Classification of geothermal systems

The temperature of the earth essentially depends on its contemplated depth. While the temperature level in areas up to 20 m below the surface is in large parts determined by sunlight and leachate, weather conditions have no effect on the earth's heat at greater depths. Instead, the heat level in deeper areas is impacted by the geothermal heat flux from the interior of the earth as well as the natural radioactive decay of isotopes (238U, 235U, 232Th, 40K) within the earth's crust.

As a result, the temperature rises with increasing depth according to the geothermal gradient of approx. 3 K per 100 m. This means that, at a depth of 400 m, a temperature of about 20 °C is reached on average. At depths of several km, significantly higher temperatures prevail, which render geothermal heat and power production feasible by means of suitable system technology. Due to geological anomalies, a higher temperature rise can be observed in some regions of the world that are particularly attractive for geothermal use. Depending on the depth of the heat reservoir, a distinction between near-surface and deep geothermics is made.

Fig. 1: Comparison between near surface and deep geothermics (Graphic: TU Bergakademie Freiberg)

Near-surface geothermics

Near-surface (or shallow) geothermics refer to the exploitation of geothermal energy at depths up to 400 m and temperatures up to 25 °C. The heating of buildings therefore requires the application of heat pumps in order to generate useful heat of temperatures between 30 - 60 °C. On the contrary, the cooling of buildings can be realized either with (active cooling) or without (passive cooling) heat pump technology.

The usable amount of geothermal energy mainly depends on the subsurface temperature - and thus on the depth of a plant. When employing closed loop systems, the thermal conductivity of the surrounding rock is of major importance. Through an open system - comprising an extraction and injection well - groundwater can be applied as heat source for the provision of energy. In that regard, the withdrawable amount of water is crucial.

Additionally, the temperature change of the underground due to the use of geothermal energy has to be taken into account. Ergo, the extractable amount of energy depends on the thermal regeneration of the soil, rock, and groundwater. Legal regulations of the respective German federal states stipulate how strongly the subsurface temperature may alter over time. Furthermore, potentially interfering adjacent geothermal systems in the neighborhood must also be considered. For a sensible and expedient utilization of geothermal energy, the logging of heat extraction data for each plant is pivotal.

Deep geothermics

Deep geothermics refer to the use of geothermal reservoirs lying more than 400 m below ground. The deepest bore of 6,400 m currently in operation for geothermal usage was realized in April 2018 by a German drilling company in Otaniemi, district of Espoo, Finland. The prevailing temperatures according to the geothermal gradient usually range from 80 °C to more than 200 °C.

At this temperature level, the extracted heat can be either provided for direct heating supply through heat plants, or converted into electricity with the help of steam power stations. In the latter case, and depending on the available temperature, steam turbines are either operated by water vapor, organic working fluids (Organic-Ranking-Cycle process), or water-ammonia mixtures (Kalina-Cycle process). Energy sources are either petrothermal (hot dry rock) or hydrothermal (deep aquifers) reservoirs.

Mine water geothermics

A special form of geothermal energy, which cannot be attributed to either shallow or deep geothermics, is the geothermal use of mine waters. Depending on the deposit and mine structure, depths ranging from a few up to several thousand meters are accessible. The man-made, partially or completely water-filled rock cavities of dead mines, present a unique possibility for geothermal application.

The artificially created mine workings offer a large, thermoconductive surface; therefore, a huge amount of heat from the rock is transferred to the mine water, which supports the thermal regeneration of the water body. By known layouts of mine workings, or, to some extent, their ensured accessibility, the difficulty of spotting mine water reservoirs - compared to conventional geothermal energy resources - is rather low. Furthermore, already existing mine structures can be drawn on to exploit the geothermal potential of mine waters.

Examples of geothermal energy usage

Fig. 2: Possible uses depending on the required flow temperature (Graphic: TU Bergakademie Freiberg)

Systems

In principle, a distinction is made between two types of systems in order to harness geothermal energy, namely open and closed systems. In the former type, groundwater is drawn from a well, cooled or heated using heat exchangers to provide energy, and then released to the environment, either via an absorption well or superficially. In a subsequent, closed intermediate circuit, the heat is transferred to and extracted by a refrigerant. In the case of the latter type, however, a heat-transfer fluid or refrigerant circulates in a closed system materially separated from the soil.

There are various technical variants for utilizing geothermal energy:

Open systems

  • Well / doublet systems for the use of near-surface ground or mine water, hydrothermal and petrothermal deposits (hot dry rock)

Literature

[1] Tholen, M. and Walker-Hertkorn, S. (2008) Arbeitshilfen Geothermie, wvgw Wirtschafts- und Verlagsgesellschaft Gas und Wasser mbH, Bonn, Germany
[2] German Geothermal Association press release (2008): Weltrekord: Deutsches Bohrunternehmen schließt tiefste Bohrung zur Energienutzung ab, published on 25 April 2018, available at https://www.geothermie.de/fileadmin/user_upload/Aktuelles/Presse/Pressemitteilungen/Pressemitteilungen_2018/PM_20180425_Weltrekord_Bohrung_Finnland.pdf , last visited on 24 January 2019
[3] Grab, T., Storch, T., and Groß, U. (2018): Energetische Nutzung von Grubenwasser aus gefluteten Bergwerken, in: Bauer, M., Freeden, W., Jacobi, H. and Neu, T. (eds.): Handbuch Oberflächennahe Geothermie, Springer Spektrum, Berlin, pp. 523–586