1. About Geothermal Energy
An enormous amount of geothermal heat is produced in the Earth’s core and moved through the molten rocks of the Earth’s mantle and aesthenosphere via processes of conduction and convection up to the Earth’s surface (the lithosphere or ‘crust’). The global technical potential of geothermal energy is enormous and renewable; a tiny fraction of the potential heat transfers involved would be sufficient to meet the world’s energy demand many times over. Problematically, most readily useable resources are not easily accessible due to the thickness of the Earth’s crust, however, or where they are, it is not typically located at points of energy demand.
The best geothermal fields including high temperature resources needed for power generation are generally found only at the boundaries of tectonic plates in subduction, divergence or rift zones. In these areas, the crust is thinner meaning that the heat in the mantle is much closer to the surface and thereby potentially economically accessible for use. Such zones are relatively rare worldwide. Furthermore, although some exploitable geothermal resources are located in populated, accessible areas where there is demand for electricity and heat, many others are found on the ocean floor or in mountainous regions.
The economic viability of geothermal energy depends not only upon the availability of suitably shallow heat sources i.e. sources at economically accessible depths from the surface, but also the presence of an effective natural heat transfer system. In the case of the latter, the ideal system is that of a hydrothermal system where meteoric (rain) water percolates through permeable rocks to a depth sufficient to form a natural geothermal reservoir that can be heated by a geothermal heat source. The presence of meteoric water is important as this allows for the natural recharge of the geothermal reservoir over time, avoiding the risk of resource depletion and allowing for its sustainable use.
A schematic of a typical hydrothermal (steam or water based) volcanic-related geothermal system is shown below (Figure 2), which are, from bottom to top:
The magmatic intrusion (also called hot body, where hot magma intrudes exceptionally far into the Earth’s crust) is often caused by tectonics of the continental plates.
The actual geothermal reservoir is where hot steam or water are trapped under high pressure beneath a tight, non-permeable layer of rocks and is heated by the magmatic intrusion below.
The geothermal wells tap into the geothermal reservoir and access the hot steam or fluid, then transfer it through pipelines to the power plant, after which the fluids are usually returned into the reservoir.
Fresh water or precipitation comes from recharge areas like lakes, rivers or the seas and provides cold meteoric waters, which slowly seep through the ground to lower layers through cracks and faults in the rocks. (after Gehringer and Loksha, 2012).
Consideration of a number of factors is required to determine the optimal use of a geothermal resource. These include the type (hot water or steam), rate of flow, temperature, chemical composition, and pressure of the geothermal fluid, and depth of the geothermal reservoir. Geothermal resources vary in temperature from 50° to 350°C, and can either be dry, mainly steam, a mixture of steam and water or just liquid water. Thus, the first consideration is to appropriately identify and characterise a geothermal resource in order to determine whether it is economically feasible to develop, and if so, for what purpose.
Figure 1. Main geothermal resource locations around the world
Courtesy of USGS
Figure 2. Schematic of a hydrothermal geothemal energy system
Source: Dickson and Fanelli, 2004
2. Identifying and Characterising Geothermal Resources
As highlighted above, geothermal energy sources become economically accessible when the Earth’s crust is thinner and the mantle is closer to the surface. A range of geological, geochemical and geophysical techniques have been developed to find and map these resources through a process of geothermal exploration. The purposes of geothermal exploration are to:
Locate areas underlain by hot rock
Estimate the volume of the reservoir, the temperature of the fluid in it, and the permeability of the formation
Predict whether the produced fluid will be dry steam, liquid or a two-phase mixture
Define the chemical nature of the geofluid
Forecast the electric power potential for a minimum of 20 years (DiPippo, 2008)
These can be divided into phases (usually seven) covering the full development cycle, the first 3-4 of which involve identifying and characterising the resource as described below (Figure 4; based on Gehringer and Loksha, 2012). These phases together can be collectively termed the “exploration” phase.
2.1 Phase 1 - Preliminary Survey
Initial resource mapping starts with reconnaissance surveys to indentify surface features that give an indication of a shallow heat source and a potential heat transfer mechanism such as fumaroles, steam vents, hot springs, boiling pools and heated earth. Remote sensing data may also be used to make a regional assessment of geological features such as faults, depressions, grabens or uplifting, heat maps (using e.g. IR) and through the use of techniques such as aeromagnetic surveys to assess depth to basement (crystalline) rocks.
2.2 Phase 2 - Exploration
The next step typically involves surface studies and initial exploration work in identified geothermal regions. This can include:
Geochemical, geological and geophysical investigations. Techniques include sampling of hot spring water to determine the temperature, origin of the water, and chemical compostion, rock sampling, and geophysical measurements such as Transient-Electro Magnetic (MET), Magneto Telluric (MT) and Bougoer gravity which can provide localised measurements by which to characterise the nature and geometry of the heat source.
Temperature gradient wells. Drilling slimline wells to a few hundred metres in the identified zone to measure the regional geothermal gradient. Under normal conditions away from the edges of tectonic plate margins (and hot spots/rift zones), the geothermal gradient is on average around 25 to 30°Celcius (C) per kilometre of depth (Goldstein et.al., 2011). Much higher gradients can give an indication of shallower heat sources at a given depth. For example, a measurement of 70°C/km would suggest a heat source of 210°C at 3 km of depth, which may be suitable for geothermal power generation (Figure 3). The use of a series of spatial geothermal gradient data can help to map the location and geometry of the heat source.
Seismic exploration. To identify geothermal reservoirs and any local and regional faults and fracture that can be used to target exploration and production drilling. Most geothermal wells look to intercept at least one fault through which hot meteoric waters are being transferred to surface or near-surface.
The next phase usually involves sketching out and delineating the potential heat resource zone and hydrothermal system, and the compilation of a preliminary reservoir model.
Figure 3. Schematic depth-temperature plot for geothermal energy
Source: EGS, undated
The outcomes of Phases 1-3, including any previously acquired subsurface information, will need to be collated into a full understanding of the nature of the geothermal resource. This will give an indication as to its potential for development, and, moving into Phase 4 and beyond, allow a more detailed feasibility study to be carried out on whether to proceed with field development, and for what purpose and what scale.
The full process by Phase for geothermal power development is shown to the right (Figure 4).
The options for utilising a geothermal resource depend on the quality and volume of the resource indentified. Hydrothermal fields are often classified into high, medium, and low temperature fields, based on inferred temperature at a depth of 1 km (Table 1).
The options for utilising the different types of resource are outlined below.
Figure 4 (right). Geothermal project development phases for a unit of approximately 50 MW
Source: Gehringher and Loksha, 2012
2.3 Phase 3 - Test drilling
The final phase of exploratory activities is test drilling. This can involve drilling a well, or a series of 3-5 wells, to confirm or otherwise the presence, volume and geometry of the target geothermal reservoir and to characterise the temperature, flow, recharge rate and geochemistry of hydrothermal fluids. It typically involves drilling full size wells (>20 cm diameter) or sometimes slimline holes (<15 cm diameter) depending on circumstances. In the case of the latter, the costs can be significantly reduced (by up to 50%; Gehringer and Loksha, 2012), although a full size bore will be needed to convert the exploration well to a producer.
Table 1. Classification of geothermal resources and use
Source: based on Dickson and Fanelli, 2004; Gehringer and Loksha, 2012
3. Utilising Geothermal Resources
The use of geothermal energy by humans has been around for thousands of years involving a range of applications such as therapeutics and balneology, cooking and washing. Industrial uses came much later, when in the 18th century geothermal muds in deposits around the town of Larderello, Italy, were used as a source for boric acid extraction. Today, modern uses principally involve the use of geothermal fluids for electricity generation, although direct use of hot geothermal waters also continues to evolve as new technologies emerge that can improve the way in which the heat may be recovered and utilised, including in the food and agriculture sector (Van Nguyen et. al., 2015).
3.1 Use for Power Generation
Geothermal power production involves accessing the geothermal fluids in the reservoir through a series of wells in a development known as a ‘steamfield’, and utilising the collected hot fluid or steam to drive a turbine for power generation, either directly, or indirectly through heating of an alternative working fluid in what is known as a ‘binary system’. While the greatest concentration of this type of resource is associated with tectonic boundaries, other lower temperature resources can be found in most countries worldwide; exploitation of these heat sources has been used for a wide range of ‘direct uses’ in homes and industry (see below). Alternatively, where a heat transfer system is absent (e.g. in arid climates or where permeable rocks are absent), a system may be developed through manmade fracturing of the rock and the injection of fluids from the surface; a process known as Enhanced Geothermal Systems (EGS). To date, EGS systems have seen only limited development at small scale around the world (e.g. in Australia and parts of the USA).
Four major types of geothermal power plant can be used to convert hydrothermal fluids to electricity: flash steam plants, binary systems, dry steam and back pressure units (Table 2; Figure 5). The choice of technology depends on the temperature and state of the geothermal fluid (i.e. wet or dry steam, a steam/water mixture or hot brine).
Countries have deployed different power generation technologies according to the nature of their geothermal resources. Single and multiple flash steam plants are currently the most common type of geothermal power generation plants in operation worldwide today, accounting for almost two thirds of installed capacity worldwide.
Dry steam technology is the ideal system as it involves less components (i.e. no condenser) and reduced needs for wastewater management, although occurrences are quite rare (e.g. the Geysers plant, CA, USA; Lardarello, Italy). The technology, however, accounts for some 22% of total capacity worldwide. Binary systems account only for around 14%, largely as a consequence of the higher costs and therefore more marginal economics of the process. It can be expected that binary systems will grow over coming years as more medium temperature geothermal fields are developed for power generation, including ‘bottoming plants’.
 A bottoming plant utilises the residual heat in wastewaters from an existing geothermal power plant
Table 2. Geothermal power plants
Source: Carbon Counts based on various sources
Figure 5. Schematic of two common types of geothermal power plant: flash steam (left) and binary (right)
4. Managing Geothermal Resources
In all cases, there is a need to carefully manage geothermal resources to avoid over use and depletion. Power plants need to be sized according to the resource available: oversizing the power plant in relation to the productive capacity of the underlying geothermal reservoir can cause unsustainable extraction rates resulting in pressure drops, temperature declines or total resource depletion. For this reason, a geothermal reservoir should be developed in a phased or step-wise approach in which increments of capacity are added sequentially as resource data and information are collected over time – instead of building one large power plant sized at the fields overall total potential. To meet this requirement, modern geothermal power systems tend to be modular in design, allowing for units of e.g. 5, 20 or 50 MW to be phased in sequentially.
In combination with undertaking a step-wise approach to field development, developing two or more geothermal fields in parallel is also recommended to accelerate the build-up of the overall geothermal development program in a country or region (Gehringer and Loksha, 2012). The combination thus helps manage resource risk at both a country/region level and the field level, whilst allowing for faster supply of new capacity. This is illustrated in Figure 7, in which one 50 MW plant built in each of two geothermal fields results in 100 MW of geothermal power being available for power generation in the same period of time needed to build a 50 MW project in one field. Under this parallel development approach, each field’s productive capacity is partially utilised only in the first phase with a “first-step” 50 MW plant. Additional plants may be added over time, however, increasing the utilization of each field’s productive capacity as demand for electric power increases. Each subsequent increment of geothermal power capacity takes advantage of the much better information about the resource, based on several years of operational data from the “first-step” plant in each field (ibid.).
3.2 Direct Uses
The direct use of geothermal resources by humans has a long history. The different applications for direct-use of geothermal energy vary according to temperature, as illustrated by the Lindal diagram below (Figure 5). Direct use is typically associated with lower-temperature geothermal resources (those with temperatures less than 150°C), although some industrial applications may require higher temperatures.
Source: Gehringer and Loksha, 2012
Figure 6. Direct use applications of geothermal energy (modified Lindal Diagram)
Source: Van Nguyen et. al., 2012, based on Lindal, 1973
Cascaded systems, whereby the same water is used (or ‘cascaded’) in successive applications at progressively lower temperatures, is possible within a single geothermal operation, improving efficiency and economic feasibility (Figure 6). The level of cascading depends on the available energy and the energy requirements of the various processes. Cascading typically combines direct uses with geothermal power production, utilizing the residual and waste heat to enhance overall project economics and resource utilisation. Cascaded, or multiple, use of geothermal energy has long been used in Iceland, where the residual heat from geothermal power generation is often utilised for nearby industries (e.g. heating, food processing, wood drying), followed by domestic heating and swimming pool use; greenhouses and fish farming; and finally ice and snow melting.
The Menengai Geothermal Project, currently being developed in Kenya, is also piloting the development of a cascaded system through provision of geothermal heat to local businesses and industry (see here for more information).
Figure 7. Schematic of a goethermal cascade
Figure 8. Parallel development of resoruces with step-wise field expansion
Source: Gehringer and Loksha, 2012
A good example of step-wise expansion is provided by the Olkaria geothermal power project in Kenya. Olkaria I (15 MW) was commissioned in 1981 and subsequently expanded to 45 MW; Olkaria II was then commissioned (2 x 35 MW units) in 2003 and expanded in 2010 by a further 35 MW. In addition, an IPP operated by Ormat Technologies operates at Olkaria III, which has been expanded from a 13 MW binary plant in 2000 to 110 MW at present. Olkaria IV was commissioned in 2014, bringing on a further 140 MW of capacity. The Kenyan Electricity Generating Company (KenGen) plans to expand the Olkaria field further over coming years as well as developing other geothermal fields for power production in parallel.
Improper steam field operation practices can also lead to pressure drops and potential depletion of the geothermal reservoir. The Geysers geothermal field in California, USA, offers a classic lesson in mismanagement of geothermal resources. During the early 1980’s, the rapid and largely uncoordinated development of the field led to multiple operators drilling multiple wells, which in many cases interfered with other operations. Since the field was not managed as a whole (i.e. ‘unitized’), and each operator maintained confidentiality over their operation with different incentives to operate based on their PPA, the effects were catastrophic with rapid depletion of the steam resource and related power output (Sanyal and Enedy, 2011).
In 1992, the California Energy Commission initiated an investigation into the problems, and it was concluded that to halt the declining production, injection augmentation (recharge) would be needed to avoid continuing pressure drop. This then became the most well-known example of artificial augmentation, when in 1997 a 46 kilometre pipeline was commissioned bringing 7.8 million gallons (30 million litres) per day of treated sewage effluent for reinjection into the reservoir.
Fluid reinjection is generally undertaken at many fields these days to avoid both environmental problems from wastewater discharge and also to maintain reservoir pressure and productivity. In all cases, reinjection must be undertaken in locations where it will not lead to cooling of the geothermal reservoir.
5. References and Further Reading
Dickson, M.H. and Fanelli, M., 2004. What is Geothermal Energy? Paper published on the website of the International Geothermal Association (www.geothermal-energy.org)
DiPippo, R., 2008. Geothermal Power Plants: Principles, Applications, Case Studies and Environmental Impact. Second Edition. Elsevier, Oxford/New York.
ESMAP, 2012. Drilling Down on Geothermal Potential: An Assessment for Central America. Report by the World Bank Energy Unit, Sustainable Development Department Latin America and the Caribbean Region/Energy Sector Management Assistance Programme. Washington D.C. March 2012.
Gehringer, M. and Loksha, V. 2012. Geothermal Handbook: Planning and Financing Power Generation. Energy Sector Management Assistance Programme (ESMAP). Technical Report 002/2012. Washington D.C. June 2012. Available at: https://www.esmap.org/Geothermal_Handbook
Van Nguyen, M., Arason, S, Gissurarson, M., and Pálsson, P., 2015. Use of geothermal energy in food and agriculture: Opportunities for developing countries. Report for the Food and Agriculture Organization of the United Nations, Rome. 2015
About Geothermal Energy Technology
This page provides a basic introduction to geothermal energy technology aimed at people new to the technology and wishing to learn more. It is organised as follows: