Environmental Engineering Reference
In-Depth Information
Figure 2.14
Irradiance on a Horizontal Area
A
hor
and an Area
A
s
Perpendicular to the Sunlight
it follows that:
E
G,hor
sin
S
E
dir,s
=
E
dir,hor
(2.26)
γ
It becomes apparent that the direct normal or
beam irradiance E
dir,s
on a
surface perpendicular to the path of the light is higher than the direct
irradiance
E
dir,hor
on a horizontal surface; this fact is taken into account when
planning solar energy systems. Inclining the surface of the system increases the
energy yield, especially at high latitudes with low solar height angles.
E
dir,tilt
cos
With
θ
tilt
from (2.24), the direct irradiance on a tilted surface is
E
dir,s
=
.
θ
tilt
The direct irradiation of a tilted surface can be calculated directly from the
direct irradiation on the horizontal surface:
cos
tilt
θ
γ
E
dir,tilt
=
E
dir,hor
•
(2.27)
sin
S
However, for low sun heights, small variations of the horizontal irradiance can
cause unrealistically high irradiances on tilted surfaces. Therefore, it should
always be checked that the calculated direct irradiance on a tilted plane is
below a maximum threshold.
Diffuse irradiance on tilted surfaces
There are two approaches for estimating the diffuse sky irradiance
E
diff,tilt
on
a tilted surface: the isotropic approach and the anisotropic approach. The