1,720,971 research outputs found
An improved model for the performance estimation of an H-Darrieus wind turbine in skewed flow
No full textSmall turbines are considered one of the most promising technologies for an effective diffusion of renewable energy sources in new installation contexts with a high degree of integration with human activity (e.g. the urban environment). In these new installations, however, the real working conditions can be far from the nominal ones. In particular, the turbine functioning can be noticeably affected by misalignments between the oncoming flow and the axis of the rotor; differently from horizontal-axis wind turbines, whose performance is decreased by a skew angle, H-Darrieus turbines are thought to take advantage from this condition in some cases.
In this study, an improved model for the performance prediction of H-Darrieus under skewed flow was developed. In detail, a theoretical approach based on Momentum Models was properly modified to account for the variations induced by the new direction of the flow which invests the rotor. In particular, the modifications in the aerodynamic characteristics of the airfoils, the swept area and the streamtubes distribution were modeled. The performance predictions of the new model were compared both with experimental data available in the technical literature and with the results of wind tunnel tests purposefully carried out on a full scale model of an H-Darrieus turbine. Notable agreement has been constantly obtained between simulations and experiments
Influence of the heat storage size on the plant performance in a Smart User case study
No full textThe increasing diffusion of renewable energy sources are posing new challenges to the power grid due to their intrinsic unpredictability causing poor power quality, line congestion and unreliable and unsecure grid operations. In the future, power grid operators may require to the customers/producers a prescribed exchange profile, leading to a diffusion of storage systems or prime movers (especially small combined heat and power for distributed resources) able to balance renewable sources fluctuations. In this latter case, one of the major issue is the efficient use of the heat co-generated: the adoption of thermal storages appears to be necessary. In this paper, the energy performance of a Smart User, i.e. a dwelling with renewable energy sources, a combined cooling, heat, and power system, and heat and cooling back-up generators, is estimated by means of a purposely developed TRNSYS unsteady model. A virtual stand-alone operating condition is imposed for the analysed building as an arbitrary profile of power exchange with the grid. The balancing of renewable sources fluctuations imposes a non-negligible part of the heat from the prime mover to be wasted and a sensible consumption for auxiliary devices (i.e. the auxiliary boiler and the compression chiller). By applying a thermal storage, and increasing its capacity, the fraction of heat wasted and the supply from other devices is remarkably changed, showing different plant performance and efficiencies. The Smart User primary energy consumptions of the different configurations are compared to several options, showing appreciable differences in the performance. © 2013 Elsevier Ltd
Energy-yield-based optimization of an H-Darrieus wind turbine
No full textSmall-size H-Darrieus wind turbines are gaining more popularity in the wind energy market, thanks to some particular benefits (simplicity, reliability and low noise emissions) and to the efforts of industrial manufacturers to propose new exterior solutions coupled with tempting rated-power offers. The actual operating conditions of a rotor over a year can be, however, very different from the nominal one and strictly dependent on the features of the installation site. Based on these considerations, a turbine optimization oriented to maximize the annual energy yield, instead of the maximum power, is thought to represent a more interesting solution. With this goal in mind, 5400 test cases of H-Darrieus rotors were compared on the basis of their energy-yield capabilities for different annual wind distributions in terms of average speed. To this purpose, the wind distributions were combined with the predicted performance maps of the rotors obtained with a specifically developed numerical code based on a Blade Element Momentum (BEM) approach. The limits related to the wind turbine start-up behavior and to the structural loads (i.e. maximum rotational speed and maximum wind velocity) were also taken into account. The analysis, developed in terms of dimensionless parameters, highlighted the configurations which ensure the largest annual energy yield for each wind distribution. In addition, the differences between the results of a design process oriented to the maximum power output or to the maximum annual-energy-yield were analyzed; in particular, the comparison showed that a design oriented to the maximum energy-yield is assumed to provide a notable increase in the extracted energy (up to around 20%, with the selected design assumptions) whenever wind distributions with low average wind speeds are considered
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
Feasibility analysis of a Darrieus VAWT installation in the rooftop of a building
No full textThe renewed interest that is being paid by architects, project developers and local governments to small-size wind turbines is mainly connected to the attractive prospects of future applications in the urban environment; the delocalized power production of these systems could indeed provide an effective answer to both the growing demand for renewable energy and the increased attention in buildings with a sustainable and low-energy design. In particular, Darrieus vertical-axis wind turbines (VAWTs) are being considered as one of the most attractive solutions due to their low visual impact, the reduced acoustic emissions and their better response to a turbulent and skewed oncoming flow. The feasibility of this scenario has, however, to be proved yet; in particular, doubts are still connected to the real producibility in a complex terrain like the urban one and to the compatibility of microeolic machines with a densely populated area. On these assumptions, the aim of this work is to critically evaluate the energetic suitability of a Darrieus VAWT installation in the rooftop of a building in a reference European city. With this goal in mind, a numerical CFD analysis was carried out to characterize the flow field in the rooftop area of buildings with different shapes and geometrical proportions: the flow velocity modulus and direction were calculated for different oncoming wind profiles and the results were projected into a net available wind distribution in the rooftop of each building. As a second step, in order to provide a reliable estimation of the real functioning of the turbine in the investigated environment, a specific numerical model has been developed to account for the effects of a skewed flow on the power performance of the Darrieus rotor. The results of these analyses were finally combined and synthesized in an energy-oriented study to evaluate the feasibility of a rooftop installatio
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
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