Generalized Nakagami-m phase crossing rate

This paper provides simple, exact, new closed-form expressions for the generalized phase crossing rate of Nakagami-m fading channels. Sample numerical results obtained by simulation are presented that validate the formulations developed here. A special case of this formulation is the Rayleigh case, whose result agrees with that obtained elsewhere in the literature. In passing, several new closed-form results concerning the statistics of the envelope, its in-phase and quadrature components, phase, and their time derivatives are obtained.     

Second-order statistics of maximal-ratio and equal-gain combining in Weibull fading

Exact expressions for the level crossing rate and average fade duration of M-branch equal-gain and maximal-ratio combining systems in a Weibull fading environment are presented. The expressions apply to unbalanced, nonidentical, independent diversity channels. In addition, new closed-form solutions for some special cases are obtained.     

Brazilian Solar Resource Atlas CD-ROM

The Brazilian Solar Resource Atlas CD-ROM is intended to be a tool for solar system design, delivering information that relies on a large interactive database, including, also, a modern device for calculating the solar radiation collected by planes inclined at different tilt angles or tracking the sun around one or two axes. The spatial distribution of the daily solar radiation, monthly and yearly averaged, that hits Brazil is presented in 13 colored maps, together with the methodological procedures used for harmonizing the information and designing the maps. A map containing the location of pyranometric and actinographic terrestrial stations, the information which was used for designing the maps, is also presented. The period of collected data runs from 1978 to 1990. Similarly, the atlas contains 13 colored maps, including daily insolation, monthly and yearly averaged, in addition to the map containing the location of the heliographic stations. The CD-ROM stores a database that includes over 500 stations located in Brazil and on the borders of its neighboring countries. The information is given either as daily, monthly average global solar radiation, or daily, monthly average insolation. A search device is included in the database, allowing to navigate over the maps or to select a desired location. The information may be printed as a report. The CD-ROM also includes a software to perform the following calculations: solar radiation estimates on a fixed plane at any inclination on North-South orientation; estimates of solar radiation collected by a plane tracking the sun about one or two axes; generation of synthetic series based on Markov’s Transition Matrixes.     

Legacy code and parallel computing: updating and parallelizing a numerical model

In this paper, we present several important details in the process of legacy code parallelization, mostly related to the problem of maintaining numerical output of a legacy code while obtaining a balanced workload for parallel processing. Since we maintained the non-uniform mesh imposed by the original finite element code, we have to develop a specially designed data distribution among processors so that data restrictions are met in the finite element method. In particular, we introduce a data distribution method that is initially used in shared memory parallel processing and obtain better performance than the previous parallel program version. Besides, this method can be extended to other parallel platforms such as distributed memory parallel computers. We present results including several problems related to performance profiling on different (development and production) parallel platforms. The use of new and old parallel computing architectures leads to different behavior of the same code, which in all cases provides better performance in multiprocessor hardware.

     

Design procedure of V-trough cavities for photovoltaic systems

The combination of photovoltaic (PV) systems with V-trough cavities has been identified as an attractive option to reduce, in the short time scale, the prices of the PV electrical energy. In places of good radiation level, the output energy of these devices can be almost doubled, compared to PV flat-plate fixed systems. Additionally, V-trough cavities are simple to manufacture and can be used with conventional (1-sun) solar cells. In this work we present a design procedure for V-trough cavities used in combination with PV generators. The main design requirements are: uniform illumination on the plane of the PV module, within a finite interval of incidence angles; minimum cost of energy; and heat dissipation by natural, passive means. The V-trough cavities depend on two parameters. We obtain a first analytical relation between the concentration ratio (C) and the V-trough angle (ψ) for concentrators with uniform illumination at the absorber. The region of minimum cost of the V-trough PV ensemble yields a second relation. Then, a unique pair of cavity parameters, satisfying the above criteria, is found. A design example of a V-trough cavity for the city of Recife, PE, Brazil, is presented. © 1998 John Wiley & Sons, Ltd.     

Dynamic behavior of water wells coupled to PV pumping systems

Evaluation of a water supply system requires an overall analysis of its behavior, including the well performance and drawdown characteristics. The response of the water well is usually characterized by a single relationship between water level and water flow, representing its quasi‐equilibrium behavior. However, for days with large and rapid variations of irradiance this relationship does not correspond to the well‐equilibrium curve. In order to simulate the well behavior it has been developed a model that takes into account the variation of water level, pressure at the entrance of the well and static pressure of the aquifer, all of them related to the water flow. The model also considers the energy conservation equation of the system through the relationship among hydraulic load, water flow, and collected irradiance. Results of simulations show that, for days with large variations in solar irradiance, the static and dynamic analysis exhibit large differences in the instantaneous water head. The dynamic analysis shows that the water well tends to attenuate the fluctuations of the water head through a negative feedback mechanism. The model helps to understand some facts observed in the field. For example, the dispersion of points in the flow rate versus solar irradiance curves. The maximum drawdown obtained with the dynamic model is considerably smaller than the one predicted with the quasi‐equilibrium analysis. That difference can represent some savings in terms of water well hardware (drilling, wiring, and piping) for programs involving the installation of a large number of systems. The slowdown of water column variations, shown by the dynamic model, signifies that the motor‐pump system is not so intensively demanded as the quasi‐equilibrium analysis would suggest, resulting in longer lifetime prospects. Copyright © 2007 John Wiley & Sons, Ltd.     

A Simple Accurate Method for Generating Autocorrelated Nakagami-m Envelope Sequences

We propose a simple accurate method for generating autocorrelated Nakagami-m envelope sequences. The method allows for arbitrary values of fading parameter and nonisotropic fading scenarios. In essence, Nakagami-m samples are first drawn and then rearranged to match the Nakagami-m autocorrelation. The rearrangement is made in accordance with the rank statistics of an underlying Rayleigh reference sequence with the desired autocorrelation. Examples illustrate the excellent performance of the new method     

The symmetrical /spl eta/-/spl kappa/ distribution: a general fading distribution

This paper specializes and parameterizes the general result presented elsewhere in the literature in order to introduce, fully characterize, and investigate the Symmetrical /spl eta/-/spl kappa/ Distribution, a general distribution used to describe the statistical variation of the envelope in a fast fading environment. It proposes estimators for the involved parameters and uses field measurements to validate the distribution. In spite of its specialization, the Symmetrical /spl eta/-/spl kappa/ Distribution still includes, as special cases, important distributions such as Rayleigh, Rice, Hoyt, Nakagami-q, and One-Sided Gaussian. The fact that the Symmetrical /spl eta/-/spl kappa/ Distribution has one more parameter than the well-known distributions renders it more flexible. Of course, in situations in which those distributions included in it give good results a better fitting is given by the Symmetrical /spl eta/-/spl kappa/ Distribution. In addition, in many other situations in which these distributions give poor results a good fitting may be found through the Symmetrical /spl eta/-/spl kappa/ Distribution. More specifically, its nonmonomodal feature finds applications in several circumstances, examples of which are given in this paper.     

Generation of daily solar irradiation by means of artificial neural net works

The present study proposes the utilization of Artificial Neural Networks (ANN) as an alternative for generating synthetic series of daily solar irradiation. The sequences were generated from the use of daily temporal series of a group of meteorological variables that were measured simultaneously. The data used were measured between the years of 1998 and 2006 in two temperate climate localities of Brazil, Ilha Solteira (São Paulo) and Pelotas (Rio Grande do Sul). The estimates were taken for the months of January, April, July and October, through two models which are distinguished regarding the use or nonuse of measured bright sunshine hours as an input variable. An evaluation of the performance of the 56 months of solar irradiation generated by way of ANN showed that by using the measured bright sunshine hours as an input variable (model 1), the RMSE obtained were less or equal to 23.2% being that of those, although 43 of those months presented RMSE less or equal to 12.3%. In the case of the model that did not use the measured bright sunshine hours but used a daylight length (model 2), RMSE were obtained that varied from 8.5% to 37.5%, although 38 of those months presented RMSE less or equal to 20.0%.A comparison of the monthly series for all of the years, achieved by means of the Kolmogorov–Smirnov test (to a confidence level of 99%), demonstrated that of the 16 series generated by ANN model only two, obtained by model 2 for the months of April and July in Pelotas, presented significant difference in relation to the distributions of the measured series and that all mean deviations obtained were inferior to 0.39 MJ/m².It was also verified that the two ANN models were able to reproduce the principal statistical characteristics of the frequency distributions of the measured series such as: mean, mode, asymmetry and Kurtosis.