Geothermal Energy for Electrical Generation
Authors: Budd C.F. 
Published: December 2007

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleum engineering.

Introduction
In the past decades, geothermal energy has come of age as a vital energy resource. Since 1979, the capacity of geothermal-electric generating plants worldwide has more than doubled to 3571 MW and is projected to nearly double again by 1985, providing projected to nearly double again by 1985, providing a potential saving of nearly 75 million bbl (11.9 × 10–6 m3) of oil annually. In the U.S., geothermal developments currently provide the energy equivalent of 10 million bbl provide the energy equivalent of 10 million bbl (1.59 × 10–6 m3) of oil annually to generate electricity at the Geysers geothermal field in California. Twenty-five percent of the electricity generated on Luzon, the mostpopulous island in the Philippines, is produced in plants fueled by geothermal energy. In Mexico, Japan, plants fueled by geothermal energy. In Mexico, Japan, New Zealand, and elsewhere new geothermal energy resources are being developed to generate electricity. Geothermal energy is derived from heat contained in the earth's crust. Harnessing a small fraction of this heat would supply the world's energy needs forever. The difficulty in extracting this energy lies in the diffuse nature of its occurrence. Only in areas where this heat is concentrated anomalously close to the surface, within drillable depths, is there potential for extracting some of the energy contained in this heat and converting it into useful work. Regions where this occurs generally lie along boundaries of tectonic plates of the earth's crust. The near-surface heat source of most interest in geothermal development is where the molten magma ofthe earth's interior has intruded into the surface crust. Heat is transmitted to surrounding rock strata, forming an area of anomalously high temperature. When water is present within the rock overlying or adjacent to the intrusion, the heated water can be extracted through wells to capture heat energy. Vertical convective circulation of the water carries it upward to the surface where it spreads laterally, cools, and descends to be heated and raised again. Depending on geologic, hydrologic, and thermodynamic factors, production fromwells drilled into these active geothermal convection systems includes steam, hot water, or a mixture of both, and may be used to power electrical generators.

History
For centuries, geothermal energy was known widely and used in the form ofhot pools and springs for bathing. In Europe ad Japan, entire communities were established around the spas that catered to vacationers and those seeking themedicinal and other mysterious beneficial qualities attributed to the sulfurous waters. In 1904 in Larderello, Italy, Prince Piero Ginari Conti made the firstattempt to harness geothermal steam from vents to drive a reciprocating engine. This early attempt led to the commissioning in 1913 of a 250-kW turbine-drivenelectrical power station, tiny compared with today's plants. Progressive expansion and development of the dry steam field followed until 130 MW of electrical generating capacity had been installed by 1940. After World War II, when plant installations were destroyed, reconstruction began. Today more than440 MW of geothermal generating capacity is installed.
JPT
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