Triple-junction solar cell performance under Fresnel-based concentrators taking into account chromatic aberration and off-axis operation

Concentration photovoltaic (CPV) systems might produce quite uneven irradiance distributions (both on their level and on their spectral distribution) on the solar cell. This effect can be even more evident when the CPV system is slightly off-axis, since they are often designed to assure good uniformity only at normal incidence. The non-uniformities both in absolute irradiance and spectral content produced by the CPV systems, can originate electrical losses in multi-junction solar cells (MJSC). This works is focused on the integration of ray-tracing methods for simulating the irradiance and spectrum maps produced by different optic systems throughout the solar cell surface, with a 3D fully distributed circuit model which simulates the electrical behavior of a state-of-the-art triple-junction solar cell under the different light distributions obtained with ray-tracing. In this study four different CPV system (SILO, XTP, RTP, and FK) comprising Fresnel lenses concentrating sunlight onto the same solar cell are modeled when working on-axis and 0.6 degrees off-axis. In this study the impact of non-uniformities on a CPV system behavior is revealed. The FK outperforms other Fresnel-based CPV systems in both on-axis and off-axis conditions.

The Dome-Shaped Fresnel-Köhler Concentrator

Manufacturing tolerances, along with a high concentration ratio, are key issues in order to obtain cheap CPV systems for mass production. Consequently, this manuscript presents a novel tolerant and cost effective concentrator optic: the domed-shaped Fresnel-Köhler, presenting a curved Fresnel lens as Primary Optical Element (POE). This concentrator is based on two previous successful CPV designs: the FK concentrator, based on a flat Fresnel lens, and the dome-shaped Fresnel lens system developed by Daido Steel, resulting on a superior concentrator. The manuscript shows outstanding simulation results for geometrical concentration factor of Cg? = ?1,230x: high tolerance and high optical efficiency, achieving acceptance angles of 1.18° (dealing to a CAP?=0.72) and efficiencies over 85% (without any anti-reflective coating). Moreover, Köhler integration provides good irradiance uniformity on the cell surface without increasing system complexity by means of any extra element. Daido Steel advanced technique for demolding injected plastic pieces will allow for easy manufacture of the dome-shaped POE of DFK concentrator.

Photovoltaic performance of the dome-shaped Fresnel-Köhler concentrator

In order to have a cost-effective CPV system, two key issues must be ensured: high concentration factor and high tolerance. The novel concentrator we are presenting, the dome-shaped Fresnel-Köhler, can widely fulfill these two and other essential issues in a CPV module. This concentrator is based on two previous successful CPV designs: the FK concentrator with a flat Fresnel lens and the dome-shaped Fresnel lens system developed by Daido Steel, resulting on a superior concentrator. The concentrator has shown outstanding simulation results, achieving an effective concentration-acceptance product (CAP) value of 0.72, and an optical efficiency of 85% on-axis (no anti-reflective coating has been used). Moreover, Köhler integration provides good irradiance uniformity on the cell surface and low spectral aberration of this irradiance. This ensures an optimal performance of the solar cell, maximizing its efficiency. Besides, the dome-shaped FK shows optimal results for very compact designs, especially in the f/0.7-1.0 range. The dome-shaped Fresnel-Köhler concentrator, natural and enhanced evolution of the flat FK concentrator, is a cost-effective CPV optical design, mainly due to its high tolerances. Daido Steel advanced technique for demolding injected plastic pieces will allow for easy manufacture of the dome-shaped POE of DFK concentrator.

Dome-shaped Fresnel-Köhler: a novel high performance optical concentrator

The dome-shaped Fresnel-Köhler concentrator is a novel optical design for photovoltaic applications. It is based on two previous successful CPV optical designs: the FK concentrator with a flat Fresnel lens and the dome-shaped Fresnel lens system developed by Daido Steel, resulting on a superior concentrator. This optical concentrator will be able to achieve large concentration factors, high tolerance (i.e. acceptance angle) and high optical efficiency, three key issues when dealing with photovoltaic applications. Besides, its irradiance is distributed on the cell surface in a very even way. The concentrator has shown outstanding simulation results, achieving an effective concentration-acceptance product (CAP) value of 0.72, on-axis optical efficiency over 85% and good irradiance uniformity on the cell provided by Köhler integration. Furthermore, due to its high tolerance, we will present the dome-shaped Fresnel-Köhler concentrator as a cost-effective CPV optical design. All this makes this concentrator superior to other conventional competitors in the current market.

Aplanatic thin TIR lens

Aplanatic designs present great interest in the optics field since they are free from spherical aberration and linear coma at the axial direction. Nevertheless nowadays it cannot be found on literature any thin aplanatic design based on a lens. This work presents the first aplanatic thin lens (in this case a dome-shaped faceted TIR lens performing light collimation), designed for LED illumination applications. This device, due to its TIR structure (defined as an anomalous microstructure as we will see) presents good color-mixing properties. We will show this by means of raytrace simulations, as well as high optical efficiency.

Design and Development of 35 % Efficient and lOOOX CPV Module with Sufficient Optical Alignment Tolerance

Modules are an important part of the CPV system. By pursing, in our objective of a 35% efficiency module, we need to look forward a significant improvement in the state of the art of CPV modules since no commercial module is capable of achieving that efficiency. Achieving this efficiency will require high efficiency cells, progress in the optics lenses that are implemented in these modules, and also integration into module. Basic design of 35% CPV module is presented considering for practical and rapid industry application. The output is 385 W while its weight is only 18 kg. In spite of its high concentration ratio reaching 1,000 X, it acceptance angle is as high as 1.1 degree.

Freeform optics applications in photovoltaic concentration

Freeform surfaces are the key of the state-of-the-art nonimaging optics to solve the challenges in concentration photovoltaics. Different families (FK, XR, FRXI) will be presented, based on the SMS 3D design method and Köhler homogenization.

Monitorización y seguimiento del esfuerzo realizado por los estudiantes y de su asistencia a actividades presenciales

Este artículo documenta el planteamiento, la metodología y los primeros resultados de un plan de monitorización detallada del esfuerzo y de asistencia a actividades presenciales por parte de los estudiantes de las titulaciones ofertadas por la Escuela Técnica Superior de Ingenieros Navales de la Universidad Politécnica de Madrid durante el segundo cuatrimestre del curso 2011-2012. Se ha establecido un sistema mecánico de recogida de datos de esfuerzo por parte de los estudiantes utilizando una hoja tipo test especialmente configurada al efecto. Se pasa una hoja en todas y cada una de las actividades presenciales realizadas y en la hoja se solicita información sobre el trabajo "fuera de clase". Se documenta en este artículo cómo se ha estructurado esa hoja, qué tipo de datos se recogen, cómo se tratan mediante una base de datos creada al efecto, qué tipo de análisis se puede realizar y qué resultados preliminares obtenemos de dichos análisis.

Variante de la carretera CL-605 en Barroman(Avila)

La carretera CL-605 de Segovia a Zamora se engloba en la Red Básica de Carreteras de la Junta de Castilla y León, con una longitud de unos 170 kms que se desarrollan en las provincias de Segovia, Ávila, Salamanca y Zamora. Su paso por la provincia de Ávila se sitúa entre sus P.P.K.K. aproximados 54+350 y 97+130, atravesando los términos municipales de Arévalo, Aldeaseca, Villanueva del Aceral, Barromán y Madrigal de las Altas Torres. En los cinco casos, las correspondientes localidades se sitúan en las inmediaciones de la carretera, aunque esta atraviesa la zona urbana definiendo una travesía de población sólo en los casos de Arévalo, Aldeaseca y Barromán. Este proyecto tiene por objeto definir y valorar las obras necesarias para la construcción de la variante de la última de las poblaciones indicadas, Barromán, desarrollando la alternativa elegida de las planteadas en el “Estudio de Alternativas” redactado en el mes de enero de 2012. En dicho estudio se procedió a definir cuatro posibles trazados, estimando la valoración de las obras a realizar para su construcción y definiendo y evaluando los criterios que permitieron realizar una comparación del carácter más objetivo posible entre las cuatro alternativas.

Free-form optics for Fresnel-lens-based photovoltaic concentrators

The Concentrated Photovoltaics (CPV) promise relies upon the use of high-efficiency triple-junction solar cells (with proven efficiencies of over 44%) and upon high-performance optics that allow for high concentration concurrent with relaxed manufacturing tolerances (all key elements for low-cost mass production). Additionally, uniform illumination is highly desirable for efficiency and reliability reasons. All of these features have to be achieved with inexpensive optics containing only a few (in general no more than 2) optical elements. In this paper we show that the degrees of freedom using free-forms allow the introduction of multiple functionalities required for CPV with just 2 optical elements, one of which is a Fresnel lens.

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.

Wide-angle, high-concentration photovoltaics to compete with flat plate systems

A novel photovoltaic concentrator enables highly uniform irradiance on a small number of efficient solar cells. The maximum electrical power of a photovoltaic (PV) energy installation depends on three factors: the available irradiance, the size of the systems collecting sunlight, and the rate at which the device transforms light into electricity (the conversion efficiency). Developers can maximize the irradiance by carefully selecting the site and orientation of the solar facility. But they can only expand their sunlight collection systems for standard flat plate PV devices by increasing the number of solar cells, at greater cost. Here, we consider the advantages of an alternative PV system that produces more energy without increasing the number of cells used (actually, reducing it), by improving the conversion rates.We also present a new device that may enhance the commercial viability of such technologies.

Development and characterization of an FK photovoltaic concentrator for maximum conversion efficiency

The outdoor measurements of a single-cell concentrator PV module reaching a regressed 35.6% efficiency and a maximum peak efficiency of 36.0% (both corrected @Tcell=25ºC) are presented. This is the result of the joint effort by LPI and Solar Junction to demonstrate the potential of combining their respective state-of-the-art concentrator optics and solar cells. The LPI concentrator used is an FK, which is an advanced nonimaging concentrator using 4-channel Köhler homogenization, with a primary Fresnel lens and a refractive secondary made of glass. Solar Junction’s cell is a triplejunction solar cell with the A-SLAMTM architecture using dilute-nitrides.

9-fold Fresnel-Köhler concentrator for increased uniform irradiance on high concentrations

Non-uniform irradiance patterns created by Concentrated Photovoltaics (CPV) concentrators over Multi-Junction Cells (MJC) can originate significant power losses, especially when there are different spectral irradiance distributions over the different MJC junctions. This fact has an increased importance considering the recent advances in 4 and 5 junction cells. The spectral irradiance distributions are especially affected with thermal effects on Silicone-on-Glass (SoG) CPV systems. This work presents a new CPV optical design, the 9-fold Fresnel Köhler concentrator, prepared to overcome these effects at high concentrations while maintaining a large acceptance angle, paving the way for a future generation of high efficiency CPV systems of 4 and 5 junction cells.

VentanaTM power train features and performance

Most CPV systems are based on Fresnel lenses. Among these, LPI-patented Fresnel-Köhler (FK) concentrator outstands owing to performance and practical reasons. The VentanaTM power train is the first off-the-shelf commercial product based on the FK and comprises both the primary (POE) lenses (a 36-units 1×1 m2 acrylic panel manufactured by EVONIK and 10×) and glass (or Savosil) secondary optics (SOE). This high concentration optical train (Cg=1,024×, ~250mm optical depth) fits with 5×5 mm2 (at least) solar cells. The optical train is the fruit of a 1-year development that has included design, modeling, prototyping and characterization, and through the process LPI had the opportunity to find out how well the actual performance correlates with models, but also learned practical aspects of a CPV system of this kind, some of which have very positive impact on system performance and reliability.