Utilisation of Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 485

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vapour is produced and a fluid that is weak in ammonia remains. The vapour is
then transported into a turbine and is released producing work. Afterwards, the
remaining vapour and ammonia-weak fluid are again mixed and conveyed to-
wards the condenser to liquefy the compound mixture. With the help of a pump,
the liquid is then brought to evaporation pressure. To improve energetic effi-
ciency, recuperators are installed within the cycle. One of them is shown in Fig.
10.20 between the hot ammonia-weak working fluid and the cold basic working
fluid.
For a geothermal fluid temperature of approximately 80 °C the achievable effi-
ciency amounts to about 8.5 %, and to 12 % for a geothermal fluid temperature of
approximately 160 °C. Hence, resource exploitation rates between 500 (t/h)/MW
and 70 (t/h)/MW, respectively (Fig. 10.15), are reached. However, these compara-
tively high efficiencies have only rarely been achieved by plant operation because
the Kalina cycle is still under development, and there are very few demonstration
plants under operation so far.
The big advantage of this cycle is that evaporation and condensation are not re-
alised isothermally, as for pure substances (as used within ORC process). In fact,
due to the mixture of two components a sliding temperature transition during
evaporation and condensation is possible.
− The temperature curve of the geothermal fluid extracted from the underground
(e.g. a temperature reduction from 150 to 85 °C), and the working fluid used
within the Kalina cycle (a rise in temperature from 75 to 145 °C, provided that
the water/ammonia solution is mixed to meet these characteristics) can thus be
ideally adapted to one another. This helps to reduce the average temperature
between both mass streams, and thus the heat transfer losses.
− Compared to the application of pure substances the average evaporation tem-
perature is increased, whereas average condensation temperature is reduced.
This is improving the Carnot efficiency of the cycle (i.e. theoretical maximum
efficiency).
Besides energetic advantages this process also offers civil engineering benefits.
Since ammonia and water have similar pressure release properties steam turbines
can be applied. Additionally, ammonia/water mixtures have for quite some time
been used on a large scale for other technical purposes (such as refrigeration). It
should thus be possible to handle the ammonia/water solution within a large-scale
cycle without any substantial technical problems. However, the low temperature
differences within the heat exchangers and the poor heat transmission properties
require much bigger equipment compared to conventional power plants based on
a steam cycle.
Combined systems. Here, different processes are combined according to the site-
specific circumstances; for instance, a single flash process is combined with a
binary process. In general, various cycles are possible. For example, once the
steam has passed the steam turbine and pressure is reduced, it can serve as a heat