Figure 4.3 shows that
Gbc = G∗b cosѲz (4.1)
where is the angle between the beam and the normal to the collector surface. In particular,
Gbh = G∗b cosѲz (4.2)
where Ѳz is the (solar) zenith angle between the beam and the vertical.
The total irradiance on any plane is the sum of the beam and diffuse components, as detailed in Section 4.8.5, so:
Gt = Gb + Gd (4.3)
See Section 4.8 for more discussion about the ratio of beam and diffuse insolation.