Direct and diffuse solar radiation correlations for tilted surfaces in Athens, Greece

Abstract The solar energy available in the Athens, Greece area was determined in a previous publication by a processing of hourly measurements corresponding to a period of 20 years. Here, the same measurements are employed in conjunction with appropriate models for analysing total radiation in direct and diffuse components and for estimating the energy incident on tilted surfaces. Results include instant (kW/m 2 ) and integrated (kJ/m 2 per day, month or year) values of the average direct, diffuse and total energy incident on tilted surfaces. These quantities are presented in the form of algebraic correlations having as independent variable time (hour, day, month), surface azimuth and angle of inclination to the horizontal plane.