Measurements show that the ground temperature below a certain depth remains relatively constant throughout the year. This is because the temperature fluctuations at the surface of the ground are diminished as the depth of the ground increases due to the high thermal inertia of the soil.
There are different geothermal energy sources. They may be classified in terms of the measured temperature as low (<100°C), medium (100-150°C), and high temperature (>150°C). The thermal gradient in the earth varies between 15 and 75°C per km depth; nevertheless, the heat flux is anomalous in different continental areas. The cost of electrical energy is generally competitive, 0.6-2.8 U.S. cents/MJ (2-10 U.S. cents/kWh), and 0.3%, or 177.5 billion MJ/a (49.3 billion kWh/a), of the world total electrical energy was generated in the year 2000 from geothermal resources (Baldacci et al., 1998).
Geothermal power based on current hydrothermal technology can be locally significant in those parts of the world where there are favorable resources. About 6 GWe of geothermal power were produced in the early 1990s and 15 GWe may be added during the next decade. If hot dry rock geothermal technology is successfully developed, the global geothermal potential will be much larger.
Deep geothermal heat plants operate with one- or two-hole systems. The high expenditure incurred in drilling holes discourages one from using this method in gaining thermal energy. The one-hole injection system or the use of existing single holes, made during crude oil or natural gas exploration, reduces the capital cost. In one-hole systems, the hole is adapted to locate in it a vertical exchanger with a double-pipe heat exchanger, in which the geothermal water is extracted via the inside pipe. Published characteristics allow the estimation of the gained geothermal heat energy flux as a function of the difference between the temperatures of extracted and injected water at different volume fluxes of the geothermal water. In general, the two-layer systems and two-hole systems are more advantageous than one-hole systems. More details of geothermal systems related to desalination are given in Chapter 8.
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