Utilisation of Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 482

Separator 2 HD-Turbine/Generator

Flash vessel 2

High
pressure
turbine

ND-Turbine/Generator Kühlturm

Low pressure
turbine

Cooling tower

Separator 1

Flash
vessel 1

G G

Generator

Gen er at or

Condenser

Sepa-
ration
of gases

Condenser

Gasab-
scheidung

Kondensator 1 Kondensator 2

Feinfilter Filter

Production well Injection well

Förderbohrung Injektionsbohrung

Fig. 10.17 Simplified schematic diagram of a geothermal power production facility operat-
ing according to the double flash technology

Closed systems. The term closed system refers to geothermal power plants that do
not directly exploit the geothermal resources, and thus the extracted steam for
power generation by a pressure reduction within a turbine, but rather apply a sec-
ondary medium. Within such closed systems the geothermal energy is transferred
to a secondary medium via appropriate heat exchangers (evaporators). Due to the
low temperature of the geothermal fluid or fluid-steam mixture in general, the
secondary medium has to be characterised by a low boiling temperature.
Under the described circumstances the Rankine process, using organic working
fluid, or the Kalina process are most suitable. They will be discussed within the
following sections.
Such cycles are applied if the primary medium is not hot enough or if its pres-
sure is too low to allow for the generation of the required pressure parameters for
a thermodynamically wise pressure reduction. Furthermore, the application of a
second working fluid is sensible if the hot water produced from the underground
is characterised by unfavourable chemical properties (such as mineralisation, gas
content, etc.), which can either not be directly controlled, or only at unreasonably
high costs.

Organic Rankine Cycle. Except for the used working fluid, and thus the realised
temperature and pressure parameters, the Organic Rankine Cycle (ORC) only
differs slightly from the classic Rankine process based on steam realised in nu-
merous conventional power plants.
As with common power plants cycles, the working fluid is preheated (in this
case with the geothermal energy provided by the geothermal fluid), evaporated,
and relieved by means of a turbine. It is then cooled by a recuperative heat ex-