Page 64 - 4167

sea floor and coastlines.
Tidal power is the only technology that draws on energy inherent in
the orbital characteristics of the Earth–Moon system, and to a lesser extent
in the Earth–Sun system. Other natural energies exploited by human

technology originate directly or indirectly with the Sun, including fossil
fuel, conventional hydroelectric, wind, biofuel, wave and solar energy.
Nuclear energy makes use of Earth's mineral deposits of fissionable

elements, while geothermal power taps the Earth's internal heat, which
comes from a combination of residual heat from planetary accretion
(about 20 %) and heat produced through radioactive decay (80 %).
A tidal generator converts the energy of tidal flows into electricity.

Greater tidal variation and higher tidal current velocities can dramatically
increase the potential of a site for tidal electricity generation.
Because the Earth's tides are ultimately due to gravitational

interaction with the Moon and the Sun and the Earth's rotation, tidal
power is practically inexhaustible and classified as a renewable energy
resource. Movement of tides causes a loss of mechanical energy in the

Earth–Moon system: this is a result of water pumping through natural
restrictions around coastlines and consequent viscous dissipation at the
seabed and in turbulence. This loss of energy has caused the rotation of

the Earth to slow in the 4.5 billion years since its formation. During the
last 620 million years the period of rotation of the Earth (duration of a
day) has increased from 21.9 hours to 24 hours; in this period the Earth
has lost 17 % of its rotational energy. While tidal power may take

additional energy from the system, the effect is negligible and would only
be noticed over millions of years.
The first tidal power station was the Rance tidal power plant

built over a period
of 6 years from 1960
to 1966 in France.
The dam of this

stanion has length
800 m and at the
same time serve as

bridge on the Rance
(figure 5. 14)

Figure 5.14

63