Advanced Fresnel Köhler concentrators for photovoltaic applications

La concentración fotovoltaica (CPV) es una de las formas más prometedoras de reducir el coste de la energía proveniente del sol. Esto es posible gracias a células solares de alta eficiencia y a una significativa reducción del tamaño de la misma, que está fabricada con costosos materiales semiconductores. Ambos aspectos están íntimamente ligados ya que las altas eficiencias solamente son posibles con materiales y tecnologías de célula caros, lo que forzosamente conlleva una reducción del tamaño de la célula si se quiere lograr un sistema rentable. La reducción en el tamaño de las células requiere que la luz proveniente del sol ha de ser redirigida (es decir, concentrada) hacia la posición de la célula. Esto se logra colocando un concentrador óptico encima de la célula. Estos concentradores para CPV están formados por diferentes elementos ópticos fabricados en materiales baratos, con el fin de reducir los costes de producción. El marco óptimo para el diseño de concentradores es la óptica anidólica u óptica nonimaging. La óptica nonimaging fue desarrollada por primera vez en la década de los años sesenta y ha ido evolucionando significativamente desde entonces. El objetivo de los diseños nonimaging es la transferencia eficiente de energía entre la fuente y el receptor (sol y célula respectivamente, en el caso de la CPV), sin tener en cuenta la formación de imagen. Los sistemas nonimaging suelen ser simples, están compuestos de un menor número de superficies que los sistemas formadores de imagen y son más tolerantes a errores de fabricación. Esto hace de los sistemas nonimaging una herramienta fundamental, no sólo en el diseño de concentradores fotovoltaicos, sino también en el diseño de otras aplicaciones como iluminación, proyección y comunicaciones inalámbricas ópticas. Los concentradores ópticos nonimaging son adecuados para aplicaciones CPV porque el objetivo no es la reproducción de una imagen exacta del sol (como sería el caso de las ópticas formadoras de imagen), sino simplemente la colección de su energía sobre la célula solar. Los concentradores para CPV pueden presentar muy diferentes arquitecturas y elementos ópticos, dando lugar a una gran variedad de posibles diseños. El primer elemento óptico que es atravesado por la luz del sol se llama Elemento Óptico Primario (POE en su nomenclatura anglosajona) y es el elemento más determinante a la hora de definir la forma y las propiedades del concentrador. El POE puede ser refractivo (lente) o reflexivo (espejo). Esta tesis se centra en los sistemas CPV que presentan lentes de Fresnel como POE, que son lentes refractivas delgadas y de bajo coste de producción que son capaces de concentrar la luz solar. El capítulo 1 expone una breve introducción a la óptica geométrica y no formadora de imagen (nonimaging), explicando sus fundamentos y conceptos básicos. Tras ello, la integración Köhler es presentada en detalle, explicando sus principios, válidos tanto para aplicaciones CPV como para iluminación. Una introducción a los conceptos fundamentales de CPV también ha sido incluida en este capítulo, donde se analizan las propiedades de las células solares multiunión y de los concentradores ópticos empleados en los sistemas CPV. El capítulo se cierra con una descripción de las tecnologías existentes empleadas para la fabricación de elementos ópticos que componen los concentradores. El capítulo 2 se centra principalmente en el diseño y desarrollo de los tres concentradores ópticos avanzados Fresnel Köhler que se presentan en esta tesis: Fresnel-Köhler (FK), Fresnel-Köhler curvo (DFK) y Fresnel-Köhler con cavidad (CFK). Todos ellos llevan a cabo integración Köhler y presentan una lente de Fresnel como su elemento óptico primario. Cada uno de estos concentradores CPV presenta sus propias propiedades y su propio procedimiento de diseño. Además, presentan todas las características que todo concentrador ha de tener: elevado factor de concentración, alta tolerancia de fabricación, alta eficiencia óptica, irradiancia uniforme sobre la superficie de la célula y bajo coste de producción. Los concentradores FK y DFK presentan una configuración de cuatro sectores para lograr la integración Köhler. Esto quiere decir que POE y SOE se dividen en cuatro sectores simétricos cada uno, y cada sector del POE trabaja conjuntamente con su correspondiente sector de SOE. La principal diferencia entre los dos concentradores es que el POE del FK es una lente de Fresnel plana, mientras que una lente curva de Fresnel es empleada como POE del DFK. El concentrador CFK incluye una cavidad de confinamiento externo integrada, que es un elemento óptico capaz de recuperar los rayos reflejados por la superficie de la célula con el fin de ser reabsorbidos por la misma. Por tanto, se aumenta la absorción de la luz, lo que implica un aumento en la eficiencia del módulo. Además, este capítulo también explica un método de diseño alternativo para los elementos faceteados, especialmente adecuado para las lentes curvas como el POE del DFK. El capítulo 3 se centra en la caracterización y medidas experimentales de los concentradores ópticos presentados en el capítulo 2, y describe sus procedimientos. Estos procedimientos son en general aplicables a cualquier concentrador basado en una lente de Fresnel, e incluyen tres tipos principales de medidas experimentales: eficiencia eléctrica, ángulo de aceptancia y uniformidad de la irradiancia en el plano de la célula. Los resultados que se muestran a lo largo de este capítulo validarán a través de medidas a sol real las características avanzadas que presentan los concentradores Köhler, y que se demuestran en el capítulo 2 mediante simulaciones de rayos. Cada concentrador (FK, DFK y CFK) está diseñado y optimizado teniendo en cuenta condiciones de operación realistas. Su rendimiento se modela de forma exhaustiva mediante el trazado de rayos en combinación con modelos distribuidos para la célula. La tolerancia es un asunto crítico de cara al proceso de fabricación, y ha de ser máxima para obtener sistemas de producción en masa rentables. Concentradores con tolerancias limitadas generan bajadas significativas de eficiencia a nivel de array, causadas por el desajuste de corrientes entre los diferentes módulos (principalmente debido a errores de alineación en la fabricación). En este sentido, la sección 3.5 presenta dos métodos matemáticos que estiman estas pérdidas por desajuste a nivel de array mediante un análisis de sus curvas I-V, y por tanto siendo innecesarias las medidas a nivel de mono-módulo. El capítulo 3 también describe la caracterización indoor de los elementos ópticos que componen los concentradores, es decir, de las lentes de Fresnel que actúan como POE y de los secundarios free-form. El objetivo de esta caracterización es el de evaluar los adecuados perfiles de las superficies y las transmisiones ópticas de los diferentes elementos analizados, y así hacer que el rendimiento del módulo sea el esperado. Esta tesis la cierra el capítulo 4, en el que la integración Köhler se presenta como una buena alternativa para obtener distribuciones uniformes en aplicaciones de iluminación de estado sólido (iluminación con LED), siendo particularmente eficaz cuando se requiere adicionalmente una buena mezcla de colores. En este capítulo esto se muestra a través del ejemplo particular de un concentrador DFK, el cual se ha utilizado para aplicaciones CPV en los capítulos anteriores. Otra alternativa para lograr mezclas cromáticas apropiadas está basada en un método ya conocido (deflexiones anómalas), y también se ha utilizado aquí para diseñar una lente TIR aplanética delgada. Esta lente cumple la conservación de étendue, asegurando así que no hay bloqueo ni dilución de luz simultáneamente. Ambos enfoques presentan claras ventajas sobre las técnicas clásicas empleadas en iluminación para obtener distribuciones de iluminación uniforme: difusores y mezcla caleidoscópica mediante guías de luz. ABSTRACT Concentrating Photovoltaics (CPV) is one of the most promising ways of reducing the cost of energy collected from the sun. This is possible thanks to both, very high-efficiency solar cells and a large decrease in the size of cells, which are made of costly semiconductor materials. Both issues are closely linked since high efficiency values are only possible with expensive cell materials and technologies, implying a compulsory area reduction if cost-effectiveness is desired. The reduction in the cell size requires that light coming from the sun must be redirected (i.e. concentrated) towards the cell position. This is achieved by placing an optical concentrator system on top of the cell. These CPV concentrators consist of different optical elements manufactured on cheap materials in order to maintain low production costs. The optimal framework for the design of concentrators is nonimaging optics. Nonimaging optics was first developed in the 60s decade and has been largely developed ever since. The aim of nonimaging devices is the efficient transfer of light power between the source and the receiver (sun and cell respectively in the case of CPV), disregarding image formation. Nonimaging systems are usually simple, comprised of fewer surfaces than imaging systems and are more tolerant to manufacturing errors. This renders nonimaging optics a fundamental tool, not only in the design of photovoltaic concentrators, but also in the design of other applications as illumination, projection and wireless optical communications. Nonimaging optical concentrators are well suited for CPV applications because the goal is not the reproduction of an exact image of the sun (as imaging optics would provide), but simply the collection of its energy on the solar cell. Concentrators for CPV may present very different architectures and optical elements, resulting in a vast variety of possible designs. The first optical element that sunlight goes through is called the Primary Optical Element (POE) and is the most determinant element in order to define the shape and properties of the whole concentrator. The POE can be either refractive (lens) or reflective (mirror). This thesis focuses on CPV systems based on Fresnel lenses as POE, which are thin and inexpensive refractive lenses able to concentrate sunlight. Chapter 1 exposes a short introduction to geometrical and nonimaging optics, explaining their fundamentals and basic concepts. Then, the Köhler integration is presented in detail, explaining its principles, valid for both applications: CPV and illumination. An introduction to CPV fundamental concepts is also included in this chapter, analyzing the properties of multijunction solar cells and optical concentrators employed in CPV systems. The chapter is closed with a description of the existing technologies employed for the manufacture of optical elements composing the concentrator. Chapter 2 is mainly devoted to the design and development of the three advanced Fresnel Köhler optical concentrators presented in this thesis work: Fresnel-Köhler (FK), Dome-shaped Fresnel-Köhler (DFK) and Cavity Fresnel-Köhler (CFK). They all perform Köhler integration and comprise a Fresnel lens as their Primary Optical Element. Each one of these CPV concentrators presents its own characteristics, properties and its own design procedure. Their performances include all the key issues in a concentrator: high concentration factor, large tolerances, high optical efficiency, uniform irradiance on the cell surface and low production cost. The FK and DFK concentrators present a 4-fold configuration in order to perform the Köhler integration. This means that POE and SOE are divided into four symmetric sectors each one, working each POE sector with its corresponding SOE sector by pairs. The main difference between both concentrators is that the POE of the FK is a flat Fresnel lens, while a dome-shaped (curved) Fresnel lens performs as the DFK’s POE. The CFK concentrator includes an integrated external confinement cavity, which is an optical element able to recover rays reflected by the cell surface in order to be re-absorbed by the cell. It increases the light absorption, entailing an increase in the efficiency of the module. Additionally, an alternative design method for faceted elements will also be explained, especially suitable for dome-shaped lenses as the POE of the DFK. Chapter 3 focuses on the characterization and experimental measurements of the optical concentrators presented in Chapter 2, describing their procedures. These procedures are in general applicable to any Fresnel-based concentrator as well and include three main types of experimental measurements: electrical efficiency, acceptance angle and irradiance uniformity at the solar cell plane. The results shown along this chapter will validate through outdoor measurements under real sun operation the advanced characteristics presented by the Köhler concentrators, which are demonstrated in Chapter 2 through raytrace simulation: high optical efficiency, large acceptance angle, insensitivity to manufacturing tolerances and very good irradiance uniformity on the cell surface. Each concentrator (FK, DFK and CFK) is designed and optimized looking at realistic performance characteristics. Their performances are modeled exhaustively using ray tracing combined with cell modeling, taking into account the major relevant factors. The tolerance is a critical issue when coming to the manufacturing process in order to obtain cost-effective mass-production systems. Concentrators with tight tolerances result in significant efficiency drops at array level caused by current mismatch among different modules (mainly due to manufacturing alignment errors). In this sense, Section 3.5 presents two mathematical methods that estimate these mismatch losses for a given array just by analyzing its full-array I-V curve, hence being unnecessary any single mono-module measurement. Chapter 3 also describes the indoor characterization of the optical elements composing the concentrators, i.e. the Fresnel lenses acting as POEs and the free-form SOEs. The aim of this characterization is to assess the proper surface profiles and optical transmissions of the different elements analyzed, so they will allow for the expected module performance. This thesis is closed by Chapter 4, in which Köhler integration is presented as a good approach to obtain uniform distributions in Solid State Lighting applications (i.e. illumination with LEDs), being particularly effective when dealing with color mixing requirements. This chapter shows it through the particular example of a DFK concentrator, which has been used for CPV applications in the previous chapters. An alternative known method for color mixing purposes (anomalous deflections) has also been used to design a thin aplanatic TIR lens. This lens fulfills conservation of étendue, thus ensuring no light blocking and no light dilution at the same time. Both approaches present clear advantages over the classical techniques employed in lighting to obtain uniform illumination distributions: diffusers and kaleidoscopic lightpipe mixing.

Optically triggered infrared photodetector

We demonstrate a new class of semiconductor device: the optically triggered infrared photodetector (OTIP). This photodetector is based on a new physical principle that allows the detection of infrared light to be switched ON and OFF by means of an external light. Our experimental device, fabricated using InAs/AlGaAs quantum-dot technology, demonstrates normal incidence infrared detection in the 2−6 μm range. The detection is optically triggered by a 590 nm light-emitting diode. Furthermore, the detection gain is achieved in our device without an increase of the noise level. The novel characteristics of OTIPs open up new possibilities for third generation infrared imaging systems

Optically triggered infrared photodetector

We demonstrate a new class of semiconductor device: the optically triggered infrared photodetector (OTIP). This photodetector is based on a new physical principle that allows the detection of infrared light to be switched ON and OFF by means of an external light. Our experimental device, fabricated using InAs/AlGaAs quantum-dot technology, demonstrates normal incidence infrared detection in the 2−6 μm range. The detection is optically triggered by a 590 nm light-emitting diode. Furthermore, the detection gain is achieved in our device without an increase of the noise level. The novel characteristics of OTIPs open up new possibilities for third generation infrared imaging systems

Aladdin's Magic Lamp: Developing Methods for Calibration and Geolocation Accuracy Assessment of the DMSP OLS

Nighttime satellite imagery from the Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS) has a unique capability to observe nocturnal light emissions from sources including cities, wild fires, and gas flares. Data from the DMSP OLS is used in a wide range of studies including mapping urban areas, estimating informal economies, and estimating urban populations. Given the extensive and increasing list of applications a repeatable method for assessing geolocation accuracy, performing inter-calibration, and defining the minimum detectable brightness would be beneficial. An array of portable lights was designed and taken to multiple field sites known to have no other light sources. The lights were operated during nighttime overpasses by the DMSP OLS and observed in the imagery. A first estimate of the minimum detectable brightness is presented based on the field experiments conducted. An assessment of the geolocation accuracy was performed by measuring the distance between the GPS measured location of the lights and the observed location in the imagery. A systematic shift was observed and the mean distance was measured at 2.9km. A method for in situ radiance calibration of the DMSP OLS using a ground based light source as an active target is presented. The wattage of light used by the active target strongly correlates with the signal measured by the DMSP OLS. This approach can be used to enhance our ability to make inter-temporal and inter-satellite comparisons of DMSP OLS imagery. Exploring the possibility of establishing a permanent active target for the calibration of nocturnal imaging systems is recommended. The methods used to assess the minimum detectable brightness, assess the geolocation accuracy, and build inter-calibration models lay the ground work for assessing the energy expended on light emitted into the sky at night. An estimate of the total energy consumed to light the night sky globally is presented.

Software for "Realistic limits for subpixel movement detection"

Object tracking with subpixel accuracy is of fundamental importance in many fields since it provides optimal performance at relatively low-cost. Although there are many theoretical proposals that lead to resolution increments of several orders of magnitude, in practice, this resolution is limited by the imaging systems. In this paper we propose and demonstrate through numerical models a realistic limit for subpixel accuracy. The final result is that maximum achievable resolution enhancement is connected with the dynamic range of the image, i.e. the detection limit is 1/2^(nr.bits). Results here presented may help to proper design of superresolution experiments in microscopy, surveillance, defense and other fields.

Realistic limits for subpixel movement detection

Object tracking with subpixel accuracy is of fundamental importance in many fields since it provides optimal performance at relatively low cost. Although there are many theoretical proposals that lead to resolution increments of several orders of magnitude, in practice this resolution is limited by the imaging systems. In this paper we propose and demonstrate through simple numerical models a realistic limit for subpixel accuracy. The final result is that maximum achievable resolution enhancement is connected with the dynamic range of the image, i.e., the detection limit is 1/2∧(nr.bits). The results here presented may aid in proper design of superresolution experiments in microscopy, surveillance, defense, and other fields.

Shell-based chronologies for the Irish Sea

We demonstrate here that the growth increment variability in the shell of the long-lived bivalve mollusc Arctica islandica can be interpreted as an indicator of marine environmental change in the climatically important North Atlantic shelf seas. Multi-centennial (up to 489-year) chronologies were constructed using five detrending techniques and their characteristics compared. The strength of the common environmental signal expressed in the chronologies was found to be fully comparable with equivalent statistics for tree-ring chronologies. The negative exponential function using truncated increment-width series from which the first thirty years have been removed was chosen as the optimal detrending technique. Chronology indices were compared with the Central England Temperature record and with seawater temperature records from stations close to the study site in the Irish Sea. Statistically significant correlations were found between the chronology indices and (a) mean air temperature for the 14-month period beginning in the January preceding the year of growth, (b) mean seawater temperatures for February-October in the year preceding the year of growth (c) late summer and autumn air temperatures and sea surface temperatures for the year of growth and (d) the timing of the autumn decline in SST. Changes through time in the correlations with air and seawater temperatures and changes towards a deeper water origin for the shells in the chronology were interpreted as an indication that shell growth may respond to stratification dynamics.

Procedures for analyzing data obtained from LANDSAT multispectral imagery /

Thesis (M. S.)--University of Illinois at Urbana-Champaign.

LAWS : laser atmospheric wind sounder : instrument panel report.

Shipping list no.: 87-731-P.

Magnetic Devices and Techniques for the Study of Viscoelasticity of Biomaterials and Myocardial Forces

Thesis (Ph.D.)--University of Washington, 2016-06

Remotely sensed data for ecosystem analyses: Combining hierarchy and scene models

Remotely sensed data have been used extensively for environmental monitoring and modeling at a number of spatial scales; however, a limited range of satellite imaging systems often. constrained the scales of these analyses. A wider variety of data sets is now available, allowing image data to be selected to match the scale of environmental structure(s) or process(es) being examined. A framework is presented for use by environmental scientists and managers, enabling their spatial data collection needs to be linked to a suitable form of remotely sensed data. A six-step approach is used, combining image spatial analysis and scaling tools, within the context of hierarchy theory. The main steps involved are: (1) identification of information requirements for the monitoring or management problem; (2) development of ideal image dimensions (scene model), (3) exploratory analysis of existing remotely sensed data using scaling techniques, (4) selection and evaluation of suitable remotely sensed data based on the scene model, (5) selection of suitable spatial analytic techniques to meet information requirements, and (6) cost-benefit analysis. Results from a case study show that the framework provided an objective mechanism to identify relevant aspects of the monitoring problem and environmental characteristics for selecting remotely sensed data and analysis techniques.

Mud, algae or seagrass: An assessment of remote sensing techniques for mapping inter-tidal seagrass cover and composition in turbid tropical environments

Large areas of tropical sub- and inter-tidal seagrass beds occur in highly turbid environments and cannot be mapped through the water column. The purpose of this project was to determine if and how airborne and satellite imaging systems could be used to map inter-tidal seagrass properties along the wet-tropics coast in north Queensland, Australia. The work aimed to: (1) identify the minimum level of seagrass foliage cover that could be detected from airborne and satellite imagery; and (2) define the minimum detectable differences in seagrass foliage cover in exposed intertidal seagrass beds. High resolution spectral-reflectance data (2040 bands, 350 – 2500nm) were collected over 40cm diameter plots from 240 sites on Magnetic Island, Pallarenda Beach and Green Island in North Queensland at spring low tides in April 2006. The seagrass species sampled were: Thalassia hemprechii, Halophila ovalis, Halodule uninerivs; Syringodium isoetifolium, Cymodocea serrulata, and Cymodoea rotundata. Digital photos were captured for each plot and used to derive estimates of seagrass species cover, epiphytic growth, micro- and macro-algal cover, and substrate colour. Sediment samples were also collected and analysed to measure the concentration of Chlorophyll-a associated with benthic micro-algae. The field reflectance spectra were analysed in combination with their corresponding seagrass species foliage cover levels to establish the minimum foliage projective cover required for each seagrass to be significantly different from bare substrate and substrate with algal cover. This analysis was repeated with reflectance spectra resampled to the bandpass functions of Quickbird, Ikonos, SPOT 5 and Landsat 7 ETM. Preliminary results indicate that conservative minimum detectable seagrass cover levels across most the species sampled were between 30%- 35% on dark substrates. Further analysis of these results will be conducted to determine their separability and satellite images and to assess the effects epiphytes and algal cover.

Imprinted hydrogels for tunable hemispherical microlenses

We report on the development of an ultraviolet curable hydrogel, based on combinations of poly(ethylene glycol) dimethacrylate (PEGMA), acrylic acid (AA) and N-Isopropylacrylamide (NIPPAm) for imprint lithography processes. The hydrogel was successfully imprinted to form dynamic microlens arrays. The response rate of the microlenses by volume change to water absorption was studied optically showing tunable focalisation of the light. Important optical refractive index change was measured between the dry and wet state of the microlenses. Our work suggests the use of this newly developed printable hydrogel for various imprinted components for sensing and imaging systems. © 2013 Elsevier B.V. All rights reserved.

Image Sensors in Security and Medical Applications

This paper briefly reviews CMOS image sensor technology and its utilization in security and medical applications. The role and future trends of image sensors in each of the applications are discussed. To provide the reader deeper understanding of the technology aspects the paper concentrates on the selected applications such as surveillance, biometrics, capsule endoscopy and artificial retina. The reasons for concentrating on these applications are due to their importance in our daily life and because they present leading-edge applications for imaging systems research and development. In addition, review of image sensors implementation in these applications allows the reader to investigate image sensor technology from the technical and from other views as well.