The Hay and Davies diffuse model  divides the sky diffuse irradiance into isotropic and circumsolar components.  Horizon brightening is not included.  An anisotropy index, , is defined as:

,

where is the direct normal irradiance and is the extraterrestrial radiation.

The Hay and Davies model formulation for sky diffuse radiation is:

,

where is the diffuse horizontal irradiance, , is the angle of incidence, is the solar zenith, and is the tilt angle of the array.

This model is implemented in the PV_LIB Toolbox as the function: pvl_haydavies1980.

References:

• [1] Loutzenhiser P.G. et al. “Empirical validation of models to compute solar irradiance on inclined surfaces for building energy simulation” 2007, Solar Energy vol. 81. pp. 254-267
• [2] Hay, J.E., Davies, J.A., 1980. Calculations of the solar radiation incident on an inclined surface. In: Hay, J.E., Won, T.K. (Eds.), Proc. of First Canadian Solar Radiation Data Workshop, 59. Ministry of Supply and Services, Canada.

Content contributed by Sandia National Laboratories