• Direct dry steam: The conversion plant of this technology is a steam turbine engine that directly uses steam extracted from underground reservoirs. The steam, which has to be at least 99.995% dry to avoid scaling and erosion of the pipes and the turbine (DiPippo 2016[1]), is directly piped to the power plant, turning the turbine and the generator. The steam is, accordingly, condensed and reinjected to the reservoir via another well. To date, direct dry steam plants are quite rare and favourable cases with typical examples Larderello in Italy and the Geysers in northern California. Typical sizes of direct dry steam plants range from 8-140 MW. This is a very rare and favourable situation, as reported by (DiPippo 2016[1]).
  • Flash-steam power plants are the most common type of geothermal plants in operation today and they involve the extraction of steam from the geothermal fluid following a separation process (flashing). The steam is subsequently directed to the steam turbine, which is connected to an electricity generator. The condensate is forwarded either for further flashing to lower pressures and temperatures, or reinjected into the underground reservoir. The reinjection has two advantages: (i) the reinjected liquid maintains the pressure in the reservoir and (ii) avoids land subsidence especially in the case of a shallow reservoir. Flash-steam power plants work best with high-enthalpy resources. The fluid fraction that exits the condenser is either reinjected, if not evaporated, through a wet cooling tower. Typical sizes of flash steam plants range from 0.2-150 MW plants according to the number of flashing processes.
  • Binary cycle power plants are usually appropriate for low and medium enthalpy geothermal fields, where the heat content of the geothermal fluid can be exploited to heat a working fluid via heat exchangers in a closed loop. The working (or else “binary”) fluid is typically an organic compound (often isopentane) with a low boiling point (Mannvit hf 2013[2]). Heat from the geothermal fluid – most existing binary plants recover heat of geothermal fluid in the range of 100–200 C (Tomarov and Shipkov 2017[3]) causes the working fluid to vapour and accordingly turns the turbines and in extension the generators. Binary power plants are closed-loop systems, mitigating drastically the emissions to the atmosphere.

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[1] DiPippo, Ronald. 2016. Geothermal Power Plants. 4th: Elsevier.

[2] Mannvit hf. 2013. Environmental Study on Geothermal Power GEOELEC Project – WP4 D4.2.

[3] Tomarov, G. V., and A. A. Shipkov. 2017. “Modern Geothermal Power: Binary Cycle Geothermal Power Plants.” Thermal Engineering 64 (4): 243–50. https://doi.org/10.1134/S0040601517040097.