Solar Energy Perspectives - IEA

Chapter 2: The solar resource and its possible uses
Figure 2.9 Global horizontal irradiance (GHI) in Potsdam (Germany) and moving averages
1 400
Annual sum of global irradiance (kWh)
1 200
1 000
800
600
400
200
0
1930 1940 1950 1960 1970 1980 1990 2000 2010
20%
Min and max deviations of the annual sums
15%
10%
5%
0%
-5%
-10%
-15%
Mean annual sum: 1 017 kWh
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Number of averaged years
Note: Datasource: The bottom DWD figure GHI Data shows fromthe 1937 deviations to 2003. from long-term average GHI of the moving averages across 1 to 22 years.
Sources: Datasource: DWD GHI Data from 1937 to 2003 (top); Volker Quaschning, DLR/Hoyer-Klick et al. 2010 (bottom).
Key point
Solar energy resource varies from year to year, as well as day to day.
Where DNI is important, one may want to concentrate the sun’s rays, usually by reflection
from a large area to a smaller one. The main reason for doing this could be to increase the
energy flux per collector surface area and thus efficiencies in collecting and converting the
sun’s energy. It thus opens up a broader range of possibilities, as shown in Part B. Another
reason could be to substitute large expensive collector areas with a combination of less costly
reflective areas and expensive but smaller “receiving” areas.
Concentrating the sun’s rays on a receiver requires reflective surface(s) to track the diurnal
(daily) movement of the apparent sun in the sky, to keep the receiver in the focus of
reflector(s). The concentration factor, i.e. the ratio of the reflector area to the receiver area, is
casually measured in “suns”: ten “suns” means a concentration of a factor ten.
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© OECD/IEA, 2011