Utilisation of Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 483

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changer (compared to relieved steam this medium is still superheated), condensed
and lifted again to evaporator pressure by means of a pump. The corresponding
schematic diagram is illustrated in Fig. 10.18.

Cooling
tower

Kühlturm

Turbine

Gene-
rator

G

Verdampfer

Evaporat or

Condenser

Kondensator

Vor-
wärmer

Econo-
miser

Grobfilter Feinfilter

Filter

Filter

Fig. 10.18 Simplified schematic diagram of a geothermal power production facility operat-
ing according to the ORC (Organic Rankine Cycle)

Production well Injection well

Ijk i bh

Within the temperature range of fluids extracted from geothermal wells, usu-
ally hydrocarbons (such as n-pentane, isobutane) are applied as working fluids. In
the past, also fluorocarbons (such as C 5 F 12 ) have been used. Furthermore, the
application of mixtures of hydrocarbons is under consideration, as they promise to
enhance efficiencies due to their smooth evaporation temperatures.
In order to apply working fluids of organic origin the plant design needs to be
adapted accordingly. The turbines differ from those used for water vapour due to
the different molecular weight and the lower specific heat capacity. Furthermore,
precautions have to be taken with regard to the higher corrosion of turbines and
heat exchangers as well as for sealing the system with regard to the atmosphere.
Fig. 10.19 shows the power generation efficiencies of selected ORC-plants
and, additionally, an average efficiency curve /10-24/; according to this illustra-
tion average efficiencies amount to between 5.5 % for a geothermal fluid tempera-
ture of approximately 80 °C and 12 % for a geothermal fluid temperature of ap-
proximately 180 °C. This corresponds to a resource exploitation exceeding
500 (t/h)/MW at 80 °C and 80 (t/h)/MW at 180 °C (Fig. 10.15).
Up to a temperature of the geothermal resource of about 135 °C net power gen-
eration efficiencies are below 10 %. At the upper limit of the temperature range
under consideration (200 °C), efficiencies amount to between 13 to 14 %, pro-
vided that the heat content of the geothermal fluid is exploited exhaustively and
that the desired cooling temperature is reached.