Geothermal Energy

In most parts of the world the vast amount of heat in the earth's interior is too deep to be tapped. In some areas, however, hot springs or geysers and molten lava streams are close enough to the surface to be used. Thermal energy from hot springs has been used for many years for producing electricity, starting in 1904 in Italy. In the USA the major geothermal power plants are located in northern California on a natural steam field called the Geysers. Steam from a number of wells is passed through turbines. The present utilization is about 900 MW and the total estimated capacity is about 2000 MW. Because of the lower pressure and temperatures the efficiency is less than with fossil-fuelled plants, but the capital costs are less and, of course, the fuel is free. New Zealand and Iceland also exploit their geothermal energy resources.

Other Renewable Resources

Tides

An effective method of utilizing the tides is to allow the incoming tide to flow into a basin, thus operating a set of turbines, and then at low tide to release the stored water, again operating the turbines. If the tidal range from high to low water is h (m) and the area of water enclosed in the basin is A (m2), then the energy in the full basin with the tide outside at its lowest level is:

Table 1.3 Sites that have been studied for tidal range generation

Site

Tidal Range(m)

Area (km2)

Generators (MW)

Passamaquoddy Bay (N. America)

5.5

262

1800

Minas-Cohequid (N. America)

10.7

777

19900

San Jose (S. America)

5.9

750

5870

Severn (U.K.)

9.8

700

8000

The maximum total energy for both flows is therefore twice this value, and the maximum average power is pgAh2/T, where T is the period of tidal cycle, normally 12 h 44 min. In practice not all this energy can be utilized. The number of sites with good potential for tidal range generation is small. Typical examples of those which have been studied are listed in Table 1.3 together with the size of generating plant considered.

A 200 MW installation using tidal flow has been constructed on the La Rance Estuary in northern France, where the tidal height range is 9.2 m (30 ft) and the tidal flow is estimated at 18 000 m3/s. Proposals for a 8000 MW tidal barrage in the Severn Estuary (UK) were first discussed in the nineteenth century and are still awaiting funding.

The utilization of the energy in tidal flows has long been the subject of attention and now a number of prototype devices are undergoing trials. In some aspects, these resemble underwater wind turbines, Figure 1.11. The technical and economic difficulties are considerable and there are only a limited number of locations where such schemes are feasible.

Wave Power

The energy content of sea waves is very high. The Atlantic waves along the northwest coast of Britain have an average energy value of 80 kW/m of wave crest length. The energy is obviously very variable, ranging from greater than 1 MW/m for 1% of the year to near zero for a further 1%. Over several hundreds of kilometres a vast source of energy is available.

The sea motion can be converted into mechanical energy in several ways with a number of innovative solutions being trialled, Figure 1.12. An essential attribute of any wave power device is its survivability against the extreme loads encountered during storms.

 
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