Encontrados 27 documentos, a visualizar página 1 de 3
On the computational studies of deterministic global optimization of head depen...
Lima, Ricardo M.; Marcovecchio, Marian G.; Novais, Augusto Q.; Grossmann, Ignacio E.This paper addresses the global optimization of the short term scheduling for hydroelectric power generation. A tailored deterministic global optimization approach, denominated sHBB, is developed and its performance is analyzed. This approach is applied to the optimization of a mixed integer nonlinear programming (MINLP) model for cascades of hydro plants, each one with multiple turbines, and characterized by a...
Reactive scheduling in a make-to-order flexible job shop with re-entrant proces...
Gomes, Marta C.; Barbosa-Póvoa, Ana Paula; Novais, Augusto Q.A mixed-integer linear programming model is presented for the scheduling of flexible job shops, a production mode characteristic of make-to-order industries. Re-entrant process (multiple visits to the same machine group) and a final assembly stage are simultaneously considered in the model. The formulation uses a continuous time representation and optimises an objective function that is a weighted sum of order ...
Modelling and identification of the dominant phenomena in hydrogen fuel-cells b...
Lopes, Vitor V.; Rangel, C. M.; Novais, Augusto Q.This paper proposes the use of the distribution of relaxation times (DRT) for the analysis of the dynamic behavior of hydrogen fuel-cells. With this aim it presents a new method to compute the DRT directly from electrochemical impedance spectroscopy (EIS) data. The method is formulated as a nonconvex complex domain optimization problem where each EIS spectra is approximated by a general fractional-order Legen...
Assessing cell polarity reversal degradation phenomena in PEM fuel cells by ele...
Travassos, Maria Antónia; Lopes, Vitor V.; Silva, R. A.; Novais, Augusto Q.; Rangel, C. M.Electrochemical impedance spectroscopy (EIS) is identified as one of the most promising in-situ diagnostics tools available for assessing fuel cell ageing and degradation. In this work, the degradation phenomena caused by cell polarity reversal due to fuel starvation of an open cathode 16 membrane electrode assembly (MEA) – low power (PEM) fuel cell (15 W nominal power) – is reported using EIS as a base techni...
Electrochemical impedance spectroscopy modeling using the dis-tribution of rela...
Lopes, Vitor V.; Rangel, C. M.; Novais, Augusto Q.This paper proposes a new approach to determine the distribution of relaxation times (DRT) directly from the electro-chemical impedance spectroscopy (EIS) data, i.e. without the use of an equivalent electrical circuit model. The method uses a generalized fractional-order Laguerre basis to represent EIS where both the parameters of the basis and their co-efficients are estimated by solving a nonconvex minimizati...
Kinetics of sodium borohydride hydrolysis in aqueous-basic solutions
Retnamma, Rajasree; Rangel, C. M.; Novais, Augusto Q.; Matthews, Michael A.Liquid-phase catalytic hydrolysis of sodium borohydride (NaBH4) for hydrogen production necessitates long-term stability of base-stabilized NaBH4 solutions at higher temperatures. The present paper reports the kinetics of aqueous-basic solutions containing 20 wt% NaBH4 with 1-15 wt% sodium hydroxide (NaOH) at 80 0C. The established kinetic model employs a modified isoconversional method assuming single-step kin...
Heuristic algorithm for the piecewise linear segmentation of multiple time-seri...
Lopes, Vitor V.; Ferro, Filipa; Carvalho, M. J.; Novais, Augusto Q.This paper presents a novel algorithm for the piecewise linear segmentation of multivariate time-series and proposes its application to the analysis of hot water thermal solar systems (TSS). The ISO 9459-5:2007 norm describes a non-intrusive dynamic test for the performance assessment of TSS. This allows to characterize the system heat losses and the thermal stratification properties, as well as to predict its ...
On the use of Markov chain models for the analysis of wind power time-series
Lopes, Vitor V.; Scholz, Teresa; Estanqueiro, Ana; Novais, Augusto Q.Wind energy is becoming a top contributor to the renewable energy mix, which raises potential reliability issues for the grid due to the fluctuating and intermittent nature of its source. This paper explores the use of Markov chain models for the analysis of wind power time-series. The proposed Markov chain model is based on a 2yr dataset collected from a wind turbine located in Portugal. The wind speed, direct...
Relaxation time distribution analysis of a polymer electrolyte fuel cell stack ...
Lopes, Vitor V.; Silva, R. A.; Novais, Augusto Q.; Rangel, C. M.Electrochemical impedance spectroscopy (EIS) is an analysis technique that is commonly used as a base diagnostics technique for the in-situ analysis of the kinetic and transport properties of proton exchange membrane (PEM) fuel cells. This work proposes to use the distribution of relaxation times (DRT) as a complementary analysis technique for the interpretation of EIS data. For this purpose, the DRT is deduced...
Assessing cell polarity reversal degradation phenomena in PEM Fuel Cells by ele...
Travassos, Maria Antónia; Lopes, Vitor V.; Novais, Augusto Q.; Rangel, C. M.The mechanisms of fuel cell degradation are multiple and not well understood. Irreversible changes in the kinetic and/or transport properties of the cell are fostered by thermal, chemical and mechanical issues which constrain stability, power and fuel cell lifetime. Within the in-situ diagnostics methods and tools available, in-situ electrochemical impedance spectroscopy (EIS) is within the most promising to be...
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