Utilisation of Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 461

In most cases water is used as heat transfer medium. However, the water is
generally treated with inhibitors to minimise corrosion of the downhole compo-
nents. For this purpose the experience gained within existing district heating sys-
tems are applied.
Subsequently, the ascending pipeline respectively the production tube re-
transfers the heat transfer medium - "charged" with geothermal heat - upward
from the bottom of the borehole to aboveground. To ensure high temperatures at
the well exit and in order to minimise heat losses, the entire production tube is
heat-insulated.
The heat carrier which passes through the well decreases the temperature at the
surface of the casing cemented into the underground. In spite of the relatively low
heat conductivity of the rocks available within the underground, due to the tem-
perature difference between the heat transfer medium and the surrounding rocks,
heat is transferred into the heat carrier, which may amount to 200 W/m /10-2/. As
the surrounding rocks do not keep their initial temperature, the heat transfer me-
dium can only reach temperatures considerably lower than those of the uninflu-
enced rocks.
The thermal capacity of such a closed system (i.e. a geothermal deep well) is
influenced mainly by
− geological parameters, such as the local geothermal gradient and the existing
thermal physical properties of the rocks available at the respective depth
around the well,
− the technical configuration of the well (i.e. diameter and materials, insulation
properties of the applied tubes, heat transfer between primary rocks, cement
and casing) and above all
− the operating principle of the entire system.
For common well depths within the range of 1,000 to 4,000 m and average geo-
logical conditions, geothermal capacities between 50 and 400 kW are expected.
The heat transfer medium is circulated within the deep well by means of a
pump which acts as the major uphole component of such a system. The required
pump capacity is lower than that of a circulation pump used for hydro-geothermal
utilisation, since there are no pressure losses in the actual heat exchanger and,
contrary to geothermal fluid extraction, the medium flows through a closed pipe-
line.
Since the temperature at the well exit is generally lower than 40 °C, a heat
pump is indispensable. Due to the relatively low thermal capacity of several
100 kW for such a deep well, an electric or gas motor-driven compression heat
pump is usually applied. Such a system configuration allows for an extensive
cooling of the heat carrier circulated within the deep well. The produced heat is at
the same time lifted to a temperature level appropriate to be used in small district
heating networks. To obtain a favourable coefficient of performance (COP) mod-
erate temperatures at the heat pump exit are advantageous (i.e. inlet and outlet
temperatures of the district heating network). If the outlet temperature of the heat-