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Fig. 3. Geothermal wellhead and casing schematic. tane through heat exchangers [9], [10]. The discussion below will focus on common industry practice in high temperature, liquid-dominated fields. The most common means of separating brine and steam is in a vertical vessel with a tangential entry using centrifugal force to separate the steam and brine. After separation of the steam from the brine, typically at pressures around 1.0 MPa, the steam is transported to a power plant through pipelines up to 1 m in diameter and up to 4 km long. A pressure drop of 0.25 MPa between wellhead and turbine, which is typical in long pipelines, represents a 7% loss of available energy. Minor condensation of steam occurs in the pipelines and is removed in a centrifugal separator, or a vane-type de-mister, prior to entering the power plant. In cases of high chloride content in the steam, steam washing can be employed to re- move the contaminant before admission to the turbine. Separated brine is usually injected directly back into the reservoir, but in some fields is disposed of on the surface (Wairakei, New Zealand; Cerro Prieto, Mexico). In other cases, it is used for process heating (Kawerau, New Zealand) to generate additional power in a binary unit (Mak-Ban, Philippines), or as feedstock for mineral extraction before being injected back into the reservoir. A plant designed to recover 30 000 metric tons of zinc annually from geothermal brines has been constructed at the Salton Sea geothermal field in California. Returning the brine to the reservoir has both advantages and disadvantages. The principal advantages are that the net withdrawal of mass from the system is greatly reduced and reservoir pressure declines more slowly, so that well outputs can be maintained for longer time. In addition it ensures that no environmental damage can occur from chemical species (e.g., As, B) in the brine. The principal disadvantage is that the cool brine may flow directly to certain production wells before it has been in contact with hot rock long enough to reheat, causing a reduction in steam output from the produc- tion wells. This is a common problem because a strong pres- sure difference builds up between injectors and producers, and the fractured nature of the rocks in geothermal systems often allows an unpredictable highly permeable path from injector to producer. It is usually mitigated by increasing the separation between injection and production wells. This is a fruitful area for research, since a better understanding of permeability structure and heat transfer characteristics in the reservoir will permit a more efficient use of the resource. V. POWER PLANTS Condensing steam turbines normally operate with inlet pressures between 0.5 and 1.0 MPa, at an isentropic effi- ciency of 78% to 83%, and with a steam usage rate in the range of 6–9 kg/kWh. Steam turbines without condensers are sometimes used during the early development of a field, but the lack of a condenser leads to the loss of 50% of the avail- able energy. Fig. 4 is a process flow diagram for a typical geothermal turbine with a condenser. In this illustration, steam is sup- plied from the flash separation process at a single pressure. PROCEEDINGS OF THE IEEE, VOL. 89, NO. 12, DECEMBER 2001 a large portion of the increased cost relative to oil and gas wells drilled to the same depth. A geothermal drilling rig typically used by the industry is powered by diesel driven generators capable of 3 MW . This power is distributed through a silicon-controlled recti- fier system to 1.5 MW electric motors that drive a hoisting and rotating mechanism, and to the 1.5 MW electric-driven pumps of the mud-circulating system. A typical drilling loca- tionmayhaveanadditionalrequirementof750kW formis- cellaneous rig equipment and personnel housing. The power requirements are approximately twice the requirement for oil and gas drilling to the same depth. To reduce the cost of geothermal wells the two main chal- lenges are: 1) to devise a more efficient excavation tech- nology to remove the large quantities of hard, fractured rock in less time and 2) to develop an improved process for pene- trating low-pressure formations while removing the rock cut- tings and cooling the well. IV. STEAM PRODUCTION Approximately 90% of geothermal fields in the world are high temperature and liquid-dominated. From these, a mix- ture of steam and brine is brought to the surface through the production wells, and the brine must be separated before delivering steam to the turbine. In vapor-dominated fields the wells produce steam only and no brine handling is re- quired. In lower temperature fields, binary systems are com- monly used, where hot water is produced, sometimes using downhole pumps, maintained as a liquid under pressure, and used to flash a secondary working fluid such as iso-pen- 1786 Authorized licensed use limited to: National United University. Downloaded on October 10, 2009 at 14:17 from IEEE Xplore. Restrictions apply.PDF Image | Geothermal Power Technology
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