Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 26

On the Northern hemisphere, however, the solar radiation incidents at a gener-
ally flatter angle during this season with comparably short days. Areas close to the
North Pole are sometimes not facing the sun throughout the entire day. During
winter solstice, all places between 66.5° N and the pole have "eternal polar night".
Correspondingly, on the Southern hemisphere the sun never disappears below the
horizon south of 66.5° S ("midnight sun").
As the earth continues its orbit around the sun, its relative position changes.
For the Northern hemisphere the sun starts to rise higher and higher, whereas the
midday altitudes get increasingly lower on the Southern hemisphere. On March,
21 st , solar radiation incidents on both poles. The Northern hemisphere is now
more sun-facing, i.e. the mean solar position above the horizon gets increasingly
higher. This continues until summer solstice (June, 21 st ), when the midnight sun
then lights up the North Pole areas, and the Antarctic region sinks into "eternal
night".
Due to these interrelations, and thus, primarily due to the angle of the earth axis
towards the ecliptic, the solar radiation in different regions of this earth is subject
to significant seasonal fluctuations.
Geothermal energy. The energy flowing from the interior of the earth to its sur-
face is fed by three different sources. On the one hand this is the energy stored in
the interior of the earth resulting from the gravitational energy generated during
the formation of the earth. The primordial heat that had even existed before that
time is added as a second source. Thirdly, the process of decay of radioactive
isotopes in the earth (in particular in the earth's crust) releases heat. Due to the
generally low heat conductivity of rocks, this heat resulting from these three
sources is to a large extent still stored in the earth.
The formation of the earth took place approximately 4.5 Billion years ago. It
was a step-by-step accumulation of matter (rocks, gases, dust) within an existing
fog. This process started off at low temperatures, which changed due to the in-
creasing mechanical force of the matter amassing. During this aggregation of mat-
ter, gravitational energy was probably converted almost entirely into heat. To-
wards the end of this accumulation of mass, after approximately 200 Mio. years,
the top level of the earth had melted. Due to this melting process, a large amount
of the released heat was emitted into space again. In spite of all uncertainties
about the accumulation of mass and the energy emission during this phase, the
energy that remained in the earth during this phase was between 15 and 35 10 30 J
/2-4/. The smaller value reflects a cold to warm primordial earth, the higher value
a warm to hot primordial earth.
The earth contains radioactive elements (i.e. uranium (U 238 , U 235 ), thorium
(Th 232 ), potassium (K 40 )). Due to radioactive decay processes, they release energy
over a period of millions of years. The mass fraction of uranium or thorium in
granite is, for example, approximately 20 ppm and in basalt 2.7 ppm. With the
appropriate half-life, a released energy of approximately 5.55 MeV for a decay
event and approximately 6 (thorium) or 8 (uranium) decay events until a stable