The temperature of the Earth's gradually increases with depth, reaching 4500 °C at its core. Some of this heat is a relic of the formation of our planet about 4.5 billion years ago, but most is a result of the decay of naturally occurring radioactive elements. As heat flows from warmer to cooler regions, the Earth's heat gradually flows from the core to the surface, where an estimated 42 million thermal MWare continually radiated away, averaging merely 0.087 W m-2. The majority of this immense heat cannot be practically captured. However, in some locations heat reaches the surface more rapidly, generally on the margins of the tectonic plates. There, the concentrated energy can be released by natural vents: volcanoes, hot steam or springs. It is in these locations that geothermal plants can preferably be inserted and are the most efficient. While hot springs and pools for bathing were used long ago by the Romans, the utilization of geothermal resources for other uses is relatively recent. In 1812, steam from geothermal fields in Larderel-lo, Italy, was used for the manufacture of boric acid. The same geothermal sources were first exploited to produce electricity in 1904, but significant development in other parts of the world did not start before the second half of the 20th century. Despite its short history, geothermal energy is now relatively common in many parts of the globe (Figs. 8.5 and 8.6). In New Zealand, two power plants were built in the early 1960s to produce 170 MW, while in the United States, California's Geysers (Fig. 8.7), the world's largest geothermal power development, came online in 1960. Today, the United States is the leading producer of geothermal electricity, followed by the Philippines, Mexico and Italy. The total generating
capacity is however, only close to 10 000 MW, and production stands around 50 TWh, which represents 0.3% of the global electricity production. Nevertheless, in some developing countries geothermal power plays a key role accounting in 1999 for 26% of the electricity generated in the Philippines, 16% in El Salvador, and 13% in Costa Rica .
The conventional type of natural geothermal reservoir used for electricity generation is hydrothermal, consisting of the accumulation of hot water or steam trapped in fractured porous rock. The most profitable and valuable - but also the rarest - are vapor reservoirs which yield mainly high-temperature, superheated steam above 220 °C, also called dry steam. This is produced from wells up to 4 km deep and is able to power directly gas turbines to generate electricity. Notable examples are Larderello in Italy, Geysers in northern California, and Mat-sukawa in Japan. More common are systems based on hot water at temperatures in the range 150 to 300 °C. In this case the hot water, when brought to the surface, depressurizes and boils explosively, forming large quantities of steam which is fed to turbines to produce electricity in so called flash-steam power plants. In both cases, residual water as well as the condensed steam after utilization are re-injected into the reservoir to maintain pressure and prolong productivity. Lower-temperature geothermal systems between 100 and 150 °C do not allow efficient flash-steam power production. There, the extracted hot water can be used to vaporize another fluid with a lower boiling point (e.g., isopentane) that will drive the power-producing turbine/generator units in so-called binary-cycle plants.
Even if some geothermally active areas have extensive hydrothermal systems, most of them consist only of hot dry rocks with no water or steam. Collecting the heat present in these formations is much more challenging. A pair of wells must be drilled at depths typically more than 4 km, and the rock artificially fractured to allow water to circulate into the injection well. Steam or hot water are then returned to the surface via the production well. The potential for this resource is enormous, and technical feasibility studies are under investigation in Japan, United States, and Europe. The most advanced research on this topic at this time is being conducted on a hot dry rock unit in Alsace, France .
Low- to moderate-temperature (from 35 to 150 °C) geothermal resources can also make direct use of the thermal energy to heat buildings, offices and greenhouses, and even for fish-farming in colder areas. For some time now, most of Iceland, which sits right on the geologically very active Mid-Atlantic Ridge, has been heated by geothermal energy. Geothermal heat can also be produced by heat pumps that use the relatively constant soil temperature to provide heating for buildings in winter and cooling in summer. Today, geothermal heat is used by more than 55 countries with an installed total capacity over 17 000 MW of thermal power. The largest installed capacity in the world is presently in the United States, followed by China, Iceland, and Japan.
Geothermal energy is, in the strictest sense, not a renewable resource. The energy that is taken out of the Earth will not be replaced in the future. At a given location, a resource can be depleted relatively quickly if too much hot water or steam is extracted and not re-injected after heat extraction, as happened for the Geysers in California. However, geothermal resources around the globe are so widespread and immense that there is little possibility of their exhaustion on a human timescale. Today, for electricity generation, only the highest grade geother-mal resources can be utilized economically, averaging a production cost of $0.03 to $0.10 per kWh , the lower end of this scale being quite competitive with fossil fuels. Geothermal resources have the advantage of being available all the time, with little or no fluctuation in power output, allowing reliable and predictable electricity production. Their environmental impact is low because the water used is generally re-injected in the reservoir, while gas emissions are limited and addressed by strict regulations. Small amounts of solid byproduct materials such as salts or heavy metals require disposal, while others such as silica, sulfur, or zinc can be extracted for sale. The principal barriers to faster worldwide geothermal development are mostly technological with high costs of exploration, drilling and plant construction, but there are also limited numbers of sites that are economically exploitable with present technologies. Most near-future development is expected in already known fields, principally along the tectonically active margins of the Pacific region, notably in East Asia. As mentioned, geothermal energy represents today only 0.3% of the global electricity generation. Despite forecasted regular growth, this share is only expected to increase slightly during the next decades. Therefore, from a global aspect, geothermal energy will remain a marginal source of electricity, despite playing an important role on a local scale in some countries. The successful development of hot dry rock technology might alter this picture, however, by allowing the installation of geothermal units in a larger number of formerly unsuitable locations.
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