Directsteam power plant

Dry-steam geothermal reservoirs are extremely rare. Where they exist the steam, with a temperature of 180°C to 350°C, can be extracted from the reservoir through a borehole and fed directly into a steam turbine. Steam from several wells will normally be fed to a single turbine. The pipes which carry the steam from the wellheads to the turbine contain various filters to remove particles of rock and any steam which condenses en route.

The steam turbine in a direct-steam geothermal plant is usually a standard reaction turbine. Efficiency is low at around 30%. Unit size in modern plants is typically between 20 and 120 MW. In some cases the steam exiting the turbine may be released directly into the atmosphere. However the steam usually contains between 2% and 10% of other gases such as carbon dioxide and hydrogen sulphide. Under these circumstances the exhaust from the steam turbine must be condensed to remove the water and then treated to remove any pollutants such as hydrogen sulphide before release into the atmosphere. At the Geysers plant in the USA, sulphur is produced as a by-product of the treatment. Condensing the steam also extracts more energy, so increasing plant efficiency.

Generator Air and water vapour

Production well

Injection well

Figure 12.1 A direct-steam geothermal power plant

Generator Air and water vapour

Condenser | | Cooling

Production well

Injection well

Figure 12.1 A direct-steam geothermal power plant

Ideally the geothermal fluid should be returned to the underground reservoir but it is often more economical to release the gas and dispose of the water produced as a result of condensing the steam from the turbine at the surface. Continual removal of fluid without replenishment eventually leads to a depletion in the quality of fluid available from the reservoir. At the Geysers geothermal field in southern California, urban wastewater is now being pumped into the underground reservoir in an attempt to maintain and eventually boost output from the resource.

Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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