Utilisation of Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 471

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However, from a current standpoint only geothermal power generation concepts
based on open systems seem to be promising from a techno-economic point of
view. Thus within the following paragraphs only open systems are discussed.
10.3.1 Technical description
Systems for geothermal power generation consist of two major parts: The subsur-
face system for heat extraction (for drilling technique and well completion of a
geothermal borehole see Chapter 10.1) and the power generating system at the
surface (see /10-4/, /10-14/, /10-15/, /10-16/, /10-17/, /10-18/, /10-19/).
10.3.1.1 Subsurface system
The following explanations are aimed to discuss all aspects associated with the
subsurface system of plants allowing for the production of electricity or heat and
electricity from geothermal energy.
Exploitation schemes. To unlock the underground successfully different options
are available which meet the needs of the respective geological conditions. Within
the following explanations different appropriate exploitation schemes are de-
scribed.
Geothermal fields. Most of the electricity produced from geothermal energy to-
day, comes from vapour-dominated fields. Such geothermal fields are the most
easy to exploit. A more or less dense array of vertical production wells is drilled
in these areas and the vapour produced from these boreholes is, after removing the
liquid phase, transported by insulated pipes above surface to a power station. The
liquid phase, and the condensed geothermal steam from the power station are in
many cases disposed of in surface waters. This caused environmental problems in
some areas, and have led to a continuous decrease of the reservoir pressure, for
instance in the geothermal fields of "The Geysers" and at Lardarello. For this rea-
son the operators started to drill re-injection wells and to re-inject the geothermal
fluids and/or make-up water (in case of Lardarello sea water) into the reservoir.
This measure not only stopped the decrease of steam production observed before,
but led to a continuous recovery of the production flow and geothermal power
production.
The exploitation scheme in liquid-dominated fields is essentially the same.
Similar to the vapour-dominated fields, the geothermal fluid is in most cases not
actively produced by down-hole production pumps but by the buoyancy effect
originating from the boiling of the geothermal fluid in the production wells. In
liquid dominated fields, re-injection of the geothermal fluid is even more desired
because of the higher mineral content of the produced fluids.