1,721,072 research outputs found
Evolutive waves in electrochemical batteries
No full textWe are now entering an epoch-making evolution in energy supply, in the framework of decarbonization and ecological transition, which are strongly addressed in Europe and other countries and continents. The phase-out of fossil fuels and the expansion of renewable sources, notably photovoltaic and wind that are intermittent and unpredictable, on one side, and the advent of electric vehicles on the other side, will imply a growing need for electrochemical batteries in both stationary and mobile versions. Huge research programs are under development to provide better and cheaper cells able to meet the future demand.
However, when we look at the past of electrotechnology, we see that batteries have already played major roles in the evolution of electrotechnology, starting with the first cell, invented by Alessandro Volta in 1799, that allowed electricity to get rid of the narrow limitations of electrostatics. Over more than half a century, primary cells derived from Volta’s invention powered the early exploitations of electricity. When reliable electromechanical generators appeared, capable of delivering cheap and large power, secondary (rechargeable) cells were developed, which were instrumental in the first season of electric vehicles, at the turn of the century, both above ground and underwater.
Just after the mid of the 20th century the advent of the transistor and solid-state electronics allowed the downsizing of many appliances. However, devices like hearing aids and radio receivers called for compact and better performing cells to became portable and the answer consisted in a new generation of primary cells, in particular alkaline.
Shortly after, space exploration called for new generations of rechargeable batteries, mostly based on Nickel-based chemistries (Ni-Cd, Ni-H2, Ni-MH), which found application also in other fields.
Toward of the end of the century, new scientific horizons were opened by the lithium technologies, first in primary cells and then in secondary ones. The evolution of rechargeable lithium-ion battery was gradual, being implemented first in low-power portable electronics (mobile phones, laptops, organizers, media players, ... rated 5-60 Wh), then in middle-power electric vehicles (HEV, PHEV, BEV, with power and energy ranging as 5-600 kW and 1–100 kWh), and in both fields they quickly became dominating. Still afterward, large stationary Li-ion (up to 150 MW and 300 MWh) started to be installed in support of electric grids with power for discharge durations up to 4 hours
A glance at hydroelectricity evolution
No full textThe ecological transition that many governments are implementing, notably the EU, will stand on the phase-out of fossil fuels and the consequent expansion of renewable sources, which will develop as a technological revolution.
However, hydropower is a major renewable source already at a mature stage and widely exploited. It early uses appeared quite soon in early isolated plants when mechanical generator became available, but in limited cases were flowing water was easily accessible. Plants and lines had limited extension at that time and thermal engines (both steam and internal combustion) allowed a more flexible adoption.
The advent of alternating current, which allowed the use of stepping-up and stepping-down transformer and thus power transmission at high voltage over long distances opened the way to the exploitation of large water resources in remote areas. An early hydroelectric power plant of this kind was put into service at Willamette Falls, Oregon, in 1889, to supply Portland through a 4-kV 125 Hz 22-km line. The soundness of the technology was proven in the 1890s. A major step ahead occurred in 1891, on occasion of the International Electrotechnical Exhibition at Frankfort, where the first three-phase power line, rated 240 kW at 15 kV and 40 Hz, extending over 175 km between the Lauffen waterfalls, were a hydrogenator was installed, and Frankfurt, was presented. Shortly after, some hydropower stations powering long lines were opened, e.g.:
1892: Aniene–Rome, Italy (1.2 MW, 5 kV, 42 Hz single-phase, 28-km)
1893: Lake Hellsjön–Grängesberg, Sweden (300 kW, 9.5 kV three-phase, 14 km)
1893: Mill Creek n.1 hydroelectric plant, California US (250 kW, 2.4 kV 50 Hz three-phase,12 km)
In 1895, the Niagara hydropower station was started, with three (increased to 10 by 1898) two-phase alternators each rated 3.7 MW 2.2 kV 25 Hz. Step-up transformers with Scott connection fed the three-phase 11 kV line powering Buffalo 35 km apart. Two similar systems appeared in the Alps, Europe, in 1898, namely the Paderno d’Adda three-phase hydropower station, rated 9 MW at 13.5 kV 42 Hz that fed Milan, 30 km apart; and the Rheinfelden three-phase hydroelectric power plant, rated 12.5 MW at 50 Hz (Germany-Switzerland). Following these achievements, countries and regions rich with water resources exploited them increasingly in the early decades of the 20th century, while extending their power lines and network which eventually were interconnected into national grids.
In the first decade of the century, also pumped hydro power station appeared, with early notable installations in Germany and Switzerland, in 1908. US followed starting in 1929 and major developments occurred in different countries after World War Two.
The growth of hydropower was massive in countries rich with water resources, notably Switzerland, Italy, Canada, Sweden, Norway, Soviet Union, and, more recently, Brazil and China, as long as more water resources were exploitable. By 1920, 40% of the electricity produced in the United States was hydroelectric and in the mid 20th century, 96% of the installed power in Canada, 94% in Switzerland, 90% in Italy, and 80% in Sweden 80%, came from hydroelectricity.
However, when the water resources were saturated, the growing demand was satisfied with thermoelectricity fed with fossil fuels. For the sake of example, hydroelectric energy share in Norway was 96.2% of the total production and 117.9% of the domestic demand in 2016; conversely, in Italy hydroelectric energy production remained substantially constant in the last 60 years, counting 44,257 GWh in 2016, but hydroelectric energy share had dropped to 15.3% of the total domestic electricity production, although flanked by 22% of other renewables. On the other hand, pumped hydro is today by far the largest-capacity form of grid electric energy storage worldwide, accounting for 181 GW of power capacity and 1.6 TWh of storage capacity, which correspond to 95% of the global figures for energy storage
Some facts on History Activities with a Glance at Notable Women in Science and Technology
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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
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