Environmental Engineering Reference
In-Depth Information
A central heat system controls the amount of output depending on the heat require-
ment. If heat requirements are particularly high, a peak-load boiler can cover the
heat peaks. A backup boiler is also helpful to guarantee reliable heat supply in case
problems occur with the extraction pump or the well.
10.2.2 Geothermal Power Plants
Using geothermal energy to generate electricity is somewhat more complex than
providing thermal heat. For one thing, the relatively low temperatures that power
plant technology requires for thermal energy present new concepts such as:
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Direct steam use
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Flash power plants
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ORC (Organic Rankine Cycle) power plants
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Kalina power plants
Normal steam turbine systems can be used in geothermally optimal locations with
temperatures between 200 °C and 300 °C. If hot steam deposits exist underground,
they can be used directly to drive the turbines.
If hot thermal water is under pressure, it can be evaporated through an expansion
stage. The steam from the water that is still hot can in turn be transferred directly
to a steam turbine. This technique is called a fl ash process.
At temperatures of 100 °C or less geothermal water is not hot enough to vaporize
water. A normal steam turbine using water as the work medium is not suitable in
this case. An ORC (Organic Rankine Cycle) system is used instead (Figure 10.7).
Figure 10.7
Principle of a geothermal ORC system.
A steam turbine also forms the core of this kind of system. However, instead of
water, the steam turbine uses an organic material such as Isopentan or PF5050. A
heat exchanger transfers the heat from the geothermal cycle to the organic working
fl uid. This material also evaporates under high pressure at temperatures lower than
