ON THE RELATIONSHIP OF BEAM TRANSMITTANCE ON CLEARNESS INDEX FOR ATHENS, GREECE

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Abstract
Proper design and performance prediction of solar energy systems requires accurate information on the availability of solar radiation. However, the complexity and cost of a reliable radiation measuring network dictates the need for developing alternatives that can replace and/or complement observations. Dependable solar radiation models are an alternative. In this paper, two years of solar radiation data from the National Observatory of Athens, Greece (37.5°N, 23.5°E, 107 m above MSL) are used to develop a continuous piecewise linear regression model that predicts hourly direct irradiation values when only global irradiation data are available. The applicability of the Randall-Whitson [7] and Jeter-Balaras [9] models in the area of Athens is also evaluated. For the urban atmospheric conditions of Athens the developed two year correlation provides a better fit to the empirical data.