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Ribeiro H.S.,Telecommunications Institute of Portugal | Ribeiro H.S.,University of Lisbon | Ribeiro H.S.,Polytechnic Institute of Tomar | Borges B.V.,Telecommunications Institute of Portugal | Borges B.V.,University of Lisbon
IEEE Transactions on Industrial Electronics | Year: 2011

The inclusion of a few additional diodes and passive elements in the high-frequency full-bridge acdc converter with galvanic isolation permits one to achieve sinusoidal input-current wave shaping and output-voltage regulation simultaneously without adding any auxiliary transistors. Recently, this procedure, together with an appropriate control process, has been used to obtain low-cost high-efficiency single-stage converters. In an attempt to improve the performance of such converters, this paper introduces three new single-stage full-bridge acdc topologies with some optimized characteristics and compares them with the ones of the existing full-bridge single-stage topologies. The approach used consists in the definition of the operating principles identifying the boost function for each topology, their operating limits, and the dependence between the two involved conversion processes. Experimental results for each topology were obtained in a 500-W modular laboratory prototype that was built with the necessary flexibility to allow the realization of each different topology. Comparison was evaluated in terms of input-current distortion, efficiency, and output-voltage disturbances that result from the input-current wave-shaping process. © 2010 IEEE.

Ribeiro H.S.,Telecommunications Institute of Portugal | Ribeiro H.S.,Polytechnic Institute of Tomar | Borges B.V.,Telecommunications Institute of Portugal | Borges B.V.,University of Lisbon
IEEE Transactions on Power Electronics | Year: 2010

This paper presents a single-stage circuit topology consisting of the association of a full-bridge isolated dc-dc converter and two input inductors and two input diodes connected to the mains network, in order to obtain an isolated ac/dc switch mode power supply, with sinusoidal input current. The proposed topology does not use an input bridge rectifier, common in similar applications. The current in the two input inductors can, therefore, flow in both directions. Consequently, the proposed topology equally distributes the current by the four-bridge transistors that provide four input parallel boost power factor correctors (PFCs). The use of the four-bridge transistors to obtain the PFC function and regulate the output voltage with galvanic isolation is a new technique that makes this topology unique, which also contributes to improve the converter efficiency. The definition of appropriate control strategies permitting the accurate simultaneous regulation of output voltage and input current is hereby described. The interdependency between these two conversion processes is completely analysed, allowing for useful design rules. Experimental results were obtained in a 650-W laboratory prototype to verify the theoretical study. A maximum efficiency of 94% was obtained. © 2011 IEEE.

Barros C.,University of Lisbon | Antunes O.S.,Polytechnic Institute of Tomar
Energy Sources, Part B: Economics, Planning and Policy | Year: 2014

This artical analyzes the efficiency and productivity growth of a representative sample of Angolan oil blocks from 2002 to 2008, using the directional distance function and the Luenberger productivity indicator. The primary advantage of this approach is that both input contractions and output expansions are considered. The model generates a productivity indicator that is decomposed into the two usual constituents of productivity growth: technological change and efficiency change. For comparative purpose a Malmquist productivity index is presented. The results show that, on average, Angolan oil blocks did experience some productivity growth during the period analyzed. In addition, the incidence of technological change was positive. Copyright © Taylor & Francis Group, LLC.

Ribeiro H.S.,University of Lisbon | Ribeiro H.S.,Polytechnic Institute of Tomar | Borges B.V.,University of Lisbon
IEEE Transactions on Industrial Electronics | Year: 2014

AC-DC conversion with power factor correction using full-bridge single-stage (FBSS) converters may introduce various complex operating limits and control problems that are not usually described in literature. This is mainly due to the fact that they devaluate and restrict the number of applications of those topologies. These problems are identified as primary transformer saturation and output voltage perturbations. The operation at light loads and with wide load variation is also a complex issue that leads to uncontrollable input current and dc bus voltage. This paper clarifies such technical problems referring their impact in the FBSS topologies operation, and presents design criteria and control solutions to minimize their effects. A low cost free wheeling circuit (FWC) that allows null input current to solve the light load operation problem is also presented. © 2013 IEEE.

Campos R.,Polytechnic Institute of Tomar | Campos R.,University of Beira Interior | Dias G.,University of Caen Lower Normandy | Jorge A.M.,INESC Porto | And 2 more authors.
ACM Computing Surveys | Year: 2014

Temporal information retrieval has been a topic of great interest in recent years. Its purpose is to improve the effectiveness of information retrieval methods by exploiting temporal information in documents and queries. In this article, we present a survey of the existing literature on temporal information retrieval. In addition to giving an overview of the field, we categorize the relevant research, describe the main contributions, and compare different approaches.We organize existing research to provide a coherent view, discuss several open issues, and point out some possible future research directions in this area. Despite significant advances, the area lacks a systematic arrangement of prior efforts and an overview of state-of-The-Art approaches.Moreover, an effective end-To-end temporal retrieval system that exploits temporal information to improve the quality of the presented results remains undeveloped. © 2014 ACM.

Mateus D.M.R.,Polytechnic Institute of Tomar | Pinho H.J.O.,Polytechnic Institute of Tomar
Water Environment Research | Year: 2010

Constructed wetlands, which facilitate phosphorus removal via precipitation, adsorption, and biological assimilation, offer a promising appropriate technology for advanced treatment in wastewater treatment plants. Because adsorption and precipitation are pointed out as the major phosphorus-removal mechanisms, the selection of a medium with high phosphorus-sorption capacity is important to obtain a sustained phosphorus removal. The objective of this study was to evaluate two kinds of lightweight expanded clay aggregates (LWAs) -Filtralite NR and Filtralite MR (Maxit Group, Avelar, Portugal) -as substrates in constructed wetlands to improve phosphorus-removal performance. Laboratory experiments were performed to test the potential of the LWAs to remove phosphorus from a phosphate solution. The experimental data were well-fitted by both the Langmuir and Freundlich isotherm models. Pilot-scale investigations were carried out to evaluate the phosphorus removal under field conditions. Four subsurface constructed wetlands were operated since June 2002; two of them were planted with Phragmites australis, and the other two were unplanted. The beds were filled with the two kinds of LWAs. Total phosphorous and pH were monitored since 2003, at a mean hydraulic load of 50 ± 4 L/(m2.d), during 6 years. The inflow phosphorus concentration was in the range 4 to 13 mg/L. Under the conditions used, beds with Filtralite MR had better efficiency, and the bed with Filtralite MR planted with Phragmites australis provided a phosphorus effluent mean concentration of 0.7 ± 0.2 mg/L, during the trial period. This study presents the first long-term pilot-scale data for constructed wetlands using LWAs.

Gomes M.H.,Polytechnic Institute of Tomar | Saraiva J.T.,INESC Porto
Electric Power Systems Research | Year: 2010

MicroGrids represent a new paradigm for the operation of distribution systems and there are several advantages as well as challenges regarding their development. One of the advantages is related with the participation of MicroGrid agents in electricity markets and in the provision of ancillary services. This paper describes two optimization models to allocate three ancillary services among MicroGrid agents - reactive power/voltage control, active loss balancing and demand interruption. These models assume that MicroGrid agents participate in the day-ahead market sending their bids to the MicroGrid Central Controller, MGCC, that acts as an interface with the Market Operator. Once the Market Operator returns the economic dispatch of the MicroGrid agents, the MGCC checks its technical feasibility (namely voltage magnitude and branch flow limits) and activates an adjustment market to change the initial schedule and to allocate these three ancillary services. One of the models has crisp nature considering that voltage and branch flow limits are rigid while the second one admits that voltage and branch flow limits are modeled in a soft way using Fuzzy Set concepts. Finally, the paper illustrates the application of these models with a Case Study using a 55 node MV/LV network. © 2010 Elsevier B.V. All rights reserved.

Gamelas J.A.F.,University of Coimbra | Ferraz E.,Polytechnic Institute of Tomar | Rocha F.,University of Aveiro
Colloids and Surfaces A: Physicochemical and Engineering Aspects | Year: 2014

The present work aimed characterizing in a systematic way the surface of metakaolinitic materials produced by calcination of a kaolinitic clay at different temperatures and to study the effect of grinding on the surface properties of metakaolinitic materials. Using X-ray photoelectron spectroscopy, it was found for all materials a Si/Al atomic ratio close to 1, confirming the presence of the 1:1 clay structure. By inverse gas chromatography, an increase of the Lewis basic properties of the surfaces of metakaolinitic materials in comparison to the original clay was found, which was due to the condensation of hydroxyl groups in the structure of the clay. The grinding of the metakaolinitic materials afforded a decrease of the dispersive component of the surface energy (γsd) as well as an increase of the specific interaction with sterically hindered molecules, caused by the diminishing of the materials surface nanoroughness. The Lewis basic properties of the materials surface also increased with grinding. Noticeably, for all studied materials a good inverse relation could be found between the γsd and the specific interaction of trichloromethane (but not with dichloromethane), showing the importance of surface nanoroughness on the adsorption process of bulky molecules. © 2014 Elsevier B.V.

Mateus D.M.R.,Polytechnic Institute of Tomar | Vaz M.M.N.,Polytechnic Institute of Tomar | Pinho H.J.O.,Polytechnic Institute of Tomar
Ecological Engineering | Year: 2012

This study investigated the potential of fragmented Moleanos limestone (FML) as substrate in constructed wetlands for phosphorus removal. Being a waste material which results from industries of extraction and production of decorative stones for construction activities, FML is quite inexpensive. Laboratory experiments were performed in a packed bed column with recycle to test the FML's capacity to sorb phosphorus from a phosphate solution. The experimental data were found to follow the Langmuir model better than the Freundlich model. From the isotherm parameters obtained it can be concluded that the sorption capacity of FML is similar to those quoted in the literature for mineral materials. A subsurface flow constructed wetland filled with FML and planted with Phragmites australis was monitored during 20 months, at a mean hydraulic load of 40±4L/(m 2day). Results demonstrated that average phosphorus removal efficiencies of 61%±7 can be achieved during the trial period, and a global first order kinetic constant of 0.062±0.001m/day was fitted to experimental data using the well mixed serial tanks model. Morphological aspects of reed growth were also studied and indicated high plant yield. The use of FML as a substrate media in constructed wetlands can be a promising solution in developing a cost effective, environmental friendly and sustainable technology for phosphorus removal, particularly where this material is readily available. © 2012 Elsevier B.V.

Pires G.,University of Coimbra | Pires G.,Polytechnic Institute of Tomar | Nunes U.,University of Coimbra | Castelo-Branco M.,University of Coimbra
Clinical Neurophysiology | Year: 2012

Objective: Non-invasive brain-computer interface (BCI) based on electroencephalography (EEG) offers a new communication channel for people suffering from severe motor disorders. This paper presents a novel P300-based speller called lateral single-character (LSC). The LSC performance is compared to that of the standard row-column (RC) speller. Methods: We developed LSC, a single-character paradigm comprising all letters of the alphabet following an event strategy that significantly reduces the time for symbol selection, and explores the intrinsic hemispheric asymmetries in visual perception to improve the performance of the BCI. RC and LSC paradigms were tested by 10 able-bodied participants, seven participants with amyotrophic lateral sclerosis (ALS), five participants with cerebral palsy (CP), one participant with Duchenne muscular dystrophy (DMD), and one participant with spinal cord injury (SCI). Results: The averaged results, taking into account all participants who were able to control the BCI online, were significantly higher for LSC, 26.11. bit/min and 89.90% accuracy, than for RC, 21.91. bit/min and 88.36% accuracy. The two paradigms produced different waveforms and the signal-to-noise ratio was significantly higher for LSC. Finally, the novel LSC also showed new discriminative features. Conclusions: The results suggest that LSC is an effective alternative to RC, and that LSC still has a margin for potential improvement in bit rate and accuracy. Significance: The high bit rates and accuracy of LSC are a step forward for the effective use of BCI in clinical applications. © 2011 International Federation of Clinical Neurophysiology.

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