Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 32

within geological periods. This mainly concerns the plankton growing within the
sea, which partly sinks down to the sea bottom. On the other hand, more energy
can be released using fossil biogenous and fossil mineral energy carriers in the
short term than is added to the earth by the described renewable energy flows.

2.2 Solar radiation

Part of the energy incident on the earth from the sun can be directly received as
radiation on the surface on the earth and be converted into different utilisable
forms of energy. Therefore, the main principles of solar radiation and its main
characteristics will be discussed below.

2.2.1 Principles

Optical windows. The atmosphere is to a large extent impermeable for solar ra-
diation. Only within the optical spectral range (0.3 to 5.0 µm; window I) and
within the low-frequency range (10 -2 to 10 2 m, window II) radiation can pass the
atmosphere (so-called optical windows of the atmosphere; Fig. 2.7). Due to ener-
getic reasons only window I is relevant for the technical use of solar energy. The
most important part of the optical window I covers the range of visible light be-
tween 0.38 and 0.78 µm.

Radio-
longwave

Te l e f o n y
telegraphy

Radio-
shortwave Gamma radiation

Micro wave
radar

X-radiation

Cosmic
radiation

Window I Window II

High energy Low energy

-12 -6 -2 2 6

10 10 10 1 10 10

Wavelength in m

Medium and
longwave
ultraviolet

Violet Blue Green Red Shortwave

infrared

0.3 0.4 0.5

0.6 0.7 5

0.78 Wavelength in µm

0.29 0.38 0.44 0.57

Fig. 2.7 Optical windows of the atmosphere (see /2-2/, /2-3/, /2-4/)

Weakening of radiation. Within the atmosphere radiation is weakened; this proc-
ess is called extinction. Various mechanisms are involved.
− Diffusion. Diffusion is diversion of radiation from its original radiation angle
without energy transfer and thus without a loss of energy. Such diffusion takes