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.     

Exact solutions for multilayer optical structures.: Application to PV modules

The analysis of multilayer optical devices is important in solar technology as they can be found in a large variety of equipment, particularly in photovoltaic modules. Absorbed and transmitted light by a set of thick layers are usually obtained by ray tracing procedures. When the optical structure is formed by more than one layer, the calculation of both the absorbed and transmitted light is rather complex due to multiple interactions between the several interfaces of the optical device. This paper describes a general analytic procedure, valid for any number of layers, to calculate the absorbed and transmitted light, e.g. solar radiation, through a set of thick optical layers. Consideration of incoming and outgoing light fluxes at each interface, and the assumption that the last interface acts as a light sink, leads to a closed system of equations that can be solved sequentially. Results are applied to analyze the optical behavior of an encapsulated solar cell and a photovoltaic module.     

Throughput performance of parallel and repetition coding in incremental decode-and-forward relaying

The throughput performance of cooperative repetition and parallel coding in incremental decode-and-forward is investigated. Four transmission methods are considered: parallel coding with and without distributed space-time coding (PC-ST and PC, respectively); and repetition coding with and without Chase combining at the destination (RC and SC, respectively). The analysis is based on the mutual information seen at the receiver for each scheme. Exact expressions for the outage probability and throughput for all methods are derived. Both ad-hoc and infra-structured relaying scenarios are investigated. Results show that SC can perform very close to RC, PC and PC-ST in terms of throughput, specially in the case of infra-structured relaying or adequate power and rate allocation. The conclusion is that SC would be a better option in practice, since it requires a simpler receiver than PC-ST, PC, and RC.     

Comparison of the performance of PV water pumping systems driven by fixed, tracking and V-trough generators

Photovoltaic pumping systems with solar tracking, coupled to low concentration cavities, have been proposed as a viable alternative to reduce the final cost of the pumped water volume. V-trough concentrators are particularly appropriate for photovoltaic applications since, for certain combinations of the concentration ratio (C) and vertex angle (Ψ), they provide uniform illumination on the region where the modules are located. Water pumping systems are only operational when the irradiance is larger than a minimum irradiance level (I_C). Solar tracking increases the average collected irradiance and, for a system operating with a given critical irradiance level (I_C), it is verified that the smaller the relationship , the larger the useful energy. Thus, the gain, in terms of pumped water volume, provided by solar tracking systems, can be larger than the gain in collected solar radiation. The combination of both devices, tracking and concentration provides an additional increase of the benefits resulting from the use of solar trackers. By means of the “Utilizability Method”, we estimate the long-term gains of pumped water volume, for tracking systems, with and without concentration, against fixed systems. The long-term water volume has been calculated using the characteristic curve of a tested PVP system with a tracking V-trough concentrator. Results show that, for the climate of the city of Recife (PE-Brazil), the annual pumped water volume of the tracking system is 1.41 times the value obtained with the fixed system. In that case, the gains observed for the collected solar energy were around 1.23. For the PVP system with tracking V-trough concentrator the annual benefits for pumped water volume are around 2.49, while for collected solar radiation we found 1.74. The annualized cost of the cubic meter of pumped water has been estimated for the three configurations. Results show a cost reduction of the order of 19% for the tracking system and of 48% for the concentrating system, when compared to the fixed configuration.     

Analysis of monthly time series of solar radiation and sunshine hours in tropical climates

This paper analyses the interannual variability of solar radiation and sunshine hours for a large tropical region (Brazil), located between latitude 0°S and 30°S, in order to improve knowledge on solar resources, generate statistical parameters for model checking or to be used as input data of synthetic time series generation. The statistics for the daily, monthly average solar radiation deviations, and daily, monthly average sunshine hours, for the various localities in Brazil, tested with the Kolmogorov–Smirnov method, show that they are random variables, normally distributed. On the other hand, the sequential properties analysis shows that the auto-correlation coefficients with lag 1 are statistically significant only for a few locals: Fortaleza, São Luís, Manaus and Belém. But it is necessary to emphasize that the auto-correlation coefficients with lag 1, though not usually statistically significant, are positive for almost all the locals. The AR-1 is the suggested procedure for monthly solar radiation synthetic time series generation, with auto-correlation coefficients varying from 0.30 to 0.47 for the localities in the north of Brazil and zero for other regions.     

Technical note On the development of spatialtemporal solar radiation maps: a Brazilian case study

This work summarizes recently published information on the solar resource of Brazil. We describe the spatial distribution of solar radiation and its relationship with climatic and geographical conditions. In order to harmonize the information in terms of type of instruments, time recording period and data processing methods, a careful selection of records from the data base was made. Density of recording stations is reasonable in the south, southeast and northeast regions of the country, while in the west center and north regions the density of stations is rather poor. The procedure to elaborate the maps of daily solar radiation, monthly and annual average is described. Consideration of the measuring period of the monthly averages, used to elaborate the contour maps, shows that they meet the requirement that 90% of averages are inside the strip of ±7.5%, centralized on the average of very long period measurements. We present one map with the localization of the recording stations and one annual and 12 monthly contour maps, describing daily solar radiation levels over the whole territory. Spacing among the contour lines is (±2 MJm² day). Annual average of solar radiation lies within the interval of ((18±2) MJm² day), except in the northeast region where values higher than (20 MJm² day) are found. Two regions with levels of (16 MJm² day) are also observed. The highest monthly average values (24 MJm² day) are observed in the state of Rio Grande do Sul, southern end of the country, in the summer season (December and January). The lowest values in the country (8 MJm² day) are observed in June and July (winter in the southern hemisphere), on the extreme south coastline of the same state, Rio Grande do Sul, below 32° south latitude.     

Analysis and evaluation of a novel quality assurance device for ultrasonic medical imaging systems

A novel test device that has the potential to facilitate day-to-day maintenance and quality assurance of ultrasonic medical imaging systems was developed and tested. The device consists of small echogenic particles deposited on a thin film substrate in patterns similar to those found on conventional eye charts, which can be scanned to produce recognizable images that can be quantitatively evaluated in a quick and repeatable fashion. An evaluation system consisting of this device and a related assessment methodology was tested at several locations by both physicians and sonographers. The results of the testing and responses of the users were both encouraging and insightful.