geothermal energy

geothermal energy Hot springs and volcanic eruptions indicated to ancient man that the interior of the Earth must be hot. In most parts of the world deep drilling has revealed that temperatures increase with depth by 20–100 °C for every kilometre, sometimes more. The highest thermal gradients are observed in areas of active volcanism. Hot springs and fumaroles are most abundant in these areas. They represent discharge from underground bodies of hot water, and sometimes steam, that are termed geothermal systems or geothermal reservoirs. In most instances the source of supply of the heat is cooling molten rock in a magma chamber. The rocks in the uppermost 2–4 km of the Earth's crust are generally porous and the pores are filled with water (groundwater). When a magma is intruded into the crust it heats up its surroundings, particularly those above its roof, including the groundwater. When heated, the groundwater expands and, as a results, it tends to rise. The rising hot groundwater is replaced by cooler groundwater from the sides, which in turn gains heat over the magma heat source. In this way a circulation (convection) of groundwater is established. It is this circulation that brings about cooling of the magma by extracting heat from it. Thus, a part of the Earth's interior heat is brought to the surface, first by rising magma and, secondly by geothermal water and steam.

A volcano, particularly one that develops a caldera by collapse of its summit, typically forms above a shallow magma chamber. The magma chamber is fed by a larger magma reservoir at the base of the crust or in the mantle. Thus, geothermal reservoirs are usually, but not at all always, found in the vicinity of active volcanoes. The ages of individual volcanoes and their associated magma chambers are usually in the range of 0.1 to 1 million years, and so are the ages of individual geothermal systems.

Not all geothermal systems have a magmatic heat source. The temperature of groundwater always increases with depth in harmony with the increased temperature in the crust. Hotter groundwater underlying colder and, therefore, denser groundwater represents an unstable condition. If the permeability of the rock is sufficiently high, and the density difference of the hot and cold groundwater is large enough, the groundwater body will start convecting in much the same way as happens above magmatic intrusions. In some areas of rapid sedimentation, as, for example, north of the Gulf of Mexico, the porosity of the sediments and the temperatures at deep levels may be high enough to make hot-water geothermal reservoirs exploitable through deep drilling although no convective groundwater systems exist.

Surface geothermal manifestations include hot steaming ground, fumaroles, mud pools, and warm to boiling hot springs. The ground surrounding some hot springs may be intensely altered, forming colourful deposits, such as native sulphur (yellow) and haematite (red). Periodic explosive boiling in the feeder pipes of hot springs leads to geyser action, in which a jet of water and steam can be thrown as much as 50–60 m into the air. The most famous geyser in the world today is Old Faithful in the Yellowstone National Park in Wyoming, USA.

The primary requisites for the formation of geothermal systems, i.e. a heat source (either magma or hot rock) and sufficiently permeable rocks, are mostly found in areas of active volcanism. Such areas generally coincide with areas of active Earth movements, as witnessed by earthquakes. Hot springs and geothermal energy sources can also occur in association with earthquake faults, even in areas free of active volcanoes (e.g. Dixie Valley, Nevada, USA and the Menderes graben, western Turkey). The earth movements create and maintain rock permeability by fracturing of the rock. Open fractures do not extend to great depths; they may be not much more than about 4 km deep in most places. Under the load of the overburden, particularly when temperatures are high, the rock yields, behaving more as a plastic material than a brittle one, with the result that pores and fractures cannot remain open. Thus in any specific area the base of a geothermal system coincides with the lowest level of permeable rocks.

Geothermal waters generally contain more dissolved gases and solids than ordinary groundwaters. The source of supply for these dissolved substances is mostly the rock in contact with the water. Some substances, particularly those forming volatile compounds, at least at high temperatures (such as carbon dioxide, boron, and hydrochloric acid), may be partly derived from the magmatic heat source through its degassing. Although 99 per cent of most common rocks consists of relatively few elements (ten to be exact), these rocks host a large number of trace elements, some of which form soluble salts, such as chlorine and boron. Many trace elements once transferred to the water, remain in solution and, as a result, become concentrated in the water. Most major elements, on the other hand, are removed from solution through deposition of hydrothermal minerals. The solubility of many minerals in water increases with temperature. Chemical reaction rates are also enhanced by rising temperature. This, together with the long residence time (tens to thousands of years) of geothermal waters underground, explains their higher content of dissolved substances in comparison with cold groundwaters, particularly of those elements forming soluble salts. The content of soluble salts varies enormously between rock types. It is highest in evaporative and marine sedimentary rocks but lowest in basalts. Geothermal waters associated with the former types of rocks, such as the Salton Sea field in southern California, are very salty (30 per cent dissolved solids, or almost ten times the salt content of sea water), whereas those in basaltic terrain, such as the Krafia geothermal system in Iceland, are very dilute (0.1 per cent dissolved solids). Hot brines, such as those in Salton Sea, are relatively rich in many metals, including lead, zinc, manganese, copper, and silver. These metals are relatively mobile in hot, saline waters because they complex with chloride, the main soluble salt-forming anion. The Salton Sea brine is considered to represent a modern ore-forming fluid. Studies of this brine, and other similar ones, are therefore not only of interest to those engaged in developing geothermal resources but also to those involved in mining of metalliferous deposits.

Modern use of geothermal energy is based on drilling for hot water and steam. The deepest geothermal drillholes approach 4 km, but a common depth is in the region of 2 km. The highest temperatures measured in wells are close to 400 °C, on the northern periphery of the Geysers field in California and at Nesjavellir in Iceland. Temperatures commonly approach or exceed 300 °C. The fluid in some drilled geothermal reservoirs is largely steam. They are then said to be vapour dominated. In others the fluid is mostly water. Such reservoirs are said to be liquid dominated. Vapour-dominated reservoirs are much less common than the liquid-dominated ones, the ratio probably being 1 to 20. Examples of the former include the Geysers field in California, Larderello in Italy, and Kamojang on the island of Java in Indonesia. Examples of liquid-dominated reservoirs include Wairakei in New Zealand, Tongonan in the Philippines, and Cerro Prieto in Mexico.

The technical possibilities of exploiting geothermal fluids for various uses are largely dictated by the temperature of the reservoir (Fig. 1). The boiling point of water is pressure-dependent. It is 100 °C at atmospheric pressure but it increases with pressure, being, for example, 300 °C at 85 bars (equivalent to about 1000 m depth). When water above 100 °C ascends a well it will begin to boil when the pressure created by the load of the overlying fluid equals the boiling point pressure. Because of this, steam can be produced from liquid water geothermal reservoirs. However, reservoir temperatures well in excess of 200 °C are desirable if steam at 140–190 °C is to be produced for electrical power generation.

Drilling for geothermal water and steam employs mostly the same technology as drilling for oil. For conventional turbines, about 2 kilograms per second (kg s⁻¹) of steam are required to generate 1 megawatt (MW) of electricity. Many geothermal wells yield about 10 kg s⁻¹ of steam and occasionally as much as 40 kg s⁻¹. The cost of drilling a typical geothermal well (2000 m deep with a diameter of 50 cm at the top and 20 cm at the bottom) is in the region of US$1.5 million.

Geothermal energy is extensively used in many countries, including China, Hungary, Iceland, Italy, Japan, Mexico, New Zealand, the Philippines, and the USA (California). Table 1 provides information on the worldwide use of geothermal energy for power generation and direct use. The most important category is the generation of electric power from geothermal steam. The steam is used in much the same way as steam produced from water by burning coal or oil. Its pressure is used to rotate a turbine connected to a generator that, in turn, produces electricity by converting the mechanical work carried out by the flowing steam into electrical energy. On a worldwide scale the electrical energy produced by geothermal power is very small. Thus, for example, the integrated production capacity of the world's geothermal power plants in 1997 (8240 MW) was only about 7 per cent of annual electric energy demand in Great Britain. In 1970, power generation by geothermal steam was only 675 MW, showing that development of this natural resource has been growing rapidly. Other uses of geothermal energy, which totalled 9686 MW thermal in 1997, are for greenhouse farming, space heating, balneology, and various industrial uses. Balneological use is the oldest. For example the Romans used hot spring waters extensively for bathing in many circum-Mediterranean countries, including even Britain. Industrial uses include paper production in New Zealand and drying of diatomaceous sediments in Iceland. Geothermal water and steam have also been used as sources of various chemicals. The classic example includes production of boron from geothermal steam in Larderello, near Pisa in Italy, although this production ceased some years ago. Common salt and carbon dioxide gas are today produced from brine and steam at the Reykjanes geothermal field in Iceland.

Only a very minor part of the Earth's heat is conveyed to the surface by rising magma and convecting groundwater. Most of it is transferred conductively. By far the larger part of the Earth's heat is therefore to be found in hot rock at depth and not in shallow crustal magma chambers or geothermal systems. This fact has stimulated projects aimed at developing techniques to extract and exploit economically the heat stored in rocks at shallow depths in the crust although the rock may be quite impermeable. The technology so far developed entails drilling a pair of appropriately spaced wells in an area of high thermal gradients. In order to create permeability the rock at depth is hydrofractured by pumping water (commonly containing sand to keep cracks open) under high pressure into one of the wells until the rock ruptures. Water is subsequently injected and it flows through the rock to the other well. Hot water is thus produced as the injected water gains heat from the hot rock through which it flows. Projects of this kind have been termed hot dry rock projects. An example is furnished by a drilling into one of the Cornish granites in south-west England. Exploitation of thermal energy from hot dry rock does not yet seem to be economically attractive, although it is technological achievable. Future projects may drill into fractured, and therefore permeable, rocks, although they do not host a convecting geothermal fluid.

Geothermal energy is preferred to fossil fuel (coal and oil) from an environmental point of view: much less carbon dioxide and sulphur are discharged into the atmosphere for every generated unit of electricity when using geothermal steam as compared to fossil fuel. The advantage of geothermal energy is even more pronounced when it is being used directly, that is, as a source of heat, as is the case with space heating and greenhouse farming. However, geothermal water and steam commonly contain appreciable amounts of some substances that are harmful to the environment, such as sulphur, arsenic, boron, and mercury. Exploitation of geothermal resources may thus create local pollution problems. In order to reduce this pollution as much as possible, a technique has been developed during the past 20 years by which the used geothermal water is reinjected into the ground through wells drilled specifically for this purpose. In the Paris basin, for example, hot water pumped from deep wells is being exploited for house heating. The water in the wells is saline and thus chemically undesirable for surface disposal. After passing through heat exchangers to extract the heat it is therefore injected back into the ground through injection wells.

Table 1. Use of geothermal energy in the world by country in 1997

Country	Electric production	Direct use
Power	Annual	Power	Annual
MW1	use GWh2	MW1	use GWh2
1 Megawatts. The number are valid for 1998 and are based on information from the International Geothermal Association. 2 Gigawatthours. (From Fridleifsson and Stefánsson (1998).)
Algeria			1	5
Argentina	0.7	3.5
Australia	0.4	0.8
Austria			21.1	84
Belgium	3.9	19
Bosnia-Herzegov	33	230
Bulgaria	95	346
Canada	3	13
China	32	175	1914	4717
Costa Rica	120	447
Croatia			11	50
Czech Republic	2	15.4
Denmark	3.2	15
El Salvador	105	486
France	4	24	309	1359
Georgia			245	2145
Germany	307	806
Greece	22.6	37.3
Guatemala	5
Hungary		750	3286
Iceland	140	375	1443	5878
Indonesia	590	4385
Ireland			0.7	1
Israel	42	332
Italy	768	3762	314	1026
Japan	530	3530	1159	7500
Kenya	45	390
Macedonia			75	151
Mexico	743	5682	28	74
New Zealand	345	2900	264	1837
Nicaragua	70	250
Philippines	1848	8000
Poland			44	144
Portugal	11	52	0.8	6.5
Romania	2	?	137	528
Russia	11	25	210	673
Serbia			86	670
Slovakia	75	375
Slovenia	34	217
Sweden	47	351
Switzerland	190	420
Thailand	0.3	2	2	8
Tunisia			70	350
Turkey	20	71	160	1232
Ukraine			12	92
United States	2850	14600	1905	3971
Europe	936	4309	4368	20505
America	3883	21529	1908	3984
Asia	3031	16092	3075	12225
Oceania	345	2901	264	1837
Africa	45	390	71	355
Total	8240	45220	9686	38906

Stefán Arnórsson

Bibliography

Armstead, H. C. H. (1978) Geothermal energy. John Wiley and Sons, New York.
Rinehart, J. S. (1980) Geysers and geothermal energy. Springer-Verlag, New York.