Geothermal energy extraction involves drilling wells into hot rocks that lie close to the Earth’s surface. Given the right geology, i.e. permeable rocks that contain very hot water, the hot water or steam flows to the surface under its own steam (to coin a phrase). Temperatures are typically in the range 200-350˚C, and at surface pressure, this super-heated water will flash to steam that may drive a turbine. The cooled “waste water” is then returned to the geothermal reservoir where it heats up again given time.
Geothermal therefore involves drilling wells, very much like the oil and gas industry and managing very hot water under pressure.
Geothermal applications are confined to areas of the world where hot water filled rocks lie close to the surface and that invariably means areas with active volcanism (USA, Philippines, Indonesia, Mexico, Italy, New Zealand, Iceland, Japan) or at least high heat flow linked to plate tectonics (Iran, Turkey, China, France).
[Inset image: Hellisheidi Geothermal Power Plant, Iceland (123 MW). Iceland exists because a hot spot on the North Atlantic mid-ocean-ridge, a rather unique setting.]
- drilling into hot rocks and confining the excess pressure is inherently dangerous but drilling companies seem by and large to be able to mange these risks
- This study found no excess mortality among workers at a geothermal plant in Italy.
- Geothermal energy does release some nasty gases like CO2 and H2S. But since the plants are normally located in active volcanic areas, they are also sparsely populated. The study linked above found no adverse effects among workers.
External environmental costs
- The release of gas mentioned above is small. Beyond that, the footprint of geothermal is relatively small compared with the power produced.
Footprint of energy system per unit of energy produced
- associated with small power stations located at the well head
- associated with access roads
- associated with cables / power distribution network
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