Environmental Engineering Reference
In-Depth Information
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Kisangani beam
Kisangani diffuse
Sutton bonnington, beam
Sutton bonnington, diffuse
J
FMAMJ
J
ASOND
Month
Figure 2.12
Comparison of Irradiation at Kisangani (Congo) and Sutton Bonnington (UK)
Sutton Bonnington experiences a strong seasonal variation of
±
73%, the irradiation in Kisan-
gani varies only by
13% around the yearly average.
The annual average irradiation peaks in the north-east of Africa at around 300 W/m
2
. Other
favoured regions can be found all over Africa, the north of South America and the south of
North America, and South-West Asia. Australia experiences solar irradiation levels above
250 W/m
2
across nearly half the continent. The irradiation reduces, generally speaking, the
further the distance away from the equator. It can be well below 100 W/m
2
on a 24 hour
average. Obviously, these values are averaged over the year and will vary signifi cantly with
the seasons. Most northern European, North American and north Asian countries are within
this latter region and present and future systems are and likely to be rooftop installations in
the kilowatt scale connected to the local 230 or 115 V network.
±
2.5.2 The Technology
There are two main technologies for the conversion of sunlight into electricity. Photovoltaic
(PV) cells depend on the use of semiconductor devices for the direct conversion of the solar
radiation into electrical energy. Effi ciencies of the typical commercial crystalline PV cells
are in the range 12-18% although experimental cells have been constructed that are capable
of over 30%. In contrast, solar thermal systems depend on intermediate conversion of solar
energy into thermal energy in the form of steam, which in turn is used to drive a turbogenera-
tor. To obtain high temperatures, thermal systems invariably use concentrators either in the
form of parabolic troughs or thermal towers. At present, generation of electricity by either
technology is substantially more expensive than traditional means. Due to the considerable
potential of cost reductions in PV systems it is believed that in the future, perhaps in a decade
