Estimation of the ground albedo for the Athens area, Greece

Ground albedo can severely influence the estimation of solar radiation incident on tilted surfaces. Depending on the model used to calculate the reflected radiation, a significant error in the estimation of total radiation on an inclined surface is likely to be introduced. In the present work, several models for calculating the ground albedo are evaluated; the derived values are further used for estimating the ground-reflected radiation on a tilted Surface. Four of these models are taken from the international literature and four new correlations are developed for the purpose of this study. All eight models are evaluated against measurements of ground-reflected radiation performed at the National Observatory of Athens in the period 1999-2008. The evaluation shows that the existing models, including that with a constant albedo value (0.2), perform relatively well. Nevertheless, if more accuracy is needed for scientific applications, site-specific models must be developed. Due to this demand new albedo models are derived for the Athens area that can perform efficiently all year round. The results of the work show that: (a) The use of the constant albedo Value of 0.2, which is widely accepted and used in most applications when ground-albedo measurements are lacking, is unrealistic for the case of Athens: it is too high. (b) Satisfactory results can be obtained when applying a new constant albedo value estimated or measured at the specific site. (c) Nevertheless, more complicated albedo models adapted to the site characteristics can give better results than any of those using a constant albedo value, if ground-albedo estimations are to be made for a short period of say 5-10 min. (C) 2009 Elsevier Ltd. All rights reserved.