ESTG - Artigos em revistas internacionais
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- The Effect of Intermittent Renewable Energy Generation on Electricity Prices: A Literature SurveyPublication . Oliveira, Blandina C. R.; Fortunato, AdelinoDespite increasing deployment of intermittent renewable energies at lower generation costs, wholesale electricity price has been falling while retail electricity prices go up. This has triggered the debate on the cost-effectiveness of this source of energy. Therefore, the aim of this paper is to present a literature survey on the effect of intermittent renewable energy generation on electricity prices. Researches have used different methodological approaches, different periods and countries to examine the impacts of intermittent renewable energy on electricity prices. Most of the studies found evidence of the merit-order effect, which means that an increase in intermittent source generation would reduce the spot electricity market price. Finally, the few studies that address the retail market found that retail electricity could either increase or decrease.
- AlineaGA - a genetic algorithm with local search optimization for multiple sequence alignmentPublication . Silva, Fernando José Mateus da; Pérez, Juan Manuel Sánchez; Pulido, Juan Antonio Gómez; Rodríguez, Miguel A. VegaThe alignment and comparison of DNA, RNA and Protein sequences is one of the most common and important tasks in Bioinformatics. However, due to the size and complexity of the search space involved, the search for the best possible alignment for a set of sequences is not trivial. Genetic Algorithms have a predisposition for optimizing general combinatorial problems and therefore are serious candidates for solving multiple sequence alignment tasks. Local search optimization can be used to refine the solutions explored by Genetic Algorithms. We have designed a Genetic Algorithm which incorporates local search for this purpose: AlineaGA. We have tested AlineaGA with representative sequence sets of the globin family. We also compare the achieved results with the results provided by T-COFFEE.
- The small world of efficient solutions: empirical evidence from the bi-objective {0,1}-knapsack problemPublication . Silva, Carlos Gomes da; Clímaco, João; Filho, Adiel AlmeidaThe small world phenomenon, Milgram (1967) has inspired the study of real networks such as cellular networks, telephone call networks, citation networks, power and neural networks, etc. The present work is about the study of the graphs produced by efficient solutions of the bi-objective {0,1}-knapsack problem. The experiments show that these graphs exhibit properties of small world networks. The importance of the supported and non-supported solutions in the entire efficient graph is investigated. The present research could be useful for developing more effective search strategies in both exact and approximate solution methods of {0,1} multi-objective combinatorial optimization problems.
- Financial risk exposures and risk management: evidence from european nonfinancial firmsPublication . Jorge, Maria João Da Silva ; Augusto, Mário António GomesPrevious empirical studies concerning corporate risk management have attempted to show that the use of derivatives as a hedging mechanism can be value enhancing. Implicit to these tests has been the assumption that firms use derivatives solely for the purpose of hedging. There is substantial literature concerning nonfinancial firms that suggest that changes in financial prices affect firms' value. Furthermore, it is a common belief that financial price exposures are created via firms' real operations and are reduced through the implementation of financial hedging strategies. We use monthly returns of 304 European firms traded in Euronext over the period from 2006-2008 to analyse whether risk management practices are associated with lower levels of risk. We pursue Jorion (1990) and Allayannis and Ofek (2001) two stages framework to investigate, firstly, the relationship between firm value and financial risk exposures; subsequently, the risk behaviour inherent to firms' real operations and to the use of derivatives and other risk management instruments. So, we argue that hedging policies affect the firm's financial risk exposures; however, we do not discard the fact that the magnitude of a firm's exposure to risks affects hedging activities. The interaction between financial price exposures and hedging activities is tested by using the Seemingly Unrelated Regression (SUR) procedure. Our major findings are as follows: Firstly, we find evidence that the sample firms exhibit higher percentages of exposure to the three categories of risks analysed when compared to previous empirical studies. Secondly, we find that hedging is significantly associated with financial price exposure. Our results are also consistent with the idea that financial risk exposure and hedging activities are endogenously related, but only in what respects the exchange risk and commodity risk exposure.
- The (Tetra) Category of Pseudocategories in an Additive 2-category with KernelsPublication . Martins-Ferreira, N.We describe the (tetra) category of pseudo-categories, pseudo-functors, natural transformations, pseudo-natural transformations, and modifications, as introduced in Martins-Ferreira (JHRS 1:47-78, 2006), internal to an additive 2-categorywith kernels, as formalized in Martins-Ferreira (Fields Inst Commun 43:387-410, 2004). In the context of a 2-Ab-category, we introduce the notion of a pseudomorphism and prove the equivalence of categories: PsCat(A) ̃PsMor(A) between pseudo-categories and pseudo-morphisms in an additive 2-category, A, with kernels- extending thus the well known equivalence Cat(Ab)̃Mor(Ab) between internal categories and morphisms of abelian groups. The leading example of an additive 2-category with kernels is Cat(Ab). In the case A=Cat(Ab) we obtain a description of the (tetra) category of internal pseudo-double categories in Ab, and particularize it to a description of the (tetra) category of internal bicategories in abelian groups. As expected, pseudo-natural transformations coincide with homotopies of 2-chain complexes (as in Bourn, J Pure Appl Algebra 66:229-249, 1990).
- Accelerating floating-point fitness functions in evolutionary algorithms: a FPGA-CPU-GPU performance comparisonPublication . Gomez-Pulido, Juan A.; Vega-Rodriguez, Miguel A.; Sanchez-Perez, Juan M.; Priem-Mendes, Silvio; Carreira, VitorMany large combinatorial optimization problems tackled with evolutionary algorithms often require very high computational times, usually due to the fitness evaluation. This fact forces programmers to use clusters of computers, a computational solution very useful for running applications of intensive calculus but having a high acquisition price and operation cost, mainly due to the Central Processing Unit (CPU) power consumption and refrigeration devices. A low-cost and high-performance alternative comes from reconfigurable computing, a hardware technology based on Field Programmable Gate Array devices (FPGAs). The main objective of the work presented in this paper is to compare implementations on FPGAs and CPUs of different fitness functions in evolutionary algorithms in order to study the performance of the floating-point arithmetic in FPGAs and CPUs that is often present in the optimization problems tackled by these algorithms. We have taken advantage of the parallelism at chip-level of FPGAs pursuing the acceleration of the fitness functions (and consequently, of the evolutionary algorithms) and showing the parallel scalability to reach low cost, low power and high performance computational solutions based on FPGA. Finally, the recent popularity of GPUs as computational units has moved us to introduce these devices in our performance comparisons. We analyze performance in terms of computation times and economic cost.
- Sustainable Electrospinning of Nanoscale FibresPublication . Alazab, Mohamed; Mitchell, Geoffrey R.; Davis, Fred J.; Mohan, Saeed D.Electrospinning is an effective technology for the preparation of nano and micro scale fibres for diverse application in oil recovery, medical devices, and filters. It is achieved by injecting a charged solution of polymeric material through a needle into a region of high electric field. Under these conditions, the expelled jet follows a chaotic, whip like trajectory towards a grounded collection plate. At low polymer concentrations, the high forces experienced by the jet prior to becoming grounded on the collection plate, result in the formation of undesirable discrete droplets of material, rather than fibres. At higher concentrations, above the critical entanglement limit for the polymer, the polymer chains are stretched and orientated whilst the solvent rapidly evaporates, delivering high aspect ratio fibres. The resulting mesh of overlapping fibres frequently has useful properties such as high surface area and porosity, which has led to their investigation for a range of applications including filtration membranes and tissue scaffolds. One of the major challenges in the development of electrospinning as a manufacturing technology is the use of organic solvents. Typically, fibres are spun from relatively dilute solutions containing 95% solvent. It is clear that systems which use water as a solvent offer many advantages in terms of safety, cost and sustainability. In this work we optimise the conditions for effectively preparing nano/micro fibres of polyethylene oxide from aqueous solutions. We contrast the fibres produced with those prepared using volatile organic solvents.
- What does low energy physics tell us about the zero momentum gluon propagatorPublication . Costa, P.; Oliveira, O.; Silva, P.J.The connection between QCD, a nonlocal Nambu–Jona-Lasinio type model and the Landau gauge gluon propagator is explored. This two point function is parameterized by a functional form which is compatible with Dyson–Schwinger and lattice QCD results. Demanding the nonlocal model to reproduce the experimental values for the pion mass, the pion decay constant, and the light quark condensate we conclude that low energy physics does not distinguish between the so-called decoupling and scaling solutions of the Dyson–Schwinger equations. This result means that, provided that the model parameters are chosen appropriately, one is free to choose any of the above scenarios. Furthermore, the nonlocal Nambu–Jona-Lasinio quark model considered here is chiral invariant and satisfies the GMOR relation at the 1% level of precision.
- A multi-objective genetic algorithm applied to autonomous underwater vehicles for sewage outfall plume dispersion observationsPublication . Moura, Ana; Rijo, Rui; Silva, Pedro; Crespo, SidónioThis work presents a multi-objective genetic algorithm to solve route planning problem for multiple autonomous underwater vehicles (AUVs) for interdisciplinary coastal research. AUVs are mobile unmanned platforms that carry their own energy and are able to move themselves in the water without intervention from an external operator. Using AUVs one can provide high-quality measurements of physical properties of effluent plumes in a very effective manner under real oceanic conditions. The AUV's route planning problem is a combinatorial optimization problem, where the vehicles must travel through a three-dimensional irregular space with all dimensions known. Therefore, minimization of the total travel distance while considering the maximum number of water samples is the main objective. Besides the AUV kinematics restrictions other considerations must be taken into account to the problem, like the ocean currents. The practical applications of this approach are the environmental monitoring missions which typically require the sampling of a volume of water with non-trivial geometry for which parallel line sweeping might be a costly solution. Some real-life test problems and related solutions are presented.
- Part Specific Applications of Additive ManufacturingPublication . Khan, Imran; Mateus, Artur; Lorger, Christina S. Kamma.; Mitchell, Geoffrey R.Additive manufacturing is one of the most important technological advances which has been implemented and recognised as a modern manufacturing technology with many advantages over conventional approaches. Fused deposition modelling is an additive manufacturing technology commonly used for modelling, prototyping, and production applications. In this work sample holding grips are designed and printed using fused deposition modelling. These are used in time-resolved experiments which require a dedicated system to study one to one structure-property relationships in electrically conductive nanocomposites under uniaxial strain. The grips serve not only to hold the sample during stretching but also have electrodes to measure the electric current and the voltage drop across the sample under uniaxial strain, as they are insulated from the rest of the tensiometer assembly. In such kind of experimental work, the success of the experiment strongly depends upon the grips as the fracture or slip of the sample during the experiment can ruin the data and lead to a loss of confidence on measurement. The use of additive manufacturing was a particular advantage in the optimization of the design of the grips.