Hydrothermal Systems, Geothermal Doublets
Hydrothermal systems use the thermal energy of an aqueous fluid at greater depths. Depending on the heat content of the fluid, systems with high enthalpy can be distinguished from low enthalpy systems. High enthalpy systems produce electrical power direct
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Hydrothermal Systems, Geothermal Doublets
Production test at a geothermal power plant
I. Stober and K. Bucher, Geothermal Energy, DOI: 10.1007/978-3-642-13352-7_8, © Springer-Verlag Berlin Heidelberg 2013
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8 Hydrothermal Systems, Geothermal Doublets
Hydrothermal systems use the thermal energy of an aqueous fluid at greater depths. Depending on the heat content of the fluid, systems with high enthalpy can be distinguished from low enthalpy systems. High enthalpy systems produce electrical power directly from hot steam or from a high-temperature two-phase fluid (Sect. 4.2). Low-enthalpy systems use the warm or hot water directly or via a heat exchanger to feed local or district heating systems, for industrial or agricultural utilization or for balneological purposes. Profitable electrical power production is possible at fluid temperatures above 120 °C. The thermal water is produced from deep groundwater reservoirs (aquifers). In principle, thermal water may also be retrieved from water conducting faults and fault zones, however, hydrothermal systems typically connect to aquifers. High-enthalpy hydrothermal systems are related to regions with extreme geothermal gradients and very high ground temperature at shallow depth typically found in volcanic active areas, young rift systems and similar geological conditions. Low-enthalpy systems can be developed in any region with average or slightly elevated geothermal gradient. Therefore, the potential for low-enthalpy systems is evident because they can be installed in normal continental crust. The present day situation is, however, strongly focused on high-enthalpy systems and most of the worldwide installed electrical power capacity from geothermal sources relates to high-enthalpy systems (Sects. 1.3 and 3.4). Deep aquifers that permit the installation of geothermal doublets may also be used as seasonal heat storage systems. This can be attractive if for example the seasonal excess heat collected by a photovoltaic system can be transferred to the deep ground for use in the cold season. An aquifer heat storage system uses, in contrast to a geothermal probe storage system (Sect. 6.8), the heat capacity of water and rock of a natural aquifer that is hydraulically sealed at the bottom and the top of the conductive layer. The aquifer heat storage system is developed by means of a production and an injection well, similar to geothermal doublets. For charging the system, water is produced from one well, heated in a heat exchanger and injected into the aquifer through the second well. The process is reversed for discharging the system (Hasnaina 1998a, b).
8.1 Geologic and Tectonic Structure of the Underground Hydrothermal systems use natural deep groundwater residing in geological reservoirs with high hydraulic conductivity. The reservoirs are embedded in other geological units with different properties. Therefore detailed and thorough knowledge of the geological structure of the underground is absolutely compulsory for exploring and constructing hydrothermal syste
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