Other 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 contribution to overall world energy demand. We should also mention briefly other renewable energy technologies that 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 temperature of the crust increases with depth and in favourable locations the energy available may be employed directly for heating purposes or for generating electrical power. Although very important in particular places, for instance in Iceland, it is currently only a small contributor (about 0.3%) to total world electricity; its contribution is estimated to rise by about a factor of 10 by 2050 in the IEA BLUE Map scenario.55

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 energy production. It has the advantage over wind energy of being precisely predictable and of presenting few environmental or amenity problems. 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 the world have been extensively studied as potential sites for tidal energy installations. The Severn Estuary 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 and provide at least 6% of the total UK electricity Figure 11.20 A tidal stream turbine. demand. Other estuaries in the UK with tidal maxima at different times of day from the Severn could help to fill in the gaps when Severn power would not be available - so providing for a more continuous energy supply. Although the long-term cost of the electricity generated from the largest schemes could be competitive, the main deterrents to such schemes are seen to be the high capital upfront cost and the possibility of environmental impacts. However, the opportunity of harnessing such substantial quantities of long-term carbon-free power is now being taken seriously in the UK and in other countries such as China where also there are large tidal ranges.

Other proposals for tidal energy have been based on the construction of tidal 'lagoons' in suitable shallow regions offshore where there is a large tidal range.56 Turbines in the lagoon walls would generate electricity as water flows in and out of the lagoons. Some of the environmental and economic problems of barrages built in estuaries might therefore be avoided.

The energy in tidal streams in coastal areas can be exploited in much the same way as wind energy from the atmosphere is harnessed (Figure 11.20). Although

Figure 11.21 A prototype of the Wave-Dragon - a device that generates ~7 MW of electrical power from the energy available in waves is in process of installation off the coast of southwest Wales, near Milford Haven. Waves break as they rise up a ramp facing them and enter a reservoir creating a small head. Energy is generated by running water down through turbines lower in the structure. The turbines are the only moving parts.

Figure 11.21 A prototype of the Wave-Dragon - a device that generates ~7 MW of electrical power from the energy available in waves is in process of installation off the coast of southwest Wales, near Milford Haven. Waves break as they rise up a ramp facing them and enter a reservoir creating a small head. Energy is generated by running water down through turbines lower in the structure. The turbines are the only moving parts.

the speeds of water are lower than that of the wind, the greater density of sea water results in higher energy densities and requires smaller turbine diameters for similar power output. Substantial energy is also present in ocean waves. A number of ingenious devices have been designed to turn this into electrical energy (Figure 11.21)57 and some are beginning to provide commercial power. However, because of the hostile ocean environment, early exploitation is comparatively costly. What is urgently needed for both tidal and wave energy is an adequate level of research, development and initial investment.

The waters around the coasts of Western Europe provide some of the best opportunities to exploit wave energy; for instance, tidal and wave energy together have the potential to provide up to 20% of the UK's electricity.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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