Like all conventional thermal power plants, a geothermal plant uses a heat source to expand a liquid to vapour/steam. This high pressure vapour/steam is used to mechanically turn a turbine-generator. At a geothermal plant, fuel is geothermal water heated naturally in the earth, so no burning of fuel is required.
At many power plants, a steam turbine is used to convert the thermal energy extracted from pressurised steam into useful mechanical energy. Mechanical energy is then converted into electricity by the generator. Geothermal plants rely upon one or a combination of three types of conversion technology— binary, steam, and flash—to utilise the thermal energy from the hot subsurface fluids and produce electricity.
Each of these processes is described in greater detail in the next section of this chapter, ‘conversion technologies’.
After the thermal energy has been used to turn the turbine, spent steam is condensed back to a liquid and injected into the ground where it is reused in the geothermal system, prolonging the lifetime of a geothermal plant. Electricity is then transported by transmission lines into the regional grid.