18.2.2
the angle of incidence (AOI).
As derived in Appendix E.3, cos γ is given by
Thus we obtain for the irradiance
Note that this equation only holds when the Sun is above the horizon (a S > 0) and the
azimuth of the Sun is within ±90° of A M , A S ∈ [A S − 90°, A S + 90°]. Otherwise, = 0.
If the azimuth of the solar position is the same as the azimuth of the module normal
A M = A S , Eq. (18.4) becomes
When using the tilt angle θ = 90° − a M we find
Very often, PV modules are not installed on a horizontal plane, but on a tilted roof,
which makes the calculation of cos γ more complex. The derivations for modules on a
tilted roof as well as some examples are shown in Appendix E.4.
Figure 18.6: Illustrating the angles used to describe the orientation of a PV module installed on a horizontal plane.
Shading
As we have seen in Section 15.3 shading can have a detrimental effect on the power
output of a PV module. Therefore, mutual shading between different rows of PV modules
must be kept in mind when designing a PV system.