Types of geothermal power plant

A geothermal power plant uses hot water or steam from underground reservoirs to drive a turbine and generator. There are three main types.

1. Dry steam plant

• Uses natural dry steam (> 235 °C) directly from the production well.
• The steam passes through a turbine, then is condensed and re-injected.
• Simplest and oldest design; rare resource (The Geysers, California; Larderello, Italy).

 Steam well -> Turbine -> Generator
                |
            Condenser -> Injection well

2. Flash steam plant

• Most common type. High-pressure hot water (> 180 °C) from the well is released into a low-pressure flash tank, where part of it "flashes" into steam.
• The steam runs the turbine; the remaining water is re-injected (or flashed again in double flash).

 Hot water well -> Flash tank -> steam -> Turbine -> Gen
                        |water              |
                        +-> Injection <- Condenser

3. Binary cycle plant

• Uses moderate-temperature water (100–180 °C).
• The hot geothermal water passes through a heat exchanger and heats a secondary working fluid with a low boiling point (isobutane, isopentane, or an organic fluid in an organic Rankine cycle). Its vapour drives the turbine.
• The geothermal water never contacts the turbine and is fully re-injected, giving almost zero emission; works with lower-temperature resources, so it has the widest potential.

 Geothermal water -> [Heat exchanger] -> Injection
                       | vapour of working fluid
                    Turbine -> Gen -> Condenser -> pump

Other types are enhanced geothermal systems (EGS, hot dry rock) where water is injected into hot rock fractures. For Nepal, low and medium temperature resources (hot springs) make binary cycle plants the most suitable option, although none is operating yet.