Spectral Analysis of Hourly Solar Radiation

Abstract

Τhis research aims at the precise estimation of solar radiation, on an hourly and daily basis, by using measured data of solar radiation and sunshine duration. Data is collected from the automatic meteorological station of Amynteo, West Macedonia, Greece. For the correlation of solar radiation and sunshine duration, the Angstrom–Prescott equation is selected. Two limit conditions of cloud coverage are examined, overcast and clear sky, for the estimation of the factors as and bs of the Angstrom–Prescott equation. The factor as is the minimum transmissivity for overcast sky, while as + bs is expressing the clear sky transmissivity. In addition, spectral analysis is performed to investigate the periodicities of the factors as and bs. The Lomb–Scargle periodogram is applied, as data are unevenly distributed in time, and reveals the existence of two periodicities for the bs factor. More specifically, annual periodicity with an estimated width equal to 0.032, and daily periodicity with a width equal to 0.184. As a result of this research, a sinusoidal equation with two harmonics is developed to calculate the bs factor, which coincides with the clearness index (Rs/Ra) under clear sky. In overcast conditions, the clearness index does not exhibit periodicities. The derived equation is used to produce estimations of solar radiation values on an hourly and daily basis. The daily values estimated from the new equation gives better results when compared to typical regression analysis.