Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 34

− Some specific wave lengths show deep cuts in the energy distribution curve
("dark ranges"). They are caused by selective absorption of the sunlight by se-
lected elements within the atmosphere.

2,500

T heoretical radiation of a
black body at 5,700 °C

E xoatmosheric
solar radiation

2,000

E nergy component mainly reflected by
atmospheric elements

O 3

O 2

1,500

S olar radiation
after penetrating the atmosphere

O 2

1,000

Absorption bands

O 3

H 2

H 2

500

H 2

H 2

CO 2

0

2.0

0 0.4 0.8 1.2 1.6 2.4 2.8 3.2

Ultraviolet Visible Close infrared Far infrared

Wavelength in µm

Fig. 2.8 Energy distribution spectra of solar radiation before and after passing through the
atmosphere of the earth (see /2-3/)

Direct, diffuse and global radiation. The diffusion mechanisms within the at-
mosphere cause diffuse and direct radiation to incident on the surface of the earth.
Direct radiation is the radiation incident on a particular spot, having travelled a
straight path from the sun. In contrast, diffuse radiation is the radiation emerged
by diffusion in the atmosphere and thus indirectly reaching a particular spot on the
earth's surface. The sum of direct (beam) radiation G b and diffuse radiation G d ,
always related to the horizontal receiving surface, is called global radiation G g
(Equation (2.5)). The diffuse radiation G d consists of the radiation diffused in the
atmosphere, the atmospheric counter-radiation, and the radiation reflected by the
neighbourhood.

G = G + G

(2.5) g b d

For calculations of the overall solar radiation on a particular receiving surface
(e.g. surface of a solar collector), direct and diffuse radiation have to be differen-
tiated as they incident at different mean angles on the receiving surface. The at-
mospheric counter-radiation and the radiation reflected by the neighbourhood on
the receiving surface generally have little impact. In wintertime or in mountainous
areas, reflected radiation can, however, contribute to a larger extent to global ra-
diation, for example caused by a snow blanket.