The word Geothermal comes from the Greek words: Geo = Earth; Thermal = Heat
Thus, the energy that can be extracted from the heat in the earth is geothermal energy.
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In the earth’s core, about 4000 miles below the surface of earth, geothermal energy is continuously produced by the slow decay of radioactive particles. The regions where the earth’s tectonic plates collide and one slides beneath another create the conditions which are most favourable for the geothermal activity.
The geothermal heat gets up to earth’s surface by following mechanisms:
In some regions, the mantle beneath the crust may be hot enough to partly melt and create magma. Magma rising upward out of the mantle (convection) can bring intense shallow heat into the crust.
Through pores and crevices in the crust, rain water seeps down to depths of a mile or more and gets heated. The heated water may be stored at depth in geothermal reservoirs, or the hot water may flow upwards out the reservoir to the surface as hot springs, or boil near the surface to create geysers, mudpots, and fumeroles.
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In the past people have used geothermal energy for following purposes
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(i) Bathing Hot springs were used by ancient civilizations for bathing,
(ii) Heating Geothermal energy was used by early Romans to heat their homes.
(iii) Cooking Geothermal water was used for cooking by Native Americans.
(iv) Medical Therapy Geothermal water was used by early Romans to treat eye and skin disease.
The present uses of geothermal energy :
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Electricity generation, space heating/cooling, Greenhouses, Aquaculture, Drying of fruits/ vegetables, and Industrial uses like manufacture of paper, washing wool, drying of cloth, etc
Electricity Generation
(i) Dry steam Plants:
They use underground steam to directly turn the turbines
(ii) Flash steam Plants:
These plants pull deep, high pressured hot water (T = 360° F) to the surface. This hot water is transported to low pressure chambers and the resulting steam drives the turbines. The remaining water and steam are then injected back into the source from which they were taken.