The centre of the Earth is around 6000 degrees Celsius; easily hot enough to melt rock. Even a few kilometres down, the temperature can be over 250 degrees Celsius if the Earth's crust is thin. In general, the temperature rises one degree Celsius for every 30 - 50 metres you go down, but this does vary depending on location. In volcanic areas, molten rock can be very close to the surface making it possible to use the heat energy.
Geothermal energy has been used for thousands of years in some locations for cooking and heating. The name "geothermal" comes from two Greek words: "geo" meaning "Earth" and "thermal" meaning "heat".
Types of Geothermal energy sources include:
- Volcanic Areas: Several types of rock contain radioactive substances such as uranium. Radioactive decay of these substances releases heat energy, which warms up the rocks. In volcanic areas, the rocks may heat water so that it rises to the surface naturally as hot water and steam. Here the steam can be used to drive turbines and electricity generators. This type of geothermal power station exists in a number of places including Iceland, California and Italy.
- Hot Rocks:In some places, the rocks are hot, but no hot water or steam rises to the surface. In this situation, deep wells can be drilled down to the hot rocks and cold water pumped in . The water runs through fractures in the rocks and is heated up, it returns to the surface as hot water and steam, where its energy can be used to drive turbines and electricity generators. The diagram below shows how the latter works.
Geothermal energy is a renewable energy resource and there are no fuel costs. No harmful polluting gases are produced. However, most parts of the world do not have suitable areas where geothermal energy can be exploited.