世界における地熱開発の趨勢

抄録

Geothermal energy has becoming rapidly perceived as a viable alternative energy source available with the current technology as well as solar energy. For a long time, geothermal energy has been utilized in various forms. High temperature steam and hot water have been mostly used for an electric power generation. World-wide today, total power of geothermally fired electric generating plant amounts to approximately 1, 100 MW. On the other hand, intermediate and low temperature geothermal fluids also have been consumed for space-heating and industrial processes in certain locations of the world.
However, the full potential of electric and non-electric application of geothermal resources has not yet been realized, and only a small part of geothermal energy stored within the earth's crust has been tapped. Remaining enormous amounts of natural heat will be made available with the further exploration and technological research and development.
Under present technology, use of geothermal energy for electric power generation is limited to the hydrothermal convection system having the temperature above 180°C. Among this, energy of natural steam has best quality, but the existence of this type of energy may be expected within only a part of whole high temperature convection system. Other hydrothermal convection systems are much more abundant geothermal resources.
Research and development of technology for the efficient utilization of hydrothermal system having temperature below 180°C as well as hot water system (above 180°C) are needed. Especially, development of technology of electric power generation from intermediate temperature (90°C-180°C) geothermal fluid and multiple use of this type of geothermal resource must be enforced.
Geothermal deposites in sedimentary basin are also potentially valuable geothermal resources which contain enormous amounts heat. In a high heat-flow area, deep-seated layered strata produce hot-water having intermediate temperature above 90°C, but, in a normal heat-flow area, water-bearing strata at depths of more than 2 km show temperature above 60°C, which is hot enough for space-heating and other non-electric use. In former case, reservoirs in subsiding sedimentary basin sometimes show abnormally high fluid pressure, and said to be geopressured.
Hot dry rock deposit without associated hydrothermal system is expected as other potential geothermal deposit and may become an attractive geothermal resource if technology extracting the stored heat is developped. Research and development of exploitation technology for this type of deposit are now world widely under way.