Environmental Engineering Reference
In-Depth Information
Figure 10.9
Diagram
of an HDR power
plant.
10.3 Planning and Design
The most important factor in planning geothermal power plants is the temperatures
that can be achieved. The design of the heat exchangers, district heating grids and
plant processes are all based on projected temperatures. Geologists try to determine
in advance at which depths the desired temperatures can be found. To an extent they
are able to rely on the knowledge gained from existing boreholes.
In addition to achievable temperatures, extractable water quantities also play a major
role. Large volumes of water are needed to achieve high output. The diameter of
the borehole as well as the pumps both have to be designed accordingly. Last but
not least, it is important that the temperature of the thermal water is not allowed to
drop too signifi cantly during the extraction process. Large-scale power plants
usually extract more heat from the depth than regularly fl ows back into an exploited
area. Therefore, a slow cooling of the exploited area cannot really be prevented.
The goal is to plan the intervals between the drilling so that the desired temperatures
can be sustained for about 30 years. After this period the temperatures drop below
the desired target values, and, as a result, the geothermal plant performance also
decreases. A new site, which should not be more than a few kilometres from the
existing site, must be developed if any further exploitation is planned.