| 1.GENERAL
the type of energy stored in such elements. The high temperature observed in deeper substrates of the planet is attributed to the decay of radioactive isotopes of the elements Uranium, Thorium and Potassium, as well as the earth’s field of gravity. Vast amounts of thermal energy are stored inside the Earth. Nevertheless, exploitation of such energy is financially viable only where local geological, hydrological and geophysical conditions permit the development of a geothermal system. A geothermal system consists of ground-water circulating at depths reaching up to a few kilometers; there it is heated through contact with hot rocks. As hot water is less dense that cold water, it tends to ascend to the surface due to buoyancy. Geothermal systems are commonly developed in areas with active volcanism, where the heat from rocks presents an increase above the standard rate of temperature rise of rock for Earth (2,5~3°C/100m), allowing the development of high temperatures at relatively shallow depths and ensuring financial viability of exploiting underground thermal energy. When such areas include water reservoirs whose cap is sufficiently sealed to prevent heat dissipation, then this reservoir becomes hot and allows the use of the geothermal field (the area where hot fluids are situated), where energy is extracted either through natural hot springs or – normally – through boreholes (Figure 1). Nevertheless,geothermal reservoirs of economic interest are also found in areas devoid of recent volcanism, areas where severe active tensile tectonics (aided by thinning of the earth crust and large and/or deep faults) can create irregular heat conditions and allow the development of such systems.
Some areas also attempt to extract the heat contained in hot dry rocks, through pairs of boreholes that introduce cold water at one end and extract warmer water from the other end. So far, results from such attempts have been unprofitable. Uses of the geothermal fluid may significantly vary depending on their temperature level; thus they are classified as follows: – High temperature (“high enthalpy”), developing temperatures in excess of 150οC, for electric power generation. – Medium temperature (“medium enthalpy”), developing temperatures between 100-150οC; and – Low temperature (“low enthalpy”), developing temperatures lower than 100οC, for ‘direct uses’ (heating of buildings, drying of produce etc). Geothermal fluids from hot springs were utilized all over the world in ancient times, albeit only for shower / spa purposes. Utilization of geothermal power in the industrial age commenced in July 4, 1904, when the first electric power station using geothermal steam started operations in Larderello, Italy. The established electrical power generation in Larderello in 1944 was already 127 MW. The energy crisis of the 70s and the aggravation of environment – related issues resulted in a re- orientation of the global energy policy to pursue the development and utilization of renewable energy technologies; since then, geothermal power has been developing fast. Only during the last decade (1990-2000), established electrical power generation using geothermal power increased by approximately 45%. The current global status of geothermal power use is summarized inFigure 2 and Table 1 attached.
Currently, geothermal power accounts for a mere 0.5% of global energy requirements. In this sense, it does not represent a significant source of power. Nevertheless, it may become a key factor of development on a local and regional scale, with regard to areas rich in hot fluids. Iceland, for instance, covers 50% of its energy requirements by geothermal power. The salts contained in geothermic fluids are occasionally a useful by-product. They are used for boron extraction in Larderello or cesium in Japan, or mercury and other basic metals in California. |
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