Intrinsic mode functions and fractal characteristics of the solar Incoming Shortwave Radiation observed at Nalohou (Benin Republic)

Abstract

Solar energy is clean and renewable energy obtained from solar radiation. However, the quantity of the Incoming Shortwave Radiation (ISR) reaching the ground depends on meteorological conditions (e.g. clouds covers) which make it intermittent, randomness and nonlinear. These complex properties lead to strong fluctuation of the available solar radiation power. In this work, we have studied fractal properties and determine the intrinsic oscillations of ISR using simultaneously the methods of: (a) Empirical Mode Decomposition (EMD), (b) Multifractal Spectra (MFS) analysis and (c) Multifractal Detrended Fluctuation Analysis (MFDFA). Data of ISR recorded at ½ hour time scale, measured in 2008 and 2009 at Nalohou AMMA-CATCH Station (Benin) are used. The results show that ISR time series can be decomposed into twelve (12) oscillatory Intrinsic Mode Functions (IMFs) with a decreasing polynomial trend. ISR is not a white noise and can be predicted. The multifractals properties obtained vary from oneIMF to another. Moreover, there is disparity between multifractal properties of each IMF and those of the original time series. So, none of the IMFs can represent the original time series and the strength of their multifractality does not depend on the oscillation period of the IMFs.