767 resultados para clearness index


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Pós-graduação em Agronomia (Energia na Agricultura) - FCA

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Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

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Pós-graduação em Agronomia (Energia na Agricultura) - FCA

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Pós-graduação em Agronomia (Energia na Agricultura) - FCA

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O presente trabalho apresenta equações de estimativa da radiação fotossinteticamente ativa global () e difusa () em função das respectivas radiações global () e difusa () do espectro solar total, bem como a estimativa da fração PAR difusa da PAR global () em função do índice de transmissividade atmosférica (). A base de dados foi adquirida no período de 01/06/1999 a 31/09/2000 na Estação de Radiometria Solar da FCA-UNESP, Botucatu. Foram utilizados dados adicionais, diferentes dos utilizados na geração dos modelos, de forma a possibilitar uma validação adequada dos modelos propostos.

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Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

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This work presents an analysis of the pattern variation of the sky conditions in Botucatu/SP. The classification was based on the clarness index KT. It was used in the classification a database of global solar irradiance in the horizontal, measured at Radiometric Station of UNESP in Botucatu (latitude 22.85°S, longitude 48.43°W, altitude 786 m) from 1996 to 2005. We analyzed the variations in frequency, during the photoperiod and over the year, of the sky conditions: I - Cloudy (KT < 0.35); II - party cloudy with dominance for the diffuse (0.35 < 0.55 < KT)) III - party cloudy with dominance for the clear (0.55 < KT < 0.65) and IV - clear (KT > 0.65). The frequency of sky conditions I and II are higher during the rainy season of the year and in hours close to sunrise and sunset. And it is lower during the dry season an in hours near noon, in wich there is a predominance of sky conditions III and IV. The monthly hourly frequency of sky condition I ranged from 46,91% of the photoperiod in January to 25.27% in August. And the frequency monthly daily ranged from nine days in January to three days in August. To the sky condition IV, the frequency monthly hourly ranged from 38.81% of the photoperiod in August to 18.17% im January. And the frequency monthly daily ranged from 17 days in August to three days in January.

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Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

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The study introduces a new regression model developed to estimate the hourly values of diffuse solar radiation at the surface. The model is based on the clearness index and diffuse fraction relationship, and includes the effects of cloud (cloudiness and cloud type), traditional meteorological variables (air temperature, relative humidity and atmospheric pressure observed at the surface) and air pollution (concentration of particulate matter observed at the surface). The new model is capable of predicting hourly values of diffuse solar radiation better than the previously developed ones (R-2 = 0.93 and RMSE = 0.085). A simple version with a large applicability is proposed that takes into consideration cloud effects only (cloudiness and cloud height) and shows a R-2 = 0.92. (C) 2011 Elsevier Ltd. All rights reserved.

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This paper analyzes the correlation between the fluctuations of the electrical power generated by the ensemble of 70 DC/AC inverters from a 45.6 MW PV plant. The use of real electrical power time series from a large collection of photovoltaic inverters of a same plant is an impor- tant contribution in the context of models built upon simplified assumptions to overcome the absence of such data. This data set is divided into three different fluctuation categories with a clustering proce- dure which performs correctly with the clearness index and the wavelet variances. Afterwards, the time dependent correlation between the electrical power time series of the inverters is esti- mated with the wavelet transform. The wavelet correlation depends on the distance between the inverters, the wavelet time scales and the daily fluctuation level. Correlation values for time scales below one minute are low without dependence on the daily fluctuation level. For time scales above 20 minutes, positive high correlation values are obtained, and the decay rate with the distance depends on the daily fluctuation level. At intermediate time scales the correlation depends strongly on the daily fluctuation level. The proposed methods have been implemented using free software. Source code is available as supplementary material.

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Cereal-legume intercropping plays an important role in subsistence food production in developing countries, especially in situations of limited water resources. Crop simulation can be used to assess risk for intercrop productivity over time and space. In this study, a simple model for intercropping was developed for cereal and legume growth and yield, under semi-arid conditions. The model is based on radiation interception and use, and incorporates a water stress factor. Total dry matter and yield are functions of photosynthetically active radiation (PAR), the fraction of radiation intercepted and radiation use efficiency (RUE). One of two PAR sub-models was used to estimate PAR from solar radiation; either PAR is 50% of solar radiation or the ratio of PAR to solar radiation (PAR/SR) is a function of the clearness index (K-T). The fraction of radiation intercepted was calculated either based on Beer's Law with crop extinction coefficients (K) from field experiments or from previous reports. RUE was calculated as a function of available soil water to a depth of 900 mm (ASW). Either the soil water balance method or the decay curve approach was used to determine ASW. Thus, two alternatives for each of three factors, i.e., PAR/SR, K and ASW, were considered, giving eight possible models (2 methods x 3 factors). The model calibration and validation were carried out with maize-bean intercropping systems using data collected in a semi-arid region (Bloemfontein, Free State, South Africa) during seven growing seasons (1996/1997-2002/2003). The combination of PAR estimated from the clearness index, a crop extinction coefficient from the field experiment and the decay curve model gave the most reasonable and acceptable result. The intercrop model developed in this study is simple, so this modelling approach can be employed to develop other cereal-legume intercrop models for semi-arid regions. (c) 2004 Elsevier B.V. All rights reserved.

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Solar radiation data is crucial for the design of energy systems based on the solar resource. Since diffuse radiation measurements are not always available in the archive data series, either due to the inexistence of measuring equipment, shading device misplacement or missing data, models to generate these data are needed. In this work, one year of hourly and daily horizontal solar global and diffuse irradiation measurements in Évora are used to establish a new relation between the diffuse radiation and the clearness index. The proposed model includes a fitting parameter, which was adjusted through a simple optimization procedure to minimize the Least Square Error as compared to measurements. A comparison against several other fitting models presented in the literature was also carried out using the Root Mean Square Error as statistical indicator, and it was found that the present model is more accurate than the previous fitting models for the diffuse radiation data in Évora.