Splitting the solar radiation in direct and diffuse components; Insights and constrains on the clearness-diffuse fraction representation

In many engineering applications, it is mandatory to know separately the solar radiation diffuse and direct components. Examples regard the assessment of the energy potentially exploitable by a system of solar thermal or photovoltaic panels and, in general, all the cases where it is necessary to calculate the radiative solar power collected by a surface. In fact, radiation components will differently project on the surface of interest and will weigh in a different manner, depending on the surface orientation, in the computation of the effective incident radiation. To perform this decomposition starting from data relative to a horizontal plane, two non-dimensional quantities, namely, the diffuse fraction, kd, and the clearness, kt, are usually put in mutual relation by correlating experimental data on a graphical ground rather than using physical considerations. In the present study, some insights are given on the shape of this correlation starting from geometric and physical considerations. It is shown that many results and graphs presented in literature have not physical meaning; rather they are simply artifacts due to geometrical or other constraints. These evidences open the way to a new approach to solar radiation decomposition founded on physical-based correlations.