Other renewableenergies 305Other renewable energies
We have so far covered the renewable energy sources for which there
is potential for growth on a scale that can make a substantial contribu-
tion to overall world energy demand. We should also mention briefly
other renewable energy technologies which contribute to global energy
production and which are of particular importance in certain regions,
namely geothermal energy from deep in the ground and energies from
the tides, currents or waves in the ocean.
The presence of geothermal energy from deep down in the Earth’s
crust makes itself apparent in volcanic eruptions and less dramatically
in geysers and hot springs. The energy available in favourable loca-
tions may be employed directly for heating purposes or for generating
electrical power. Although very important in particular places, for in-
stance in Iceland, it is currently only a small contributor (about 0.3%)
to total world energy; its contribution could rise to the order of one per
cent during the next few decades (Table 11.5).
Large amounts of energy are in principle available in movements
of the ocean; but in general they are not easy to exploit. Tidal energy
is the only one currently contributing significantly to commercial en-
ergy production. The largest tidal energy installation is a barrage across
the estuary at La Rance in France; the flow from the barrage is directed
through turbines as the tide ebbs so generating electricity with a capacity
of up to 240 MW. Several estuaries in theworld have been extensively
studied as potential sites for tidal energy installations. The Severn Estu-
ary in the UK, for instance, possesses one of the largest tidal ranges in
the world and has the potential to generate a peak power of over 8000
MW or about six per cent of the total UK electricity demand. Although
the long-term cost of the electricity generated from the largest schemes
could be competitive, the main deterrents to such schemes are the high
capital up-front cost and the significant environmental impacts that can
be associated with them. More recent proposals, rather than making use
of estuaries, have been based on the construction of tidal ‘lagoons’ in
suitable shallow regions off shore where there is a large tidal range.^57
Turbines in the lagoon walls generate electricity as water flows in and
out of the lagoons. Some of the environmental and economic problems
of barrages built in estuaries are therefore avoided.
The energy in currents (including tidal streams) in the ocean can be
exploited in much the same way as wind energy from the atmosphere is
harnessed. Although the speeds in water are lower than that of the wind,
the greater density of sea water results in higher energy densities and
requires smaller turbine diameters. Substantial energy is also present in
ocean waves and a number of ingenious devices have been designed to