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وبلاگ بزرگ مقالات زمین شناسی - انرژی زمین گرمایی - Geothermal power

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انرژی زمین گرمایی - Geothermal power

Geothermal power

 

Thermal or electrical power produced from the thermal energy contained in the Earth (geothermal energy). Use of geothermal energy is based thermodynamically on the temperature difference between a mass of subsurface rock and water and a mass of water or air at the Earth's surface. This temperature difference allows production of thermal energy that can be either used directly or converted to mechanical or electrical energy.

Characteristics

 Temperatures in the Earth in general increase with increasing depth, to 400–1800°F (200-1000°C) at the base of the Earth's crust and to perhaps 6300–8100°F (3500–4500°C) at the center of the Earth. Average conductive geothermal gradients to 6 mi (10 km; the depth of the deepest wells drilled to date) are shown in Fig. 1 for representative heat-flow provinces of the United States. The heat that produces these gradients comes from two sources: flow of heat from the deep crust and mantle; and thermal energy generated in the upper crust by radioactive decay of isotopes of uranium, thorium, and potassium. The gradients of Fig. 1 represent regions of different conductive heat flow from the mantle or deep crust. Some granitic rocks in the upper crust, however, have abnormally high contents of uranium and thorium and thus produce anomalously great amounts of thermal energy and enhanced flow of heat toward the Earth's surface. Consequently thermal gradients at shallow levels above these granitic plutons can be somewhat greater than shown on Fig. 1. See also: Earth, heat flow in

 

 

Fig. 1  Calculated average conductive temperature gradients in representative heat-flow provinces of the United States. 1 km = 0.6 mi. °F = (°C × 1.8) + 32. (After D. E. White and D. L. Williams, eds., Assessment of Geothermal Resources of the United States–1975, USGS Circ. 726, 1975)

 

 

 The thermal gradients of Fig. 1 are calculated under the assumption that heat moves toward the Earth's surface only by thermal conduction through solid rock. However, thermal energy is also transmitted toward the Earth's surface by movement of molten rock (magma) and by circulation of water through interconnected pores and fractures. These processes are superimposed on the regional conduction-dominated gradients of Fig. 1 and give rise to very high temperatures near the Earth's surface. Areas characterized by such high temperatures are the primary targets for geothermal exploration and development.

Commercial exploration and development of geothermal energy to date have focused on natural geothermal reservoirs—volumes of rock at high temperatures (up to 662°F or 350°C) and with both high porosity (pore space, usually filled with water) and high permeability (ability to transmit fluid). The thermal energy is tapped by drilling wells into the reservoirs. The thermal energy in the rock is transferred by conduction to the fluid, which subsequently flows to the well and then to the Earth's surface.

Natural geothermal reservoirs, however, make up only a small fraction of the upper 6 mi (10 km) of the Earth's crust. The remainder is rock of relatively low permeability whose thermal energy cannot be produced without fracturing the rock artificially by means of explosives or hydrofracturing. Experiments involving artificial fracturing of hot rock have been performed, and extraction of energy by circulation of water through a network of these artificial fractures may someday prove economically feasible.

There are several types of natural geothermal reservoirs. All the reservoirs developed to date for electrical energy are termed hydrothermal convection systems and are characterized by circulation of meteoric (surface) water to depth. The driving force of the convection systems is gravity, effective because of the density difference between cold, downward-moving, recharge water and heated, upward-moving, thermal water. A hydrothermal convection system can be driven either by an underlying young igneous intrusion or by merely deep circulation of water along faults and fractures. Depending on the physical state of the pore fluid, there are two kinds of hydrothermal convection systems: liquid-dominated, in which all the pores and fractures are filled with liquid water that exists at temperatures well above boiling at atmospheric pressure, owing to the pressure of overlying water; and vapor-dominated, in which the larger pores and fractures are filled with steam. Liquid-dominated reservoirs produce either water or a mixture of water and steam, whereas vapor-dominated reservoirs produce only steam, in most cases superheated.

Natural geothermal reservoirs also occur as regional aquifers, such as the Dogger Limestone of the Paris Basin in France and the sandstones of the Pannonian series of central Hungary. In some rapidly subsiding young sedimentary basins such as the northern Gulf of Mexico Basin, porous reservoir sandstones are compartmentalized by growth faults into individual reservoirs that can have fluid pressures exceeding that of a column of water and approaching that of the overlying rock. The pore water is prevented from escaping by the impermeable shale that surrounds the compartmented sandstone. The energy in these geopressured reservoirs consists not only of thermal energy, but also of an equal amount of energy from methane dissolved in the waters plus a small amount of mechanical energy due to the high fluid pressures. See also: Aquifer; Ground-water hydrology

 

Use of Geothermal Energy

Although geothermal energy is present everywhere beneath the Earth's surface, its use is possible only when certain conditions are met: (1) The energy must be accessible to drilling, usually at depths of less than 2 mi (3 km) but possibly at depths of 4 mi (6–7 km) in particularly favorable environments (such as in the northern Gulf of Mexico Basin of the United States). (2) Pending demonstration of the technology and economics for fracturing and producing energy from rock of low permeability, the reservoir porosity and permeability must be sufficiently high to allow production of large quantities of thermal water. (3) Since a major cost in geothermal development is drilling and since costs per meter increase with increasing depth, the shallower the concentration of geothermal energy the better. (4) Geothermal fluids can be transported economically by pipeline on the Earth's surface only a few tens of kilometers, and thus any generating or direct-use facility must be located at or near the geothermal anomaly.

Direct use

Equally important worldwide is the direct use of geothermal energy, often at reservoir temperatures less than 212°F (100°C). Geothermal energy is used directly in a number of ways: to heat buildings (individual houses, apartment complexes, and even whole communities); to cool buildings (using lithium bromide absorption units); to heat greenhouses and soil; and to provide hot or warm water for domestic use, for product processing (for example, the production of paper), for the culture of shellfish and fish, for swimming pools, and for therapeutic (healing) purposes.

Major localities where geothermal energy is directly used include Iceland (30% of net energy consumption, primarily as domestic heating), the Paris Basin of France (where 140–160°F or 60–70°C water is used in district heating systems for the communities of Melun, Creil, and Villeneuve la Garenne), and the Pannonian Basin of Hungary.

 

Electric power generation

The use of geothermal energy for electric power generation has become widespread because of several factors. Countries where geothermal resources are prevalent have desired to develop their own resources in contrast to importing fuel for power generation. In countries where many resource alternatives are available for power generation, including geothermal, geothermal has been a preferred resource because it cannot be transported for sale, and the use of geothermal energy enables fossil fuels to be used for higher and better purposes than power generation. Also, geothermal steam has become an attractive power generation alternative because of environmental benefits and because the unit sizes are small (normally less than 100 MW). Moreover, geothermal plants can be built much more rapidly than plants using fossil fuel and nuclear resources, which, for economic purposes, have to be very large in size. Electrical utility systems are also more reliable if their power sources are not concentrated in a small number of large units.

In the United States a law was passed in 1978 that required the output from geothermal power generation projects (and others not based on fossil fuel resources, and cogeneration projects) to be purchased by electrical utilities at the cost that was avoided by the utility as a result of obtaining the power from a geothermal power plant. The legislation is called the Federal Public Utility Regulatory Policies Act (PURPA) and has created an incentive for the development of geothermal power projects.

The process used for generating power varies in accordance with the characteristics of the geothermal resource. The characteristics that affect the process are the temperature, the suspended and dissolved solids in the resource, and the level of noncondensable gases (primarily carbon dioxide) entrained in the geothermal brine, or steam. Almost all resources discovered to date are of the hydrothermal type (pressurized hot water) which can be produced from a well by two methods. If the temperature of a hydrothermal resource is below 400°F (204°C), a geothermal well can be produced with a pump, which maintains sufficient pressure on the geothermal brine to keep it as pressurized hot water. For hydrothermal resources over 400°F, the more suitable method of production is to flow the wells naturally, yielding a flashing mixture of brine and steam from the wells.

Steam flash process

The most common process is the steam flash process (Fig. 2), which incorporates steam separators to take the steam from a flashing geothermal well and passes the steam through a turbine that drives an electric generator. For the greatest efficiency in this process, a double-entry turbine is utilized which enables the most amount of steam available in the production from the geothermal well to be converted to electric power. If the resource has a high level of suspended and dissolved solids, it may be necessary to incorporate scaling control equipment in the steam flash vessel at the front of the plant and solids-settling equipment at the tail end of the plant. This will keep the process equipment from becoming plugged and allows a clean residual brine to be maintained for reinjection into the reservoir. If there are significant amounts of noncondensable gases, it may be necessary to install equipment to eject these gases out of the condenser to keep the back pressure on the system from rising and thereby cutting down on the efficiency of the process.

 

Fig. 2  Schematic diagram of the steam flash process.

 

 

 

 

 Fig. 3  Schematic diagram of the binary process.

 

 

 There are, at present, two resources in operation that have “dry steam,” which is produced from the wells directly. These are very easy to convert to electric power and use the above described process without the necessity of the separation and brine injection equipment.

 

Binary process

A more efficient utilization of the resource can be obtained by using the binary process (Fig. 3) on resources with a temperature less than 360°F (180°C). This process is normally used when wells are pumped. The pressurized geothermal brine yields its heat energy to a second fluid in heat exchangers and is reinjected into the reservoir. The second fluid (commonly referred to as the power fluid) has a lower boiling temperature than the geothermal brine and therefore becomes a vapor on the exit of the heat exchangers. It is separately pumped as a liquid before going through the heat exchangers. The vaporized, high-pressure gas then passes through a turbine that drives an electric generator. The vapor exhaust from the turbine is then condensed in conventional condensers and is pumped back through the heat exchangers. There is a distinct environmental advantage to this process since both the geothermal brine and power fluid systems are closed from the atmosphere. Hydrocarbons, such as isobutane and propane, are common power fluids used in this process. See also: Electric power generation

 

Production and Pollution Problems

The chief problems in producing geothermal power involve mineral deposition, changes in hydrological conditions, and corrosion of equipment. Pollution problems arise in handling geothermal effluents, both water and steam.

Mineral deposition

In some water-dominated fields there may be mineral deposition from boiling geothermal fluid. Silica deposition in wells caused problems in the Salton Sea, California, field; more commonly, calcium carbonate scale formation in wells or in the country rock may limit field developments, for example, in Turkey and the Philippines. Fields with hot waters high in total carbonate are now regarded with suspicion for simple development. In the disposal of hot wastewaters at the surface, silica deposition in flumes and waterways can be troublesome.

Hydrological changes

Extensive production from wells changes the local hydrological conditions. Decreasing aquifer pressures may cause boiling water in the rocks (leading to changes in well fluid characteristics), encroachment of cool water from the outskirts of the field, or changes in water chemistry through lowered temperatures and gas concentrations. After an extensive withdrawal of hot water from rocks of low strength, localized ground subsidence may occur (up to several meters) and the original natural thermal activity may diminish in intensity. Some changes occur in all fields, and a good understanding of the geology and hydrology of a system is needed so that the well withdrawal rate can be matched to the well's long-term capacity to supply fluid.

 Corrosion

Geothermal waters cause an accelerated corrosion of most metal alloys, but this is not a serious utilization problem except, very rarely, in areas where wells tap high-temperature acidic waters (for example, in active volcanic zones.) The usual deep geothermal water is of near-neutral pH. The principal metal corrosion effects to be avoided are sulfide and chloride stress corrosion of certain stainless and high-strength steels and the rapid corrosion of copper-based alloys. Hydrogen sulfide, or its oxidation products, also causes a more rapid degradation than normal of building materials, such as concrete, plastics, and paints. See also: Corrosion

 Pollution

 A high noise level can arise from unsilenced discharging wells (up to 120 decibels adjusted), and well discharges may spray saline and silica-containing fluids on vegetation and buildings. Good engineering practice can reduce these effects to acceptable levels.

Because of the lower efficiency of geothermal power stations, they emit more water vapor per unit capacity than fossil-fuel stations. Steam from wellhead silencers and power station cooling towers may cause an increasing tendency for local fog and winter ice formation. Geothermal effluent waters liberated into waterways may cause a thermal pollution problem unless diluted by at least 100:1.

Geothermal power stations may have four major effluent streams. Large volumes of hot saline effluent water are produced in liquid-dominated fields. Impure water vapor rises from the station cooling towers, which also produce a condensate stream containing varying concentrations of ammonia, sulfide, carbonate, and boron. Waste gases flow from the gas extraction pump vent.

Pollutants in geothermal steam

Geothermal steam supplies differ widely in gas content (often 0.1–5%). The gas is predominantly carbon dioxide, hydrogen sulfide, methane, and ammonia. Venting of hydrogen sulfide gas may cause local objections if it is not adequately dispersed, and a major geothermal station near communities with a low tolerance to odor may require a sulfur recovery unit (such as the Stretford process unit). Sulfide dispersal effects on trees and plants appear to be small. The low radon concentrations in steam (3–200 nanocuries/kg or 0.1–7.4 kilobecquerels/kg), when dispersed, are unlikely to be of health significance. The mercury in geothermal stream (often 1–10 microgram/kg) is finally released into the atmosphere, but the concentrations created are unlikely to be hazardous. See also: Air pollution

 

Geothermal waters 

The compositions of geothermal waters vary widely. Those in recent volcanic areas are commonly dilute (<0.5%) saline solutions, but waters in sedimentary basins or active volcanic areas range upward to concentrated brines. In comparison with surface waters, most geothermal waters contain exceptional concentrations of boron, fluoride, ammonia, silica, hydrogen sulfide, and arsenic. In the common dilute geothermal waters, the concentrations of heavy metals such as iron, manganese, lead, zinc, cadmium, and thallium seldom exceed the levels permissible in drinking waters. However, the concentrated brines may contain appreciable levels of heavy metals (parts per million or greater).

Because of their composition, effluent geothermal waters or condensates may adversely affect potable or irrigation water supplies and aquatic life. Ammonia can increase weed growth in waterways and promote eutrophication, while the entry of boron to irrigation waters may affect sensitive plants such as citrus. Small quantities of metal sulfide precipitates from waters, containing arsenic, antimony, and mercury, can accumulate in stream sediments and cause fish to derive undesirably high (over 0.5 ppm) mercury concentrations. See also: Water pollution

 Reinjection

The problem of surface disposal may be avoided by reinjection of wastewaters or condensates back into the countryside through disposal wells. Steam condensate reinjection has few problems and is practiced in Italy and the United States. The much larger volumes of separated waste hot water (about 55 tons or 50 metric tons per megawatt-electric) from water-dominated fields present a more difficult reinjection situation. Silica and carbonate deposition may cause blockages in rock fissures if appropriate temperature, chemical, and hydrological regimes are not met at the disposal depth. In some cases, chemical processing of brines may be necessary before reinjection. Selective reinjection of water into the thermal system may help to retain aquifer pressures and to extract further heat from the rock. A successful water reinjection system has operated for several years at Ahuachapan, El Salvador.

 

Bibliography

  •  H. C. H. Armstead, Geothermal Energy, 2d ed., 1983
  • Ali Fazeli = egeology.blogfa.com
  • R. Bowen, Geothermal Resources, 2d ed., 1989
  • Ali Fazeli = egeology.blogfa.com
  • M. Economides and P. Ungemach (eds.), Applied Geothermics, 1987
  • Ali Fazeli = egeology.blogfa.com
  • L. Edwards et al. (eds.), Handbook of Geothermal Energy, 1982
  • Ali Fazeli = egeology.blogfa.com
  • J. Elder, Geothermal Systems, 1981
  • Ali Fazeli = egeology.blogfa.com
  • M. A. Grant et al., Geothermal Reservoir Engineering, 1983
  • Ali Fazeli = egeology.blogfa.com
  • K. Wohletz and G. Heiken,Volcanology and Geothermal Energy, 1992
  • Ali Fazeli = egeology.blogfa.com

 

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