Environmental Engineering Reference
In-Depth Information
Table 2.11
Albedo for Different Types of Surface
Surface
Albedo
A
Surface
Albedo
A
Grass (Summer)
0.25
Asphalt
0.15
Lawn
0.18-0.23
Woods
0.05-0.18
Dry grass
0.28-0.32
Heathland and sand
0.10-0.25
Uncultivated fields
0.26
Water surface (
γ
S
> 45°)
0.05
Soil
0.17
Water surface (
γ
S
> 30°)
0.08
Gravel
0.18
Water surface (
γ
S
> 20°)
0.12
Concrete, weathered
0.20
Water surface (
γ
S
> 10°)
0.22
Concrete, clean
0.30
Fresh snow cover
0.80-0.90
Cement, clean
0.55
Old snow cover
0.45-0.70
Source:
Dietze, 1957, TÜV 1984
The albedo value
A
influences the accuracy of the calculations noticeably.
Table 2.11 shows approximate albedo values for different types of surface. For
unknown surfaces, the value
A
= 0.2 is often used.
Irradiance gain for surface tilt or tracking
If a solar energy system tracks the sun so that the angle of incidence is virtually
zero, the energy yield increases significantly. The higher direct irradiance on a
surface perpendicular to the solar radiation causes the increased energy yield.
During days with high direct irradiation, tracking can achieve energy gains
over horizontal orientation in the order of 50 per cent in summer and up to
300 per cent in winter, depending on the latitude of the location (see Figure
2.15). However, tracking can cause a reduction in energy yield in overcast
conditions because the contribution of diffuse irradiation from behind the
surface is lost. Tracking achieves the main energy gain in summer. On the one
hand, the absolute energy gain in summer is higher than in winter; on the other
hand the number of overcast days is usually lower in summer.
There are two principle options for solar energy system tracking: one-axis
and two-axis tracking. Two-axis tracking systems move a surface always into
an ideal position; however, two-axis tracking systems are relatively
complicated and thus one-axis tracking systems are preferred in some
instances. One-axis tracking systems can follow the daily or annual path of
the sun. Tracking to the annual path of the sun is relatively simple: the surface
tilt angle need only be changed once a week or even once a month.
A
two-axis tracked solar energy system
installed in middle-European
latitudes can obtain an energy gain in the order of 30 per cent compared to
inclined non-tracking systems. The energy gain of a one-axis tracking system
is slightly lower; it is closer to 20 per cent. Regions with higher annual
irradiation also have a higher absolute energy gain because typically the direct
beam contribution is much higher. However, tracking systems are more
complicated, more expensive and have higher operating and maintenance
costs. The tracking system must also resist strong winds. There are two
