A new approach for assessing the clear-sky direct normal irradiance in real time

In the present paper, a new approach is proposed to assess the clear-sky direct normal irradiance (DNI) in real time. This approach combines an existing empirical model, proposed by Ineichen and Perez in 2002, with a new methodology for the computation of atmospheric turbidity. This methodology takes advantage of the fact that changes in atmospheric turbidity are relatively small throughout the day in comparison to changes in DNI, even when the sky is free of clouds. We considered data from two experimental sites (Golden, in the USA, and Perpignan, in France) and compared the proposed approach with several combinations of empirical models and ways of computing atmospheric turbidity. A polynomial of the cosine of the solar zenith angle has also been included in the comparative study. In both sites, our approach outperforms the other approaches. It has proven to be well adapted to the real-time assessment of the clear-sky DNI, in particular when the Sun is occulted by clouds during less than eight consecutive hours.