1,720,983 research outputs found
Grid Interaction of Electrolysers and Fuel Cells for Utilisation of Renewable Energy Surplus
No full textThis thesis investigates the grid interaction of electrolysers (ELs) and fuel cells (FCs) combined as green hydrogen storage for utilisation of renewable energy surplus. The integration of renewable energy sources (RESs), such as solar PV and wind, with power grids is becoming increasingly popular to reduce greenhouse emissions and mitigate climate change. However, integrating RESs with power grids can produce challenges, such as the impacts of sudden variations in weather conditions including say cloud passing and wind gusts on PV and wind systems respectively, and sudden variations in load demands. These sudden variations are usually accompanied by high ramp rates, where the power outputs of renewable energy sources or the load demands can ramp up or down at a very high rate. Besides these, the harmonic distortions caused by modern power electronic loads can threaten the power quality and reliability. To minimise above mentioned problems, a hybrid energy storage system (HESS) is considered for the future power grid infrastructure. HESS can mitigate not only the system challenges but also, with sufficient capacity, the fast-rising energy demand, by storing surplus when there is excess generation and discharging the energy when there are insufficient power outputs from renewables to meet the load demand. Traditionally, battery energy storage (BES) is considered to supplement uncertain renewable generation. However, BES alone may have unanticipated environmental and financial consequences. Recently, with the increasing global interest in producing green hydrogen energy (GHE) to transition to a zero-carbon economy, several research papers in the literature have proposed to convert any surplus output from RES (whenever the electricity marginal price is zero or negative) to hydrogen using an EL. The produced hydrogen gas is transferred and stored in a controlled manner. Whenever there is a need for energy say due to unavailability of supply from renewables, the stored hydrogen gas is used to produce electrical energy using a FC to supply to the grid. To make the integrated power system including RESs, and HESS comprising ELs and FCs, an optimal, efficient, reliable, and cost-effective power management control strategy (PMCS) with advanced control decisions needs to be formulated. The main aim is to utilise the surplus RES to generate and store hydrogen gas and convert the stored hydrogen gas into electrical power with the aid of FC. Furthermore, it is important to optimally share the energy between the grid and HESS.This thesis conducts a detailed review on FCs/ELs comprising GHE storage technologies for utilising the RES surplus because the state of the art is still limited in terms of topologies integrated with different types of grids (i.e., AC or DC), multiple types of FCs/ELs, and a variety of power electronic interfaces with appropriate PMCS. The thesis proposes a mathematical and novel electrical equivalent circuit model of proton exchange membrane electrolyser (PEMEL) where the dynamic cell performance has been evaluated by considering the fluctuations in the external current profile. To demonstrate its adaptive feature, the proposed electrical equivalent circuit model is then scaled up to a 1 MW array by series and parallel connections, and the resulting system is validated by comparing it with the other reported experimental results of a 1 MW stack.Furthermore, the dynamic features of the PEMEL stack are used to investigate the impact of introducing the PEMEL stack's control loop into a single-area power system populated with solar PV units, while accounting for the communication time lag during the control signal transfer process. The dynamic performance of the proposed power system as well as steady-state errors are subsequently investigated to demonstrate the PEMEL stack's effective contribution to frequency regulation services. Furthermore, the potential resilience benefits of frequency control services and frequency sensitivity analysis for a modified IEEE-13-bus distribution system have been investigated. The results show that the system with PEMEL control loop responds to frequency changes faster than standard synchronous generators, implying the proposed PEMEL stack has a significant potential for enhancing frequency stability, resilience, and robustness.The thesis also includes a system modelling and performance analysis of a renewable hydrogen energy hub (RHEH) connected to an AC/DC hybrid microgrid. When power is in excess, a coordinated power flow control strategy minimises generation and demand mismatches while also producing green hydrogen. A modified hybrid approach involving perturb and observe, and particle swarm optimization techniques is suggested for tracking maximal power points. The thesis also presents a novel compensation technique for reducing intermittencies in a PV-powered microgrid that employs a hybrid multilevel energy storage system. This method solves memory and temporal delays through improved storage capacity. Simulations using a 24-hour solar irradiance profile are used to evaluate the technique on a 100-kW grid-interactive PV-dominated microgrid system.The thesis presents a grid-integrated analytical model for the co-production of hydrogen from an offshore hybrid energy system. The system supplies a percentage of its capacity to the onshore grid facility while producing hydrogen. The electricity is quantified based on a market price and total offshore generation. A case study is presented for a hypothetical 10 MW hybrid offshore energy system in NSW, Australia. The thesis proposes a probabilistic approach to size the HESS and a two-layer distributed energy management strategy for the HESS. Simulation results show that the supercapacitor bank can handle high-frequency fluctuations and avoid round-trip losses associated with HESS. The two-layer distributed energy management strategy extends operating lifetime and reduces operation costs of HESS, and also, maximizes the utilization HESS by avoiding excessive switching between FCs and Els, and maintaining an equal number of ON/OFF operations by ELs and FCs.</p
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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