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Unformatted text preview: 12-1 Chapter 12 GEOTHERMAL POWER 12.1 Introduction The word geothermal comes from the Greek words geomeaning earth and thermemeaning heat. So, geothermal energy is the heat from the depths of the earth. Volcanoes, geysers, hot springs and boiling mud pots are visible evidences of the great reservoir of heat that lies within the earth. We can use the steam and hot water produced inside the earth for the purpose of direct heating or to generate electricity. Although the amount of thermal energy within the earth is very large, most of this heat is too deeply buried to be exploited. The depth from which heat may be extracted economically is unlikely to exceed 10 km. Its recovery for useful purpose is feasible at certain sites only, where favorable geological conditions exist. The sites where it is available near the surface and is relatively more concentrated its extraction and use may be considered feasible. Such sites are known as geothermal fields. Most geothermal resources produce low-grade heat at about 50-70oC, which can be used directly for thermal applications. Occasionally, geothermal heat is available at temperatures above 90oC, and so electrical power production using steam turbines can be contemplated. The countries currently producing the most electricity from geothermal reservoirs are the United States, New Zealand, Italy, Iceland, Mexico, the Philippines, Indonesia and Japan, but geothermal energy is also being used in many other countries. As per U.S. Geological Survey, the entire heat content of the earth’s crust up to a depth of 10 km above 15 oC is defined as geothermal resource. As such the geothermal resource is estimated to be more than 2.11 ×1025J, which is equivalent to 109MTOE (million tons of oil equivalent). This is a huge amount of energy, enough to supply our energy needs at current rates for 3,50,000 years. Thus it is considered practically inexhaustible source though technically not renewable. But, generally it is a low-grade thermal energy form and its economic recovery is not feasible every where on the surface of the earth. Practically it is not the size of the resource that limits its use but the availability of technology that can tap the resource in an economic manner. It is common to use heat from the near surface ground, up to a depth of approx. 10 ft., as input to heat pump. Although this may be interpreted as a ‘geothermal’ source, such systems are not included in this chapter. It is more meaningful to consider such sources as stored heat from sunshine, since replenishment will be more from the environment above than below the location. Low temperature resources i.e. geysers have been used from time immemorial for applications such as therapeutic hot baths, cooking, space and water heating. About 10,000 years ago, Paleo-Indians used hot springs in North American for cooking. Every major hot spring in the United States can be associated with Native American tribes. California hot springs, like at the Geysers in the Napa area, were important 12-2...
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This note was uploaded on 03/16/2012 for the course ECE 5374G taught by Professor Srahman during the Spring '12 term at Virginia Tech.
- Spring '12