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   Our earth's interior - like the sun - provides heat energy from nature. This so-called geothermal energy means the use of the earth's steam and hot water reservoirs in order to produce energy. The energy source either can be used directly as heat source (i.e. near-surface geothermal energy) or it is used to power turbines for the production of electricity (i.e. deep-seated geothermal energy). At current, around 300 geothermal power plants worldwide with a capacity of 8,400 MW produce geothermal electricity. The most important users of this source of energy are the United States, the Philippines, Indonesia, Iceland, and Mexico.
Regarding deep-seated geothermal energy supplies, geologists distinguish between two different processes to produce electrical power:
- Hydro-Geothermal Process
It implies the search for hot water as deep as 5,000 metres which is brought up to the surface. The geothermal water then is passed through a heat exchanger where its heat is being extracted and converted. The most established ways are the so-called ORC-Technique as well as the Kalina-Technique, most recently.
- Hot-Dry-Rock Process (HDR)
It uses dry and hot rock formations as deep as 7,000 metres. Water from the surface is being turned down an injection well into hot rock formations with great pressure, thus creating gaps and fissures and subsequently heating up within these niches. As per a second drilling, the extremely hot water is being brought up to the surface then where its heat is being extracted via a heat exchanger.
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