Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 27

condition is reached, an amount of heat of around 1 J/(g a) is generated. This re-
sults, for example, in a radiogenic heat production efficiency of approximately
2.5 µW/m 3 in granite rock and of approximately 0.5 µW/m 3 in basalt rock.
The decay of such natural, long-living isotopes in the earth permanently gener-
ates heat. The involved isotopes in the near-surface layers of the earth are mainly
enriched in the continental earth crust. Due to such radioactive decay processes,
the earth has received around 7 10 30 J of radiogenic heat since its formation. The
potential radiogenic heat of still existing radioactive isotopes is approximately
12 10 30 J /2-4/. These figures are rather vague as little is known about the distribu-
tion of radioactive isotopes in the interior of the earth.
The currently available heat resulting from the earth's formation, respectively
the primordial heat, as well as the heat already released to date and the heat attrib-
utable to the further decay of radioactive isotopes, all result in a total heat of the
earth between 12 and 24 10 30 J; in the exterior earth crust up to a depth of
10,000 m this amounts to approximately 10 26 J. This energy potential is equal to
the solar radiation incident on the earth in the course of many millions of years.
Energy from planetary gravitation and planetary motion. The earth and the
moon rotate around a mutual centre of gravity. Due to the disproportional charac-
ter of the overall mass between these two celestial bodies it is located within the
body of the earth. When earth and moon rotate around this gravitational centre, all
points on these celestial bodies move in circles with the same radius around the
gravitational centre. Within the centre of the earth, the gravitational force of the
moon equals the centripetal force required for the rotational motion of the earth.
On the side facing the moon, the gravitational force is stronger, therefore all mat-
ter on this side of the earth attempts to move towards the moon. In contrast, on the
side facing away from the moon, the mass gravitational force of the moon is
smaller than the centripetal force required for the movement of all matter in this
orbit. Therefore, all matter on earth attempts to move away from the moon. This
effect can, for example, be observed with the tides of movable water masses on
the surface of the earth.
The earth's body is stretched to a certain extent under the influence of these
forces. The response time of this deformation that change its direction by 360
degrees within 24 hours is too long to allow the earth's body to stretch fully.
Therefore a complete formation of the theoretical distortion does not take place.
The water, however, follows this deformation with a small delay due to the inner
friction of the water masses, the friction with the ground of the sea, the clash with
the continental rims, and from entering straits and bays. These delaying forces
thus lead to a phase shift between the highest moon position and the high tide, and
thus also to a reduction in the speed of the rotation of the earth.
The energy source causing the tides is mainly a result of the combined plane-
tary motions and the mass gravitational effect that the celestial bodies, earth and
moon have on each other.