The energy conversion in geothermal power plants is based on the Rankine cycle. The basic layout and T-s diagram for a vapour-dominated geothermal power plant are shown in Fig. 7.1 and 7.2 respectively. These systems are most suitable for electricity generation with lowest cost and the least number of serious problems. But these are the rarest form of geothermal systems. Examples are Geysers in USA, Larderello in Italy and Matsukawa in Japan.
Dry steam at 35 bar and 200°C is available at the bottom of the well gets superheated due to pressure drop to 7 bar. The particulate matter is separated in a centrifugal separator. The process 6-7 and 7-1 are throttling processes so that enthalpy of steam remains constant. After passing through steam turbine and condenser, it is re-injected back to earth.
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H2S Removal:
The recent environmental regulation, the toxic and noxious H2S is removed in the shell-and-tube condenser by a process called Stratford process. 90% H2S is removed and commercial-grade sulphur is produced as a by-product.
Example:
A 100 MW vapour-dominated, system uses saturated steam from a well with a shut-off pressure of 28 bars. Steam enters the turbine at 5.5 bars and condenses at 0.15 bars. The turbine poly tropic efficiency is 0.82 and the turbine-generator combined mechanical efficiency is 0.9. The cooling tower exist is at 20°C. Calculate the necessary steam flow, the cooling water flow and the plant efficiency and heat rate if reinjection occurs prior to cooling tower.
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Solution:
