Modélisation du Rayonnement Solaire à l’Echelle Journalière et Horaire

Abstract

In order to estimate the sun rays, we defined first the astronomical and radiometric parameters which are included in the modelling calculus of sun rays. Then, we presented several measuring instruments of sun rays used or not for data gathering used in the modelling. After that, we presented the estimation models; Perrin de Brichambaut model, fitted to clear sky, used in the estimation of sun rays on a horizontal plane, Liu & Jordan model, fitted to clear sky, used in the estimation of sun rays on an inclined plane and Mohamed Rédha Yaïche approach, fitted to all types of skies, for sun rays estimation. This approach consists of using Perrin de Brichambaut model for estimating irradiations values on a horizontal plane by combining this model with the approach method in order to determine the type of sky; the approach consist of subdividing the celestial sphere, theoretically, into eight identical portions, each portion represents 1/8 octa, by definition a sky is totally clear if its cloud cover is equal to 0/8 octa and the sky is totally obscured if its cloud cover is equal to 8/8 octas, but during the estimation calculus it was commode to consider the opposite assumption. After, we subdivided the global ray into two parts; direct and diffuse, if the direct irradiation increases the diffuse irradiation decreases and reciprocally, the method consists of multiplying the direct irradiation by a n/8 factor and therefore the diffuse irradiation was multiplied by a 8/n factor for a number n varying from 0 to 8 thus reflecting different types of skies, as a consequence a factors series follows so that the series has a kind of symmetry. For the intermediate types of skies, another factors series is imperative, having likewise the samev symmetry characteristic. The global estimated irradiation is the sum of direct and diffuse irradiations estimated according to their factors. After, we calculated the algebraic relative error of the global irradiation for each factor, we chose the lowest value of the relative error and hence, the latter yields the required factor and therefore the type of sky, the relative error is used too to value the method. The same approach is applied to Liu & Jordan model to value the estimation but also to establish a base of calculated data for several angles of inclination and direction. We worked with hourly and daily measured data provided by URERMS of Adrar. We noticed that the approach got good results when it is applied to both models. By the other hand, Liu & Jordan model uses the formula of Perrin de Brichambaut model by combining the latter with the albedo and inclination factor, this factor depends only on astronomical parameters and inclination angle. Consequently, it is enough to have got only one horizontal pyranometer to get a calculated data base for any angle of inclination and direction. This shows the economic impact that this approach may have. Finally, the calculations were performed using the software in Excel realized by Mohamed Rédha Yaïche.