Geothermal energy is the heat contained within Earth’s body. The origins of this heat are found in the processes that led to the formation of Earth from the consolidation of stellar gas and dust some 4 billion years ago into a high-temperature ball of matter. Over time the outer regions of that ball cooled but the core remains at a very high temperature. Radioactive decay within Earth’s body continually generates additional heat that augments that already present.

The distance from Earth’s surface to its core is 6500 km. At the core the temperature may be close to 6000 oC, creating a temperature gradient between the center and the much cooler outer regions. As a consequence, heat flows continuously toward the surface. Most of this heat reaches the surface at a low temperature and cannot be exploited, but in some places a geothermal anomaly creates a region of high temperature close to the surface. In such regions it may be possible to use the energy, either for heating or in some cases to generate electricity.

The geothermal energy that is capable of being exploited at the surface is contained within Earth’s solid outer shell, called its crust. Earth’s crust is generally around 56 km thick. Starting from the ambient surface temperature, the temperature within the crust increases on average by 17–30 oC for each kilometer below the surface. Based on this, it has been estimated that the top 3 km of the crust contain around 4.3 x 107 EJ of energy, around 10,000 times more than annual global energy consumption. Below the crust is the mantle, a viscous semi-molten rock that has a temperature between 650 oC and 1250 oC. Inside the mantle is the core. Earth’s core consists of a liquid outer core and a solid inner core where the highest temperatures are found.

The Earth’s crust is not a shell of uniform thickness. Exploitable geothermal temperature anomalies occur where molten magma in the mantle comes closer than normal to the surface. In such regions the temperature gradient within the rock may be 100 oC/km, or more. Sometimes water can travel down through fractured rock to such anomalies and by convective flow carry the heat back to the surface. More dramatically, 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. However, direct exploitation of this energy source is likely to be difficult. The magma also intrudes into the crust at the boundaries between the tectonic plates that make up Earth’s surface. These boundaries can be identified by earthquake regions such as the Pacific basin “ring of fire.”

Power Generation Technologies

The most obvious surface 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 ancient Chinese used hot springs for bathing and 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 14th century; this system is still in existence.

Industrial exploitation of hot springs dates from the discovery of boric acid in spring waters at Larderello, 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 from geothermal heat took place in 1904. This led, in 1915, to a 250 kW power plant that 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 United States that began operating in 1960. Global geothermal generating capacity has grown slowly since then. In 2012 there was 11,224 MW of installed geothermal capacity worldwide.

The principal exploiters of geothermal power generation, by country, are shown in Table 12.1. The largest user is the United States with 3187 MW of generating capacity in 2012. The Philippines has 1904 MW and Indonesia 1222 MW. There are also large geothermal capacities in Mexico, Italy, New Zealand, Iceland, Japan, Kenya, and several countries in Central America. According to the Geothermal Energy Association, from which these figures are derived, there are 25 nations exploiting geothermal power. Two that exploited it in the past, Greece and Argentina, no longer do so.

As Table 12.1 suggests, easily accessible geothermal resources suitable for power generation are not widely distributed; neither are they large. Consequently, geothermal power contributes only a small amount to global generation. Even so, 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 as the schematic in Figure 12.1 illustrates. Such easily exploited geothermal resources are rare but others can be used with little more complexity. The virtual absence of atmospheric emissions (although geothermal wells can release carbon dioxide) means that geothermal energy is also clean compared to fossil fuel–fired power.

While natural geothermal fields are relatively rare, there is a much larger geothermal potential linked to deep, hot underground rock within the crust. This heat is more expensive to exploit but could potentially offer a far larger generating capacity. Today, however, only experimental exploitation of this resource has been carried out.

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