Holes drilled in the earth's crust can produce water as steam, as liquid or as mixtures of the two, and use can be made of this thermal source of electrical power generation, which has developed in twenty one countries during the present century. The world's 6300 MW electrical power capacity is provided by a number of different power cycles which have to contend with many difficulties in the technology, including solid deposition from the fluid, corrosion of plant and environmental pollution.
The continental crust has an average depth of 35 km, while the oceanic crust has 5 km of water plus 5 km of rock. The temperature rises with depth in the crust, commonly with gradients between 20 and 30°C/km but thermal gradients of several hundred degrees per kilometer depth are observed in some cases. Heat flows through the rock by conduction and more rapidly by fluid convection within rock fissures.
An economical geothermal field requires a source of heat from below, a permeable Aquifer holding the water or steam, a source of water recharge to make up for fluid losses and caprock to prevent uncontrolled loss of fluid into the atmosphere [Bowen (1989)]. The temperature of geothermal fluids in hydrothermal convection systems should be higher than 180°C if they are to be used directly for power generation.
Hydrothermal sources producing geothermal fields with high temperature gradients are comparatively rare, however, as they are confined mainly to the earth's tectonic plate boundaries.
Since drilling is a large component of the capital cost of a geothermal power station, it is not often worth developing systems deeper than 2-3 km, and depths of 500-1500 m cover most operations.
There are two types of geothermal field. The first is the wet (or "liquid dominated") field which produces water under pressure at temperatures over 100°C. On reaching the surface, the pressure is reduced, and part of the water is "flashed" to steam, leaving a larger fraction as boiling water. The second is the dry (or "vapor dominated") field, which produces dry saturated, or superheated, steam at pressures higher than that of the atmosphere.
Lower temperature fluids are used for direct heating in buildings and industrial processes.
Electrical power was first generated from geothermal energy in 1904 at Larderello in Italy. Although the installed electrical generation capacity was about 130 MW by the time of the Second World War, no other country had, by then, exploited geothermal energy for power production. In the 1950s, the United States, Japan and New Zealand became more interested in the technology. The Wairakei power station was built in New Zealand between 1956 and 1963, with several units giving a total power rating of almost 200 MW. Development in the Geysers (California, USA) started with a unit rated at 11 MW commissioned in 1960, and the 22 MW Matsukawa plant inaugurated commercial electricity production in Japan in 1966. Mexico, Iceland and the Soviet Union also built plants about this period.
In the period up to 1985, there was a high growth in geothermal power exploitation following the oil crises in 1973 and 1979, in the Geysers and Imperial Valley in California, Central America, Japan and the Philippines—all located in the seismic zones of the world, where the geology supports flow of high temperature fluids close to the earth's surface. Countries in these regions which are heavily dependent upon imported oil have been particularly active in this growth (see Table 1).
Source: Fridleifsson and Freeston, 1994.
Several types of power generation cycle are used for geothermal systems. Steam extracted from dry wells, or after separation from wet wells, can be passed directly through a turbine and exhausted to atmosphere. This direct noncondensing cycle is the simplest and has the lowest capital cost. However, it uses twice as much steam as the condensing cycle for a given rate of electrical energy generation. If the field is liquid-dominated, the water can be flashed in a vessel operating at a pressure lower than that at which the main steam is introduced to the turbine. The flash steam is passed through the lower pressure stages of the turbine, and the unleashed hot water is discharged to waste, or may be used in industrial or district heating.
In other plants, power generation is achieved by passing fluid through well-head separators and then using the steam in a condensing turbine, using a tower cooled by natural ventilation. Small "portable" turbines are used directly at well-heads. Although they have lower efficiency, they are low cost and can be used to test the behavior of a reservoir at the same time as drilling activities aimed at supplying heat to a central power plant. Although some plants use single flash systems, it is possible to use a second flash vessel to extract extra power. Flash steam systems can be rated at between 10 and 120 MW.
Binary cycle units employ low boiling point fluids in a heat exchanger which extracts the heat from the geothermal fluid. This allows more heat to be extracted because it rejects the geothermal fluid at lower temperature than the flash steam units. The technique allows use of geothermal fluids produced at lower temperatures, and restricts corrosion and scaling problems to the heat exchanger. The first binary cycle plant was commissioned in 1979 at East Mesa (Imperial Valley, California), and was rated at 10 MW. A number of other binary cycle plants since have been constructed, but this type of power generation system has not been used as often as the steam turbine units.
The efficiencies of geothermal power plants are low because of low fluid temperatures and pressures. Conventional fossil fuel plants are twice as efficient, so geothermal heat must be very cheap to compete with fossil fuels as a source of electrical power.
One of the problems encountered by the industry has been the drilling of "dry" wells, which are found to produce insufficient hot fluid to run a geothermal plant. In other cases, wells show a rapid decline in production rate. Much effort has been put into the modeling of geothermal reservoir performance to try to understand these problems. Nevertheless, it is to be expected that a borehole will have a useful life of only about 10 years, after which it may be deepened or new boreholes drilled to access other parts of the reservoir.
The chemistry of water/rock interaction has been the subject of extensive studies. Dissolution of rock components can cause changes in the permeability of the reservoir, and can produce highly saline geothermal fluids. Changes in temperature and pressure can subsequently cause precipitation of the dissolved solids (for example, silica or calcium salts).
This scaling phenomenon is particularly serious where water flashes to steam, and may result in premature blockage of the reservoir, or of the borehole walls and the surface equipment. The unwanted deposits can be removed from the borehole by periodic reaming operations, but in a few cases the scaling is so rapid and persistent that the well has to be abandoned.
Dissolved substances may corrode pipework and other equipment, and when discharged to the environment they may be an unacceptable source of pollution. Plants can use special reactors to remove dissolved solids before the fluid enters the main surface plant. Gas ejector discharges contain hydrogen sulfide, and the Geysers in California is an example where plant has been installed to extract this noxious component, before discharge.
The fluids leaving the reservoir may also contain dust, which may erode turbines.
The more restrictive environmental legislation becomes, the more difficulty the geothermal power industry has in meeting these requirements. Hot water containing dissolved solids may be difficult to discharge into local water courses, and modern plants reinject the discharge water into the rock formation from which it came (as at the Geysers). Care has to be taken to locate this reinjection so that the cold water does not mix with the hot fluids in the reservoir too close to the extraction point, otherwise the temperature of the production fluid will be lowered.
The technology is well established, and geothermal plants are generally both efficient and reliable. A number of plants are planned and under construction world-wide but, like any alternative power source, further growth will depend on the oil price. There are a number of countries in which little or no exploitation of possible resources has taken place. In particular, the rate of exploitation in South America has been low.
This entry has not covered the use of geothermal energy for direct heating, and has therefore ignored such developments in countries such as Iceland, France and Japan [Harrison et al. (1990)].
Experiments have been carried out in the USA, in Europe and in Japan to extract geothermal energy in dry geological formations by artificially introducing water as a heat transfer medium. This "Hot Dry Rock" technology is still being developed [Baria (1990)].
The ultimate process in geothermal energy exploitation could be the direct extraction of heat from a magma chamber. There are such chambers close to the surface of the earth, in volcanic regions (e.g., 1000°C at 4 km depth). The problems of drilling, and of designing a heat exchanger for these extreme circumstances have not been overcome. Nevertheless, the US Department of Energy has funded a study of this intense magma energy source.
Baria, R., Ed. (1990) Hot Dry Rock Geothermal Energy. Robertson Scientific Publications, London.
Bowen, R. (1989) Geothermal Resources. 2nd edn., Elsevier Applied Science, London and New York.
Fridleifsson, I. B., Freeston D. H. (1994) Geothermal Energy Research and Development, Geothermics, 23, 2, 175-214. DOI: 10.1016/0375-6505(94)90037-X
Harrison R., Mortimer N. D., Smarason O. B. (1990) Geothermal Heating. Pergamon Press, Oxford.