1,720,962 research outputs found
A NUMERICAL APPROACH TO OHMIC LOSSES ASSESSMENT IN CONCENTRATING PHOTOVOLTAIC SYSTEMS
No full textIt is well known that concentrator solar cells operating under concentration experience a number of physical effects which affect their performances. In particular, ohmic losses can determine a noticeable performance worsening of concentrator solar cells. The goal of this dissertation is to develop a distributed electrical model of solar cell in order to simulate the operation of concentrator solar cells in a number of working conditions characteristic of Concentrating Photovoltaic (CPV) systems, such as uneven illumination profiles with arbitrary spectral distributions. To this end a mixed optical-electrical simulation tool has been developed in order to assess the performances of a typical concentrator solar cell in the case of illumination provided by different kinds of concentrators; in particular a Fresnel lens, a parabolic mirror and a freeform mirror have been considered and compared. At high concentration factors front contact grid pattern has a key role in extracting photogenerated charges, and hence it is another factor that can strongly affect the cell performances; for this reason the above mentioned distributed electrical model has also been applied to the assessment of ohmic losses impact on concentrator silicon solar cells performances in the presence of different kinds of front contact grid patterns. In particular, a comb-like geometry, a square-like geometry and a novel fractal autosimilar geometry have been simulated and compared. Another aspect investigated in this dissertation is the formation of voids in the solder joint region, during soldering process of concentrator solar cells to Metal Core Printed Circuit Boards (MC-PCB). Some commercially available silicon solar cells have been soldered in such a way that a great number of voids have arisen and their distribution has been revealed by X-ray inspection. Electrical and thermal behaviour of one of the cells has been assessed by a joint thermal-electrical simulation tool. In this thesis electrical, optical and thermal simulations have been performed by means of ORCAD PSPICE software, ZEMAX software and ADINA 8.7 software, respectively
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
A joint thermal-electrical analysis of void formation effects on concentrator silicon solar cells solder layer
No full textIt is known that the formation of voids in solar cell solder joints leads to a worsening of their heat sinking capabilities, causing an increase in the average device temperature and thermal resistance. This phenomenon can be detrimental for the solar cell's performance, since the open-circuit voltage linearly decreases with temperature. The performances of silicon solar cells, in the presence of voids in the cell solder joint, are studied by means of numerical simulations and of an analytical thermal model which can assess the local temperature increase at cell surface due to a single isolated void in the solder joint. The results show that for small isolated voids the analytical model gives temperature peaks above the voids which match very well with the simulations result, within 5% of relative error. The analytical model also gives an estimation of the whole device thermal resistance, in the presence of a regular pattern of non-interacting voids all with the same surface area. For a 10×7 pattern of small area voids the analytical value for the device thermal resistance matches well with the results of numerical simulations, with a maximum error of 17.3% at 70% void coverage. To determine the temperature profile of the device surface we have implemented a thermal finite element analysis (FEA) which employs a detailed 3D model of the real morphology of voids in the solar cell, obtained by X-ray inspection. The resulting temperature map has been used as an input parameter for the subsequent electrical simulations performed by means of PSPICE software, which is based on a distributed 2.5 D electrical model of the solar cell. Results show that, for a concentrating factor of 100×, a real void pattern with 36.6% void coverage does not noticeably affect the performances of a concentrator silicon solar cell
Finite Element Model for Thermal Analysis of a High-Concentration Modular CPV System
No full textAiming at assess and optimize the thermal budget during operative condition of an innovative modular point-focus concentrating photovoltaic (CPV) system, a finite element thermal analysis have been carried out in this work. The CPV system has been designed and developed at University of Ferrara, as a part of Europen APOLLON project. Each module is comprised of a free-form 20 x 20 cm2 polycarbonate reflector and a secondary reflector in Cassegrain configuration. An aluminum high reflection homogenizer is placed in front of the solar cell and provides a further concentration level. The particular concentrating optic design offers unique possibility to tune the concentrating factor, ranging from 100x up to 400x, while maintaining an outstanding evenness of the irradiance on the cells, only by selecting the aperture angle of the aluminum homogeneizer. 3D simulations have been performed by means of a detailed finite element (FE) model in which both structure and fluid element are included. Solar cell temperature has been simulated and optimized as a function of thermal coupling technical solution with the heat sink. Heat transfer due to natural air convection induced by the thermal buoyancy force inside the CPV enclosure has been also taken into account. Results predicts a quasi equilibrium temperature of solar cell in operative conditions always under 80°C, even without any finned heat sink on the back of the module
Effects of Irradiance Distribution Unevenness on the Ohmic Losses of CPV Receivers
No full textConcentrating photovoltaic (CPV) systems use solar collectors to focus the incoming solar radiation
onto PV receivers equipped with small area solar cells. The irradiance profile at the PV receiver plane is strongly
dependent on the concentrator architecture and has deep influence on the performances of the solar cells. In this
work we will describe an approach matching ray-tracing simulations and circuit analysis to assess the ohmic losses of
the PV receiver operating under uneven illumination.
In this investigation we considered three types of solar concentrator optics (Fresnel lens, parabolic mirror and
freeform mirror) having different flux distribution in the focal region. The optical simulations have been carried out
taking into account the real radial distribution (sunshape) of the incident solar energy. The irradiance profile at the
PV cells plane has then been used as a map for the photo-generated current to be collected and drawn from the
devices. The cells have been binned into a grid of generators connected between them by a resistive network
simulating the metal contacts and the semiconductor layers. The current-voltage characteristics of real solar cells has
been measured and modeled by an array of diodes connected in parallel to each generator. The nonlinear circuit has
been solved by a simulation program with integrated circuit emphasis (SPICE
Analysis of Non-Conventional Front Contact Patterns Impact on Concentrator Solar Cells Performances Through a 2.5-D Distributed Electrical Model
No full textA 2.5-D distributed circuit model for concentrator silicon solar cells has been developed in order to assess cell performance in the presence of particular top contact grid pattern geometries. Three kinds of front contact patterns have been considered: a conventional comb-like pattern, a square-like pattern and a fractal (auto-similar) one. Typical electrical parameters have been chosen for bulk resistivity, emitter resistivity, metal-semiconductor contact resistivity and metal resistivity. In this way an accurate assessment of ohmic losses can be carried out. Simulations have been performed by means of ORCAD PSPICE software. A 100 μm-wide, 5 μm-thick contact frame has been applied to the cell perimeter and the current was supposed to be extracted by four points, in correspondence of the mid points of the four sides of the cell. The simulations have been carried out under the assumption of a uniform irradiance distribution incident onto the cell surface (an arbitrary irradiance distribution however can be simulated) with a concentration factor of 50. For each of the three structures geometrical parameters such as finger width, finger spacing, total coverage and number of autosimilarity levels have been varied in order to achieve the optimal performance in terms of power delivered by the cell, for three different values of cell dimensions, 1 cm × 1 cm, 1.5 cm × 1.5 cm and 2 cm × 2 cm. Simulations results show that fractal contact grid pattern provides the best performance in all three cases and the power difference increases with increasing cell dimensions; for a 2 cm × 2 cm cell, difference between fractal and square patterns reaches 13%
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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