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in Figure 2, namely direct, reflected and diffuse radiation. The various types of radiation
can occur in isolation but in most cases the radiation incident on a solar thermal collec-
tor is a combination thereof.
The solar radiation available outside the earth's atmosphere, the so called extraterres-
trial radiation, has a density of 1367 W/m². Depending on:
- the location of the solar system
- the time of day and year
- the "obstacles" in the atmosphere such as cloud cover and pollution
- and the inclination of the solar system in relation to the sun
this value varies strongly. A map of the United Kingdom and Ireland is shown in Figure
3, indicating average annual solar energy gains on the horizontal surface.
<1000 W/m²
<1200 W/m²
Figure 2 – Forms of incident solar radiation
<200 W/m²
<600 W/m²
SOLAR
strong
diffuse
diffuse
direct
direct + reflected
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ST0133 – A 02/09
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