Geothermal power generation

Geothermal power production has a relatively long history. Just 38 years after the invention of the electric power generator by Werner von Siemens and 22 years after the start of the first power station by Thomas A. Edison in New York in

1882, geothermal power production was invented by Prince P. G. Conti in Lardarello, Italy in 1904. Geothermal power production in Tuscany has continued since then and amounted to 128 MW of installed electrical power in 1942 and to about 790 MW in 2003. In 1958, a small geothermal power plant began operating in New Zealand, in 1959 another in Mexico, and in 1960 commercial production of geothermal power began in the USA within the Geysers Field in California. Today 25 countries use geothermal energy for power production, and the worldwide installed electrical capacity has increased to about 8,930 MW in the year 2004 with an average annual increase of about 17 % between the year 1995 and the year 2000 /10-6/, /10-11/, /10-12/. One of the main reasons for this success is the base load ability of geothermal power generation.

Today, geothermal power production is economic viable only when high temperatures are found at relatively shallow depth. In regions with a normal or a slightly above normal geothermal gradient of about 3 K per 100 m, one has to drill more than 5,000 m deep in order to achieve temperatures above 150 °C. Such deep wells are expensive (generally more than 5 Mio. €) and there is a high risk of failure. For this reason under economic considerations geothermal power production is mainly restricted to geothermal fields with extremely high temperature gradients and high heat flows. Such fields often show surface manifestations of geothermal activity like fumaroles or hot springs. A scheme of such a geothermal field is shown in Fig. 10.13. The heat source are hot magmatic bodies that have risen up in the earth crust from a greater depth (often several tens of kilometres) to more shallow regions. The high heat flow in the overlaying crust, resulting from these magmatic bodies, heats up water of meteoric or marine origin in porous or fractured rock formations, that are in most cases covered by a cap rock of low permeability. Due to buoyancy effects the water starts to converge within the host rock bringing high temperatures closer to the surface. Depending on temperature and pressure the fluid may start to boil at certain depths and vapour is produced. The amount of vapour characterises those systems as liquid or vapour dominated reservoirs. Typical fluid temperatures range from 150 to 300 °C at depths between a few hundred and 3,000 m. Even higher temperatures are encountered at greater depths.

Most of the geothermal fields used today are in zones with active volcanism. Generally, not the volcanoes, which cool down rapidly, but the magma chambers buried underneath the volcanoes are the heat source for geothermal manifestations over prolonged time periods and are the indirect sources for geothermal power production. Magma chambers contain silitic or basaltic magma. Their volume measures from about 1 to 105 km3. Their heat content is tremendous (up to 1023 J) /10-6/.

Geothermal fields accompanied with volcanism /10-6/ are found along subduction zones at active continental margins where oceanic crust is pushed underneath the continental crust. Examples of these areas are aligned along the Pacific Ring of Fire, such as the Altiplano of the Andes, the Taupo Region in New Zea land, Kamchatka, and parts of Japan, Indonesia and the Philippines. Volcanic areas are also found above "hot spots" or mantle plumes outside the subduction zones. Examples are the Yellowstone volcanic fields or the Clear Lake Volcanic Field with the geothermal field of the Geysers. Continental rifts are also zones with recent and young volcanism. Rifting (spreading) of continental crust occurs in regions where the magma flow in the upper mantle is in direction of the earth surface causing an uplift of the mantle crust boundary and a thinning of the crust. A prominent example of a continental rift system with a high geothermal potential is the East African Rift or Graben. This Graben is extending from the Red Sea to Mozambique in the southern part of Africa. The largest mass of hot magma is ascending along the mid-ocean ridges. Most of the spreading centres are under water. There are, however, a few regions astride these ridges that are above water and accessible for extraction of geothermal heat. The best-known example is Iceland.

Fig. 10.13 Scheme of a geothermal system /10-11/

The potential for geothermal power production from geothermal fields accompanied with volcanism is tremendous. Recent estimates reach up to 22,000 TWh/a /10-11/. For comparison, the geothermal power production in the year 2004 amounted to about 57 TWh/a. This shows that geothermal power production from geothermal areas can be increased by at least two or three orders of magnitude compared to the current state. Many developing countries in South and Central America, in Asia, and Africa could and some do supply a major portion of their electricity consumption by geothermal power production from such geothermal fields associated with volcanism.

Such geothermal fields are generally characterised on the one hand by temperatures above 150 °C at relatively shallow depths (frequently under 1,000 m), and on the other hand by a high degree of hydraulic permeability. Though their poten tial is tremendous they are pinheads on a map of global scale. Most areas of the continents are lacking such favourable conditions and many of the geothermal fields are in remote areas far from any consumer. For this reason geothermal power generation faces two major challenges.

- Firstly, geothermal reservoirs of relatively low temperatures at relatively great depth have to be exploited.

- Secondly, concepts need to be developed that allow to cost-effectively access geothermal energy from hot rock formations at great depths.

Due to the relatively low thermal conductivity of rock, which is generally between 1 and 5 W/(m K), the well itself is a too small heat exchanger to withdraw heat at rates of economic interest. Deep wells are expensive and a thermal power in the order of magnitude of at least 10 MW is required to justify the investment. This can only be achieved if fluid is produced from the reservoir at a flow rate exceeding 100 m3/h per production well.

The major technical problem encountered with such a geothermal power generation is the generally low hydraulic permeability of deep rock formations. There are only a few types of rock formation, such as highly porous sandstone, intensely fractured rock, or karstic limestone that provides sufficient permeability to achieve the production flow rates required for cost-efficient power generation. This type of reservoir is referred to as hot water aquifer. Their temperatures only rarely exceed 150 °C (i.e. only in case of a geothermal anomaly); hence, generally temperatures between 100 and 150 °C are expected. In general, higher temperatures are only achieved from depths of 5,000 m onwards. However, at this depth rock permeability is generally very low and thus insufficient for geothermal power generation.

There are two possibilities of tapping such tight rock formations. Firstly, tapping of fault zones which reach down far below and allow for natural water movements, and, secondly, creation of artificial heat exchanging surfaces according to the Hot-Dry-Rock (HDR) concept. At present geothermal power production in the regions with normal to slightly above normal temperature gradients appears feasible only for these three types of reservoirs: hot water aquifers, fault zones, and crystalline bedrock.

The geothermal power potential of these reservoirs is practically inexhaustible. A recently performed study for Germany for instance /10-13/ estimated the geo-thermal power potential for this relatively small country lacking major temperature anomalies to more than 1,100 EJ of electric energy. In many other countries the crystalline basement was and will by far be the biggest resource. The geo-thermal power potential of deep reaching faults of 45 EJ, and of the hot water aquifers of 9 EJ, was much smaller but, if compared to the annual power consumption of Germany of 2 EJ/a, they are nevertheless very interesting resources. It has to be mentioned, however, that the hydraulic properties of the hot water aquifers and of the faults at great depths are not known in wide areas and that there is a high probability not to meet the desired productivity. For this reason stimulation methods, such as acid injection or hydraulic fracturing, may become very important when exploiting these kinds of reservoirs. In many cases the situation may not differ very much from the situation in the crystalline basement, where large artificial fracture surfaces will have to be created in order to achieve the desired flow rates. Experiments performed in Hot-Dry-Rock projects have shown that the waterfrac-technique is most suitable and possibly the only possibility for this purpose.

On the basis of the above mentioned framework conditions, the following approaches of geothermal power generation are distinguished.

- Power generation by open systems. Within open systems the heat carrier is circulated within an open circuit (i.e. the heat transfer medium is pumped into the underground, mixes up with potentially available geothermal fluid, and is produced again). In this respect, the following kinds of reservoirs are distinguished.

• Hot water aquifers (i.e. fissured porous reservoirs). By means of a two or more well system, aquifers containing a hot geothermal fluid can be tapped in sedimentary basins. Assuming that the aquifer has a sufficiently high temperature and that a sufficient production rate is either naturally available or produced by stimulation, such a two or more well system can either serve for combined heat and power (CHP) generation, or exclusively for power generation.

• Faults. Faults are potential flow paths of waters and are tapped e.g. by two wells similar to the hot water aquifers mentioned above, provided that sufficient permeability is available. As they generally reach deep into the underground, they allow achievement of very high temperatures.

• Crystalline rocks (i.e. Hot-Dry-Rock (HDR) or Hot-Fractured-Rock (HFR)). By fracturing new or enlarging already existing small faults respectively the existing network of fissures within the basement rocks, the Hot-Dry-Rock (HDR) or Hot-Fractured-Rock (HFR) technologies artificially create a new heat exchanger in the underground. If this heat exchanger is connected with the surface by means of e.g. two wells, water can circulate and heat up. It is thus available for geothermal heat and/or power generation.

- Power generation by closed systems. Within closed systems the heat carrier is circulated within a closed circuit. Hence, the heat transfer medium that is pumped into the underground is entirely separated from any fluids possibly available in the ground. Two types of closed systems are distinguished.

• One-way system. The underground is opened by means of a flow-through system, where a heat transfer medium is pumped one-way within a well into the underground and produced within the same well some kilometres away as a feed for heat and/or electricity generation.

• Two-way system. Energy can also be withdrawn from the underground by means of a deep coaxial well, through which the heat transfer medium circulates as discussed in Chapter 10.2.

However, from a current standpoint only geothermal power generation concepts based on open systems seem to be promising from a techno-economic point of view. Thus within the following paragraphs only open systems are discussed.

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