A Manual on Computing Direct Solar Irradiance on Plane Terrestrial Surfaces

This precise and concise manual is written as a reference for solar energy scientists and engineers and can also be used to supplement self-instruction, short training courses, and community college or university- level courses. We first discuss the radiation laws of Planck, Wien, and Kirchhoff in relation to solar radiant energy. We then explain relevant earth-sun relationships and the characteristics and disposition of solar radiant energy fluxes reaching the earth's surface. We use classical earth-sun spherical astronomical relationships to derive and develop algorithms for estimating direct solar irradiance (in the absence of an atmosphere) on a plane horizontal terrestrial surface. We then extend these algorithms to any arbitrarily inclined and east-west oriented (relative to an observer's local meridian directly facing the equator) plane terrestrial surface. These algorithms are useful to assess the feasibility of installing solar energy collection systems on existing, arbitrarily-oriented roofs and surfaces. We also explain how Angstrom's approach to account for atmospheric attenuation of solar shortwave radiation can be applied to estimate net direct irradiance. We discuss how net long-wave radiation can be estimated and combined with net direct irradiance to obtain net all-wave irradiance on plane terrestrial surfaces. We include this latter discussion since technology is still being developed to permit direct measurements of net all-wave radiation using wide band radiometers. We provide numerous worked examples of the application of the estimating algorithms that will permit the reader to cross-check their precision and accuracy. As demonstrated in addenda to some worked examples, readers can easily extend these algorithms to develop spreadsheet-based calculators for estimating direct solar irradiance on any plane terrestrial surface at a given location over the course of the year. Or they can contact the author to request a Microsoft Excel-based calculator package he has developed for this purpose.