Environmental Engineering Reference
In-Depth Information
10.2
Heat supply by deep wells
If geothermal fluid cannot be tapped by deep wells, the borehole can still be util-
ised by means of a geothermal deep well; whose functionality is similar to that of
geothermal probes applied near the surface (see Chapter 9.2). Thus the following
paragraphs contain a brief description of geothermal deep well technology. Sub-
sequently, economic and environmental aspects are discussed.
10.2.1
Technical description
To exploit heat stored in the underground by means of a deep well a cased bore-
hole is required (see Chapter 10.1). Additionally, the hole must be equipped with
a double coaxial casing. To access the heat of the deep underground a heat trans-
fer medium is pumped into the underground from aboveground through the annu-
lus section between the production tube and the well casing. From the bottom of
the well the heated heat transfer medium is pumped to aboveground within the
production well (Fig. 10.10). The well casing seals the borehole fully against the
bedrock (i.e. closed system).
Heat carrier
inlet
Heat carrier
outlet
Geothermal heat flow
Fig. 10.10 Operation principle of a geothermal deep well
As the rock temperature rises with increasing depths in proportion to the geo-
thermal gradient, the heat transfer medium heats up on its way to the bottom of
the borehole, and thus withdraws energy from the underground making the geo-
thermal heat technically available. However, only as much heat can be withdrawn
from the underground as is provided by the natural geothermal heat flow (on av-
erage 65 mW/m 2 ). As the heat transfer medium does not penetrate the tube casing
there is no mass transfer between the deep well and the underground (i.e. closed
system) as it is the case for hydro-geothermal heat utilisation (i.e. open system).
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