[en] In this research work the solar radiation for Lahore (Latitude 31.56)has been studied for efficient utilization of solar energy employing sunshine hour data. The results obtained exhibit the variation of direct and diffuse radiation at Lahore. The diffuse radiation is maximum in the month of July and minimum during the month of April, May and June. The K Value indicates the clear sky during the month of January, February, of April, May and September to December. Where as during the month of Jun to August the sky is mostly cloudy which is the monsoon months. From the estimated results it is found that with the exception of monsoon months solar energy can be utilized very efficiently through out the year. (author)

[en] For many solar-related applications, it is important to separately predict the direct and diffuse components of irradiance or illuminance. Under clear skies, turbidity plays a determinant role in quantitatively affecting these components. In this paper, various aspects of the effect of turbidity on both spectral and broadband radiation are addressed, as well as the uncertainty in irradiance predictions due to inaccurate turbidity data, and the current improvements in obtaining the necessary turbidity data

[en] The measured values of monthly average daily global H-bar and diffuse H-bar_d solar radiation on a horizontal surface, as well as the number of bright sunshine hours n-bar, for four Egyptian locations are analyzed. The selected locations include Matruh, Al-Arish, Rafah and Aswan to represent the weather conditions of the north and south of Egypt. First, second and third order correlations between the diffuse fraction H-bar_d/H-bar and the clearness index K-bar_t and also between the diffuse transmittance H-bar_d/H-bar₀ and the maximum relative number of bright sunshine hours n-bar/N-bar have been proposed for the selected locations using the method of regression analysis. Comparisons between the measured and calculated values of H-bar_d show that a first order correlation between H-bar_d/H-bar and K-bar_t can be used for estimating H-bar_d at the present locations with good accuracy. However, second order correlations between H-bar_d/H-bar or H-bar_d/H-bar₀ and n-bar/N-bar are recommended for estimating H-bar_d at these locations. For all locations, the relative percentage errors for a single month rarely exceed ±10%. Furthermore, the data available from the four locations are combined and employed to develop correlations that may be applied for any Egyptian location. All Egypt correlations are found to be able to predict the annual averages of horizontal diffuse radiation with excellent accuracy. Therefore, the long term performance of solar energy devices can be estimated

[en] In Part II, more detailed observations of solar radiation hourly averages of Hamburg were analysed. Global solar radiation, strongly influenced by clouds, decreased with a low significance between 1964 and 1989. The significance of the trend of increasing direct solar radiation in the same period is very weak, because the clouds play the dominant role. The diffuse solar radiation, which is a safe indicator for trends in solar irradiance, because it is relatively independent of the weather, decreased between 1964 and 1989. The reasons for this decrease are the measures to clean the air. Between 1975 and 1987 the diffuse solar radiation increased slightly. The reason for this fact is a doubling of optically active aerosol particles in the atmospheric boundary layer in this time period. (orig.)

[en] Solar radiation over the Central Black Sea Region of Turkey, covering the five provinces (Amasya, Corum, Ordu, Samsun and Tokat), was analyzed to guide future projects. The maximum monthly average global solar radiation ranged from 20.05 (for Ordu) to 23.71 MJ m^-2 (for Tokat). The maximum monthly average daily measured sunshine duration ranged from 6.89 (for Ordu) to 11 h (for Corum). A quadratic polynomial equation was empirically developed to predict the monthly average daily global radiation. A hybrid model was also developed based on the predictions of six existing models to predict the monthly average daily diffuse and beam radiation. Various statistical tests (analysis of variance for model adequacy and t-test for significance of model parameter etc.) have shown that the models developed in this study are adequate for solar radiation prediction

[en] A harmonic model for the hourly global and diffuse solar radiation fractions is developed. The Fourier coefficients of the model are determined by using measured data. In testing the generality of this approach, similar analysis is applied to data collected in five cities in different countries. The results demonstrate a consistency in the level of accuracy of the model. Computations by the model are in excellent agreement with the data. The percentage error between the data and the Fourier coefficients are used in the model to make it general. The errors of the general model remain within acceptable limits, especially for time range of about ten hours around the solar noon. Computations of the hourly global and diffuse solar radiation fractions by the harmonic model developed in this research demonstrate a significant improvement over computations by existing models. (author). 10 refs., figs., tabs

[en] This paper presents a new method for estimating hourly direct and diffuse solar radiation. The essence of the method is the estimation of two important factors related to solar radiation, atmospheric transmittance and a dimensionless parameter, using empirical and physical equations and data from general meteorological observation stations. An equation for atmospheric transmittance of direct solar radiation and a dimensionless parameter representing diffuse solar radiation are developed. The equation is based on multiple regression analysis and uses three parameters as explanatory variates: calculated hourly extraterrestrial solar radiation on a horizontal plane, observed hourly sunshine duration and hourly precipitation as observed at a local meteorological observatory. The dimensionless parameter for estimating a diffuse solar radiation is then determined by linear least squares using observed hourly solar radiation at a local meteorological observatory. The estimated root mean square error (RMSE) of hourly direct and diffuse solar radiation is about 0.0-0.2 MJ¥m(-2)¥h(-1) in each mean period. The RMSE of the ten-day and monthly means of these quantities is about 0.0-0.2 MJ¥m(-2)¥h(-1), based on comparisons with AMeDAS station data, located at a distance of 6 km

[en] A daily average diffuse sky radiation regression model based on daily global radiation was developed utilizing two year data taken near Blytheville, Arkansas (Lat. =35.9⁰N, Long. = 89.9⁰W), U.S.A. The model has a determination coefficient of 0.91 and 0.092 standard error of estimate. The data were also analyzed for a seasonal dependence and four seasonal average daily models were developed for the spring, summer, fall and winter seasons. The coefficient of determination is 0.93, 0.81, 0.94 and 0.93, whereas the standard error of estimate is 0.08, 0.102, 0.042 and 0.075 for spring, summer, fall and winter, respectively. A monthly average daily diffuse sky radiation model was also developed. The coefficient of determination is 0.92 and the standard error of estimate is 0.083. A seasonal monthly average model was also developed which has 0.91 coefficient of determination and 0.085 standard error of estimate. The developed monthly daily average and daily models compare well with a selected number of previously developed models. (author). 11 ref., figs., tabs

[en] This document shows the result of an exhaustive study about the theoretical and numerical models available in the literature about solar radiation modelling. The purpose of this study is to develop or adapt mathematical models describing the solar radiation specifically for Spain locations as well as to create computer tools able to support the labour of researchers or engineers needing solar radiation data to solve or improve the technical or energetic performance of solar systems. As result of this study and revision, a C library (SRADLIB) is presented as a key tool for the compilation of the mathematical models from different authors, for the comparison among the different approaches and for its application in computer programs. Different topics related to solar radiation and its modelling are first discussed, including the assumptions and conventions adopted and describing the most accepted and used current state-of-the-art models. Some typical problems in the numerical calculation of radiation values are also posed with the proposed solution. The document includes next a complete reference of the developed functions, with many examples of application and calculus. (Author) 24 refs

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