1,721,090 research outputs found
Dealing with uncertainty in planning of energy storage systems in power distribution networks
No full textIn the last decades, power systems are undergoing major modifications, mostly driven by the climate change. This increasing awareness has brought national policies to meet the necessities of reducing CO2 emissions and the use of fossil fuels by incentivizing renewable energy sources. At the same time, the latest advances in the field of distributed energy resources, including more efficient photovoltaic, wind and energy storage systems, have raised the interest in taking full advantage of these technologies, for instance to reduce grid power losses.
As a consequence, these new challenging environmental and economic targets set by government policies are driving a dramatic transformation of power supply in electricity systems, with traditional thermal power plants getting off the stage, replaced by distributed generation. Moreover, non-dispatchable distributed energy resources such as wind and photovoltaic systems, with their dependance on weather and climate conditions, introduce new sources of uncertainty in power system operation.
Energy storage systems represent a solution for enhancing grid performance, reliability, flexibility and security in the presence of renewables. Charging of the storage devices installed in a distribution grid may help avoid or mitigate the reverse power flow upstream the transformers in case of an excess of generation from distributed generators, without resort to curtailment of generated power. Moreover, use of ESSs may help smooth the fast voltage variations arising as one of the tangible effects of sudden changes in energy production and/or demand patterns.
This thesis addresses the problem of the optimal energy storage system allo- cation in power distribution networks with high penetration of renewables. This is a problem which has received increasing attention in the literature, due to the numerous benefits that the use of energy storage systems brings to the power system and its stakeholders. The considered decision problem consists of defining the number of storage units to be deployed, their locations (placement) and sizes (sizing). This decision is made at the planning stage, when the future realizations of demand and generation in the network are unknown.
In this thesis uncertainty is accounted for by formulating the allocation problem in a scenario-based stochastic optimal power flow framework. The high dimensionality of the sources of uncertainty involved, puts additional burden on the solution of the allocation problem which, even in the deterministic case, is subject to the combinatorial complexity intrinsic in the placement problem, and the nonconvexity of power flow equations. The main contribution of this thesis is therefore to cope with the prohibitive computational complexity of the large-scale mixed-integer nonconvex optimization problems formulated for optimal allocation. This is done along two directions. On the one hand, the thesis proposes a scenario- reduction technique which makes it possible to solve at the optimum the sizing problem over a large set of scenarios by solving a smaller problem over a selected set of scenarios. On the other hand, the combinatorial nature of optimal placement problems is overcome through a near-optimal placement strategy with well-defined performance guarantees based on submodularity theory and greedy methods. The proposed techniques take advantage of recent developments on convex relaxations and linear approximations of power flow equations to tackle the solution of the considered optimal power flow problems.
Numerical results and sensitivity analyses are reported to show the effectiveness of all the procedures and approaches presented in this thesis
Fostering the understanding of multi-representational examples by self-explanation prompts
No full textBerthold K, Renkl A. Fostering the understanding of multi-representational examples by self-explanation prompts. In: Bara BG, Barsalou L, Bucciarelli M, eds. Proceedings of the CogSci 2005. Mahwah, NJ: Erlbaum; 2005: 250-255
Experimental demonstration of ship target detection in GNSS-based passive radar combining target motion compensation and track-before-detect strategies
Full text linkThis work discusses methods and experimental results on passive radar detection of moving ships using navigation satellites as transmitters of opportunity. The reported study highlights as the adoption of proper strategies combining target motion compensation and track-before-detect methods to achieve long time integration can be fruitfully exploited in GNSS-based passive radar for the detection of maritime targets. The proposed detection strategy reduces the sensitivity of long-time integration methods to the adopted motion models and can save the computational complexity, making it appealing for real-time implementations. Experimental results obtained in three different scenarios (port operations, navigation in open area, and river shipping) comprising maritime targets belonging to different classes show as this combined approach can be employed with success in several operative scenarios of practical interest for this technology
Numerical Analysis of the Dynamics of Reed Valves Taking into account the Acoustic Coupling with the Fluid. Application to Compressors for Domestic Refrigeration
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Stochastic optimization for energy storage allocation in smart grids in the presence of uncertainty
No full textA key subject in the study of smart grids is to accommodate uncertainty in various contexts, including planning and operation of electricity grids in the presence of distributed generation from renewable energy sources, stochastic demand patterns, and varying network configurations. The impact of uncertainty on the solution of different problems formulated in the optimal power flow framework calls for stochastic programming paradigms in the form of two- or multi-stage problems, or optimization programs with chance constraints. In this chapter, we focus on the problem of optimally siting and sizing the energy storage systems in a distribution network. These devices are recognized as good candidates to tackle different issues, such as voltage/frequency regulation, minimal curtailment of renewable generation, peak shaving, or others. For the sizing problem, a scenario-based approach is adopted to cope with uncertain demand and generation profiles at the different buses of the network. A novel scenario reduction technique is presented to make the resulting stochastic optimization problem computationally tractable. A heuristic strategy based on network voltage sensitivity analysis is adopted to deal with the combinatorial nature of the energy storage siting problem. The overall procedure is tested on a IEEE benchmark network, highlighting good performance on a realistic case study
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
On the greedy placement of energy storage systems in distribution grids
No full textThis paper considers power distribution grids and addresses the problem of finding optimal placement strategies for energy storage systems. The placement problem is, unfortunately, combinatorially complex. A placement strategy with performance guarantees based on submodularity theory and greedy methods is proposed. The greedy method is then compared with a heuristic strategy that relies on voltage sensitivity analysis. The features of both techniques are discussed and illustrated through an application to a test feeder
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