Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 23

2 Basics of Renewable Energy Supply

2.1 Energy balance of the earth

The energy flows of the earth are fed from various sources described below. Solar
energy has a share of more than 99.9 % of all the energy converted on earth. The
solar radiation incident on the earth is weakened within the atmosphere and par-
tially converted into other energy forms (e.g. wind, hydro power). Therefore the
structure and the main attributes of the earth's atmosphere will also be described
in more detail, followed by balancing the global energy flows.

2.1.1 Renewable energy sources

Solar energy. The sun is the central body of our planetary system; it is the star
closest to the earth. Its schematic structure with its main parameters is shown in
Fig. 2.1. Accordingly, the nucleus has temperatures of approximately 15 Mio. K.
Energy is released by nuclear fusion where hydrogen is melted to helium. The
resulting mass loss is converted into energy E . According to Einstein, it can be
calculated multiplying mass m and the square of the speed of light v c (Equation
(2.1)). Approximately 650 Mio. t/s of hydrogen are converted into approximately
646 Mio. t/s of helium. The difference of approximately 4 Mio. t/s is converted
into energy.

2
c

E = m v (2.1)

The energy released within the nucleus of the sun is initially transported by ra-
diation to approximately 0.7 times the solar radius. Further transport to the surface
of the sun takes place through convection. Afterwards, the energy is released into
space. This energy stream released by the sun is differentiated as radiation of mat-
ter on the one hand and electromagnetic radiation on the other hand (see /2-1/).
− The radiation of matter consists of protons and electrons released by the sun at
a speed of approximately 500 km/s. However, only a few of these electrically
charged particles reach the earth's surface, as most of them are deflected by the
terrestrial magnetic field. This is of particular importance for life on earth, as
this harsh matter radiation would not allow organic life in its current form.
− Electromagnetic radiation mainly released by the photosphere (Fig. 2.1) covers
the entire frequency from short-wave to long-wave radiation. This type of solar