In this article, the author discusses the use of view shed models to calculate intercepted solar radiation. They have developed an algorithm for rapid calculation of intercepted solar radiation using a combination of view shed analysis and a lookup table of irradiance from each sky direction. First, the angular distribution of sky obstruction is calculated for the location of interest and stored in a hemispherical coordinate system. Then lookup table values for all unobstructed sky directions are summed to determine total irradiance. Angle of incidence is accounted for using a cosine correction. The process can be repeated across a topographic surface to determine the spatial distribution of intercepted solar radiation. The results can be expressed as an energy flux or as sky view factors. This lookup table approach accounts for anisotropic irradiance distributions and can treat different spectral bands separately. The same view shed approach can be used for optimizing calculation of reflected radiation from topographic surfaces. This approach will be especially valuable for studies of ecological processes, by permitting assessment of topographic influences on energy balance and microclimate.
The image above is an example of the sky split into 160 sectors, with 20 equal ranges of zenith angles and 8 ranges of azimuth angles.
Solar radiation is the primary source of energy at the earth's surface, and topography is a primary factor modifying the radiation reaching any particular spatial location. Herein, we have presented a general algorithm that permits rapid and effective calculation of topographic influences on intercepted solar radiation based upon knowledge of sky obstruction, irradiance as a function of sky direction, and surface orientation.
LINK TO ARTICLE: http://professorpaul.com/publications/rich_et_al_1994_asprs.pdf
Rich, P., Dubayah, R. C., Hetrick, W., & Saving, S. (1994). Using viewshed models to calculate intercepted solar radiation: applications in ecology. American Society for Photogrammetry and Remote Sensing Technical Papers. In American Society of Photogrammetry and Remote Sensing (pp. 524-529).