Geology Reference
In-Depth Information
In addition to the possibility of a power plant catastrophically failing, there
is also the issue of social and political threats stemming from the use of nuclear
material as a weapon or as a terrorist target. The safe burial of radioactive waste
must also be faced, since some of the spent matter will retain dangerous levels of
radioactivity for thousands of years.
That said, nuclear energy has an advantage over fossil fuels in that it does not
emit greenhouse gases and its reserves are greater. Some renowned scientists such as
Lovelock (2006) claim that: “There is no alternative but nuclear fission energy until
fusion energy and sensible forms of renewable energy arrive as a truly long-term
provider”.
6.5 Tidal energy
Tidal energy represents the smallest source of energy on Earth. Tides result from
the gravitational attraction exerted upon the planet by the moon and to a lesser
extent by the sun (Tarbuck and Lutgens, 1984). As the Earth spins on its axis, its
bulges move and produce two high and two low tides every 24 hours. Tidal heights
are not uniform everywhere and whilst rarely exceed a metre in the deep ocean,
over continental shelves they may reach 20 metres. Movement of such vast masses
of water requires a great deal of exergy, estimated at 2.7 TW. Through the course
of a year, this amounts to 0:85 10
20
J (Skinner, 1986).
Tidal electricity generation involves the construction of a barrage across a delta,
estuary, beach, or other places that are influenced by the tides. As in a hydropower
plant, turbines produce electricity as water flows through them. Tidal stations do
however differ from their hydraulic counterparts in the sense that they permit and
are able to take advantage of a two-dimensional flow. They are consequently able
to produce electricity when water enters the basin and when it leaves (Charlier and
Justus, 1993). Tidal provides a non-polluting and inexhaustible supply of energy
and ensures the regularity of power production from year to year with a less than
5% annual variation. The specific tidal exergy is about 110 kJ for each m
2
of
reservoir and each metre difference in height (Hermann, 2006). The worldwide tidal
power production is about 300 MW (Martinot, 2006). Projects worldwide have
been estimated to have a potential energy output of around 166 GW, according to
the World Energy Council.
Tidal heights of 5 metres or more and easily dammed bays or estuaries are needed
to make tidal plants more effective. The high capital cost for construction and the
limited number of potential sites worldwide (about 20) are its main drawbacks and
ultimately mean that relatively few tidal power plants have been constructed. Of
these, the oldest and by far the largest is the La Rance 240 MW barrage located
near St. Malo, in Brittany, Northern France. A similar sized tidal power plant
should be completed in South Korea by 2016.
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