1,720,967 research outputs found
Autonomous Airborne Wind Energy Systems: Accomplishments and Challenges
No full textAirborne wind energy (AWE) is a fascinating technology to convert wind power into electricity with an autonomous tethered aircraft. Deemed a potentially game-changing solution, AWE is attracting the attention of policy makers and stakeholders with the promise of producing large amounts of cost-competitive electricity with wide applicability worldwide. Since the pioneering experimental endeavors in the years 2000-2010, there has been a clear technology convergence trend and steady progress in the field. Today, AWE systems can operate automatically with minimal supervision in all operational phases. A first product is also being commercialized. However, all-around fully autonomous operation still presents important fundamental challenges that are conceptually similar to those of other systems that promise to change our lives, such as fully autonomous passenger cars or service drones. At the same time, autonomous operation is necessary to enable large-scale AWE, thus combining challenging fundamental problems with high potential impact on society and the economy. This article describes the state of the art of this technology from a system perspective and with a critical view on some fundamental aspects, presents the latest automatic control results by prominent industrial players, and finally points out the most important challenges on the road to fully autonomous AWE systems
Record VOC-values for thin-film polysilicon solar cells on foreign substrates using a heterojunction emitter
No full textThin-film polysillicon solar cells on foreign substrates are often considered as promising low cost alternative to bulk silicon solar cells. Until now however, the obtained efficiencies and open-circuit voltages are far below those of other technologies. In this paper, we show how the open-circuit voltage can be enhanced significantly using an amorphous silicon-crystalline silicon heterojunction emitter instead of a diffused homojunction emitter. Open-circuit voltages up to 536 mV were obtained for polysilicon solar cells with a p-n structure on foreign substrates
Record VOC-values for thin-film polysilicon solar cells on foreign substrates using a heterojunction emitter
No full textThin-film polysillicon solar cells on foreign substrates are often considered as promising low cost alternative to bulk silicon solar cells. Until now however, the obtained efficiencies and open-circuit voltages are far below those of other technologies. In this paper, we show how the open-circuit voltage can be enhanced significantly using an amorphous silicon-crystalline silicon heterojunction emitter instead of a diffused homojunction emitter. Open-circuit voltages up to 536 mV were obtained for polysilicon solar cells with a p-n structure on foreign substrates
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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
Influence of seed layer morphology on the epitaxial growth of polycrystalline-silicon solar cells
No full textThin-film silicon solar cells on low-cost foreign substrates could lead to a large cost reduction of photovoltaic electricity if sufficiently high efficiencies could be obtained. A possible approach is to make polycrystalline-silicon solar cells by epitaxial thickening of large-grained thin seed layers made by aluminium-induced crystallization (AIC) of silicon. Until now however, obtained efficiencies are too low to lead to the desired cost reduction. We report on the influence of the AIC seed layer morphology (grain size and presence/absence of secondary crystallites on top of the surface) on the epitaxial growth of absorber layers and on the resulting cell parameters. To increase the grain size of the seed layers, we investigated the use of a nitric acid treatment to oxidize the Al layers prior to the amorphous silicon deposition. We compared seed layers oxidized by nitric acid treatment to seed layers oxidized by a short exposure to the ambient air. The nitric acid treatment led to a larger grain size and strongly affected the structure of the secondary crystallites on most investigated samples. Removing these crystallites before epitaxial growth clearly led to larger grains and increased the open-circuit voltages (Voc) of our solar cells. Using the nitric acid treatment, absorber layers with grain diameters up to 50 μm were made, that reached efficiencies of 4.9%, with Voc values around 500 mV. However, seed layers that were made through oxidation by air exposure and had grain diameters below 12 μm led to similar Voc values and even higher efficiencies (5.3%). We believe that regions with very small grain size on the large-grained samples are responsible for the relative poor behaviour of the large-grained solar cells. Our results show that increasing the average grain size of AIC seed layers does not automatically lead to better solar cells
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