GEOTHERMAL ENERGY

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Geothermal energy is a proven resource for direct heat and power generation. In over 30 countries geothermal resources provide directly used heat capacity of 12,000 MW and electric power generation capacity of over 8,000 MW. It meets a significant portion of the electrical power demand in several developing countries. For example, in the Philippines geothermal provides 27% of that country's total electrical generation, from power plant complexes as large as 700 MW.

Individual geothermal power plants can be as small as 100 kW or as large as 100 MW depending on the energy resource and power demand. The technology is suitable for rural electrification and mini-grid applications in addition to national grid applications. Direct use of geothermal heat can boost agricultural and aqua-culture production in colder climates and supply heat for industrial processes that can add value to local primary products. Geothermal resources may be especially important and significant in developing nations where no indigeneous fossil fuel resources exist such as oil, coal or natural gas. For example in Tibet, where no readily available fossil fuels exist, the Nagqu geothermal field (Tibet Autonomous Region, PRC) provides a useful energy source for the local population. With the help of the UN, a 1 MWe binary plant was built in 1993.

Costs of geothermal electric power are very dependent on the character of the resource and project size. The unit costs of power currently range from 2.5 to over 10 US cents per kilowatt-hour while steams costs may be as low as US$3.5 per tonne. Major factors affecting 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.

I. Geothermal Resources

Geothermal means "Heat from the Earth". The heat that flows from the Earth's hot interior due to crustal plate movements, zones of high heat flow may be located close to the surface where convective circulation plays a signifcant role in bringing the heat close to the surface.

Deep circulation of groundwater along fracture zones will bring heat to shallower levels, collecting the heatflow from a broad area and concentrating it into shallow reservoirs or discharging as hot springs. These reservoirs may contain hot water and/or stream. By drilling into these reservoirs, the hot water and/or steam is piped to the surface where it is used for direct use applications, or the high pressure steam is separated to drive turbines for power generation. The low energy waste water form such power generaiton is then usually re-injected back into the reservoir, or further utilised for direct heat applications. This technology enables it to be utilised to generate electricity and provide domestic and industrial heat. Geothermal energy has proved to be reliable, economic, environmentally friendly and renewable.

In general there are two main categories, the high temperature resources and the moderate/low temperature resources. The high temperature geothermal resources - 220 degrees Celsius and up - are predominantly found in volcanic regions and island chains. The moderate to low temperature resources are found on all continents. The high temperature are almost always used for power production while most of the low temperature resources are used for direct heating purposes or agriculture and aquaculture.

II. Resource Evaluation and Risk Assessment

Geothermal projects typically progress through stages of reconnaissance, exploration and development with various decision points along the way. In the early exploration stage when there are uncertainties of finding a useable heat resource after expending effort on early reconnaissance, surface exploration and/or drilling exploration wells. Carefully implemented regional reconnaissance surveys can, however, lead to a sound prioritisation of target areas by the filtering out of less promising prospects. Good exploration surveys of targeted prospect areas have shown they deliver high success rates for exploration drilling. Examples of such successful programmes include those undertaken in Indonesia, New Zealand and the Philippines where exploration drilling achieved success rates of over 80%.

III. Applications

The range of potential methods for utilising any geothermal resource is very dependent on the temperature of the resource.

Direct Uses of Geothermal Heat

Lower temperature geothermal resources are found in many regions of the World. They can provide useful energy for heating buildings and agricultural and industrial processes. Such heat can also be available as a by-product of geothermal power generation projects that use higher temperature resources. Some greenhouses in the central region of Algeria are reported to be using 60°C geothermal water for heating. In China most of their geothermal use (of an estimated 181 fields) has been for space heating, agriculture, aquaculture, balneology and medicinal purposes. Romania has a total of 130 MWt installed thermal capacity for direct uses, which are: district heating for about 3000 dwellings, 47 ha of greenhouses, sanitary hot water for 16000 dwellings, preparation of industrial hot water for about 10 factories and balneological uses.

Space/District Heating.

Schemes utilising geothermal heat provide over 80% of the central heating needs of Reykjavik city in Iceland and are employed in many towns in USA, Poland and Hungary. The World Bank is currently supporting a program in Poland for using hot water from unsuccessful oil wells to displace the use of coal for district heating.

Heat pump technology utilising the earth as a heat source and sink to provide central heating in winter and cooling in summer. In many countries such heating or cooling may be supplemented by electric power and has been widely applied in the USA and Europe.

Agriculture and Aquaculture.

In temperate and colder climates, greatly improved plant and fish growth can be achieved by heating soils, greenhouses and fish ponds using geothermal heat.

Power Generation

High temperature geothermal reservoirs containing water and/or steam can provide steam to directly drive steam turbines and electrical generation plant. Binary cycle sytems using heat transfer media of lower boiling point than water (such as organic fluids), enable power to be generated from lower temperature resources.

With over 8000 MW of installed capacity, geothermal electric power generation is a well-proven technology that has been especially successful in countries and islands that have a high reliance on imported fossil fuels.

Small and Mini-grid Power Generation.

Power plants as small as 100kW, but commonly 1-5MW, may provide distributed generation on larger grids or they may be a major generation source for smaller power grids. There is a perception that geothermal power plants are base load stations that operate 24 hours a day and 365 days a year. This is not necessarily the case. Indeed geothermal power plants can be designed to follow load demand if necessary such as may be required in mini-grid applications. Small power plants are usually built using a modular approach that reduces site construction costs and can be placed adjacent to the wells so that the overall project has a minimal environmental impact. The Ribeira Grande Geothermal field on Sao Miguel Island, Azores has had a 5 MWe binary plant in operation since 1994. In Fang, Thailand there is a 300 KW binary module which has been in use since 1989 and utilises fluids of 116°C.

Grid-Based Power Generation.

Power plants with generation units up to 100MW in size are connected to national power grids and usually operated in a base load mode, operating at full capacity continuously 365 days of the year. This type of generation is widespread in Indonesia and the Philippines.

Costs

A summary of typical current costs are provided in the following tables (see Economics Section for detail).

Unit cost of Steam or Water (US$/tonne)

Some Geothermal Issues….

Markets

Geothermal plants can be particularly suitable for smaller power grid systems that otherwise would have a high dependence on fossil fuels. Where generation capacity growth is required because of grid expansion into rural areas or other economic development, existing thermal power plant can provide backup or peaking capacity while new geothermal capacity is installed in stages to meet growing base load demand.

Policy Framework

Many developing countries have adopted energy policies that focus on: improving access to electricity for rural households, creating an environmentally sound energy sector, making optimal use of local resources by diversifying the primary energy sources for electricity production and stimulating private sector involvement. Geothermal energy development is compatible with these policy priorities.

Power sector reforms open the sector up to private investment and competition, stimulating power utilities to enhance their operating efficiency and improve the quality of service that they provide to their customers. These reforms have opened the way for many private geothermal power projects internationally but have often weakened government owned agencies that have in recent decades taken responsibility for exploration and evaluation of geothermal resources in many countries.

Resource Identification

Most nations that have identified their indigenous geothermal potential have conducted some investigations and inventory studies of potential geothermal reserves over the past 20 years. In many cases, however, there has been limited development beyond this exploration stage. Progression to the drilling of deep exploration wells may be constrained by limited available budgets for such work. Many prospective geothermal reservoirs have been identified by surface studies and shallow drilling, but a large proportion have not had the potential energy supply confirmed by drilling of deeper wells.

Financial and Economic Feasibility

Geothermal power generation typically involves relatively high levels of capital investment. Such expenditure, required to prove the geothermal resource capacity, involves some risk. Geothermal power projects are characterised by high capital investment for exploration, drilling wells and installation of plant, but low operating costs because of the low marginal cost of fuel. Return on investment is not achieved as quickly as with cheaper fossil fuel power plant, but longer term economic benefits accrue from the use of this indigenous fuel source. The following Table details typical capital costs for various sizes of geothermal power plants for medium and hugh quality resources.

Direct Capital Costs (US$/kW installed capacity)

  
Environmental

Since geothermal is often a replacement for diesel or other fossil fuels, it has great benefits for people's health through improved air quality. There are atmospheric emissions from geothermal power plants which are predominantly CO2 and H2S. However, in the context of global climate change, geothermal has significantly lower CO2 emissions than fossil fuels. Atmospheric emissions from geothermal plants average only about 5% of the emissions from equivalent generation sized fossil fuel power plants. The actual land use for geothermal energy production is relatively small for both the fuel acquisition and the energy production. The common practice of re-injecting spent geothermal fluids means the impacts on aquatic life have been eliminated. Geothermal plants also co-exist successfully with other land uses.

Adverse environmental impacts of geothermal development may include land subsidence and increased microseismic activity. However such adverse factors need to be balanced against the more obvious advantages of geothermal over fossil fuels.

Sociological

The integration of social concerns into the decision, planning and management of any geothermal project is mandated by international agreements/protocols, individual state laws, and by the policies of bilateral agencies and international financing institutions. Greater benefit can be returned locally in countries where landowners have some control over access to the geothermal resource. The low marginal cost of the fuel source may mean that off-peak capacity from geothermal power plant can be cheaply used for regional development projects such as pumping irrigation water. Modest land requirements have meant that this energy source can provide direct benefit to local and regional communities while having a minimal impact on existing land uses.

Risks and Limitations of Geothermal Energy Development

Exploration Risk. There are risks of not finding a useable heat resource after expending effort on early reconnaissance and surface exploration works. Good exploration surveys of targeted prospect areas have proven to deliver high success rates for exploration drilling. Similarly, there is a major cost incurred drilling exploration wells which may not result in useful production. Carefully implemented regional reconnaissance surveys can, however, lead to a sound prioritisation of target areas by the filtering out of less promising prospects. Examples of such successful programmes include those undertaken in Indonesia, New Zealand and the Philippines where success rates of subsequent exploration drilling exceeded 80%.

Size of Development and Reservoir Exhaustion. How a geothermal reservoir will perform over several decades provides another significant risk in geothermal development. A complete understanding of the reservoir can only be obtained by withdrawing fluids from the reservoir over a sustained period. Subsequent assessment of resource size and production capacity is possible to reasonable levels of certainty and forms a critical part of any geothermal development.

Economic and Political Risk. Risk may be encountered in developing countries through changes in economic fortunes, as experienced in Asia in the late 1990's, and from changes in government policy, such as rescinding incentives for the development of rural and renewable energy.

International Geothermal Association

Geothermal Energy Technology (GET)

Related Links

U.S. Department of Energy - Energy Efficiency and Renewable Energy Network (EERE)

U.S. Department of Energy Information Administration (Geothermal Energy)

U.S. Department of Energy E-print Network
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Bonneville Power Administration

California Energy Commission
California Energy Commission (Geothermal Progam)

Center for Renewable Energy and Sustainable Technology - Geothermal

ECOworld

Educational Zone

Geo-Heat Center

Geothermal Education Office (Geothermal Energy)

Geothermal Resources Council

Idaho National Engineering and Environmental Laboratory (Geothermal Energy)

International Geothermal Association

Lawrence Berkeley National Laboratory (Geothermal Energy Development)

National Renewable Energy Laboratory (Geothermal Technologies)

Sandia National Laboratories (Geothermal Research Department)

Scientific and Technical Information Network - Defense Technical Information Center

Southern Methodist University (Geothermal Laboratory)

Stanford Geothermal Program

State Energy Alternatives

 

International Related Links

British Library

Linda Hall Library of Science, Eng