Comparison between measurements and models for daily solar irradiation on tilted surfaces in Athens, Greece

This paper presents a comparison of several models against measurements of daily total solar radiation on surfaces of various tilts and azimuths in Athens. Such measurements are carried out at the Institute of Meteorology and Physics of the Atmospheric Environment, National Observatory of Athens and are the first of this kind in Greece. The models encounter a constant surface albedo of 0.2 and are of two categories. Category I includes models estimating hourly diffuse radiation on tilted planes (Bugler, Gueymard, Reindl and Temps-Coulson), which obtain the necessary global and diffuse horizontal radiations from either measurements or estimations from the Meterological Radiation Model (MRM) developed at the National Observatory of Athens. Category II includes Isotropic and Klein models for daily estimations; the models obtain their input from available measurements. Three types of diffuse radiation estimation in the second category are encountered. The purpose of the present paper is two-fold. First, it aims at showing the difference in estimating daily global irradiation between hourly models (category I) and daily models (category II). Second, it intends to give an alternative way to estimating daily irradiations on tilted surfaces using just MRM for engineering purposes.