Environmental Engineering Reference
In-Depth Information
Global irradiance (W/m
2
)
900
800
700
22 December
28 December
2 July
600
500
400
300
200
100
0
00
02
04
06
08
10
12
14
16
18
20
22
00
Time
Figure 2.5
Global Irradiance throughout the Day in Karlsruhe (Germany) for
2 July and 22 and 28 December 1991
the Earth's sunbelt the figure is around 2500 kWh/(m
2
year). However, the
latitude can only give a rough indication of the annual irradiation because
local effects have a major impact on the energy reaching Earth's surface. For
instance, the annual irradiation in Stockholm (Sweden) and Berlin (Germany)
are nearly the same, although Stockholm's latitude is 7° higher than Berlin. On
the other hand, the annual irradiation in London, which is south of Berlin, is
significantly lower.
Table 2.5 gives an overview of monthly average global irradiation values
for some locations around the world. It clearly demonstrates that there are
significant variations between different locations. However, the precise
irradiance at the given site is required for planning solar energy systems. This
can be estimated using existing databases. Some free Internet databases offer
monthly irradiation values for many locations in the world; some even offer
hourly irradiance datasets for some sites (e.g.
www.satellight.com
,
eosweb.larc.nasa.gov/sse
or
rredc.nrel.gov/solar
). Computer programs (see also
CD-ROM) such as the Meteonorm program can also be used for interpolating
the meteorological parameters of a given site based on measurements taken at
locations close to the proposed site.
The annual irradiation in the Sahara is about 2350 kWh/(m
2
year). The
total annual irradiation received by the surface of the Sahara (around 8.7
million km
2
) is nearly 200 times higher than the global annual primary energy
demand; in fact the global primary energy demand could be provided by
collecting the solar energy received by 48,500 km
2
of the Sahara, an area
slightly larger than Switzerland, or one-ninth that of California. These
numbers clearly show that it is possible to provide the whole global energy
demand solely by solar energy.
