1.) Compute the beam component on the tilted plane as a function of the incidence angle and the tilt angle. Due to symmetry, incidence angles greater than 90° will have the same effect as those less than 90°. To avoid negative numbers, the absolute value is applied.

2.) Compute the surface reflected irradiance on the tilted plane as a function of albedo and tilt angle.

3.) Sum the component irradiance to get the total irradiance on the tilted plane.

 

Reference

Muneer, T. Solar Radiation and Daylight Models, Elsevier, 2004, pp 102, 155, 156.

R. Perez, P.Ineichen, R. Seals, J. Michalsky, R. Stewart. Modeling Daylight Availability and Irradiance Component from Direct and Global Irradiance. Solar Energy 44, no 5, pp 271-289, 1990.

Hay, J.E., Davies, J.A., Calculations of the solar radiation incident on an inclined surface. Proc. of First Canadian Solar Radiation Data Workshop, 1980, 59. Ministry of Supply and Services, Canada.