Utilisation of Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 480

duced. This is the only difference compared to flash systems, because the flash
vaporisation, and thus the energetically unfavourable pressure decrease, are not
required where direct steam utilisation is possible.
Single flash process without condensation. The pressure of the produced hot water
or water-steam mixture from the underground is fed into a flash container, where
the pressure is slightly reduced. By this measure, the portion of the steam is in-
creased. Afterwards the gaseous and liquid phases are separated from each other.
While in most cases the separated liquid is re-injected into the underground, the
produced steam is fed into the turbine to produce work. Parallel to this, the pres-
sure is reduced to atmospheric conditions and the steam is subsequently released
to the atmosphere via a diffuser.
Due to pressure reduction to atmospheric pressure, the energetic efficiency of
such systems is very unfavourable in general. Nevertheless, such geothermal
power plants are very cost-efficient since condensers and cooling towers are not
required.
Single flash processes are thus sensible if high thermal fluid volume flows can
be achieved or if thermal fluid temperatures are high (or if a high portion of non-
condensable gases cannot be otherwise controlled).
Single flash process with condensation. Fig. 10.16 illustrates the principle of a
single flash process with condensation.
The boiling liquid produced from the underground is thus evacuated into the
flash vessel. Under a pressure level below the pressure level of the production
well, a small quantity of dry saturated steam and a large quantity of boiling water
is produced. Once the steam has been separated from the liquid, it is transported to
the turbine where it performs work.
The cooling parameters of the condenser determine the minimum final pressure
of the steam released, which in most cases is clearly within the vacuum area.
Whether such a low pressure level can be utilised depends on the cost of evacuat-
ing non-condensable gases from the condenser.
At a temperature of approximately 160 °C the maximum resource exploitation
amounts to 150 (t/h)/MW and to 50 (t/h)/MW at a geothermal fluid temperature of
approximately 240 °C (Fig. 10.15).
Besides the technical and possibly environmental problem of releasing incon-
densable gases into the atmosphere, single flash processes are often characterised
by the disadvantage of producing solid precipitations during flashing. Such pre-
cipitations remain as coatings on the equipment and have a detrimental effect on
safety, or need to be removed and disposed.
A major energy portion of the geothermal fluid produced originally from the
underground (i.e. the fluid that has been separated within the flash vessel and re-
transferred to the underground) is not used for energetic purposes. This is the
reason why the utilisation ratio of the entire system is relatively low.