Análise harmônica da radiação solar global mensal na região de Pelotas-RS

Abstract

Solar radiation, besides being our planet’s main source of energy, has been increasingly studied and used by several areas of the knowledge. Considering its importance, this work aims to expand information on the subject, by applying the harmonic analysis technique to data of total monthly global solar radiation of the region of Pelotas-RS. Such technique decomposes the series of data in independent components, called harmonics. The objectives are: to analyze the behavior of radiation and to establish two models in order to estimate the total monthly global solar radiation in a medium year. The first one, starting from 10 years (1993-2002) of measured data, and the second, due to the lack of measured data for a larger period, starting from 30 years (1973-2002) of estimated data by the Ångström-Prèscott equation. To consolidate the results, the technique has also been applied to a 10-year period (1993-2002) of estimated data by Ångström-Prèscott. In the three cases, it was tested the normality of data and homogeneity of data variances, being verified that these follow the normal distribution and present homogeneous variances, with no need to be transformed. For the 10 years of measured data, it was used to determine the waves that would compose the final model, a test of stability (variation analysis) in which the only wave considered stable was the annual wave (1st harmonic), with a representative percentile of 98,74% of the total variation of the data. The obtained final model, based on the measured data, was: ( o ) am 48 , 108 t . 30 sen . 16 , 205 71 , 431 Yˆ t = + + . The analyses results with the estimated data by Ångström-Prèscott, in the periods of 10 and 30 years, were almost identical to those with the measured data. In a qualitative analysis, the annual wave, due to its highly representative percentile, 98,87% (1993-2002) and 99,66% (1973-2002), was also the only one indicated to compose a final estimate model. The obtained final model, based on the 30 years of estimated data, was: ( o ) t am 48 , 108 t . 30 sen . 56 , 201 01 , 427 Y ˆ 1 = + + . Considering the estimate results obtained by the two final models, the conclusion was that they are equivalent. By the three harmonic analyses, it was verified that the behavior of the total monthly global solar radiation, in the region of Pelotas-RS, follows the natural annual cycle of the Earth’s movement around the Sun, presenting maximum levels in summer and minima in winter, with intermediate values in spring and autumn. The great variation between radiation’s levels in summer and winter was attributed to the large distance of the region (latitude) from the equator line.