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denmarks energy program

Part 2: Denmark's Wind Energy Program

 
Denmark

One of the greatest barriers to robust renewable energy support by the U.S. government is the notion that government programs necessarily create a reliance on subsidies that is ultimately self-defeating. In reality, properly designed and implemented programs can gradually coax an industry and specific technologies into commercial maturity. This is illustrated by the Danish government's coordinated and systematic efforts to build a wind industry that must now be considered the world's best.

Governmental support has made this small country not only a major center of wind power, but an industrial center for what is currently the only renewable energy technology that can compete head-to-head with fossil fuels in terms of costs—and the fastest growing energy source in the world.9 Denmark is now manufacturing about 1 gigawatt of wind capacity a year, the rough equivalent of two large nuclear or coal-fired power stations, and creating about 16,000 jobs at the same time.

Denmark's success in the development of a wind turbine industry has been measurable: in 1983, Denmark exported 20 MW of wind turbines while in 1997 Danish wind turbine exports reached 681 MW. At home, wind energy generated 27 GWH of electricity in 1983 and steadily increased in each consecutive year to 1830 GWH of electricity in 1997.

Although Germany has recently overtaken Denmark in terms of installed wind capacity, the Danish wind turbine industry still produces more equipment than all other nations combined in the fast-growing global market. After establishing both a stable home market as the basis for development of wind turbine technology and a strong export sector, the Danish government is now extending that model into other areas of energy as well, including combined heat and power. Wind turbines now constitute the nation's second most important export.

Recently, sales of wind turbines in Denmark and elsewhere have been spurred by the need to reduce carbon emissions. Denmark's "Energy 21" policy aims to bring national carbon dioxide emissions by 2005 to 20% below the levels in 1988. Launched in 1996, Energy 21's goals for the longer term are even more ambitious: to halve carbon dioxide emissions by 2030.10 Clearly, however, Danish wind power development predates current concerns over climate change.

To stimulate the development of wind energy, the Danish government has relied principally on a combination of taxes and subsidies. Relatively simple at its outset in the 1970s, wind policy became more complex in the 1990s with the introduction of new taxes designed to cope with SO2 and CO2 emissions, together with new subsidy schemes. Still, the Danish government has remained fixed on its goal of protecting the environment in a way that also shields and stimulates domestic industry.

The Original Incentives

Today's wind energy program is part of an overall energy plan published in 1976. Its main objective was to make Denmark less dependent on imported energy, especially oil. Since the mid-1980s, however, environmental concerns have propelled the Danish program. The initial steps taken in Denmark to build the turbine industry included the following:

  • R&D: A comprehensive energy research and development program started in 1976 with wind energy as only one area of work. This funded efforts to collect fundamental information regarding the planning and construction of large (more than 500 kilowatts (kW)) wind turbines. About 10% of the total Danish energy research budget since 1976 has been devoted to wind.

  • Turbine certification: The Risoe Test Station for Wind Turbines began in 1978 as a pilot project, paid for by the Danish Energy Agency. Initially, Denmark required certification only as a condition of eligibility for government subsidies, and manufacturers paid just a token fee for the procedure. Now, however, certification is required for connection to the Danish grid, and the process has evolved into a commercial activity for Risoe; manufacturers must pay the full cost of testing and certification. In addition, the government has extended the authority to issue certificates to other organizations. The certification standard is updated by the Danish Energy Agency with advice from an advisory committee that includes representatives from the Danish Wind Turbine Owners Association, the Danish Utilities Association, the Insurers Association, and the Wind Turbine Manufacturers Association. Risoe supplies a technical expert to advise the committee.

  • Capital Subsidies: A subsidy equal to 30% of the investment costs of a wind turbine started in 1979, leading to the deployment of 200—300 machines a year. After steady increases in the reliability and cost-effectiveness of turbines, the subsidy for wind power was repealed in 1989. (It was retained for other forms of renewable energy, however, and is now helping spur an increase in electricity generation from bioenergy, principally from burning straw and wood chips.)

  • Mandated electricity purchases: Danish power companies are required to pay 85% of the retail electricity price for the wind energy purchased from privately owned wind turbines.

As a result of these programs, Denmark has seized a position of clear dominance, with 58.5% of global sales in 1997, followed by Germany with 16%, the United Kingdom with 14.1%, and the United States with just 2.4% of global sales. From 1993 to 1995, the wind industry grew at a rate of 50% a year in Denmark, with annual growth rates of 10—20% projected through 2000.

The Wind Guilds

Because of the large size, location in visible wind-swept sites, noise, effect on wildlife, and other impacts of wind turbines, landowners, planners, environmentalists, and others sometimes resist these technologies. In the United States, such "environmental" opposition frequently proves the final barrier to the development of a project. The Danish solution to this—parts of which appear to have evolved independently of any overarching guidance from the government—was to allow turbine ownership by guilds or co-operatives, and to require member-owners to live within 3 kilometers of the site. The guilds eventually organized as the Danish Wind Turbine Owners Association, which became a powerful political force. Today, 100,000 Danish families own wind turbines or shares in wind co-operatives.

In the mid-1980s, this ownership rule was modified somewhat, to require that guild members live in or within 10 kilometers of the same borough as the turbine and to limit the share of any individual owner to the greater of 6,000 kilowatt-hours (kWh) per year or 135% of that person's electricity consumption. This change was made in part because of pressure from electric utilities, which were seeking to limit private ownership of generating facilities. Under pressure from the various guilds, the law was amended again in 1992 to relax ownership requirements. The geographic area of residency was expanded to include residents of the borough in which the turbine was located and those of neighboring boroughs. The ownership share was increased to the greater of 9,000 kWh per year or 150% of consumption. The rules were expanded further in 1996 to allow ownership of up to 30,000 kWh per year by any person who lived or worked in the borough or who owned a house or real estate there.

The guilds played a significant role in the steady technological improvement of Danish turbines by providing regular reports in the membership magazine, Naturlig Energi. Each month Naturlig Energi had a list of all turbines with an indication of what they produced and what technical problems were encountered. This accountability had a positive effect on development. The turbine owners themselves had then the opportunity to explain how well or how badly their turbines produced, and the manufacturers discovered that their own turbines quickly became either a good or a bad advertisement for their business. In the process of development the statistics also showed the importance of good siting.

The association and its magazine also played a pivotal role in establishing the credibility of wind power and exploding myths of unreliability and high cost. And it helped create both a market for insurance and a free-standing firm to supply it. As a result, consumers can buy insurance at the same time as a turbine, providing protection not only against damage and losses but also against the risk of a manufacturer going bankrupt within the warranty period. By 1997-, the association had a membership of 2,150 wind turbines, while the turbine guilds had 54,844 members.

Danish Taxes

With the advent of concerns over local air pollution and global warming, Danish support for wind energy has been revitalized. Denmark levies a tax on all electricity; the basic support mechanism for wind energy is a partial rebate of this tax. The actual tax structure is quite complex, for there are three environment-related taxes: an energy tax that varies for natural gas, unleaded gasoline, diesel fuel, and other energy; a carbon dioxide tax; and a sulfur dioxide tax. Denmark also applies different rates to space heating and to "heavy" and "light" uses of energy. Heavy uses include 35 different energy-intensive production process, such as cement production.

The taxes have three specific goals: to reduce CO2 emissions from 1988 levels by 20% by 2005, to reduce SO2 emissions from 1980 levels by 80% by 2000, and to increase the share of Denmark's gross energy consumption provided by renewable energy to 35%—or perhaps even 50%—by 2030. Of course, the taxes also provide income for the national government. Together, they account for about 7% of domestic revenue.

The tax system is further complicated by a set of 15 subsidy schemes related to energy production and consumption. The bulk of these are directed primarily at converting central and electric heating systems to district heating and to expanding and renovating the existing district heating network. But the largest one in terms of money is a production subsidy of 0.27 kroner (3.8¢) per kWh for electricity generated from renewable energy sources. In addition, natural gas generators receive a subsidy of 0.07 kroner (9.7¢) per kWh. These subsidies accounted for about 45% of the total subsidy budget, or about 855 million kroner ($119 million) in 1996. The share is expected to rise in coming years because of the increase in electricity production from natural gas and renewable energy sources.

Denmark's New Renewable Portfolio Standard

The liberalization of the electricity market has caused Denmark to enact a very recent and fundamental change in its support for renewables: from fixed payments to a renewable portfolio standard (RPS) system. This change is projected to happen gradually over the next decade, and the Danes are working on the details.11 Most importantly, as the shift takes place, a system is being created to insure the survival of existing projects and the continual growth of near-term future renewables. Also, the Danes will maintain current taxes on electricity and carbon dioxide will adapt them to work with the RPS.

Very briefly, the RPS works as follows: a target for renewables is legislatively set, usually in terms of a percentage contribution from renewables. Electricity distributors are held responsible for meeting this target in most RPS systems. To meet their obligation, distribution companies may either develop renewable resources themselves or purchase renewable generation credits from other renewable generators as proxies for their own renewable generation. The RPS is competitive because electricity distributors will seek the lowest cost option to meet their obligation. No matter which option is chosen, the extra costs of renewable electricity are handed down to all of the consumers of the distribution company.12

The components of an RPS are all evident in Denmark's new system: renewable implementation targets, the creation of renewable energy certificates with the goal of creating a special market for renewables, and an obligation on all consumers to purchase electricity from renewables. It is notable that a nation recognized for being a world leader in renewable energy has chosen to implement a renewable support mechanism invented in the U.S.13

It should be noted at this stage, however, that the Danish RPS is a hybrid system retaining fixed payment characteristics: it maintains a guaranteed payment of .33 kroners (4.6¢) to renewable generators and then allows the market for renewable energy credits to determine the additional value of the renewable energy credit. This renewable energy credit is bounded between .10 kroner (1.4¢) and .27 kroner (3.8¢) by law.

The adoption of an RPS represents a distinct transition from a steady, fixed premium system to a competitive system. This transition is consistent with a worldwide trend toward liberalization of markets. Denmark is moving towards a market system for its renewables to retain consistency with the liberalization in the rest of its electricity sector. The liberalization of its electricity sector, in turn, is consistent with trends in Europe to decrease market barriers between nations.

The decision to implement an RPS reflects a definite confidence in the Danish ability to achieve renewables targets, as targets are the focal point of the RPS. The benefits of competition also appear to be desired for the continual evolution of the renewables industry in Denmark: a recent policy paper by the Danish Energy Agency specifically identifies the element of competition to produce cost-effective development of renewables as a major driver for this shift in the renewable program.14

 

Renewable Energy Policy Outside the United States

   
  1. Abstract
  2. Message from REPP Staff
  3. Why Are They Doing it?
  4. Introduction & Overview
  5. Danish Wind
  6. German Encouragement
  7. Non-Fossil Fuel in Britain
  8. Dutch National Plans
  9. Japanese Efficiency
  10. Successful PVs
  11. Lessons for the U.S.
 
+ نوشته شده در  یکشنبه دوازدهم آذر 1385ساعت 21:0  توسط امیر علی باب هادی عشر  | 

Coastal North Carolina Wind Resource Assessment Project

INTRODUCTION

Recent advances in wind turbine technology combined with work done by the North Carolina State Energy Office to map the wind resource in eastern North Carolina show that the Coastal region of North Carolina holds substantial wind power potential for wind energy development. In light of the growing awareness of the wind resource and the importance this resource could play in providing the state with new renewable energy, it is now important to begin a public outreach process to determine how the wind resource can be developed consistent with community interests and other constraints on development.

First, in order to move this public process forward, recent work done to assess the wind resource in coastal North Carolina will be reviewed and made available to the public. This work will include an assessment of the current state of wind technology for on-shore and offshore applications.

Second, representatives of the areas likely to be affected by coastal wind development will be engaged to determine public attitudes towards wind development, both positive and negative. As part of this work, we will open a public process that will map out the current siting and project approval process. Important aspects of assessing coastal NC wind development include the following:

  • An initial assessment of oversight responsibilities relative to the federal and state authorities involved with siting. Determining the values important to the citizens of the state likely to be affected by the development and the potential for the process, both as it is currently configured and as it might be amended, to consider these public values.
  • A public review should help make the approval process as open and transparent to potential wind developers as possible.
  • The Project will undertake concrete steps such as placing up to three wind anemometers in highly motivated communities to determine the actual wind resource.
  • A major concern will also be to fashion ways in which local communities can benefit from wind projects, particularly near-shore and offshore projects.
  • Finally, the pipeline of potential projects for wind development will be reviewed and ranked in order of interest and potential benefits they can bring to areas of the coastal zone.

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COASTAL WIND WORKING GROUP FOCUS

The North Carolina State Energy Office, the North Carolina Solar Center, and the Renewable Energy Policy Project are initiating this public process by forming the Coastal Wind Working Group. The Coastal Wind Working Group will over the next year undertake several efforts to:

  • Improve the wind resource assessment and increase the public awareness of this resource;
  • Increase the understanding of the potential for wind energy development in coastal North Carolina and assess the community interest in specific wind developments;
  • Develop an initial assessment of the approval path for near-shore and offshore developments;
  • Establish where among all the potential sites the best opportunities exist;
  • Develop ways by which proximate or affected communities can share in the development of near shore and offshore projects, and;
  • Determine how to remove roadblocks to resource development and in particular to assess how best to provide the market access and contracts necessary for commercial development.

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BASIC RESOURCE ASSESSMENT

The North Carolina State Energy Office has undertaken the initial steps to assess the wind resource in the coastal areas of the state. The wind maps based on this initial assessment show significant areas with wind class 3 and above wind resources. The potential is found in all relevant areas: off shore, near shore, and some onshore locations. The next stage of this assessment is to look more closely at the specific areas of opportunity and to begin to overlay the wind resource potential map with other maps outlining other types of information such as geographic, historic, and environmental sensitivities in order to establish areas with the greatest potential for development. In addition, the maps will add an economic assessment of the site desirability by including factors such as distance from roads and cost of transmission interconnection. When finished, the maps will show wind resource, special development restrictions, local community reaction, and economic viability. The intent is to use the resource assessment to determine where the best opportunities for pursuing coastal development exist and structure project development towards these sites.

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ASSESS COMMUNITY INTEREST

A major outreach effort will be conducted to contact the communities in the coastal region. Particular efforts will be made to contact communities of color, educational institutions, farming communities, and businesses to determine their interest in wind power development. Outreach will also be made towards other communities to explain in advance the economic, environmental, and visual impacts of potential developments. The thrust of this effort will be to determine in advance, to the extent possible, community reaction and to work particularly with those communities that see benefits from wind development and have an interest in supporting wind projects.

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SITING AND APPROVAL

The Coastal Wind Working Group believes that a responsible approach towards siting this resource must meet two objectives. The process for siting and approval should be responsive to public demands and should allow an appropriate balance of state and federal interests. The siting and approval process must also be as transparent as possible in order to make the resource reasonably available to developers. Offshore wind development is in its earliest stages in the United States. One project is actively being pursued in the Cape Cod region but has not obtained the permits necessary for development. The uncertainty and confusion associated with the Cape development has resulted in several attempts to draft legislation streamlining the federal licensing requirements. Experience with the Cape project has shown that both federal and state authorities can influence project approval. What is not clear is the lines of authority between federal and state authorities. Desirable development can be thwarted by uncertainty. We believe that establishing at least an initial assessment of the siting process can reduce that uncertainty and make the process more open and transparent. It is important to get an understanding of the siting and approval process from the agencies with oversight of the process at the federal and state level. That assessment of the approval process can then be judged against the desires of the public. A major goal of this effort is to identify the important state interests that should be considered in the permitting process, assess the likelihood of having these interests appropriately considered in the present approval scheme, and suggesting appropriate changes in the event state interests are judged to be inadequately considered. To view the siting and approval report, "Offshore Wind Farm Approval Process, North Carolina", click here.

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PROJECT DEVELOPMENT OPPORTUNITIES

The wind maps show that the wind resource is strongest offshore but that near and on shore sites also can be economically developed. We fully expect that some of the potential sites will be identified for any number of reasons as undesirable. At the same time, we believe that the ability of wind energy development to deliver substantial economic development benefits to localities in which projects are located will be attractive to some of the coastal communities. The goal of this effort will be to establish an initial pipeline of projects that meet a minimum threshold of attractiveness for development. The pipeline will be compared to the potential demand for wind, taking into account policy mandates such as the NC GreenPower pricing targets. Once this pipeline has been established, it will be a goal of the project to examine how the maximum economic benefits can be delivered to local communities.

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COMMUNITY SHARE IN DEVELOPMENT

In rural areas that have already experienced substantial wind energy development farmers have shared in the projects by receiving royalty payments based on the installed capacity on their land. These payments have usually taken the form of royalty payments per turbine installed. For offshore projects no models exist to demonstrate how nearby or affected communities can share in the economic benefits of the developments. This project will work to develop potential models that can bring a share of the projects benefits to affected communities and will work with interested communities to select the most desirable mechanisms.

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PROJECT DEVELOPMENT AIDS

Substantial uncertainty in energy markets makes it difficult for renewable projects to be developed without support. These supports most often take the form of contracted purchase of output either to satisfy green pricing program purchases or meet renewable portfolio standards requirements. There are other potential public demands that can also be translated into supports sufficient to allow project developments. An aim of this assessment will be to assemble the public demands for wind power in the foreseeable future in North Carolina and offer those to potential developers in coastal areas.

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+ نوشته شده در  یکشنبه دوازدهم آذر 1385ساعت 20:49  توسط امیر علی باب هادی عشر  | 

این هم یک تحقیق انگلیسی درباره انرزی زمین گرمایی

Geothermal Energy for Electric Power
A REPP Issue Brief

By Masashi Shibaki
With Fredric Beck, Executive Editor1

December 2003


FORWARD
This paper provides a general background on utility-scale geothermal power and seeks to teach the readers a basic understanding of geothermal power, as well as build a solid foundation for further understanding of the technical, economic, and policy dimensions of geothermal power worldwide. Economic data and current U.S. geothermal policy help elucidate the concepts of this paper. Readers may refer to the extensive references to reports and Web links to well-established geothermal energy sources, at the end of this brief to become learn the latest developments in geothermal power’s role in clean energy generation.

INTRODUCTION
Geothermal1 energy is energy derived from the heat of the earth’s core. It is clean, abundant, and reliable. If properly developed, it can offer a renewable and sustainable energy source. There are three primary applications of geothermal energy: electricity generation, direct use of heat, and ground-source heat pumps. Direct use includes applications such as heating buildings or greenhouses and drying foods, whereas ground source heat pumps are used to heat and cool buildings using surface soils as a heat reservoir. This paper covers the use of geothermal resources for production of utility-scale electricity and provides an overview of the history, technologies, economics, environmental impacts, and policies related to geothermal power.

_____________
1The authors would like to thank Karl Gawell and Diana Bates of the Geothermal Energy Association for circulating this paper for peer review and for providing valuable comments, and Kelly Ross and Leona Kanaskie of REPP for technical editing of the document.

 

 

Geothermal Resources

Understanding geothermal energy begins with an understanding of the source of this energy—the earth’s internal heat. The Earth’s temperature increases with depth, with the temperature at the center reaching more than 4200 °C (7600 °F). A portion of this heat is a relic of the planet’s formation about 4.5 billion years ago, and a portion is generated by the continuing decay of radioactive isotopes. Heat naturally moves from hotter to cooler regions, so Earth’s heat flows from its interior toward the surface.2

Because the geologic processes known as plate tectonics, the Earth’s crust has been broken into 12 huge plates that move apart or push together at a rate of millimeters per year. Where two plates collide, one plate can thrust below the other, producing extraordinary phenomena such as ocean trenches or strong earthquakes. At great depth, just above the down going plate, temperatures become high enough to melt rock, forming magma.3 Because magma is less dense than surrounding rocks, it moves up toward the earth’s crust and carries heat from below. Sometimes magma rises to the surface through thin or fractured crust as lava.

Figure 1. Schematic of geothermal power plant production and injection wells.
Source: U.S. Department of Energy, http://www.eia.doe.gov/kids/renewable/geothermal.html.

However, most magma remains below earth’s crust and heats the surrounding rocks and subterranean water. Some of this water comes all the way up to the surface through faults and cracks in the earth as hot springs or geysers. When this rising hot water and steam is trapped in permeable rocks under a layer of impermeable rocks, it is called a geothermal reservoir. These reservoirs are sources of geothermal energy that can potentially be tapped for electricity generation or direct use. Figure 1 is a schematic of a typical geothermal power plant showing the location of magma and a geothermal reservoir.4 Here, the production well withdraws heated geothermal fluid, and the injection well returns cooled fluids to the reservoir.

Resource Identification

Geological, hydrogeological, geophysical, and geochemical techniques are used to identify and quantify geothermal resources. Geological and hydrogeological studies involve mapping any hot springs or other surface thermal features and the identification of favorable geological structures. These studies are used to recommend where production wells can be drilled with the highest probability of tapping into the geothermal resource. Geophysical surveys are implemented to figure the shape, size, depth and other important characteristics of the deep geological structures by using the following parameters: temperature (thermal survey), electrical conductivity (electrical and electromagnetic methods), propagation velocity of elastic waves (seismic survey), density (gravity survey), and magnetic susceptibility (magnetic survey).5 Geochemical surveys (including isotope geochemistry) are a useful means of determining whether the geothermal system is water or vapor-dominated, of estimating the minimum temperature expected at depth, of estimating the homogeneity of the water supply and, of determining the source of recharge water.

Geothermal exploration addresses at least nine objectives:6

1.     Identification of geothermal phenomena

2.     Ascertaining that a useful geothermal production field exists

3.     Estimation of the size of the resource

4.     Classification of the geothermal field

5.     Location of productive zones

6.     Determination of the heat content of the fluids that will be discharged by the wells in the geothermal field

7.     Compilation of a body of data against which the results of future monitoring can be viewed

8.     Assessment of the pre-exploitation values of environmentally sensitive parameters

9.     Determination of any characteristics that might cause problems during field development

Drilling
Once potential geothermal resources have been identified, exploratory drilling is carried out to further quantify the resource. Because of the high temperature and corrosive nature of geothermal fluids, as well as the hard and abrasive nature of reservoir rocks found in geothermal environments, geothermal drilling is much more difficult and expensive than conventional petroleum drilling. Each geothermal well costs $1–4 million to drill, and a geothermal field may consist of 10–100 wells. Drilling can account for 30–50% of a geothermal project’s total cost.7 Typically, geothermal wells are drilled to depths ranging from 200 to 1,500 meters depth for low- and medium-temperature systems, and from 700 to 3,000 meters depth for high-temperature systems. Wells can be drilled vertically or at an angle. Wells are drilled in a series of stages, with each stage being of smaller diameter than the previous stage, and each being secured by steel casings, which are cemented in place before drilling the subsequent stage. The final production sections of the well use an uncemented perforated liner, allowing the geothermal fluid to pass into the pipe. The objectives of this phase are to prove the existence of an exploitable resource and to delineate the extent and the characteristics of the resource. An exploratory drilling program may include shallow temperature-gradient wells, “slim-hole” exploration wells, and production-sized exploration/production wells. Temperature-gradient wells are often drilled from 2–200 meters in depth with diameters of 50–150 mm. Slim-hole exploration wells are usually drilled from 200 to 3000 meters in depth with bottom-hole diameters of 100 to 220 mm. The size and objective of the development will determine the number and type of wells to be included in exploratory drilling programs.8

 

 

 

 

 

 

 

 

 

 

 

 

 

Geothermal Power Technology

Utility-scale geothermal power production employs three main technologies. These are known as dry steam, flash steam and binary cycle systems. The technology employed depends on the temperature and pressure of the geothermal reservoir. Unlike solar, wind, and hydro-based renewable power, geothermal power plant operation is independent of fluctuations in daily and seasonal weather.

Dry steam
Dry steam power plants use very hot (>455 °F, or >235 °C) steam and little water from the geothermal reservoir.12 The steam goes directly through a pipe to a turbine to spin a generator that produces electricity. This type of geothermal power plant is the oldest, first being used at Lardarello, Italy, in 1904.13 Figure 2 is a schematic of a typical dry steam power plant.14

  

Figure 2. Dry Steam Power Plant Schematic  Figure 3. Flash Steam Power Plant Schematic
Source: National Renewable Energy Laboratory (NREL)

Flash steam 
Flash steam power plants use hot water (>360 ºF, or >182 ºC) from the geothermal reservoir.15 When the water is pumped to the generator, it is released from the pressure of the deep reservoir. The sudden drop in pressure causes some of the water to vaporize to steam, which spins a turbine to generate electricity. Both dry steam and flash steam power plants emit small amounts of carbon dioxide, nitric oxide, and sulfur, but generally 50 times less than traditional fossil-fuel power plants.16 Hot water not flashed into steam is returned to the geothermal reservoir through injection wells. Figure 3 is a schematic of a typical flash steam power plant.17

Binary-cycle
Binary-cycle power plants use moderate-temperature water (225 ºF–360 ºF, or 107 ºC–182 ºC) from the geothermal reservoir. In binary systems, hot geothermal fluids are passed through one side of a heat exchanger to heat a working fluid in a separate adjacent pipe. The working fluid, usually an organic compound with a low boiling point such as Iso-butane or Iso-pentane, is vaporized and passed through a turbine to generate electricity. An ammonia-water working fluid is also used in what is known as the Kalina Cycle. Makers claim that the Kalina Cycle system boosts geothermal plant efficiency by 20–40 percent and reduces plant construction costs by 20–30 percent, thereby lowering the cost of geothermal power generation.



Figure 4. Binary Cycle Power Plant Schematic
Source: National Renewable Energy Laboratory (NREL)

The advantages of binary cycle systems are that the working fluid boils at a lower temperature than water does, so electricity can be generated from reservoirs with lower temperature, and the binary cycle system is self-contained and therefore, produces virtually no emissions. For these reasons, some geothermal experts believe binary cycle systems could be the dominant geothermal power plants of the future. Figure 4 is a schematic of a typical binary cycle power plant.18

Geothermal Power Generation

As of 2000, approximately 8,000 megawatts (MW) of geothermal electrical generating capacity was present in more than 20 countries, led by the United States, Philippines, Italy, Mexico, and Indonesia (see Table 2 below). This represents 0.25% of worldwide installed electrical generation capacity. In the United States, geothermal power capacity was 2,228 MW, or approximately 10% of non-hydro renewable generating capacity in 2001 (see Figure 5 below).19 This capacity would meet the electricity needs of approximately 1.7 million U.S. households.20

Current geothermal use is only a fraction of the total potential of geothermal energy. U.S. geothermal resources alone are estimated at 70,000,000 quads21, equivalent to 750,000-years of total primary energy supply (TPES) for the entire nation at current rates of consumption. The geothermal energy potential in the uppermost 6 miles of the Earth’s crust amounts to 50,000 times the energy of all known oil and gas resources in the world.22 Not all of these resources are technologically or economically accessible, but tapping into even a fraction of this potential could provide significant renewable resources for years to come. The Geothermal Energy Association reports the potential for developing an additional 23,000 MW of generating capacity in the United States using conventional geothermal energy technology.23

Table 2. Installed Geothermal Generating Capacities Worldwide24

Country

1995 (MWe)

2000 (MWe)

Country

1995 (MWe)

2000 (MWe)

United States

2,817

2,228

Kenya

45

45

Philippines

1,227

1,909

Guatemala

33

33

Italy

632

785

China

29

29

Mexico

753

755

Russia

11

23

Indonesia

310

590

Turkey

20

20

Japan

414

547

Portugal

5

16

New Zealand

286

437

Ethiopia

0

8

Iceland

50

170

France

4

4

El Salvador

105

161

Thailand

0.3

0.3

Costa Rica

55

142

Australia

0.2

0.2

Nicaragua

70

70

Argentina

0.7

0

Total (MW)

 

 

 

6,833

7,974




Figure 5. U.S. Nonhydro Renewable Power Generating Capacity, 2001

Source: EIA Renewable Energy Annual 2001. Biomass excludes agriculture byproducts/crops, sludge waste, tires, and other biomass solids, liquids, and gases.

Capacity Factor
The percentage of time a power plant runs is the plants capacity factor. Geothermal power plants typically produce electricity about 90% of the time, though can be run up to 98% of the time if the contract price of power is high enough to justify increased operational and maintenance costs. In comparison, coal-fired power plants are typically run 65–75% of the time, while nuclear plants in the United States have run at very high capacity factors (95–98%) in recent years due to lucrative market and regulatory conditions.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Economics

The commercial viability of geothermal power production is influenced by capital costs for land, drilling, and physical plant; operating and maintenance costs; the amount of power generated and sold from the plant; and the market value of that power. However, because geothermal power plants incur high capital costs at the beginning of the project, they are typically at an economic disadvantage to conventional fossil fueled power plants. Fossil fuel plants have lower up-front capital costs, but incur fuel costs for the life of the plant. This section discusses capital cost, operating and maintenance cost, average cost of power production over the life of the plant (known as the levelized cost of power production), as well as the economic impacts of geothermal power such as labor creation, tax base contributions, and balance-of-trade impacts.

Capital Cost
Capital costs are the fixed costs for power plant construction. Geothermal capital costs include the cost of land, drilling of exploratory and steam field wells, and physical plant, including buildings and power-generating turbines. Geothermal plants are relatively capital-intensive, with low variable costs and no fuel costs. The capital cost for geothermal power plants ranges from $1150 to $3000 per installed KW, depending on the resource temperature, chemistry, and technology employed. These costs may decrease over time with additional technology development. Plant lifetimes are typically 30–45 years. Financing is often structured such that the project pays back its capital costs in the first 15 years. Costs then fall by 50–70%, to cover just operations and maintenance for the remaining 15–30 years that the facility operates.25 Table 3 shows the capital costs for geothermal plants, and Table 4 shows conventional baseload power direct capital costs for comparison.


Table 3. Geothermal Power Direct Capital Costs
(US$1999 /KW installed capacity)26

Plant Size

Cost

High-Quality Resource

Medium-Quality Resource

Small plants (<5 MW)

Exploration

$400–$800

$400–$1,000

Steam field

$100–$200

$300–$600

Power plant

$1,100–$1,300

$1,100–$1,400

Total

$1,600–$2300

$1,800–$3,000

Medium plants (5–30 MW)

Exploration

$250–$400

$250–$600

Steam field

$200–$500

$400–$700

Power plant

$850–$1,200

$950–$1,200

Total

$1,300–$2,100

$1,600–$2,500

Large plants (>30 MW)

Exploration

$100–$400

$100–$400

Steam field

$300–$450

$400–$700

Power plant

$750–$1,100

$850–$1,100

Total

$1,150–$1,750

$1,350–$2,200

Table 4. Conventional Baseload Power Direct Capital Costs

 

Resource

CaptialCost ($US1999/kW)

Geothermal

$1,150–$3,000

Hydropower27

$735–$4,778

Coal28

$1,070–$1,410

Nuclear29

$1,500–$4,000



Operating and Maintenance Cost

Geothermal power plant operating and maintenance costs range from $0.015 to $0.045 per KWh, depending on how often the plant runs. Geothermal plants typically run 90% of the time. They can be run up to 97–98% of the time, but this increases maintenance costs. High run times are found when contractual agreements pay high prices for power. Higher-priced electricity justifies running the plant at high-capacity factors because the resulting higher maintenance costs are recovered. Table 5 provides geothermal operating and maintenance cost by plant size. Large plants tend to have lower O&M costs due to economies of scale.


Table 5. Geothermal Operating and Maintenance Costs
by Plants Size (U.S. cents/kWh)30

Cost Component

Small Plants
(<5 MW)

Medium Plants (5–30 MW)

Large Plants
(>30 MW)

Steam field

0.35–0.7

0.25–0.35

0.15–0.25

Power plants

0.45–0.7

0.35–0.45

0.25–0.45

Total

0.8–1.4

0.6–0.8

0.4–0.7


As shown by Table 6, geothermal operating costs of 0.4–1.4 ¢/kWh are within the range of O&M costs of conventional power plants.


Table 6. Opeating and Maintenance Cost Comparison
by Baseload Power Source (U.S. cents/kWh)

Resource

O&M Cost (cents/kWh)

Geothermal

0.4–1.4

Hydropower31

0.7

Coal32

0.46

Nuclear33

1.9

Levelized Cost
The levelized cost of power production is the average cost of power production over the life of a power plant, taking into account all capital expenses and operating and maintenance costs, as well as fuel costs for power plants that rely on external fuel sources. Major factors affecting geothermal power cost are the depth and temperature of the resource, well productivity, environmental compliance, project infrastructure and economic factors such as the scale of development, and project financing costs.

Real levelized costs for geothermal electricity generation are $0.045-$0.07 per KWh, which is competitive with some fossil fuel facilities, without the pollution.34 The lowest cost of geothermal electricity is approximately $0.015 per KWh. At the Geysers, power is sold at $0.03 to $0.035 per KWh. Some geothermal power plants can charge more per KWh during some time periods, because of incentives related to reliability of generation and power provided during peak demand. The cost of generating power from geothermal resources has decreased about 25% over the past two decades.35

The goal of the geothermal industry and the U.S. Department of Energy is to achieve a geothermal energy life-cycle cost of electricity of $0.03 per KWh. It is anticipated that costs in this range will result in about 10,000 MW of new capacity installed by U.S. firms within the next decade. Table 7 presents the levelized cost comparison of power by source. It shows that in some cases, geothermal energy can compete directly with conventional baseload power sources.

Table 7. Levelized Cost Comparison
of Baseload Power by Source

Resource

Levelized Cost36
(U.S. cents/kWh)

Geothermal

1.5–7.0

Hydropower

0.5–2.4

Coal

2.0–5.0

Nuclear

1.5–3.0

Job Creation

In 1996, the U.S. geothermal energy industry as a whole provided approximately 12,300 direct jobs in the United States, and an additional 27,700 indirect jobs in the United States. The electric generation part of the industry employed about 10,000 people to install and operate geothermal power plants in the United States and abroad, including power plant construction and related activities such as exploration and drilling; indirect employment was about 20,000.37 Table 8 provides estimates of job creation from renewable energy development based on existing and planned projects in California and the market outlook of project developers and equipment manufacturers. Natural gas is included in the table because the bulk of new nonrenewable generation is expected to rely upon natural gas. The table indicates that geothermal and landfill methane energy generation yields significantly more jobs per MW of installed capacity than do natural gas plants.


Table 8. Employment Rates by Energy Technology38,39

Power Source

Construction Employment (jobs/MW)

O&M Employment (jobs/MW)

Total Employment for 500 MW Capacity

Factor Increase over Natural Gas

Wind

2.6

0.3

5,635

2.3

Geothermal

4.0

1.7

27,050

11.0

Solar PV

7.1

0.1

5,370

2.2

Solar thermal

5.7

0.2

6,155

2.5

Landfill methan/digester gas

3.7

2.3

36,055

14.7

Natural gas

1.0

0.1

2,460

1.0


Economic Impacts
One of the most important economic aspects of geothermal energy is that it is generated with indigenous resources, reducing a nation’s dependence on imported energy, thereby reducing trade deficits. Reducing trade deficits keeps wealth at home and promotes healthier economies. Nearly half of the U.S. annual trade deficit would be erased if imported oil were displaced with domestic energy resources.

Geothermal energy production in the United States is a $1.5-billion-dollar-per-year industry.40 Nevada’s geothermal plants produce about 210 MW of electricity, saving energy imports equivalent to 800,000 tons of coal or 3 million barrels of oil each year. In addition, state governments receive tax revenue. In 1993, Nevada’s geothermal power plants paid $800,000 in county taxes and $1.7 million in property taxes. The U.S. Bureau of Land Management collects nearly $20 million each year in rent and royalties from geothermal plants producing power on federal lands in Nevada—half of these revenues are returned to the state.41

Economic Impacts in Developing Countries
Nearly half of the developing countries have rich geothermal resources, which could prove to be an important source of power and revenue.42 Geothermal projects can reduce the economic pressure of developing country fuel imports and can offer local infrastructure development and employment. For example, the Philippines have exploited local geothermal resources to reduce dependence on imported oil, with installed geothermal capacity and power generation second in the world after the United States. In the late 1970s, the Philippine government instituted a comprehensive energy plan, under which hydropower, geothermal energy, coal, and other indigenous resources were developed and substituted for fuel oil, reducing their petroleum dependence from 95% in the early 1970s to 50% by the mid-1980s.43

Developing countries will likely require increasing amounts of power in the coming years. Through technology transfer programs, some industrialized countries are helping developing countries make use of their local sustainable and reliable geothermal energy resources.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Environmental Impacts

Geothermal power plants do have some environmental impacts. However, these impacts should be balanced against geothermal energy’s advantages over conventional power sources when conducting assessments of power plant project environmental impacts. The primary impacts of geothermal plant construction and energy production are gaseous emissions, land use, noise, and potential ground subsidence.

Gaseous Emissions
Geothermal fluids contain dissolved gases, mainly carbon dioxide (CO2) and hydrogen sulfide (H2S), small amounts of ammonia, hydrogen, nitrogen, methane and radon, and minor quantities of volatile species of boron, arsenic, and mercury. Geothermal power provides significant environmental advantage over fossil fuel power sources in terms of air emissions because geothermal energy production releases no nitrogen oxides (NOx), no sulfur dioxide (SO2), and much less carbon CO2 than fossil-fueled power. The reduction in nitrogen and sulfur emissions reduces local and regional impacts of acid rain, and reduction in carbon-dioxide emissions reduce contributions to potential global climate change. Geothermal power plant CO2 emissions can vary from plant to plant depending on both the characteristics of the reservoir fluid and the type of power generation plant. Binary plants have no CO2 emissions, while dry steam and flash steam plants have CO2 emissions on the order of 0.2 lb/kWh, less than one tenth of the CO2 emissions of coal-fired generation (see Table 9). According to the Geothermal Energy Association, improved and increased injection to sustain geothermal reservoirs has helped reduce CO2 emissions from geothermal power plants.

Table 9. Comparison of CO2 Emissions by Power Source44

Power Source

CO2 Emissions (lb/kWh)

Geothermal

0.20

Natural gas

1.321

Oil

1.969

Coal

2.095


Hydrogen sulfide emissions do not contribute to acid rain or global climate change but does create a sulfur smell that some people find objectionable. The range of H2S emissions from geothermal plants is 0.03–6.4 g/kWh.45 Hydrogen sulfide emissions can vary significantly from field to field, depending on the amount of hydrogen sulfide contained in the geothermal fluid and the type of plant used to exploit the reservoir. The removal of H2S from geothermal steam is mandatory in the United States. The most common process is the Stretford process, which produces pure sulfur and is capable of reducing H2S emissions by more than 90%.46 More recently developed techniques include burning the hydrogen sulfide to produce sulfur dioxide, which can be dissolved, converted to sulfuric acid and sold to provide income.

Landscape Impacts and Land Use

Geothermal power plants require relatively little land. Geothermal installations don’t require damming of rivers or harvesting of forests, and there are no mineshafts, tunnels, open pits, waste heaps or oil spills. An entire geothermal field uses only1–8 acres per MW versus 5–10 acres per MW for nuclear plants and 19 acres per MW for coal plants.47

Table 10 compares acreage requirements by technology. Geothermal power plants are clean because they neither burn fossil fuels nor produce nuclear waste. Geothermal plants can be sited in farmland and forests and can share land with cattle and local wildlife. For example, the Hell’s Gate National Park in Kenya was established around an existing 45-MWe geothermal power station, Olkaria I. Land uses in the park include livestock grazing, growing of foodstuffs and flowers, and conservation of wildlife and birds within the Park. After extensive environmental impact analysis, a second geothermal plant, Olkaria II, was approved for installation in the park in 1994, and an additional power station is under consideration.48

Table 10. Comparison of Land Requirement for Baseload
Power Genreation

Power Source

Land Requirement (Acre/MW)

Geothermal

1–8

Nuclear

5–10

Coal

19


Geothermal plants are also benign with respect to water pollution. Production and injection wells are lined with steel casing and cement to isolate fluids from the environment. Spent thermal waters are injected back into the reservoirs from which the fluids were derived. This practice neatly solves the water-disposal problem while helping to bolster reservoir pressure and prolong the resource’s productive existence.49

Noise

Noise occurs during exploration drilling and construction phases. Table 11 (next page) shows noise levels from these operations can range from 45 to 120 decibels (dBa). For comparison, noise levels in quiet suburban residences are on the order of 50 dBa, noise levels in noisy urban environments are typically 80–90 dBa, and the threshold of pain is 120 dBa at 2,000–4,000 Hz.50 Site workers can be protected by wearing ear mufflers. With best practices, noise levels can be kept to below 65 dBa, and construction noise should be practically indistinguishable from other background noises at distances of one kilometer.

Table 11. Geothermal Exploration and construction Noise Levels
by Operation51

Operation

Noise Level (dBa)

Air drilling

85–120

Mud drilling

80

Discharging wells after drilling (to remove drilling debris)

Up to 120

Well testing

70–110

Diesel engines (to operate compressors and provide electricity)

45–55

Heavy machinery (e.g., for earth moving during construction)

Up to 90

Ground Subsidence

In the early stages of a geothermal development, geothermal fluids are withdrawn from a reservoir at a rate greater than the natural inflow into the reservoir. This net outflow causes rock formations at the site to compact, particularly in the case of clays and sediments, leading to ground subsidence at the surface. Key factors causing subsidence include:

  • A pressure drop in the reservoir as a result of fluid withdrawal
  • The presence of a highly compressible geological rock formation above or in the upper part of a shallow reservoir
  • The presence of high-permeability paths between the reservoir and the formation, and between the reservoir and the ground surface

If all of these conditions are present, ground subsidence is likely to occur. In general, subsidence is greater in liquid-dominated fields because of the geological characteristics typically associated with each type of field. Ground subsidence can affect the stability of pipelines, drains, and well casings. It can also cause the formation of ponds and cracks in the ground and, if the site is close to a populated area, it can lead to instability of buildings.

The largest recorded subsidence in a geothermal field was at Wairakei in New Zealand. Here the ground subsided as much as 13 meters. Monitoring has shown that a maximum subsidence rate of 45 cm/year occurred in a small region, outside the production area, with subsidence of at least 2 cm/year occurring all over the production field.52 Effects of the subsidence in the Wairakei region included:

  • The creation of a pond about 1 km in length and 6 m in depth in what was originally a fast-flowing stream,
  • Cracking of both a nearby highway and the main waste water drain on the site,
  • Compressive buckling and tensile fracturing of steam pipelines, and
  • Fissures in surroundings fields.

Although Wairakei presents an extreme example, little is currently known about how to prevent or mitigate subsidence effects. The only action is to try to maintain pressure in the reservoir.53 Fluid re-injection can help to reduce pressure drop and hence subsidence, but its effectiveness depends on where the fluid is re-injected and the permeability conditions in the field. Typically, re-injection is done at some distance from the production well to avoid the cooler rejected waste fluid from lowering the temperature of the production fluid and may not help prevent subsidence.54

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Policy

Renewable energy can reduce dependence on fossil fuels, reduce harmful pollution from energy production and consumption, and reduce emissions of greenhouse gases. However, most renewables have very different cost structures from conventional energy generating technologies, with high up-front costs and low operating costs. This is true for geothermal energy, which has high exploration and drilling costs in aditional to capital plant expenses. With additional technology development, these costs can be lowered, and geothermal energy can become more cost-competitive with other energy sources.

To spur geothermal technology and market development, the United States has developed policies at the federal and state government level offering a variety of tax incentives for the manufacture, installation, and use of renewables. This section discusses U.S. federal and state policies to promote geothermal energy, as well as policies in other nations with significant geothermal resources.

U.S. Federal Policies
With the oil embargoes and energy crisis of the 1970s, as well as growing environmental awareness, concerns about the United States continued dependence on conventional fossil fuels, as well as energy-related health and environmental hazards were raised. Policies to promote renewable energy and energy efficiency were developed to help decrease the Nation’s dependence on fossil fuels and increase domestic energy conservation and efficiency. This section focuses on the some approaches by the U.S. government to encourage the development of geothermal energy, including R&D funding, tax credits, and regulatory policy.

Federal Research and Development (R&D)
Federal energy R&D funding is important for maintaining technological progress in energy development since private industry cannot afford to fully fund, on its own, the continued research required. Energy R&D progress reduces cost, as well as increases energy yields from existing resources. Federal energy R&D includes nuclear, fossil fuel, renewable, energy conservation, and other energy technologies. Many geothermal energy R&D projects are undertaken in conjunction with industry partners and universities to ensure rapid deployment of the new technology into the marketplace.

During the mid-1990s, ongoing deregulation of the electric and natural gas utility industry in the United States, along with lower energy prices, resulted in a significant downturn in the private sector’s support for energy R&D. President Clinton, reacting to the trends, asked his Committee of Advisors on Science and Technology (PCAST) to perform an assessment of the U.S. energy R&D effort.55 As a result of PCAST’s energy R&D assessment, a recommendation was made to set aside $51 million for geothermal energy R&D. This proposal included recommendations to expand advanced drilling R&D through the National Advanced Drilling and Excavation Technologies Institute, increase R&D on reservoir testing and modeling, and increase geothermal productivity. However, appropriations for FY’01 only amounted to $26.6 million, less than half of PCAST’s recommended funding. As demonstrated in Figure 6, appropriations for geothermal R&D have remained relatively flat from 1998 to 2003, at approximately half of the PCAST recommended level.56 Increased federal geothermal R&D appropriations would help geothermal energy is to expand to its fullest potential.

Figure 6. U.S. Geothermal Energy R&D Budget, 1998–2003
Source: Department of Energy Office of Budget

 

 

 

Public Utility Regulatory Policies Act (PURPA)

The Public Utility Regulatory Policies Act (PURPA) is one of five statutes of the National Energy Conservation Policy Act of 1978, which sought to decrease the Nation’s dependence on foreign oil. The intent of PURPA is to encourage the development of independent, non-utility, fuel-efficient cogeneration plants and small renewable energy power projects by requiring utilities to buy power from such plants at the utility’s avoided cost. An avoided cost is that amount that a utility would otherwise have to spend to generate or procure power. As state above, PURPA requires utilities to buy power from two types of independent power producers: (1) small power producers using renewable energy sources; and (2) co-generators. Under PURPA, independent power producers are designated as qualifying facilities (QFs). A QF seeking a small power producer status must produce energy with at least 75 percent of the total energy input provided by renewable energy. A QF seeking co-generator status under PURPA must produce electricity and another form of energy sequentially while using the same fuel source. One of the benefits of PURPA is that it allows a period of fixed payments for both energy and capacity via long-term contracts which then makes a favorable environment for renewables, including geothermal, to obtain financing.

Tax credits
Tax credits are used as a tool to encourage certain behaviors or influence decisions. The U.S. government has been using tax credits to influence energy production decisions for decades. The first energy tax incentives arrived on the scene in 1978 with the passage of the Energy Tax Act of 1978. Tax incentives have been created, terminated, and reactivated in the United States over the past 20 years. In 1978, the Energy Tax Act extended a 10% business energy tax credit for investments in solar, wind, geothermal, and ocean thermal technologies. In 1986, the Tax Reform Act repealed the 10% business energy tax credit. In 1992, the 10% business energy tax credit returned as a permanent tax credit under the Energy Policy Act, but the credit could only be applied to investments in solar and geothermal equipment.

Other factors influence the ebb and flow of tax credits, such as politics, economics, and energy supply. Variability in the political support of tax incentives creates uncertainly in long-term renewable markets, therefore, making it difficult for developers to maximize the opportunities for development of renewables. However, without tax credits, the penetration of renewable energy, such as geothermal, into the energy production sector would be more difficult. Including geothermal energy under the federal Production Tax Credit (PTC) could provide a significant boost to the geothermal sector.

U.S. State Policies

State governments, in addition to the federal government, have initiated programs and policies to drive the diversification of the nation’s energy portfolio by incorporating renewable energy into the energy supply. Identified below are some policy measures that are influencing energy policy decisions at the state level.

Public Benefit Funds

Public Benefit Funds (PBF) are generated from a few sources such as a customer charge on utility bills and new user access fees to fund various public programs. These programs include low-income energy assistance, energy efficiency, consumer energy education, and renewable energy technology development and demonstration. California was the first state to create a PBF. In 1996, California placed a charge on all electricity bills from 1998 through 2001 that would provide $540 million for “new and emerging” renewable energy technologies. As of 2002, at least 24 states have a Public Benefit Fund program in place. See REPP’s map of state PBF policies for specific details of these policies at http://www.repp.org/sbf_map.html.

Renewable Portfolio Standards

Renewable Portfolio Standards (RPS) mandate a state to generate a percentage of its electricity from renewable sources or meet a specific renewable capacity requirement. Each state has a choice of how to fulfill this mandate using a combination of renewable energy sources, including wind, solar, biomass, geothermal, or other renewable sources. As of 2002, 12 states have adopted an RPS as part of their restructuring processes. California, for example, has an aggressive renewable portfolio standard requiring utilities to purchase 20% of their electricity from renewable sources by 2017. In 1999, Texas initiated a capacity-based standard to ensure that 2,000 megawatts (MW) of new generating capacity from renewable energy technologies be installed by 2009. Geothermal energy will most likely help fulfill RPS requirements in western states where geothermal energy is more prevalent. See REPP’s map of state RPS policies for specific details of these policies at http://www.repp.org/rps_map.html.

 

Policies in Other Nations
The Philippines, the world’s second largest user of geothermal energy for power generation, provides an example of several incentives to attract geothermal development. They are as follows:

  • Recovery of operating expenses not exceeding 90% of the gross value in any year with carry forward of unrecovered cost,
  • Service fee of up to 40% of net proceeds,
  • Exemption from all taxes except income tax,
  • Income tax obligation paid out of government’s share,
  • Exemption from payment of tariff duties and compensating tax on the importation of machinery, equipment, share parts and all materials for geothermal operation,
  • Depreciation of capital equipment over a 10-year period,
  • Easy repatriation of capital equipment investment and remittance of earnings, and
  • Entry of alien technoical and specialized personnel (including members of immediate families)

According to the Philippine Department of Energy, an additional eight geothermal power plants will come on line from 2003 to 2010. Expected capacity additions during this time total 621 MWe.57

 

 

 

 

 

 

 

 

 

 

 

 

 

Future Developments

Renewable energy technology is continuously evolving with the goal of reducing risk and lowering cost. The goal of the geothermal industry and the U.S. Department of Energy is to achieve a geothermal energy life-cycle cost of electricity of $0.03 per KWh.58 To achieve the goal of lowering cost and risk, other types of nontraditional resources and experimental systems are being explored. Among these are hot dry rock resources, improved heat exchangers, and improved condenser efficiency.

Hot Dry Rock
Hot dry rock geothermal technology offers enormous potential for electricity production. These resources are much deeper than hydrothermal resources. Hot dry rock energy comes from relatively water-free hot rock found at a depth of about 4,000 meters or more beneath the Earth’s surface. One way to extract the energy is by circulating water through man-made fractures in the hot rock. Heat can then be extracted from the water at the surface for power generation, and the cooled water can then be recycled through the fractures to pick up more heat, creating a closed-looped system. Hot Dry Rock resources have yet to be commercially developed. One reason for this is that well costs increase exponentially with depth, and since Hot Dry Rock resources are much deeper than hydrothermal resources, they are much more expensive to develop. Figure 7 shows the projected capital cost for hot dry rock compared to traditional geothermal power technology from 1996 to 2030. The figure shows that the capital cost of hot dry rock will decrease by almost half in 30 years, but it will still be twice as expensive as other traditional geothermal technologies. If the technology can evolve to make hot dry rock resources commercially viable, hot dry rock resources are sufficiently large enough to supply a significant fraction of U.S. electric power needs for centuries.

 

Figure 7. Projected Capital Costs for Hot Dry Rock Compared to Traditional Geothermal
Power Technology, 1996–2030

Source: U.S. Department of Energy, 1997.

Heat Exchanger Liners
The highly corrosive nature of geothermal plants poses a challenge to heat exchangers by reducing their thermal conductivity. Research is currently being conducted to replace the use of expensive heat exchanger materials, such as stainless steel and titanium, with new, less expensive polymer-base coated carbon steel. The polymer-base-coated carbon steel is proving to be as resistive to corrosion as the conventional, expensive materials.59

Air-Cooled Condensers
Currently, the National Renewable Energy Laboratory (NREL) is investigating ways to improve the efficiency of air-cooled condensers that are commonly used in binary-cycle geothermal plants. Air-cooled condensers use large airflow rates to lower the temperature of the gas once it has passed through the system to produce condensation. The fluid is then collected and returned to the cycle to be vaporized. This cycle is important in binary-cycle geothermal plants because of the lack of make-up water. To increase the heat exchange efficiency, NREL is currently testing the use of perforated fins in the condensers, with all of the air flowing through the perforations, to increase the heat exchange and therefore, condensation. Initial tests have indicated a 30–40% increase in heat transfer. Such an increase in heat transfer technology could increase the efficiency of future binary-cycle geothermal plants.

As technological improvements continue to be discovered and more geothermal plants are brought online, geothermal generating capacity in the United States will continue to increase. Figure 8 shows projected geothermal power generation under these scenarios and projected generation from Annual Energy Outlook 2003. 60 Installed capacity is likely to increase via new installation, as well as technological improvement leading to increased yield. The U.S. DOE projects that U.S. geothermal generation will increase by over 160% from 2000 to 2025, from 14.1 to 36.9 billion kilowatt-hours per year.

Figure 8. DOE Annual Energy Outlook Projected Geothermal Generation 2000–2025
(Billion KWh) Projected Capital Costs for Hot Dry Rock Compared to Traditional
Geothermal Power Technology, 1996–2030


Source: EIA. Annual Energy Outlook 2003

 

 

 

Closing

Our intention has been to provide the reader with a balanced overview of the utility-scale geothermal power industry. We believe clean, reliable power can be developed from renewable resources, with geothermal power making an important contribution. Examples from the U.S. geothermal sector have been used to illustrate the costs, benefits, policies, and trends in geothermal energy today. What follows is a list or further resources available on the world-wide web to allow the reader to gain a deeper understanding of the potential of geothermal power and the issues surrounding its development. We urge the reader to seek further understanding of these issues, and the means to their resolution, in order to support the progress of geothermal energy in providing clean, reliable, and economic power.

 

 

 

Slide show

 

 

 

 

 

 

 

 

 

 

+ نوشته شده در  جمعه سوم آذر 1385ساعت 9:46  توسط امیر علی باب هادی عشر  | 

اگر درخواستی برای تحقیق دارید در قسمت نظرات وارد کنید

 

+ نوشته شده در  یکشنبه بیست و یکم آبان 1385ساعت 20:46  توسط امیر علی باب هادی عشر  | 

انرژی زمین گرمایی

زمینی كه زیر پای ما قرار دارد، منبع بسیار عظیم انرژی است. این انرژی كه به صورت حرارت از اعماق زمین به سطح آن هدایت می شود در صورت توسعه فناوری استخراج آن، به تنهایی قادر خواهد بود كلیه نیازهای انرژی امروز و آینده بشر را تامین كند. طبق محاسبه ها، مشخص شده است كه انرژی حرارتی ذخیره شده در ۱۱ كیلومتر فوقانی پوسته زمین معادل پنجاه هزار برابر كل انرژی به دست آمده از منابع نفت و گاز شناخته شده امروز جهان است. پس این منبع عظیم انرژی می تواند در آینده جایگزین قابل اطمینانی برای انرژی حاصل از سوخت های فسیلی باشد. البته بدیهی است كه بهره برداری گسترده از ذخایر انرژی زمین گرمایی، مستلزم توسعه بیشتر در زمینه تكنیك های اكتشاف و استخراج آن است.
انرژی زمین گرمایی چیست
اصطلاح زمین گرمایی ترجمه واژه
Geothermal است كه ریشه یونانی داشته و از كلمات Geo به معنای زمین و Therme
به معنی حرارت تشكیل شده است. در حقیقت انرژی زمین گرمایی، انرژی ای است كه از سیال آب داغ یا بخارداغ موجود در اعماق زمین به دست می آید.
این انرژی در مخزن زمین گرمایی متمركز شده است كه برای دسترسی به آن در محل مخزن، چاهی عمیق حفر می كنند. سیال خروجی از چاه، عامل انتقال انرژی از مخزن به سطح زمین است. البته عمق مخزن زمین گرمایی نباید بیش از سه هزار متر باشد زیرا بهره برداری از انرژی آن با فناوری كنونی بشر توجیه اقتصادی ندارد. با افزایش عمق زمین درجه حرارت افزایش می یابد. این افزایش حرارت را شیب حرارتی می نامند. تمام منابع انرژی زمین گرمایی در نقاطی واقع شده اند كه از شیب حرارتی بالایی برخوردارند.
تاریخچه
این انرژی از ابتدای خلقت مورد استفاده انسان بوده است. بدین ترتیب كه از آن برای شست وشو، پخت وپز، استحمام، كشاورزی و درمان بیماری ها استفاده می شد. اسناد و مدارك موجود ثابت می كند كه ساكنان كشورهایی نظیر چین، ژاپن، ایسلند و نیوزیلند در گذشته های دور از این انرژی استفاده می كردند. در سال
۱۸۲۸ فردی به نام لاردرللو در كشور ایتالیا برای تهیه اسید بوریك از حرارت آب های گرم به جای سوزاندن هیزم استفاده كرد. در سال ۱۹۰۸ در منطقه مذكور نخستین نیروگاه زمین گرمایی به ظرفیت ۲۰ كیلووات راه اندازی شد كه در سال ۱۹۴۰ ظرفیت آن به ۱۲۷ مگاوات افزایش یافت. تا سال ۱۹۵۰ بهره گیری از انرژی زمین گرمایی رشد چندانی نداشت، اما حد فاصل سال های ۱۹۵۰ تا ۱۹۷۳ به دلیل گران شدن بی سابقه و ناگهانی نفت، همه كشورها به فكر استفاده از انرژی های جایگزین افتادند و به تدریج كشورهایی چون آمریكا، ایسلند، فیلیپین، اندونزی و اغلب كشورهایی كه روی كمربند زمین گرمایی جهانی قرار داشتند بهره برداری از این انرژی را شروع كردند.


نشانه های انرژی زمین گرمایی
----------------------------------
مهمترین نشانه های منابع زمین گرمایی موارد زیر است:
سنگ های آتشفشانی جوان جوان تر از یك میلیون سال
چشمه های آبگرم
بخارفشان یا گازفشان
آب فشان
نواحی دگرسان شده
گل فشان
كوه های آتشفشانی فعال
البته ذكر این نكته ضروری است كه برای آغاز بررسی های اكتشافی در یك منطقه زمین گرمایی، بیش از یك نشانه باید در منطقه وجود داشته باشد.
موارد كاربرد انرژی زمین گرمایی
پس از انجام بررسی های اكتشافی و حفر چاه های اكتشافی و تولیدی در میدان زمین گرمایی، مسئله كاربرد انرژی زمین گرمایی مطرح می شود. مهمترین عامل در تعیین نوع كاربرد مخزن زمین گرمایی، درجه حرارت آن است. امروزه منابع زمین گرمایی را بر اساس درجه حرارت به سه دسته كلی حرارت بالا، حرارت متوسط و حرارت پایین تقسیم می كنند. مبنای این تقسیم بندی، درجه حرارت مخزن در عمق یك كیلومتری زمین است. به این ترتیب كه اگر درجه حرارت مخزن در عمق مذكور بیش از
۲OOC باشد آن را حرارت بالا می نامند. درجه حرارت مخازن حرارت متوسط و پایین به ترتیب بین ۱۵۰C و ۲۰۰C و كمتر از ۱۵۰C است. امروزه از مخزن های زمین گرمایی به دو صورت عمده كاربرد غیر مستقیم تولید برق و كاربرد مستقیم انرژی حرارتی استفاده می شود.

تولید برق
-----------
به منظور تولید برق از انرژی زمین گرمایی، سیال مخزن آب داغ یا بخار از طریق چاه های حفر شده به سطح زمین هدایت شده و پس از به چرخش درآوردن توربین در نیروگاه، برق تولید می كند. بدیهی است كه از مخازن حرارت بالا بیشتر برای تولید برق استفاده می شود. در حال حاضر
۲۲ كشور جهان به كمك منابع زمین گرمایی خود بیش از MW ۸۲۰۰
برق تولید می كنند. در نیروگاه های زمین گرمایی، انرژی الكتریكی به كمك چرخه های مخصوصی تولید می شود. مهمترین و رایج ترین آنها عبارتند از:
چرخه تبخیر آنی
در این دسته از چرخه های تولید برق، سیال زمین گرمایی پس از خروج از چاه، وارد یك جداكننده شده و بخار حاصل به سمت توربین و آب داغ به سمت چاه های تزریقی و برج خنك كننده روانه می شود. حال، برحسب اینكه عمل جدایش یا تبخیر آنی در یك مرحله یا دو مرحله انجام شود و برحسب وجود یا عدم وجود كندانسور، سه نوع چرخه تبخیر آنی وجود دارد: چرخه تبخیر آنی یك مرحله ای بدون كندانسور، چرخه تبخیر آنی یك مرحله ای با كندانسور، چرخه تبخیر آنی دومرحله ای.
چرخه دومداره
از این چرخه برای تولید برق از مخزن های زمین گرمایی حرارت پایین استفاده می شود. حدود
۵۰ درصد مخازن زمین گرمایی شناخته شده جهان درجه حرارتی بین ۱۵۰C تا ۲۰۰C دارند، كه اگر برای تولید برق از آنها از چرخه تبخیر آنی استفاده شود، چرخه مزبور بازده بسیار پایینی خواهد داشت. در این چرخه از سیال عامل برای تولید برق استفاده می شود بدین ترتیب كه آب داغ، سیال عامل را در یك مبدل حرارتی، گرم و به بخار تبدیل می كند. بخار حاصل، توربین را به حركت در آورده، برق تولید می كند. از جمله مزیت های مهم این چرخه، عدم وجود خوردگی یا رسوب گذاری توسط سیال عامل است. در حال حاضر مهمترین كشورهای جهان از نقطه نظر تولید برق از منابع زمین گرمایی، كشورهای آمریكا ۲۲۲۸ مگاوات، فیلیپین ۱۹۰۹ مگاوات، ایتالیا ۷۶۹ مگاوات، مكزیك ۷۵۵ مگاوات و اندونزی ۵۹۰ مگاوات هستند.

كاربرد مستقیم
--------------------
كاربرد مستقیم انرژی زمین گرمایی، بهره برداری بدون واسطه از انرژی زمین گرمایی است. در این حالت، انرژی زمین گرمایی به انرژی الكتریكی تبدیل نمی شود، بلكه فقط از انرژی حرارتی آن استفاده می شود. مخزن های زمین گرمایی كه دمای آنها بین
۶۵C تا ۱۵۰C است برای تولید برق، توجیه اقتصادی ندارد، لذا این گونه مخزن ها برای استفاده مستقیم از انرژی حرارتی، مناسب هستند. مخزن های زمین گرمایی حرارت پایین، نسبت به مخزن های حرارت بالا گستردگی بیشتری دارند. آب داغ مخزن های حرارت پایین را می توان با دستگاه های حفاری چاه های آب استخراج كرد. یك محقق ایسلندی به نام لیندال به منظور نشان دادن موارد كاربرد انرژی زمین گرمایی، نموداری تهیه كرده است كه در آن موارد مختلف كاربرد سیال زمین گرمایی بر حسب درجه حرارت آن ارائه شده است. همان گونه كه در نمودار لیندال مشخص شده است، موارد بهره برداری مستقیم از انرژی زمین گرمایی را می توان به ۶
رده كلی زیر تقسیم بندی كرد:
گرمایش ساختمان ها
كشاورزی
دامپروری
كاربردهای صنعتی
درمان بیماری ها
سایر
گرمایش ساختمان ها
این مورد متداول ترین كاربرد مستقیم انرژی زمین گرمایی است. حدود
۳۷
درصد كاربرد مستقیم انرژی زمین گرمایی در سراسر جهان را گرمایش فضاهای مختلف مسكونی، تجاری، اداری و غیره به خود اختصاص می دهد. البته در صورت نامناسب بودن كیفیت آب از نظر شیمیایی، از مبدل حرارتی برای گرمایش استفاده می شود. یكی از مزیت های مهم سیستم های گرمایشی این است كه آب داغ پس از تٲمین حرارت فضاهای مختلف، مجددا به درون مخزن زمین گرمایی تزریق می شود و در نتیجه میزان آلودگی زیست محیطی آن بسیار پایین است.
شایان ذكر آنكه امروزه انواع خاصی از مبدل های حرارتی وجود دارند كه درون چاه های زمین گرمایی تعبیه شده و حرارت آب داغ مخزن را به آب شیرین درون مبدل منتقل می كنند. درجه حرارت آب گرم مورد نیاز برای سیستم های گرمایشی حدود
۶۰C یا بالاتر است. امروزه كشورهای ایسلند، فرانسه، مجارستان و ژاپن برای تٲمین حرارت سیستم های گرمایش مركزی خود از انرژی زمین گرمایی استفاده می كنند. به عنوان مثال شهر ۱۵۰هزار نفری ریكیاویك مركز ایسلند تماما به وسیله آب داغ تولیدی از مخزن های زمین گرمایی مجاور شهر تامین می شود.

كشاورزی
-----------
عمده ترین كاربرد انرژی زمین گرمایی در زمینه فعالیت های كشاورزی، تامین گرمایش گلخانه ها است. البته در برخی از مناطق سردسیر از حرارت آب داغ مخزن های زمین گرمایی برای گرم كردن خاك های كشاورزی نیز به كار می رود. این نوع كاربرد در كشورهای سردسیر بسیار گسترش دارد. از جمله محصولاتی كه به كمك این انرژی كشت می شوند می توان به خیار، گوجه فرنگی، انواع گل ها، گیاهان خانگی، نهال درختان و انواع كاكتوس ها اشاره كرد. در بین كشورهای جهان مجارستان از نظر استفاده از گلخانه های زمین گرمایی مقام نخست را دارد. برای گرم كردن گلخانه ها معمولا یا آب داغ را از لوله های فلزی عبور می دهند یا اینكه همانند سیستم های گرمایشی خانه ها از پره های رادیاتور استفاده می كنند، یا آب داغ را از درون شبكه متراكمی از لوله ها كه در پشت آنها یك فن قوی وجود دارد، عبور می دهند. علاوه بر مجارستان كشورهایی نظیر ایسلند، چین، یونان، نیوزیلند و روسیه نیز در زمینه گلخانه های زمین گرمایی فعال هستند.

دامپروری
----------
به كمك انرژی زمین گرمایی می توان انواع مختلف آبزیان را نیز پرورش داد. امروزه در سطح جهان از انرژی زمین گرمایی برای پرورش و رشد آبزیانی نظیر میگو، قزل آلا، صدف و همچنین آبزیان آكواریومی استفاده می شود. نظر به اینكه درجه حرارت بهینه برای پرورش انواع مختلف آبزیان برای هر یك از آنها میزان مشخصی است ،با استفاده از انرژی زمین گرمایی می توان درجه حرارت حوضچه های پرورش را در حد مطلوب تامین كرد و آن را در تمام طول سال ثابت نگه داشت. بدین ترتیب می توان مقدار تولید انواع مختلف آبزیان را به میزان قابل توجهی افزایش داد. به عنوان مثال رشد بهینه ماهی قزل آلا در درجه حرارت
۵۱۵
درجه سانتیگراد است.
كشورهایی مانند ایسلند، گرجستان، تركیه، نیوزیلند، ژاپن و چین از جمله كشورهای پیشرو در زمینه استفاده از انرژی زمین گرمایی برای پرورش آبزیان هستند. در حال حاضر
۱۶ كشور از چنین تاسیساتی بهره می گیرند.

كاربردهای صنعتی
---------------------
این دسته از كاربردهای انرژی زمین گرمایی هنوز مانند سایر مصارف انرژی زمین گرمایی در سطح جهان گستردگی چشمگیری ندارد. با این وجود، در حال حاضر حدود
۱۹
كشور جهان از این انرژی در فرآیندهای مختلف صنعتی استفاده می كنند. به عنوان مثال می توان به موارد زیر اشاره كرد:
تولید برات و اسید بوریك از سیال های زمین گرمایی در ایتالیا
استحصال نفت در روسیه
پاستوریزه كردن شیر در رومانی
تولید چرم در اسلوونی و صربستان
تولید گاز دی اكسید كربن در ایسلند و تركیه
تولید كاغذ و قطعات خودرو در مقدونیه
تولید خمیر كاغذ، كاغذ و چوب در نیوزیلند

درمان بیماری ها
------------------
این كاربرد نیز بسیار قدیمی بوده و از روزگاران دور اقوامی چون رومی ها، چینی ها، ژاپنی ها، عثمانی ها و ساكنان سایر نواحی كره زمین به منظور استحمام و درمان بیماری های گوناگون از آب های گرم طبیعی زمین استفاده می كردند.
در حال حاضر حدود
۴۵ كشور جهان از چشمه های آب گرم خود برای این منظور استفاده می كنند. در ارتباط با توسعه چنین مراكزی، شواهد و نمونه های متعددی را می توان در سطح جهان معرفی كرد. به عنوان مثال، ژاپنی ها با بهره گیری از بیش از ۲۲۰۰
كانون تفریحی مرتبط با چشمه های آبگرم، سالانه قریب به صد میلیون مهمان و گردشگر را پذیرا هستند.
امروزه از آب های گرم دارای حرارت بیش از
۵۰ درجه سانتیگراد برای درمان بیماری هایی نظیر فشار خون بالا، روماتیسم، بیماری های پوستی و بیماری های دستگاه عصبی استفاده می شود.

ذوب برف جاده ها
-------------------
به كمك انرژی زمین گرمایی می توان برف یا یخ جاده ها و پیاده روها را نیز ذوب كرد. گسترش این نوع كاربرد نسبت به سایر موارد انرژی زمین گرمایی محدودتر است. امروزه در سراسر جهان به كمك انرژی زمین گرمایی حدود
۵۰۰هزار متر مربع از مسیر پیاده روها و جاده ها گرم می شوند كه بخش اعظم آنها نیز در كشور ایسلند وجود دارند. در حال حاضر به جز كشور ایسلند، كشورهایی چون آرژانتین، آمریكا و ژاپن نیز برای ذوب برف جاده های خود از انرژی زمین گرمایی بهره می گیرند.همان گونه كه پیشتر اشاره شد جنبه های گوناگون كاربرد انرژی زمین گرمایی به سرعت در حال افزایش است و مرتبا به كشورهای بهره مند از این انرژی افزوده می شود. میزان گسترش موارد كاربرد مستقیم انرژی زمین گرمایی در سراسر جهان در جدول ذیل آمده است. یادآور می شود كه پمپ های حرارتی زمین گرمایی نوعی سیستم تهویه گرمایش و سرمایش است. همان گونه كه می دانیم، درجه حرارت زیرزمین تا اعماق كم ۲ تا ۱۵ متری تقریبا در تمام طول سال ثابت است. بنابراین با استفاده از این پدیده طبیعی می توان گرمایش و سرمایش منازل را در زمستان و تابستان فراهم كرد. در واقع سازوكار اصلی این سیستم های تهویه، تبادل حرارت با بخش های كم عمق زمین است، بدین معنی كه در فصل تابستان، حرارت را از داخل منازل به زمین منتقل می كنند و در زمستان، حرارت زیرزمین را به داخل فضاهای مسكونی هدایت می كنند.

مزیت های كاربرد انرژی زمین گرمایی
----------------------------------------
امروزه تولید انرژی به كمك منابع سوخت های فسیلی یا نیروگاه های هسته ای با آلودگی قابل ملاحظه محیط زیست همراه است، ولی انرژی زمین گرمایی علاوه بر تجدیدپذیر بودن در مقایسه با سایر منابع تولید انرژی، آلایندگی كمتری دارد و جزء منابع پاك انرژی به شمار می رود. میزان آلودگی نیروگاه ها یا طرح های كاربرد مستقیم زمین گرمایی، ارتباط مستقیمی با درجه حرارت منبع زمین گرمایی دارد. به این ترتیب كه منابع حرارت بالا نسبت به انواع حرارت پایین، آلودگی بیشتری تولید می كنند و همچنین طرح های كاربرد مستقیم نیز كمتر از نیروگاه های زمین گرمایی، محیط زیست را آلوده می كنند. به طور كلی مزیت های انرژی زمین گرمایی را می توان به دو دسته كلی مزایای زیست محیطی و كاربردی تقسیم بندی كرد.
مزیت های زیست محیطی كاربرد انرژی زمین گرمایی شامل موارد زیر است:
عدم آلودگی هوا
تولید
CO2 كم، تولید H2S پایین و عدم تولید NOx

عدم آلودگی منابع آب های زیرزمینی
عدم نیاز به زمین وسیع
امروزه به دلیل تزریق سیال خروجی از نیروگاه ها و سایر طرح های كاربرد مستقیم انرژی زمین گرمایی، میزان آلایندگی این قبیل طرح ها به حداقل مقدار خود رسیده است.
مزایای كاربردی
صرفه جویی در مصرف سوخت های فسیلی
طولانی بودن زمان دسترسی
گستردگی موارد كاربرد
مستقل بودن از شرایط جوی
امكان تولید برق به وسیله واحدهای قابل حمل
میزان دی اكسید گوگرد تولید شده در نیروگاه های زمین گرمایی حدود
۸ درصد مقدار تولید شده در نیروگاه های فسیلی است. در خصوص دی اكسید كربن نیز نیروگاه های زمین گرمایی در وضعیت بسیار مناسب تری نسبت به نیروگاه های فسیلی قرار دارند. بدین معنی كه مقدار گاز CO2 تولید شده در نیروگاه های زمین گرمایی به ترتیب معادل ۱۵ درصد نیروگاه های گاز سوز، ۱۰ درصد نیروگاه های نفت سوز و ۸ درصد نیروگاه های زغال سنگ سوز است.

انواع مخزن های زمین گرمایی

سنگ مخزنی برای بخار
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توده یا حجمی از سنگ های نفوذپذیر و متخلخل است كه در اعماق مختلف زمین قرار داشته و خلل و فرج آنها را آب داغ یا بخار تحت فشار، اشغال كرده است. مخزن های زمین گرمایی تحت شرایط خاص زمین شناسی به وجود می آیند. به عنوان مثال، گسل ها یكی از عوامل كنترل كننده اندازه و شكل مخازن زمین گرمایی است. مخزن های زمین گرمایی از اجزایی تشكیل شده است كه وجود همه آنها برای تشكیل مخازن ضروری است. بدین ترتیب كه در صورت عدم وجود هر یك از آنها، مخزن زمین گرمایی به وجود نخواهد آمد. اجزای یك مخزن زمین گرمایی عبارتند از:
۱
سنگ مخزن كه سنگی نفوذپذیر و متخلخل است.
۲
سنگ پوشش كه سنگی متراكم و غیرقابل نفوذ بوده و مانع از خروج سیال از مخزن و در نتیجه افت فشار آن می شود.
۳
منبع حرارت كه ممكن است عوامل مختلفی باشد مانند یك توده نفوذی آذرین جوان و عمیق یا حرارت حاصل از حركت های زمین یا حرارت حاصل از تجزیه عنصر های رادیواكتیو و نازك شدگی پوسته زمین و غیره.
۴
منبع سیال كه اغلب بارش های جوی، منابع آب زیرزمینی و یا آب موجود در ماگما است.
انواع مخزن های زمین گرمایی به شرح زیر است:

۱ مخزن های گرمابی

در حال حاضر این دسته از مخزن ها تنها مخزن های اقتصادی زمین گرمایی جهان است. همان گونه كه از نام آنها بر می آید این مخزن ها، حاوی سیال هستند كه بر حسب نوع سیال به دو دسته مخزن ها حاوی آب داغ و بخار تقسیم می شوند. مخزن های آب داغ فراوان تر از مخزن های حاوی بخار است. كشورهای نیوزیلند و ایسلند تعداد قابل توجهی از مخزن های آب داغ دارند.
در مخزن های حاوی بخار حجم تغذیه سیال مخزن نسبت به دمای غشای حرارتی آن به قدری كم است كه به محض ورود آب، بخش عمده آن به بخار تبدیل می شود. در این دسته از مخزن ها، بخار تحت فشار با دمای زیاد
C ۲۰۰
منبسط شده و به بخار خشك فوق العاده گرم تبدیل می شود، بنابراین، این گونه مخزن ها را مخزن های بخار خشك نیز می نامند.
در مخزن های آب داغ، حجم آب داغ بیش از حجم بخار و گاز مخزن است. این قبیل مخزن ها بسیار فراوان بوده و حدود
۹۰ درصد مخازن زمین گرمایی جهان را تشكیل می دهد. دمای آنها بین ۲۵ تا C ۳۱۰ متغیر است. منبع عمده تامین كننده آب این مخزن ها، بارش های جوی و آب های سطحی است.

۲ مخزن های سنگ داغ خشك

در نواحی ای كه سنگ های متراكم آذرین، داغ ، بدون تخلخل و نفوذپذیری در عمق كم قرار دارند، می توان آب را به طور مصنوعی درون سنگ ها تزریق كرد. بدین ترتیب كه دو چاه با عمق های متفاوت و با زاویه ای نسبت به سطح قائم حفر می شود. اغلب چاه ها تا جایی حفر می شود كه درجه حرارت سنگ ها به C ۲۰۰ برسد. یكی از چاه ها عمیق تر است و درست زیر انتهای چاه كم عمق تر قرار دارد.
سپس به كمك پمپ های پیستونی قوی آب به درون چاه ها تزریق می شود. فشار آب موجب شكستگی سنگ ها شده و بدین ترتیب بین دو چاه ارتباط برقرار می شود. سپس از چاه كم عمق تر آب را به درون زمین پمپ می كنند. آب بر اثر تماس با سنگ ها بخار شده و رو به بالا حركت می كند و از طریق چاه كم عمق به سطح زمین می رسد كه به كمك آن می توان برق تولید كرد. البته این طرح هنوز اقتصادی نشده است و كماكان در مرحله پژوهش است. در حال حاضر اغلب كشورهای پیشرفته نظیر آمریكا، فرانسه، آلمان، ژاپن در حال تحقیق در زمینه این طرح هستند.

۳ مخزن های تحت فشار

این گونه مخزن ها در عمق های زیاد بیش از۴۶۰۰m قرار دارد و غالبا سنگ مخزن آنها از جنس ماسه است. روی این مخزن ها را سنگ پوشش ضخیم پوشانده است. آب داغ موجود در فضاهای خالی ماسه ها تحت فشار هیدروستاتیك بوده و حاوی مقدار مختلفی از گاز متان محلول است. به طور كلی دو عامل در تشكیل این گونه مخزن ها بسیار موثر است:
سنگ پوشش مناسب غیرقابل نفوذ و دارای ضخامت قابل ملاحظه كه از نفوذ سیال و حرارت مخزن به خارج جلوگیری میكند.
استقرار مخزن در عمق زیاد از سطح زمین.
دو عامل فوق در برخی موارد موجب افزایش درجه حرارت مخزن به میزان
C ۲۰۰ می شود. البته امروزه این گونه مخزن ها از حیث اقتصادی مقرون به صرفه نیست. به عنوان مثال در كناره ساحلی آمریكا، از ایالت تگزاس تا لوییزیانا، تعدادی از این قبیل مخزن های زمین گرمایی كشف شده است. مخزن های تحت فشار، اكثرا در حین انجام بررسی های اكتشافی ذخیره نفت و گاز شناسایی شده اند. چنانچه بهره برداری از این مخزن ها مقرون به صرفه شود، می توان از انرژی فسیلی و جنبشی آنها استفاده كرد.

۴ مخزن های ماگمایی

در برخی نقاط كره زمین و به ویژه در مناطق فعال آتشفشانی، توده های آذرین جوان كم عمقی وجود دارد كه دارای انرژی حرارتی فوق العاده زیادی است. این توده های آذرین را مخزن های ماگمایی می نامند. بهترین نمونه این دسته از مخزن های زمین گرمایی در جزایر هاوایی وجود دارند كه محققان توانسته اند با تزریق آب به درون مخزن و تولید بخار از آنها به طور آزمایشی برق تولید كنند ولی كماكان این طرح با مشكلاتی مواجه است كه از جمله آنها می توان به موارد زیر اشاره كرد:
مشكلات و خطرات حفاری در سنگ های نیمه مذاب با حرارت بیش از
C ۸۰۰

آماده سازی جداره چاه كه در درجه حرارت های بالا كاری بسیار مشكل است.
عدم كارایی تكنیك های ژئوفیزیكی كنونی برای تعیین موقعیت دقیق توده سنگ های آذرین مدفون. زیرا در درجه حرارت های زیاد، ابزارهای ژئوفیزیكی كنونی دقت كافی را ندارند.
البته در حال حاضر صرفا منابع زمین گرمایی ارزش اقتصادی دارد و از آنها به طور گسترده بهره برداری می شود. بدون شك در آینده نه چندان دور با ابداع فناوری های جدید سایر انواع منابع زمین گرمایی نیز قابل استفاده خواهند بود

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