This model calculates the direct solar radiation on a tilted surface (general model).
The direct solar radiation forms an angle with the vector normal
to the earth Zen. The same direct solar radiation
forms an angle with the normal vector of a tilted surface
Inc.
Based on trigonometric relations, the model calculates the
direct normal irradiation. IrrDirNor =
IrrDirHor/cosAngleZen, Which maximum value is limited for
numerical reasons to the solar constant 1367 W/m2.
the amount of direct solar radiation incident on a tilted
surface is equal to IrrTotTil = IrrDirNor *
cosAngleInc