Understanding deep geothermal energy
Understanding deep geothermal energy

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Understanding deep geothermal energy

5 Geothermal power plants

A typical geothermal power plant has a power output around 30-50 MW compared to several GW from conventional oil- or coal-fired plants. Therefore, between 20-33 geothermal stations are needed to replace a 1 GW conventional power station. However, once installed, geothermal operating costs are low.

Electricity generation using steam from geothermal fields is essentially the same as in fossil-fuel and nuclear power plants, with various modifications to suit the enthalpy of the field. In high-enthalpy fields, such as Larderello in Italy and the Geysers Steam Field in California, the reservoir contains large volumes of superheated water at pressures up to 30 times atmospheric pressure, similar to that used in a conventional thermal power plant. When the fluid flashes to vapour the steam escapes at speeds up to 300 m s−1 (comparable with a jet engine at full throttle) and is suitable for electricity generation using the simplest turbines (Figure 8a) from which steam is vented to the atmosphere. The efficiency can be further increased if the steam is condensed behind the turbine (Figure 8b) as this increases the pressure gradient and so increases the speed of the steam. The 'spent' water will still be very hot, and can be re-injected into the geothermal aquifer system to maintain the volume and pressure of the geothermal fluid.

Figure 8
Figure 8 Types of geothermal turbine. (a) Simple turbine used in dry steam fields that vents 'spent' steam to the atmosphere. (b) Turbine using a steam condenser behind the turbine to increase the pressure difference and therefore the steam speed through it.

Lower enthalpy fields produce fluid that flashes to steam close to the well-head or within the plant itself at commensurately lower pressures and speeds, thereby reducing power output and efficiency. To maintain efficiency requires more complex turbines and heat-management schemes. At still lower enthalpy, conventional steam turbines become less efficient.

To increase the contribution of geothermal energy to electrical power supplies, development needs to focus along two lines. One option is developing lower temperature fields. Instead of using steam to drive turbines, hot water vaporizes a more volatile fluid, typically a light hydrocarbon. The other aims at harnessing the energy of fields in which fluids are at very high temperatures and pressures, including HDR fields. However, drilling technology has a depth limit, due to decreasing strength of the drill stem and decomposition of lubricants for the drill's cutting bit at very high temperatures.

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