seguimiento del punto de máxima potencia (en inglés MPPT: Maximum Power Point Tracking) existentes a la entrada de los inversores convencionales tienen para seguir los movimientos exploratorios del seguidor. Por ello se propone la maximización de otras variables eléctricas bajo dife

Embedment of metal nanoparticles in GaAs and Si for plasmonic absorption enhancement in intermediate band solar cells

The high near-field enhancement occurring in the vicinity of metallic nanoparticles (MNPs) sustaining surface plasmons can only be fully exploited in photovoltaic devices if the MNPs are placed inside their semiconducting material, in the photoactive region. In this work an experimental procedure is studied to embed MNPs in gallium arsenide (GaAs) and silicon (Si), which can be applied to other semiconductor host materials. The approach consists in spin-coating colloidal MNPs dispersed in solution onto the substrate surface. Then a capping layer of the same material as the substrate is deposited on top to embed the MNPs in the semiconductor. The extinction spectra of silver (Ag) and gold (Au) MNPs embedded in GaAs and Si is modeled with Mie theory for comparison with optical measurements. This contribution constitutes the initial step towards the realization of quantum-dot intermediate band solar cells (QD-IBSC) with MN

Heterogeneous data source integration for smart grid ecosystems based on metadata mining

The arrival of new technologies related to smart grids and the resulting ecosystem of applications andmanagement systems pose many new problems. The databases of the traditional grid and the variousinitiatives related to new technologies have given rise to many different management systems with several formats and different architectures. A heterogeneous data source integration system is necessary toupdate these systems for the new smart grid reality. Additionally, it is necessary to take advantage of theinformation smart grids provide. In this paper, the authors propose a heterogeneous data source integration based on IEC standards and metadata mining. Additionally, an automatic data mining framework isapplied to model the integrated information.Ministerio de Economía y Competitividad TEC2013-40767-

Análisis estratégico de la producción científica española por campos: Ciencias Naturales, Médicas y de la Vida

La investigación es una misión fundamental de la universidad que determina la producción científica, que, a su vez, está asociada al desarrollo social y económico. Los indicadores bibliométricos proporcionan una medida de la producción científica. Por otro lado, la medida de dicha producción distinguiendo por campos y especialidades es mucho más útil y permite conocer interesantes detalles para la gestión, tanto de una universidad como de un siste-ma universitario. Utilizando los indicadores bibliométricos del ranking ARWU, se analiza por campos y especialidades con especial énfasis en las universidades españolas. Se identifican los puntos fuertes y débiles por especialidad en cuanto a volumen y calidad de la producción, impacto, reconocimiento y colaboración internacional, así como el posi-cionamiento internacional de las universidades españolas. De todo ello se derivan recomendaciones para la gestiónResearch is a fundamental mission of the university that determines scientific production which, in turn, is associated with social and economic development. Bibliometric indicators provide a measure of scientific production. On the other hand, the measurement of said production distinguishing by fields and subjects is much more useful and allows to know interesting details for the management, both of a university and of a university system. Using the bibliometric indicators of the ARWU ranking, it is analyzed by fields and subjects with special emphasis on Spanish universities. The strong and weak points are identified by subject in terms of volume and quality of production, impact, recognition and international collaboration, as well as the international positioning of Spanish universities. Management recommenda-tions are derived from all of thisP20-02019, financed by FEDER/Junta de Andalucía-Department of Economic Transformation, Industry, Knowledge and Universitie

Radiative thermal escape in intermediate band solar cells

To achieve high efficiency, the intermediate band (IB) solar cell must generate photocurrent from sub-bandgap photons at a voltage higher than that of a single contributing sub-bandgap photon. To achieve the latter, it is necessary that the IB levels be properly isolated from the valence and conduction bands. We prove that this is not the case for IB cells formed with the confined levels of InAs quantum dots (QDs) in GaAs grown so far due to the strong density of internal thermal photons at the transition energies involved. To counteract this, the QD must be smaller