Graphical analysis of real skies with the clear day index: Using PENN state SURFRAD data

Graphical methods are explored as an educational tool to convey the complimentary nature of clear sky models, clearness indices, and meteorological data. The hourly clearness index (kT ) and the hourly clear sky index (kc) can be used in energy simulations to convey anisotropic conditions of shortwave irradiation on oriented surfaces, used for long-term performance of solar energy conversion systems. The hourly clear sky index is the ratio of a measured global horizontal irradiation (I) relative to an estimated clear sky condition, I_c (MJ/m²), in contrast with the traditional clearness index calculated under AM0 irradiation conditions, I₀. Measured data were collected from the SURFRAD monitoring station near Penn State. Clear sky estimation used the Bird Clear Sky Model (National Renewable Energy Laboratory) for global horizontal irradiation (GHI) and direct normal (DNI), as well as components of beam, and hemispherical diffuse irradiation. Days in January and July were assessed for State College, PA. Our graphical approach explores the combined plot of the measured and clear sky components. A stronger understanding is conveyed in the graphical approach for the relationships between components of solar irradiation and the role of air chemistry, clouds, and ground conditions.