Nonlinear control of the doubly-fed induction generator in wind power systems

This paper describes the models of a wind power system, such as the turbine, generator, power electronics converters and controllers, with the aim to control the generation of wind power in order to maximize the generated power with the lowest possible impact in the grid voltage and frequency during normal operation and under the occurrence of faults. The presented work considers a wind power system equipped with the doubly-fed induction generator and a vector-controlled converter connected between the rotor and the grid. The paper presents comparative results between proportional-integral controllers and neural networks based controllers, showing that better dynamic characteristics can be obtained using neural networks based controllers.     

Melting of PCM in a thermal energy storage unit: Numerical investigation and effect of nanoparticle enhancement

The present paper describes the analysis of the melting process in a single vertical shell‐and‐tube latent heat thermal energy storage (LHTES), unit and it is directed at understanding the thermal performance of the system. The study is realized using a computational fluid‐dynamic (CFD) model that takes into account of the phase‐change phenomenon by means of the enthalpy method. Fluid flow is fully resolved in the liquid phase‐change material (PCM) in order to elucidate the role of natural convection. The unsteady evolution of the melting front and the velocity and temperature fields is detailed. Temperature profiles are analyzed and compared with experimental data available in the literature. Other relevant quantities are also monitored, including energy stored and heat flux exchanged between PCM and HTF. The results demonstrate that natural convection within PCM and inlet HTF temperature significantly affects the phase‐change process. Thermal enhancement through the dispersion of highly conductive nanoparticles in the base PCM is considered in the second part of the paper. Thermal behavior of the LHTES unit charged with nano‐enhanced PCM is numerically analyzed and compared with the original system configuration. Due to increase of thermal conductivity, augmented thermal performance is observed: melting time is reduced of 15% when nano‐enhanced PCM with particle volume fraction of 4% is adopted. Similar improvements of the heat transfer rate are also detected. Copyright © 2012 John Wiley & Sons, Ltd.     

Life cycle cost analysis of energy efficiency design options for refrigerators in Brazil

The purpose of this paper was to present the results of a life cycle cost analysis concerning the purchase and operation of a more efficient popular refrigerator model compared with a baseline design in Brazil. The summarized results may be useful for organizations working to promote sustainable energy development. This paper specifically focuses on refrigerators, since their energy consumption is predicted to constitute over 30% of the total average domestic electricity bill in Brazilian households. If all new Brazilian refrigerators had an energy efficiency at the level consistent with the least life cycle cost of ownership, it would result in an annual savings of 2.8 billion dollars (US$) in electricity bills, 45 TWh of electricity demand, and 18 Mt of CO₂ emissions, with a respective payback period of 7 years which is less than half the average estimated lifetime of a refrigerator. The analysis was conducted following the guidelines of similar analyses available from the US Department of Energy and the Collaborative Labeling and Appliance Standards Program.

Numerical investigation of several physical and geometric parameters in the natural convection into trapezoidal cavities

Natural convection in trapezoidal cavities, especially those with two internal baffles in conjunction with an insulated floor, inclined top surface, and isothermal left-heated and isothermal right-cooled vertical walls, has been investigated numerically using the Element based Finite Volume Method (EbFVM). In numerical simulations, the effect of three inclination angles of the upper surface as well as the effect of the Rayleigh number (Ra), the Prandtl number (Pr), and the baffle’s height (H_b) on the stream functions, temperature profiles, and local and average Nusselt numbers has been investigated. A parametric study was performed for a wide range of Ra numbers (10³ ? Ra ? 10⁶) H_b heights (H_b = H¹/3, 2H¹/3, and H¹), Pr numbers (Pr = 0.7, 10 and 130), and top angle (θ) ranges from 10 to 20. A correlation for the average Nusselt number in terms of Pr and Ra numbers, and the inclination of the upper surface of the cavity is proposed for each baffle height investigated.     

Natural convection in a partially open square cavity with internal heat source: An analysis of the opening mass flow

A steady buoyancy-driven flow of air in a partially open square 2D cavity with internal heat source, adiabatic bottom and top walls, and vertical walls maintained at different constant temperatures is investigated numerically in this work. A heat source with 1% of the cavity volume is present in the center of the bottom wall. The cold right wall contains a partial opening occupying 25%, 50% or 75% of the wall. The influence of the temperature gradient between the verticals walls was analyzed for Ra_e = 10³–10⁵, while the influence of the heat source was evaluated through the relation R = Ra_i/Ra_e, investigated at between 400 and 2000. Interesting results were obtained. For a low Rayleigh number, it is found that the isotherm plots are smooth and follow a parabolic shape indicating the dominance of the heat source. But as the Ra_e increases, the flow slowly becomes dominated by the temperature difference between the walls. It is also observed that multiple strong secondary circulations are formed for fluids with a small Ra_e whereas these features are absent at higher Ra_e. The comprehensive analysis is concluded with horizontal air velocity and temperature plots for the opening. The numerical results show a significant influence of the opening on the heat transfer in the cavity.     

Mechanical,Thermal, and Morphological Behavior of Silicone Rubber during Accelerated Aging

An accelerated aging study on silicone rubber exploring the effects of exposure to a functional oil (polyalkylene glycol) at elevated temperature (195°C) is reported in this paper. Variations in mechanical (tensile, tear, hardness) and thermal (conductivity, specific heat capacity) properties were monitored versus aging time while permanent deformation of the rubber was evaluated through creep and recovery measurements. Morphology and surface chemistry of the aged rubber were also investigated through scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy, respectively. Aging had a significant impact on the mechanical properties with the ultimate tensile strength and elongation at break decreasing from 7.4?MPa and 2250% in unaged samples to 1.5?MPa and 760% in 6-week aged samples, respectively. The tear strength and hardness exhibited an initial increase during the early stages of aging, followed by a decreasing trend. In contrast, the thermal properties did not change significantly and FTIR did not detect any changes in the surface chemistry of the rubber with aging. SEM however, provided evidence of an increase in brittle behavior from the morphology of the fractured surfaces.     

Spatial correlation of incidence of dengue with socioeconomic, demographic and environmental variables in a Brazilian city

Dengue is endemic in most tropical countries. The reasons for its maintenance are related to several risk factors including socioeconomic factors. Urban space and time are also two important dimensions to describe the dynamics of an outbreak. The aim of this study was to evaluate the existence of a spatial correlation of the incidence of dengue and to identify variables that explain the spatial dependence in a medium-sized city of Brazil. We analyzed registered autochthonous dengue cases from September 1994 to August 2002 correlating them with socioeconomic, demographic and environmental variables using the Moran index. Our results indicate spatial dependence of the incidence of dengue and that socioeconomic factors were critical both at the beginning of the study and later on; other variables related to the organization of urban space were also involved in the occurrence of the disease. The use of spatial analysis tools is important to identify critical control areas with several variables intimately related to the modulation of the disease dynamics.     

Composites Between Alumina and an Ester‐Ether‐Ester Bioresorbable Copolymer

Composites between alumina and the bioresorbable poly(ε‐caprolactone)‐block‐poly(oxyethylene)‐block‐poly(ε‐caprolactone) copolymer were obtained by reacting ε‐caprolactone with preformed poly(ethylene glycol), in the presence of ceramic alumina powder, at 185°C under vacuum. The mechanical properties, tested by compression and flexural strengths and Young's modulus, show that the copolymer interacts poorly with the alumina grains. Both scanning electron and atomic force microscopy show a scarce wettability between alumina and copolymer, as well as the aggregation of alumina micro‐particles into clusters of big size. Both mechanical and morphological tests seem to indicate a stronger interaction between the alumina micro‐particles than between the alumina surface and the reaction mixture during the polymerization, as well as a “compacting effect” by alumina on the forming copolymer. The FT‐IR spectra of the composites show both copolymer and alumina absorption bands. The FT‐IR analysis on the fractions of an extraction with CHCl₃ indicates the presence of traces of poly(ε‐caprolactone), stably linked to alumina. The polymerization of ε‐caprolactone with alumina alone in the same conditions gives poly(ε‐caprolactone), mainly free and in minor part linked to the alumina surface. Two polymerization mechanisms, simultaneously occurring, are proposed. The most relevant result of this work is the lack of chemical inertness of alumina towards ε‐caprolactone, which leads to reconsider also the use of alumina as a biochemically inert material.