This is used where dry steam (unaccompanied by liquid water) at a temperature of up to 350°C and a pressure of eight to nine atmospheres is found. A steam turbine is used, with steam expanding and cooling as it passes through the turbine. The low – pressure steam can then be vented to the atmosphere (in a ‘back-pressure’ turbine) or condensed (in a ‘condensing’ turbine) (see Figure 5.5 a). Modern plants require about 6.5kg of steam per kWh of electricity generated, or 100kg/steam for a 55MW plant (Brown, 1996). The best thermal efficiency achieved is around 30 per cent.
Single-flash steam powerplant
Where the geothermal resource consists of a mixture of steam and hot liquid water, the steam and liquid water must be separated. If the resource consists of hot water, one wishes to avoid flashing of the water into steam as it rises (so as to avoid deposition of minerals that will eventually clog the well), and this is done by pressurizing the well. Steam is produced at the surface in a separator and fed into a conventional turbine. The steam temperature is typically 155-165°C, so more steam is required than in the dry steam powerplant (around 8kg/kWh). Most of the fluid remains as unflashed brine, which is either re-injected into the aquifer or used in a local district heating system (see Figure 5.5b).