Geothermal energy in the state Brandenburg

Table of contents

Abstract

1. Introduction
1.1. Use of natural heat of the earth until today
1.2. Definition of geothermal energy

2. Main features of geothermal energy
2.1. The different types of geothermal energy sources
2.2. Technology of geothermal energy and technological problems in connection with possible ecological effects
2.2.1. Hydrothermal energy technology
2.2.2. Hot dry rock method
2.2.3. Shallow geothermal application systems
2.3. Legal and political aspects
2.3.1. Legal requirements
2.3.2. Energy political measures

3. Geothermal energy in Brandenburg
3.1. Geology of Brandenburg
3.1.1. Character of the landscape
3.1.2. Present day climatic condition
3.1.3. Geological and hydrological overview of Brandenburg
3.1.4. Relations between tectonic structure and temperature distribution in Brandenburg
3.1.5. The temperature distribution in Brandenburg
3.1.6. Chemical composition of thermal water in Brandenburg
3.2. Geothermal potential of Brandenburg
3.3. Application of geothermal energy in Brandenburg now and then
3.4. Economical aspects
3.5. Current projects in Brandenburg
3.6. Possible future development

4. Conclusion

5. Reference List

6. Appendix
I. Table thermal conductivity
II. Drillings with temperature surveys in Brandenburg
III. Temperatures in 2000 meters depth
IV. Temperatures in 4000 meters depth
V. Map of Brandenburg
VI. Geologic timescale

Abstract

Worldwide the demand in energy is increasing continuously. The awareness that fossil energy resources are limited and the fluctuation of crude oil- and natural gas prices were leading internationally and nationally to the recognition of geothermal energy as a possible energy source among the renewable energies (Rummel et al., 1993).

Geothermal energy is the heat of the earth. According to technical applications geothermal energy can be classified into three different natural systems, the shallow geothermal system, the hydrothermal low or high pressure systems and the hot dry rock system. These three systems of geothermal energy are described according to their possibilities of technical application. Legal requirements are playing a significant role in the application of geothermal energy as well as the political situation with regard to energy politics in the different states of Germany.

Special attention is given to the state Brandenburg. At first the geologic conditions have to be outlined. From the geologic conditions and the energy political situation as well as the supply and demand structure in Brandenburg, the potential for the use of geothermal energy is arising. Applications of geothermal energy started in the beginning of the 20th century in Brandenburg are in form of thermal springs. In the second half of the 20th century the use in Brandenburg turned to the utilization of shallow geothermal energy and later to deep geothermal energy projects. Today mainly the shallow geothermal energy systems are used commercially because they are economically wise in contrast to deep geothermal energy systems which can not be used economically wise yet. Their technology is still under development. Current projects will show their stage of development.

1. Introduction

1.1. Usage of natural heat of the earth until today

The heat in the inner of the earth has always been a matter of consideration in human life. The oldest kind to utilise geothermal energy is the use of warm water in baths. A first peak in using geothermal heat was during the Roman Empire in Europe and in the eastern Mediterranean region in form of thermal baths. In regions of cold and temperate climate like Iceland having geological favourable sites, other possibilities for the use of geothermal energy were found out like cooking and heating. Although the importance of spas and thermal baths is still increasing today, the significance of geothermal energy as substitute for fossil energy sources is much higher nowadays. Since the dependence on limited fossil energy sources was understood the relevance of renewable energies as primary energy carrier has been increasing rapidly (Buntebarth, 1980).

During the 18th century the first investigation and calculations were taking place with regard to the temperature distribution below the surface. Starting with hypothetical assumptions about the heat inside the earth by many scientists like Descartes, Leibniz and later Fourier the research of geothermal heat began which is today a part of geophysics. The first practical application of geothermal energy was carried out in 1827 in Laderello, Italy heating a tank for the production of boric acid. In Laderello the generation of electric power out of geothermal energy began in 1904 (Kühn, 1988). In 1912 the first generator with an installed capacity of 250 kW was used economically fed by hot vapour. Today two power units with each 150 MW installed capacity are in operation, using a hydrothermal steam reservoir in 3000 meters depth.

Many countries in the world are using geothermal energy for the generation of heat and power already. In countries like the USA, Japan, Italy and Island where geologic favourable sites are present a focus was set on the generation of power from geothermal energy what made them to be market leader in this area. An overview of geothermal power plants in operation today is given in figure 1.1. Furthermore, regions with a potential for geothermal energy applications in tectonic active zones are po inted out (Rummel et al., 1993).

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Figure 1.1: Installed capacity for the generation of electric power from geothermal resources worldwide (Rummel et al., 1993)

(orange: Areas with potential for geothermal power plants, red dots: Active volcanoes, purple triangle: Geothermal power plants

Figure 1.1: Installed capacity for the generation of electric power from geothermal

resources worldwide (Rummel et al., 1993)

(orange: Areas with potential for geothermal power plants, red dots:

Active volcanoes, purple triangle: Geothermal power plants)

Since 1975 the USA has the biggest concentration of power plants with a total installed capacity of 502 MW utilizing the steam originating from the Geysers. The first geothermal power plant in Japan has been set into operation in 1966. Japan has a high potential of geothermal resources due to the volcanic active zone with 600 volcanoes on which Japan is situated. Iceland is utilizing its geothermal resources since 1930 for heating purposes (Rau, 1978). In these countries the use of geothermal sources has been concentrated on regions with geothermal anomalies. Anomalies can occur when magma is located near the surface which leads to an anomal increase in temperature in the rock or the fluid contained in the rock, for example. Due to using geothermal anomalies the power plants are operating economically very efficient. Power production costs range from 2.5 to 7 Cent/kWh. Heat production costs vary between 1 and 3.5 Cent/kWh (Kleemann et al., 1993).

The first geothermal drilling in the world was carried out in Rüdersdorf (east of Berlin, Germany) in 1833. The first exact temperature survey and examination were done in Sperenberg to the south of Berlin in 1870 in a drilling of about 1272 metres depth resulting in one of the most important discoveries in the field of geothermal heat. The geothermal gradient with a value of 33.7 m/K has been discovered. This value was measured in the deepest borehole in the world within this time. Until today this value is regarded to be the average geothermal gradient (Huenges et al., 2000).

Although the first geothermal drilling was done in Germany the large scale use of geothermal heat began only in 1978 when geothermal heat from shallow regions was utilized with help of heat pumps for room heating purposes mainly. Before, the use of geothermal energy was restricted to balneological purposes in thermal baths. The first hydrothermal heating plant was set into operation in 1984 in Waren (Mecklenburg-West Pomerania) with an installed capacity of 5 MW (Kaltschmitt et al., 2003).

In Europe intensive research activities are under progress to improve the economic efficiency of power generation form geothermal resources. Six European countries including Germany are carrying out a project in France examining a hot dry rock formation. In Germany research activities are running with regard to the generation of power out of hot dry rock formation and low-enthalpy hydrothermal waters. Currently in the course of a pilot project it is tried to include an Organic Rankine Cycle into an existing geothermal heating plant in order to generate power (Huenges et al., 2000). The technology for the generation of heat and power using geothermal energy sources is still under development and capable of improvement.

1.2. Definition of geothermal energy

The literal translation of ‘geothermal’ is earth heat. Energy is the ability of a system to do work. Different forms of energy can be distinguished. There is for example mechanic, thermal, electric or chemical energy. The ability to do work is showing in form of force, heat or light. The term ‘geothermal energy’ therefore refers to stored energy below the surface in form of heat.

The heat of the earth is originating from the decay of radioactive isotopes like U238, U235, Th232 and others which are contained in small amounts in the earth crust (Rummich, 1978). A further source is the original heat stored during the formation of the earth itself and the gravitational energy set free during the earth’s formation (Rummel et al., 1993).

Inside the earth there is a continuous flow of heat from the inward towards the outward. The terrestrial heat flow is generally 63 mW/m² (Huenges et al., 2000) for continents but can differ from region to region. The temperature gradient amounts in average circa 33.7 m/K (Rau, 1978).

The heat of the earth is a geopotential that is available everywhere, either in rock material itself or as fluid in porous rock (Buntebarth, 1980).

Heat transport results from conduction and convection. Conduction of rock material under high temperature and pressure is taking place in the earth mantle. Transport of heat also occurs by the circulation of fluids. An important influence on the geothermal field is represented by the movement of water and tectonic processes where heat is transported much faster by convection than the conductive transport within rock.

The temperature distribution according to depth is influenced by different parameters as the radiogenic heat production, the heat flow density, the thermal conductivity of rock material, convection by fluids and the geologic structure of the underground. The temperature has a very big influence on all physical properties of rocks as well as on the migration and transformation of substances. On the other hand the structure and the properties of rocks, bedding formation and other geological conditions are determined by the distribution of temperature in the earth crust. Hot zones originating from magmatic regions have a positive effect on the temperature distribution. If there is a region with big differences from the average, this region is called anomaly (Huenges et al., 2000).

Different types of geothermal systems can be differentiated. Geothermal heat originating from the shallow underground is the first type. Furthermore there are hydrothermal low and high enthalpy systems as well as hot dry rock systems which are sources of geothermal energy. Other types are magmatic or volcanic systems and geo-pressurized hot water systems. Volcanic systems only occur in young crustal regimes with tension tectonics around the pacific, on islands in the Atlantic, east Africa and partly Europe (Rummel et al., 1993). Volcanic systems are used successfully in countries like the USA, Italy, Japan and Iceland (Rau, 1978).

Geothermal heat can be used in various ways with different technologies for heating or cooling purposes or the generation of power. Geothermal energy is a regenerative form of energy as long as the heat extracted from the ground is smaller than the flow of heat from the inner earth to the outer part (Buntebarth, 1980).

The geothermal potential in the earth crust is enormous. The stored geothermal energy worldwide amounts approximately to 43 million EJ in depth until 3000 meters. 85% of this stored amount of geothermal heat has a temperature level below 100°C. The McKelvey-Diagram (see figure 1.2) shows the connection between the total amount of geothermal resources and the part which can be used (Kaltschmitt et al., 1993).

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Figure 1.2: McKelvey – Diagram (Rummel et al., 1993)

The vertical axis of the diagram represents the depth of possible geothermal resources and the economical use. The horizontal axis represents the knowledge of geoscientific research which is including proven, probable and possible geothermal reserves (Rummel et al., 1993). Reserves are resources which are accessible by technical devices and whose exploitation is economical sensible. Resources are the natural occurrence of matter like coal or in this case the occurrence of geothermal heat in the earth (Schieferdecker, 2001).

Out of the total amount of geothermal energy resources only a small part can be used by the technology available today (Kaltschmitt et al., 1993).

2. Main features of geothermal energy

2.1. The different types of geothermal energy sources

In general two types of geothermal energy sources can be distinguished for the application of geothermal energy, the shallow geothermal heat and the deep geothermal heat. The deep geothermal heat can be subdivided into low-enthalpy hydrothermal heat in connection with aquifers as well as high-enthalpy hydrothermal heat, geothermal heat originating from hot dry rocks, magma and geo-pressurized hot water systems (Rummel et al., 1993). (See figure 2.1)

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Figure 2.1: Division of the different geothermal energy systems, (Rummel et al., 1993)

In the following the main types of geothermal energy systems will be described.

Shallow geothermal energy sources

The use of geothermal energy begins below the surface. In regions from 10-400 meters thermal energy can be used, though the temperature level is low with degrees only up to 20°C. Actually, there is no exact limit for the transition from shallow to deep heat sources because with geothermal probes higher depth can be reached due to technical development. The value 400 meters as a lower limit for the shallow geothermal energy went down into guidelines like VDI 4640 (Kaltschmitt et al., 2003).

In region up to 30 meters the shallow geothermal heat originates from the solar radiation stored in the ground. The solar radiation has a value of 1.35*106 mW/m² (Beer, 2000). Compared to the average terrestrial heat flow rate of 63 mW/m², the solar radiation is a much higher value which can be used economically efficient while the terrestrial heat flow rate can not be used as heat source due to economical low efficiency.

Further sources for the heat below the surface are the reflection of radiation, precipitation, circulation of ground water and thermal conduction of the soil. The temperature in the ground is undergoing daily and seasonal fluctuations. While the influence of daily temperature fluctuations can only be noticed until 1.5 meters below the surface, annual temperature periods can be proven until a depth of 30 meters. Best suitable for heat storage in shallow regions are water saturated rocks with a high heat capacity (Beer, 2000).

Deep geothermal energy sources

The water below the surface is divided into infiltration water and groundwater. The infiltration water is located in the water unsaturated zone between the surface and the groundwater zone. Beside rock and water also air can be found in this zone. In the groundwater zone which is water saturated the water leading layer also called aquifer is situated. According to the type of rock pore-, fracture or karst systems are distinguished.

The extraction and use of geothermal energy is determined by the factors temperature potential, geologic situation, purpose of use and infrastructure (Beer, 2000).

As shown in figure 2.1 hydrothermal energy resources can be divided into hydrothermal low-enthalpy systems and hydrothermal high-enthalpy systems. Hydrothermal low enthalpy systems are warm- or hot water occurrences with temperatures up to 100°C. Hydrothermal high-enthalpy systems are characterised by temperatures above 100°C as well as saturated steam and unsaturated steam with temperatures of 150 to 250°C. The low-enthalpy system has a sedimentary origin and is formed during the lowering of a sedimentation basin. The sediments in a basin have a strong water- or steam leading rock layer. The heat stored in the water or steam can be utilized for room heating purposes or the provision of warm water.

Hydrothermal high-enthalpy deposits contain hot water mixed with gases like methane. The origin of such deposits is the rise of hot mantle material and the formation of deep sedimentary basins. If subsequent impermeable sediments seal the porous rock formations, the pore fluids within the porous sediments are heated (Rummel et al., 1993).

The primary characteristics for a sedimentary reservoir are size and form as well as sediment structure, texture and composition of minerals. Consequently porosity, permeability, density and saturation of the fluid are secondary characteristics. The groundwater is characterised by its chemical composition and the groundwater flow.

The most important transport parameter of a reservoir is the permeability that is dependent on the structure of the pore volume and the presence of water bearing bed. The sufficient vertical and lateral distribution of rock also called net thickness is the part of a reservoir which proves to have characteristics needed for a successful use. To ensure high volume flows and a long-lasting, constant extraction and injection, minimum values for porosity, permeability and net thickness are demanded. Furthermore, an economical interesting temperature level has to be present and the deep water has to be suitable for technological processes (Huenges et al., 2000).

The term hot dry rock is originating from the assumption that deep crystalline basement rock formations are nearly dry and impermeable for fluids due to the pressure of overburden rock (Rummel et al., 1993). The presence of highly mineralised water in the fracture system can not be excluded (Kaltschmitt et al., 1993). The deposits are called ‘dry’ because there is not enough natural occurring water suitable for a long-term use. Most rock formations in the upper continental crust have a low porosity and poor permeability. The deposits are characterised by fractures created during the formation of joints. The existence of hot fluid and their mobility along joint- or fracture systems is limited. In depth of 4000 to 7000 meters overlying rock formations are exerting a high pressure on the deposits below. The crystalline hard rock in the basal complex is mainly granite or gneiss. Hot dry rock reservoirs have the biggest potential for a geothermal energy use, which is technical accessible at present. For the generation of power temperatures should reach above 150°C to be economically efficient (Rummel et al., 1993).

Worldwide there are intensive research activities which are concerned with the development of technical processes to make use of the stored heat in the deep located rock because it is best suitable for the generation of power. Concept for using heat from hot dry rocks was initiated by energy politics and the energy industry. This energy is interesting for a use because it can give access to a high, up to now unused, local energy source (Lund, 2000).

Volcanic systems can be differentiated in fumaroles, mofets, geysers, hot lava or magmatic chamber systems and other systems. Magmatic chamber systems are intrusions of magma which penetrated from deep underground into the earth crust. The magmatic intrusion passes the heat to the surrounding rock or to water-bearing layers. Magmatic intrusions cause the development of geothermal anomalies. The geothermal power plants operating worldwide make use of volcanic systems (Rau et al., 1978).

Geo-pressurized hot water systems exist in porous rock formations which were rapidly subsided to great depth together with enclosed pore fluids. The pore content is exposed to high overburden pressure and temperatures of up to 200°C. The energy content of highly compressed pore fluids consists of their heat content, the pressure potential and the content of hydrocarbons. The formation of these reservoirs occurs in deep marine basins. First geo-pressurized hot water systems were identified in the deep sediments of the Gulf of Mexico at depth of 6-8 kilometres. The total energy content of the system is estimated to about 5*109 MWyears at temperatures between 150 and 180°C. Similar resources may be expected in other geosynclinal regimes as the Molasse basin in the south of Germany or in the Po basin in the north of Italy (Rummel et al., 1993).

In Germany possibilities for the use of geothermal energy sources are present according to the geologic conditions. Shallow geothermal heat sources can be tapped with help of heat pumps. Hydrothermal low-enthalpy sources can be utilized in geothermal heating plants. Hot dry rock formations and geo-pressurized systems can be tapped with deep probe systems or the hot dry rock method for the generation of power or heat. Magmatic chambers are not relevant for the region of Germany as it only occurs in regions of young active volcanism (Huenges et al., 2000).

2.2. Technology of geothermal energy and technological problems in connection with possible ecological effects

There is a variety of possibilities to make use of the geothermal energy in the shallow and deep underground. Only the most important technologies which are applied in Germany will be described here. The application of the chosen technologies is dependent on parameters like geological characteristic of the underground as well as on economical and political aspects.

The technology for the use of the deep geothermal energy can be split into three significant methods. The use of low-enthalpy hydrothermal heat in connection with aquifers, the hot dry rock method and the use of geothermal energy with help of deep geothermal probes. The geothermal heating plant converts the heat for application purposes like room heating or the supply of warm water. If there is a sufficient inlet temperature the generation of electric power can be realized for extraction temperatures above 100°C (Rummich, 1978). The most significant technology for the provision of heat in Germany is the use of low-enthalpy hydrothermal heat in geothermal heating plants. The deep geothermal probe is a further possibility to use low-enthalpy hydrothermal resources. The hot dry rock method is not applied in Germany. Research activities in France are investigating if a use of hot dry rock formations is possible in Europe. The shallow geothermal heat is extracted from the ground in various application systems (Kaltschmitt et al., 1993).

2.2.1. Hydrothermal energy technology

The production of heat from low-enthalpy hydrothermal reservoirs in a geothermal heating plant is strongly dependent on the geologic site characteristics as well as on the supply and demand structure. Essential elements are the drillings. The classical method for the use of low-enthalpy hydrothermal heat is the drill doublet consisting of a productive and an injection well. Through the productive well thermal water is extracted from the aquifer and after cooling down the water is normally injected back into the aquifer if the thermal water has a high content in minerals (Kayser, 1999). (See figure 2.2)

The rotary method is worldwide used for the production of a drilling. This method requires a continuous rotation of the drilling tool. The main difference between a geothermal drillings and an oil- or gas drillings is that the drillings are carried out under a lower mountain pressure due to a lower depth but with a higher borehole temperature. Beside that, mineralized and aggressive fluids are likely to occur in the borehole. For the production of thermal water a suitable borehole diameter is especially important. A diameter chosen to small can lead to a restricted production rate or in contrast, a borehole with a diameter too big is presenting a higher financial risk (Kaltschmitt et al., 1999).

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The pressure in the reservoir zone can be above original ground level or below. If the reservoir zone is above the original ground level artesian conditions are prevailing where the thermal water is coming up without aid of pumps. Used reservoir zones are most often not under artesian conditions. The installation depth of the pump depends on the height of the water table. According to the discharge and the geological conditions a ‘driving gradient’ has to be created for the transport of thermal water. This gradient is a pressure gradient. The following drawdown of the remaining water level is called the ‘dynamic table’. The production pump has to be installed below the dynamic thermal water table in depth of 100 to 400 meters (Kaltschmitt et al., 1999).

The heat of the thermal water is utilized in the heating plant via a heat exchanger. In case the aquifer parameters and consumer demand do not correlate, the temperature level can be increased with heat pumps. The power for the heat pump can be provided by a unit-type cogenerating station fuelled with natural gas or fuel oil. The unit-type station is also suitable for peak-load supplies occurring periodically (Kayser, 1999).

The thermal water is transported in buried, heat isolated pipes between the productive well, the heating plant and the injection drill. Pipes are hardly laid above the surface because they are exposed to mechanical impacts and environmental influences. Furthermore the pipes make the respective site useless for other applications.

For the heat transport a separation of the thermal water system and heating system is necessary. The separation guarantees an independency of both systems (Huenges et al., 2000).

The thermal water can contain minerals like hydrogen sulphide or salt which cause corrosion.

The geothermal heating plant in Neustadt-Glewe (Mecklenburg-West Pomerania) for example is extracting thermal water of mesozoic origin with a mineralization content of 220 g/l. Due to the high mineralization of the water all pipes are made of steel to prevent corrosion (Kaltschmitt et al., 1999).

In general special materials have to be chosen to ensure the safety of a plant. The thermal water should not be changed in its consistence before it is injected into its original environment. The choice of material is further important to prevent the formation of secondary solids (corrosion products) in the thermal water stream. Following materials suitable to prevent corrosion:

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Table 2.1 (Huenges et al., 2000)

The material has to be chosen according to the characteristics of a geologic site and their thermal water components as well as to the financial resources (Huenges et al., 2000).

To prevent pollution of the water that is injected into the underground, the system needs to be sealed and a leakage detector has to be installed. The leakage detector controls the whole system and identifies leakages fast and precisely. In case of an accident the thermal water spill will be stopped immediately (Kayser, 1999).

Inert gas (nitrogen) can be added to the thermal water cycle to prevent oxygen from entering the facility. The reaction between thermal water and oxygen causes a change of redox potential which can lead to problems when injecting the water into the aquifer. Oxygen leads furthermore to the formation of oxidation and corrosion products like voluminous iron hydroxides. Therefore, the oxygen content of the injection water needs regular measurement. Particles cause sedimentation in components or formation of precipitants. Filters prevent particles from entering the pipe system and the injection well (Huenges et al., 2000).

The potential environmental effects of a hydrothermal heating plant are depending on the share of geothermal heat fed into the distribution network. Generally a much smaller amount of greenhouse gas emissions is taking place compared to a fossil fuelled heating plant. The drilling and the water extraction have only local and short influences on the environment. The salinity of the water can harm flora and fauna. The thermal water is most often injected into the aquifer. Only in case of an accident thermal water could come into contact with the surface environment. Due to the high safety standard accidents are not likely to occur (Huenges et al., 2000).

Geothermal heating plants are base and middle load facilities. A maximal use of the geothermal heat can be achieved by supplying industrial facilities with a high demand in heat or to link the extracted heat to a district heating net. In Germany mostly two-conductor distribution systems which are running with water are used. The district heating net is distributing heat for room- and water heating purposes. Usually district heating plants have a maximum capacity of 1700 to 2500 h/a. To gain a maximum use from the extracted heat a high amount of full load hours is needed. Therefore the maximum output of the geothermal heating plant should be smaller than the maximum capacity of the heating net. Temperature and pressure are the main parameters for operating the network. Most systems are driven with a flexible inlet temperature of 60 to 80°C and a return temperature of 30 to 40°C (Kayser, 1999).

2.2.2. Hot dry rock method

The idea of the hot dry rock method is to create artificial surfaces in impermeable rock formation where an injected fluid is acting as heat exchanging agent. The circulating fluid withdraws the heat from the rock. (See figure 2.3) The hot dry rock method consists of two deep drillings. The rock between the two endpoints is hydraulically fractured to make the rock permeable for water. The stimulation of the rock in the deep underground is the key for a successful use. To get a spacious network of combined fractures, water is pressure-injected into the ground. The stimulation has to be done more than once. By the injection of water natural fractures are hydraulically expanded and new fractures are created. If possible already existing joint systems in crystalline rock can be used. The resulting closed circulation system has a high permeability for water (Huenges et al., 2000).

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The principle of this method is that the injected water is taking thermal heat from the surrounding on its way through the fractured rock. Due to the flow resistance inside the rock a pump has to force the water from one end point to the other. The used power for the pump is reducing the efficiency of the power plant severely.

Technical measures are necessary to prevent precipitation- and corrosion problems in the surface facilities and to avoid alterations of the chemical composition in the thermal water. Therefore the fluid from the underground has to be kept in a closed system. Under these conditions a secondary cycle with a heat carrying agent is necessary. Depending on the temperature level, the heat is either used directly for heating purposes or for the generation of power. The generation of power can be realized with help of the Organic Rankine Cycle. The principle of the Organic Rankine Cycle is, that the energy of the thermal water is transferred to a organic fluid that is evaporating at a lower temperature in order to produce electric power. The heat carrying agent is an organic fluid with suitable density-, pressure- and enthalpy differences. Fluids used so far are explosive, inflammable and harmful for the environment. Nowadays environmental friendly organic fluids are tested.

At present there is no power plant that uses the hot dry rock method commercially in Germany (Kaltschmitt et al., 2003). In France and Japan power plants are operated for research purposes. The projected in Soultz-sous-Forêts (France) started in 1987. Experiments are carried out to create artificial fractures in crystalline rock in depth of 3500 to 3900 with temperatures of about 140°C. In first circulation test in 1997 244000 tons of thermal water were produced and injected within four month. A thermal capacity of 10-11 MW could be achieved. The power consumption of the plant itself ranged between 200 and 230 kW. No corrosion or precipitation products were found in the plant after the circulation test. In the fracture system no solution processes were noticed. The filter is expected to last several weeks or even month. The hot dry rock system in France is working almost fully automatic and environmentally friendly. The knowledge gained in that project is applicable to Germany as geologic conditions in the deep underground are similar.

The efficiency of geothermal power plants with temperatures below 150°C is relatively low in comparison to heating plants. In Central Europe the temperatures in the deep underground do not exceed 250°C. The resulting useful temperatures of 100 to 150°C give a relatively low efficiency in comparison to conventional thermal power stations. High amounts of waste heat can be obtained. The waste heat can alternatively be used for industries, greenhouses and fish farms, or for the supply of heating nets (Kaltschmitt et al., 1999).

A special case for the use of the geothermal heat is the deep geothermal probes. Abandoned bore holes are suitable for use. A deep geothermal probe is functioning similar to the probes which are used for shallow ground heat extraction described in 2.2.3. A deep geothermal probe has advantages in comparison to other technologies applied in deep located regions. The technology can be applied everywhere because it operates with a closed system. No substances are extracted from the ground and no emission of harmful substances can take place. The system is highly reliable in supply and it has a long lasting life. The geothermal probe has hardly any environmental effects (Schneider, 2000).

2.2.3. Shallow geothermal application systems

The energy stored until a depth of 400 meters can be used with various techniques, methods and concepts. A temperature level most often below 20°C has to be considered. In order to utilize the heat extracted from the ground a heat pump is needed to transform the low temperature level to a higher one. The ratio of used energy as motive power for the heat pump and gained energy from the surrounding is 1:3. For example, for the production of 4 kWh of energy for heating purposes 1 kWh of electric energy is needed.

A system for using heat from shallow regions is consisting of three elements. The first element is a device to extract the energy from the ground, the second one is the heat pump to increase the temperature level and the third one is the heat sink, a system that uses the produced heat. Consequently the use of energy from near surface regions is always demanding an additional energy supply. The elements of shallow geothermal systems will be described in detail along with their different variants (Kaltschmitt et al., 1999).

The heat pump is extracting heat from rock or the water contained in the rock pores. Normally water is stored in the pores. Air is a poor heat conductor. There are different systems of how the heat can be extracted from the ground. Two main systems can be distinguished, the closed system and the open system.

Open systems are groundwater wells with a constant groundwater temperature level of 8-12°C. Aquifers with a high permeability are the main prerequisite. Restrictions for the use of open systems can arise from water laws. This system was mainly applied in the 70’s. Experiences with the application of the system were showing that hydroxide clogging was causing difficulties in operating the system. For that reason closed systems like geothermal collectors were preferred in many cases. Today geothermal probes are applied increasingly due to the area geothermal collectors require (Kaltschmitt et al., 1999).

Another possibility for shallow heat extraction is the artificial cavern for example originating from former mining activities. Caverns can serve as a groundwater reservoir. Determining for the use of underground mines is the depth of the water table inside the cavern. Most often they are located to deep which can cause a high energy demand for big extraction heights (Kaltschmitt et al., 2003).

The closed system can be collectors, probes or earth-touched stilts also called ‘energy stilts’. The heat is extracted via a closed cycle. Inside the system water with antifreeze or heat pump fluid itself is flowing. The heat carrier which is extracting the heat from the ground is not in direct contact with the rock or the pore content. Theoretical these system are applicable everywhere.

Vertically installed heat probes need a smaller area in comparison to horizontally laid collectors. Heat probes are installed in drillings to a depth of several hundred meters (see table 2.2). The material used for such probes is most often high-density-polyethylene (Kaltschmitt et al., 1999). If a high amount of heat is extracted while applying geothermal probes the risk occurs that the deeper ground would possibly not be able to regenerate itself in total during summer. The surrounding area of the probe is relatively low in comparison to the tapped volume. The consequences would be a lower efficiency of the heat pump due to lower temperatures. Geothermal probes are more suitable be used for heating and cooling purposes. A heat extraction between 180 and 650 MJ/a can be achieved for collectors and probes (Kaltschmitt et al., 2003).

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