Utilisation of Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 452

− Reference system DH-II. The geothermal heating plant is equipped with one
production and one injection well of a total depth of 2,450 and 2,350 m, re-
spectively, with a distance of 1,500 m between the two boreholes (Table 10.1).
The temperature of the geothermal fluid used for energy provision is approxi-
mately 100 °C. Depending on the actual heat load 30 to 60 m 3 /h of geothermal
fluid are produced by the production well and delivered to the heating plant.
The production and injection well are connected to the heating station via an
underground pipeline of glass fibre reinforced plastic. Within the geothermal
heating station heat is supplied by two load-controlled titanium plate heat ex-
changers of a maximum thermal capacity of 1,700 kW each, which are incorpo-
rated into the overall system by means of an intermediate circuit. This permits
an optimum temperature adaptation of the geothermal fluid parameters "tem-
perature" and "mass flow" to the heat capacity required by the district heating
system. About 85 % of the heat provided throughout the year is covered by
geothermal fluid, whereas the remaining 15 % are provided by a light fuel oil-
fired peak-load heating station. The geothermal heating station is designed for
a thermal capacity of about 5 MW, of which 3.4 MW directly result from geo-
thermal fluid. Generated heat is distributed according to the heating network
defined in Table 10.1.
− Reference system DH-III. For this system geothermal fluid of a temperature of
62 °C is produced from a reservoir layer located at a depth of 1,300 m. The
geothermal fluid is delivered by means of a borehole pump installed at a depth
of 390 m within the production well (Table 10.1) at a volume flow of 72 m 3 /h.
Subsequently, the geothermal fluid is pumped to the geothermal heating sta-
tion, which, according to the specific operating conditions, withdraws thermal
capacities of 450 to 1,400 kW by heat exchangers. Two heat pumps driven by
electrical energy with a thermal capacity of 1,450 kW subsequently cool down
the geothermal fluid to about 25 °C, providing a temperature of 70 °C, avail-
able to heat up the heat transfer medium used within the district heating system.
Subsequently, the cooled geothermal fluid is re-injected to a reservoir at a
depth of about 1,250 m. The electricity required for the operation of the heat
pumps and other auxiliary units (such as pumps) are partly provided by two
natural gas-fired CHP stations with a thermal capacity of 630 kW and an elec-
tric capacity of 450 kW. The heat generated beside the electric energy is addi-
tionally used within the district heating network. In order to cover peak de-
mands, and to ensure a secure heat supply in case of malfunction of the geo-
thermal part, two natural gas-fired boilers are integrated within the overall sys-
tem. For such a geothermal heating plant with an installed thermal capacity of
10 MW about half (50.6 %) of the annual heat demand is covered by geother-
mal heat (direct heat transfer and geothermal share of the heat provided by the
heat pumps). Together with the heat from the CHP station and the overall heat
provided by the heat pumps this system accounts for almost 85 % of the total
annual heat demand. The remaining 15 % are provided by the peak-load heat-