Geothermal power

Geothermal energy is the heat contained within the body of the earth. The origins of this heat are found in the formation of the earth from the consolidation of stellar gas and dust some 4 billion years ago. Radioactive decay within the earth continually generates additional heat which augments that already present.

The distance from the surface of the earth to its core is 6500 km. Here the temperature may be as high as 7000°C. As a result of the temperature gradient between the centre and the much cooler outer regions, heat flows continuously towards the surface. An estimated 100 X 1015W of energy reaches the surface each year. Most of this heat cannot be exploited but in some places a geothermal anomaly creates a region of high temperature close to the surface. In such cases it may be possible to use the energy, either for heating or in some cases to generate electricity.

The region of the earth at the earth's surface is called its crust. The earth's crust is generally 5-km to 55-km thick. Starting from the ambient surface temperature, the temperature within the crust increases on average by 17-30°C for each kilometre below the surface. Below the crust is the mantle, a viscous semimolten rock which has a temperature of between 650°C and 1250°C. Inside the mantle is the core. The earth's core consists of a liquid outer core and a solid inner core where the highest temperatures are found.

Geothermal temperature anomalies occur where the molten magma in the mantle comes closer than normal to the surface. In such regions the temperature gradient within the rock may be 100°C/km, or more. Sometimes water can travel down through fractured rock and carry the heat back to the surface. Plumes of magma may rise to within 1-5 km of the surface and at the sites of volcanoes it actually reaches the surface from time to time. The magma also intrudes into the crust at the boundaries between the tectonic plates which make up the surface of the earth. These boundaries can be identified by earthquake regions such as the Pacific basin 'ring of fire'.

The most obvious signs of an exploitable geothermal resource are hot springs and geysers. These have been used by man for at least 10,000 years. Both the Romans and the ancient Chinese used hot springs for bathing and for therapeutic treatment. Such use continues in several parts of the world, particularly Iceland and Japan. A district heating system based on geothermal heat was inaugurated in Chaude-Aigues, France, in the fourteenth century; this system is still in existence.

Table 12.1 Main geothermal users, worldwide

Capacity (MW)

Table 12.1 Main geothermal users, worldwide

Capacity (MW)

USA

2850

Philippines

1850

Italy

770

Mexico

740

Indonesia

590

New Zealand

345

Iceland

140

Source: US Geothermal Education Office.

Source: US Geothermal Education Office.

Industrial exploitation of hot springs dates from the discovery of boric acid in spring waters at Larderello in Italy around 1770. This led to the development of a chemical industry based on the springs. It was here, too, that the first experimental electricity generation based on geothermal heat took place in 1904. This led, in 1915, to a 250-kW power plant which exported power to the local region. Exploitation elsewhere had to wait until 1958 when a plant was built at Wairakei in New Zealand and the Geysers development in the USA which began in 1960.

Geothermal generating capacity has grown slowly since then. By the beginning of the twenty-first century there was roughly 8000 MW of installed geothermal capacity worldwide.1 The largest user is the USA with around 2850 MW of installed capacity. The Philippines has 1850 MW while Italy has 770MW, Mexico has 740MW and Indonesia has 590 MW (see Table 12.1). In total 23 countries have exploited geothermal power but two, Greece and Argentina, no longer have operating capacity.

Geothermal energy is attractive for power generation because it is simple and relatively cheap to exploit. In the simplest case steam can be extracted from a borehole and used directly to drive a steam turbine. Such easily exploited geothermal resources are rare but others can be used with little more complexity. The virtual absence of atmospheric emissions means that geothermal energy is clean compared to fossil-fuel-fired power. The US Department of Energy classifies geothermal energy a renewable one.

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