Environmental Engineering Reference
In-Depth Information
10
Utilisation of Geothermal Energy
10.1
Heat supply by hydro-geothermal systems
The expression 'hydro-geothermal energy utilisation' refers to the exploitation of
the energetic potential of cold thermal (40 to 100 °C) and hot thermal (above
100 °C) water available within the deep underground.
Usually, geothermal fluid is brought up to the surface from underground via
boreholes, where the heat is used by potential consumers. Except for cases in
which geothermal fluid is consumed as such, they are re-transferred to the under-
ground via a second borehole. This closed cycle helps to conserve the mass bal-
ance and thus avoids hydraulic problems. Moreover, for environmental reasons,
especially highly mineralised geothermal fluids cannot be disposed of above the
surface.
Within the following section the technical basics, as well as economic and en-
vironmental aspects for systems for the use of hydro-geothermal energy are de-
scribed.
10.1.1 Technical description
When describing the system we distinguish between the downhole and the uphole
part of the geothermal fluid circuit, the district heating system, and the incorpora-
tion of geothermal energy into supply systems /10-1/, /10-2/, /10-3/, /10-4/.
Fig. 10.1 provides an overview of the basic concept of such an energy system,
illustrating the two-hole system (consisting of one production well and one injec-
tion well) required to tap hydro-geothermal deposits inside the ground. But firstly
the drilling of a geothermal well is described briefly.
Geothermal well drilling.
Due to severe drilling conditions at high temperatures
especially in geothermal fields, or for the drilling of hard abrasive rock types (like
for Hot-Dry-Rock reservoirs; see Chapter 10.3), the drilling of geothermal wells is
usually more challenging compared to oil and gas wells. Therefore within the
following explanations the basics of deep well drilling is discussed with a special
focus on unlocking geothermal reservoirs.
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