Geothermal Electricity

Geothermal electricity is the process of converting the geothermal energy into electrical energy.  Some of the technologies used for generating geothermal electricity are flash steam power plants, binary cycle power plants and dry steam power plants.  According to one authority, geothermal represents 42% of the electricity produced by the combination of wind, solar, tidal and geothermal. Electricity generation mainly involves conventional steam turbines which operate at a minimum of 150C.

Geothermal electricity is used in twenty four countries and the geothermal heating is used in seventy countries. Heat contained within the planet causes macrogeological events like earthquakes, volca­noes and tectonic movement. Geothermal energy in the context refers to that small fraction of the Earth's heat which can be converted to useful energy. Most of this heat is generated by decaying radioactive isotopes within the Earth's mantle.

The rate of increase of temperature according to depth in the ground is called the 'geo­thermal gradient' and averages 2.5 to 3.0°C per 100m of depth. Modern drilling techniques can penetrate up to 10 km.

Where there are active geothermal areas this gradient can increase by a factor of ten, pro­ducing temperatures above 300°C at 500 to 1000m. This occurs where there is an upward intrusion of high-temperature rocks from the magma bell. In such circumstances a tempera­ture of around 600°C can be expected at depths from 5 to 10 km. This would provide high-pressure steam. However, useful geothermal energy is available at the normal geothermal gradient.

This heat has to be brought to the surface. Geothermal springs do this spontaneously. More often, water has to be injected into the hot; permeable rocks known as the 'thermal reservoir', where it circulates, and absorbing heat in the process. If there are several geothermal wells in vicinity, this is described as a 'thermal field'.

An advantage of geothermal energy is that it is independent of climate or seasonal/diur­nal variation. The capacity factor of geothermal plants is often in excess of 90%, producing energy at a price which is lower than most other renewable technologies.

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