Figure 3. Dry steam power plant
Hot water Cold water Hot gas/steam Air/cold vapour Current
Transition
Condenser/heat
Turbine Generator Condenser Cooling tower
Main outlet power cable
In the heat exchanger heat from the water transfers to a secondary or working fluid such as isobutane, which has a lower boiling point than water. This expands into a vapour, and the resulting force drives the turbine. The vapour then returns to liquid form in a condenser and passes to the heat exchanger to begin its next circuit.
Production well
Geothermal reservoir Figure 4. Flash steam power plant
Hot water Cold water Hot gas/steam Air/cold vapour Current
Transition
Condenser/heat
Turbine Generator Condenser
Heat exchanger Cooling tower
Main outlet power cable
Injection well
• Flash/binary combined cycle power plant (Figure 6). Water at high pressure and high temperature from the geothermal well is fed into a separator, where lower pressure causes some of it to flash to steam. The steam drives the plant’s level I turbine. The remaining water passes to a preheater where it heats the secondary working fluid in a binary system. The working fluid then enters a vaporiser where it is further heated by the steam leaving the level I turbine. The vaporised working fluid drives the level II turbine.
Water pipe
Production well
Geothermal reservoir
Injection well
Figure 5. Binary cycle power plant
Hot water Cold water Hot gas/steam Air/cold vapour Current
Transition
Condenser/heat
Turbine Generator Condenser Separator Cooling tower
Direct heat use
Main outlet power cable
Geothermal power plants generate consistently and can deliver baseload power. They also have a high capacity factor. Globally, geothermal has a mean capacity factor of 74%, which compares favourably with other renewables. The figure for biomass is 55%, hydropower 43% and solar 11%. The equivalent figure for nuclear is 79%, and for fossil fuels 46%.
Geothermal power is highly scalable, and facilities require relatively little space compared to other types of power plant. This is illustrated in Table 1, which shows the footprint in square kilometers required to produce 1 GWh of electricity from different energy sources.
Production well
Geothermal reservoir Figure 6. Flash/binary combined cycle power plant
Hot water Cold water Hot gas/steam
Air/cold vapour Current
Motive vapour
Non condensible gas Transition
Condenser/heat
Turbine Generator Turbine 2 Condenser Vaporiser Preheater Separator
Table 1. Footprint (km2 ) required to
generate 1 GWh, for various energy sources. Geothermal energy is very space efficient
Production well
Geothermal reservoir
Injection well
Type of plant Geothermal Wind PV
Coal
Space requirement (km2
/GWh)
1046 3458 8384 9433
Main outlet power cable
Injection well
Geothermal power generation is generally clean with the only emission being water vapour. However, in some cases geothermal systems can emit small amounts of the greenhouse gases hydrogen sulphide and carbon dioxide, as well as very small amounts of sulphur dioxide, nitrous oxides, and particulates. There is also a risk that water flowing through underground reservoirs can become contaminated with trace amounts of elements such as arsenic, mercury, and selenium. If the geothermal system is not isolated effectively, these elements can leak into water sources.
Equipment for geothermal power plant
Large motors and generators play a variety of roles across the geothermal value chain.
| Geothermal power
www.modernpowersystems.com | March 2024 | 31
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