Simulation and applications of cumulative anisotropic sky radiance patterns

Abstract

The prediction of the incident solar energy on a surface under a partially obstructed sky is an important task for most solar energy-related architectural and engineering applications. In previous contributions, Ivanova described an approach that solves this problem using cumulative sky radiance patterns. The method considers the numerical integration of fisheye orthographic projections of these patterns on a considered surface, taking into account the shading caused by all existing obstructing objects in the environment. Further developments of the approach are proposed here to improve the method’s accuracy. The resolution system called SOLARES includes two separate units: a pre-processor and a main program. For each new site, the sky radiance patterns have to be computed once with the pre-processor. The latter prepares daily cumulative sky radiance patterns based on the PVGIS TMY solar database. The direct and diffuse components of the sky radiance are modeled separately. The values of diffuse radiance are generated using an anisotropic model for the luminance and radiance sky distributions. The results are saved in binary files for ulterior use in the main program. The fisheye orthographic projections of the cumulative radiance patterns of the sky hemisphere onto a receiving surface are discretized into matrices of pixels and corresponding direct and diffuse radiance values. The shading objects are projected onto that surface, and the results are saved in a separate shading matrix generated for a specific viewpoint or averaged over the whole surface with the help of the finite-element method. The present numerical method can be used to generate accurate long-term time series of irradiance on any tilted exterior or interior surface in complex urban environments with many neighboring shading surfaces of any tilt and orientation. Additionally, the method allows the visualization of the different system components to enhance architectural or solar applications.