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Geothermal electricity

Climate Technology Centre and Network
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Geothermal electricity is electricity generated from geothermal energy. Inside the Earth's crust there are several reservoirs with hot water which can be used for heating buildings and/or production of electricity. The process causes relatively little CO2 emissions (from the steam), which could potentially be reinjected in the earth's crust through carbon capture and storage. Contrary to conventional geothermal power plants, present generation plants re-inject the condensated steam or hot water into the underground acquifer so that the reservoir capacity could remain intact. Around half of the geothermal power capacity is located in developing countries, especially in regions with hydrothermal manifestations (e.g., hot springs, volcanos). The potential for geothermal power production is limited to a few countries/regions in the world which are in volcanic areas, particularly in the ‘ring of fire’ around the Pacific Ocean and the rift valley in Eastern Africa. Basically, a geothermal energy system requires a heat source (the Earth's core), an underground reservoir (an aqcuifer holding, e.g., meteoric water), and a fluid (water) to carry the heat from the reservoir to the energy production point. In some cases, the fluid and reservoir can be artificial. For example, water can be pumped through pipes drilled through hot underground rocks which heat the water so that it can be used for energy production. 

For information related to Eastern Africa, please view this website for a map of geothermal power plants in Eastern Africa and geothermal database.

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Geothermal electricity generation mainly takes place in conventional steam turbines and binary plants, depending on the characteristics of the geothermal resource. Geothermal energy originates from the high-temperature aquifers inside the Earth’s crust at depths of between one and four kilometres. These aquifers are surrounded by porous, soft rocks and/or sand and are heated by the Earth's heat. Hot water or steam within the acquifers could reach temperatures of over 300oC. This heat can be used for heating of buildings and/or production of electricity. 

Type of geothermal power plant

Dry steam power station: Some reservoirs have such high temperatures that they mainly contain steam ('dry steam'), which can be piped directly into a steam power plant to spin the turbine generator. Dry steam plants are the simplest and oldest design. They directly use geothermal steam of 150 °C or greater to turn turbines and are available with either atmospheric (back-pressure) or condensing exhausts. Atmospheric exhaust turbines are simpler and cheaper. The steam, direct from dry steam wells or, after separation, from wet wells, is passed through a turbine and exhausted to the atmosphere.

With this type of unit, steam consumption (from the same inlet pressure) per kilowatt-hour produced is almost double that of a condensing unit. However, the atmospheric exhaust turbines are extremely useful as pilot plants, stand-by plants, in the case of small supplies from isolated wells, and for generating electricity from test wells during field development. They are also used when the steam has a high non-condensable gas content (>12% in weight). The atmospheric exhaust units can be constructed and installed very quickly and put into operation in little more than 13-14 months from their order date. This type of machine is usually available in small sizes (2.5 - 5 MWe).

The condensing units, having more auxiliary equipment, are more complex than the atmospheric exhaust units and the bigger sizes can take twice as long to construct and install. The specific steam consumption of the condensing units is, however, about half that of the atmospheric exhaust units. 

Flash power plant: Some geothermal reservoirs produce mostly hot water ('hot water-dominated reservoirs') with a temperature ranging between 150 and 370 oC. This water can be brought up to the surface and ‘flashed’ into steam for use in a generator to produce electricity. 

Binary power plants: When the reservoir temperature is not high enough to produce sufficient steam for a flash power plant (i.e. between 120 – 180 oC), the water from the reservoirs can be pumped through a heat exchanger. Here, the heat from the water is transferred into a secondary (binary) working liquid such as isopentane which boils at a low temperature and has a higher vapour pressure at low temperatures. The vapour is subsequently used to spin the turbine generator to produce electricity. 

Generating electricity from low-to-medium temperature geothermal fluids and from the waste hot waters coming from the separators in water - dominated geothermal fields has made considerable progress since improvements were made in binary fluid technology. The binary plants utilize a secondary working fluid, usually an organic fluid (typically n-pentane), that has a low boiling point and high vapour pressure at low temperatures when compared to steam. The secondary fluid is operated through a conventional Rankine cycle (ORC): the geothermal fluid yields heat to the secondary fluid through heat exchangers, in which this fluid is heated and vaporises; the vapour produced drives a normal axial flow turbine, is then cooled and condensed, and the cycle begins again.

By selecting suitable secondary fluids, binary systems can be designed to utilise geothermal fluids in the temperature range 85-170 °C. The upper limit depends on the thermal stability of the organic binary fluid, and the lower limit on technical-economic factors: below this temperature the size of the heat exchangers required would render the project uneconomical. Apart from low-to-medium temperature geothermal fluids and waste fluids, binary systems can also be utilised where flashing of the geothermal fluids should preferably be avoided (for example, to prevent well sealing). In this case, down-hole pumps can be used to keep the fluids in a pressurised liquid state, and the energy can be extracted from the circulating fluid by means of binary units.

Binary plants are usually constructed in small modular units of a few hundred kWe to a few MWe capacity. These units can then be linked up to create power-plants of a few tens of megawatts. Their cost depends on a number of factors, but particularly on the temperature of the geothermal fluid produced, which influences the size of the turbine, heat exchangers and cooling system. The total size of the plant has little effect on the specific cost, as a series of standard modular units is joined together to obtain larger capacities. Binary plant technology is a very cost-effective and reliable means of converting into electricity the energy available from water-dominated geothermal fields (below 170 °C).

Kalina cycle: A new binary system, the Kalina cycle, which utilizes a water-ammonia mixture as working fluid, was developed in the 1990s. The working fluid is expanded, in super-heated conditions, through the high-pressure turbine and then re-heated before entering the low-pressure turbine. After the second expansion the saturated vapour moves through a recuperative boiler before being condensed in a water-cooled condenser. The Kalina cycle is more efficient than existing geothermal ORC binary power plants, but is of more complex design.

Enhanced Geothermal Systems (EGS): Enhanced or engineered geothermal systems aim at using the heat of the Earth where no or insufficient steam or hot water exists and where permeability is low. EGS technology is centred on engineering and creating large heat exchange areas in hot rock. The process involves enhancing permeability by opening pre-existing fractures and/or creating new fractures. Heat is extracted by pumping a transfer medium, typically water, down a borehole into the hot fractured rock and then pumping the heated fluid upanother borehole to a power plant, from where it is pumped back down (recirculated) to repeat the cycle.

Size of the plant and grid

Small mobile plants, conventional or not, can not only reduce the risk inherent to drilling new wells but, what is more important, they can help in meeting the energy requirements of isolated areas. The convenience of the small mobile plants is most evident for areas without ready access to conventional fuels, and for communities that would be too expensive to connect to the national electric grid, despite the presence of high voltage transmission lines in the vicinity. 

Geothermal electricity can be delivered to large grids and mini-grids. Large-scale plants are grid-connected and deliver power for baseload purpose. Geothermal electricity production has been successfully developed in regions with hydrothermal manifestations (e.g., geysers and hot springs). Almost half of the globally installed geothermal power capacity is located in developing countries.

Co-benefits of this technology

  • Geothermal power is a stable source of energy as it is independent of weather circumstances and the climate in the countries. It is therefore a reliable source of energy and commonly has a high capacity factor of between 70 and 90% of installed capacity, which makes it applicable for both base and peak load, especially because geothermal power facilities have an inherent storage capacity. Geothermal energy is an indigenous source of energy and reduces the need to import fossil fuels.
  • Small mobile geothermal plants can help in meeting the energy requirements of isolated areas. The standard of living of many communities could be considerably improved were they able to draw on local sources of energy. Electricity could facilitate many apparently banal, but extremely important operations, such as pumping water for irrigation, freezing fruit and vegetables for longer conservation.
  • Geothermal power production would have the following social benefits:
    • It could contribute to a better income distribution towards local municipalities as the operation and management of geothermal facilities bring employment and increased economic activity in the regions where they are located. The employment will involve skilled, specialised jobs, which may not yet be available in these regions. This would require training of local people and/or hiring experts from other places.
    • In the case of the LaGeo geothermal power project in El Salvador, the building up of the facility is accompanied by a research project on biodiversity and a forest conservation and reforestation programme in the areas surrounding the project. Moreover, the project has a community engagement programme with participation of the neighbouring municipalities of Berlin. The programme aims at generating local employment opportunities, social investment activities, development of sustainable small business, and protection of the local environment.  
  • Geothermal power production has the environmental benefit of being a relatively clean fuel.
  • The contribution to greenhouse gas emission reduction from geothermal electricity production would lie in the possibility that it could replace fossil fuel based electricity production capacity. A CO2 emission source for this technology is the geothermal steam. Geothermal plants could theoretically inject these gases back into the earth, as a form of carbon capture and storage.

Some disadvantages to consider: The exploration of the geothermal energy systems could be complex. In particular, the process of confirmation of the location of the acquifer, its size and temperature is rather cost intensive. Although the drilling technologies are similar to the ones used in the oil industry, difficult rock permeability could complicate the drilling of wells. Some adjustments need to be made to deal with the higher temperatures and to make sure that the drilling fluid does not contaminate with the ground water. Finally, lack of reliable data on geothermal resources implies uncertainty about the availability and characteristics of the possible aquifers. This increases the financial risk and reduces the interest of financial institutions to provide funding to projects.

Potentially negative environmental impacts of geothermal power production are:

  • The impact of the drilling on the nearby environment. This requires the installation of a drilling rig and equipment, as well as construction roads. Depending on the distance that needs to be drilled, the area needed for the drilling rig could vary from 300 m2 to 1500 m2. Drilling could also lead to surface water pollution (e.g., through blow-outs) and emission of polluting gases into the atmosphere.
  • The pipelines to transport the geothermal fluids will have an impact on the surrounding area.
  • The reduction in the pressure in the aquifers. This could lead to subsidence of the ground in the geothermal facility regions. Re-injection of the condensed and/or cooled water back into the reservoirs could neutralise the subsidence. Re-injection also reduces the risk that the steam is exhausted into the atmosphere or that used water is discharged into surface water.

Case studies

Best practise guide for geothermal exploration