1.3        Geographical assessment

Geothermal reservoirs are basically distinguished as active and passive zones with respect to their thermal potential. Active zones (New Zealand, California, Hawaii etc.) have high potentials for energy utilization.

 

The presence of active zones is explained by the volcanic and tectonic activities of the crust. The crust is made up of huge plates, which are in constant but very slow motion. In general geothermal reservoirs occur where magma rises up through the plates of the crust. Magma can move near the surface in three main geologic areas:

 

 

 

1. where large oceanic and crustal plates meet and collide, one sliding under the other to form a subduction zone (e.g. the Nazca and South American plates),

2. around rift boundaries (e.g. the North American and Eurasian plates), where plates move apart,

3. on volcanic strings forming hot spots that produce magma in the mantle, thus generating exceptionally intense heat flux.

The best example of a subduction zone is the Ring of Fire. This region clustering around the Pacific Ocean plate boundary is the most abundant in geothermal energy: the South American Andes, Central America, Mexico, the Cascade Range of the U.S. and Canada, the Aleutian Range of Alaska, the Kamchatka Peninsula of Russia, Japan, the Philippines, Indonesia and New Zealand.

Other areas, such as Iceland, the rift valleys of Africa, the mid-Atlantic Ridge and Basin, as well the Range Province in the U.S., lie on rift boundaries, whilst the chain of the Hawaiian Islands is an example of hot spots. In Europe, Italy, Greece and Iceland could be mentioned as examples, as they are volcanically the most active countries.

Since it is high temperature reservoirs (over 150°C) that are suitable for the commercial production of electricity, geothermal power plants are most often constructed where hydrothermal energy appears in the form of steam. In the recent past researches have explored more and more sites suitable for steam production, which has led to an increase in the number of geothermal power plants, first of all in Iceland, Japan, USA, New Zealand and Italy.

Source: Balogh-Völgyes

In more than 35 countries geothermal resources provide a heat capacity of 12000 MW for direct use and an electric power generation capacity of over 8200 MW. In Europe heat capacity has tripled since 1980. The majority of Europe’s capacity is provided by Dogger Limestone stratum, which is used for the district heating of Paris.

The chief producer of the world is the United States, with a 44.4% share. The second and third largest producers are Mexico and Italy.