Chapter 9
Geothermal Energy
9.1 The Origin of Geothermal Energy
The geothermal energy is basically thermal energy stored in the Earth's crust. It is clean and sustainable. It is one of the less-recognized forms of renewable energy, and this is the only form of renewable energy which is not dependent on the Sun. Geothermal energy comes from the natural generation of heat primarily due to the decay of the naturally occurring radioactive isotopes of uranium, thorium and potassium within the Earth. Resources of geothermal energy range from the shallow ground to hot water and hot rock found a few miles beneath the Earth's surface and even deeper down to the extremely high temperatures of molten rock called magma. It is estimated that the annual energy due to the internal heat generation and flowing from the interior of the Earth to the surface is about 1021 J per year.
Geothermal energy has been used by the Romans, Japanese, Turks, Icelanders, Central Europeans and the Maori of New Zealand for bathing, cooking and space heating. Baths in the Roman Empire, the middle kingdom of the Chinese and the Turkish baths of the Ottomans were some of the early users of geothermal energy [1, 2].
Geothermal energy is present everywhere below the Earth's surface, but the most desirable resources are concentrated in the regions of active or geologically young volcanoes. Most geothermal reservoirs are deep underground with no visible sign of it available on the Earth's surface. But sometimes, geothermal energy erupts to the surface in the form of
- volcanoes
- hot springs
- geysers.
The total thermal energy contained in the Earth is estimated to be 12.6 × 1012 EJ and that of the crust of the order of 5.4 × 109 EJ up to depths of 50 km. Stored thermal energy down to 3 km depth in the continents is estimated to be 42.67 × 106 EJ (Source EPRI report 1978). With the present global energy consumptions for all types of energy, the Earth's energy to a depth of 10 km could theoretically supply all of mankind's energy needs for 6 million years. At present, it is only possible to extract small portion of this stored energy due to limitations in drilling technology and rock permeability. Commercial utilization is concentrated on areas where drilling to depths up to 4 km can access fluids at temperatures of 180°C to more than 350°C.
The first use of geothermal energy for electric power production was in Italy, with experimental work by Prince Ginori Conti between 1904 and 1905. The first commercial power plant (250 kW) was commissioned in 1913 at Larderello, Italy. Now power is generated in plants installed in Thailand, Argentina, Taiwan, Australia, Costa Rica, Austria, Guatemala, Ethiopia, with the latest installations in Germany and Papua New Guinea.
9.2 Types of Geothermal Resources
The Earth from crust to core is divided into three areas: core, mantle and crust, as shown in Fig. 9.1. The core which is about 1900 km below the Earth's surface is extremely hot 5000–6000°C and is made mostly of iron (80%) and nickel (20%), whose inner half (by radius) is solid and whose outer half is liquid. After that, there is a thick layer in the middle called mantle, which makes up most of the rest (83%) of the Earth's volume and is made mostly of rocky material, whose inner part is semi-rigid and whose outer and cooler part is plastic and, therefore, can flow (thick lava). Then there is the top layer of the Earth called crust where temperatures are moderate for humans to survive. A semi-fluid material called magma originates in the lower part of the Earth's crust and in the upper portion of the mantle. There, high temperatures and pressures cause some rocks to melt and form magma. On average, the temperature of the Earth increases about 30°C/km above the mean surface ambient temperature. The temperature at the base of the crust is about 1000°C, and assuming a conductive gradient, the temperature of the Earth at 10 km inside would be over 300°C.
Figure 9.1 Earth temperatures from crust to core.
The distribution of layers is not the same everywhere inside the Earth. In certain locations, anomalies exist in composition and structure of these layers. There are certain regions where hot molten rock (magma) occurs at shallow depths due to the following: (i) intrusion of molten rock (magma) from depth, bringing up great quantities of heat; (ii) high surface heat flow, due to a thin crust and high temperature gradient; (iii) ascent of groundwater that has circulated to depths of several kilometres and been heated due to the normal temperature gradient; (iv) thermal blanketing or insulation of deep rocks by thick formation of such rocks as shale whose thermal conductivity is low; and (v) anomalous heating of shallow rock by decay of radioactive elements, perhaps augmented by thermal blanketing.
Most geothermal exploration and use occur in these places where the gradient is higher, and thus, drilling is shallower and less costly.
Magma is extremely hot – between 700 and 1300°C. When magma rises to the surface of the Earth through vents or during volcanic eruptions, it becomes lava. When lava stops flowing and cools, it hardens into igneous rock. Several of the world's most advanced geothermal sites are located in extinct volcanic areas.
Geothermal energy resources are characterized by geological settings, intrinsic properties and viability for commercial utilization. Most of the geothermal sites are located near the edges of the pacific plate known as ring of fire. The diversity of both the nature of the geothermal resource and its exploitation presents a challenge in the context of resource classification. There are different ways to classify the geothermal resources. Temperature is a fundamental measure of the quality of the resource and consequently is the primary element of most classification systems by which geothermal energy is identified and utilized. Geothermal resources suitable for different types of uses are commonly divided into two categories, high and low enthalpy, according to their energy content. High-enthalpy resources (>150°C) are suitable for electrical generation with conventional cycles, while low-enthalpy resources (<150°C) are employed for direct heat uses and electricity generation using binary cycles [3, 4].
The geothermal resources can also be classified as convective (hydrothermal), conductive and deep aquifers as shown in Table 9.1 [5–7]. The hydrothermal resources include liquid- and vapour-dominated type. Conductive sources include hard rock and magma over a wide range of temperatures. Deep aquifers are circulating fluids in porous media or fractured zones. These are either geopressured or at hydrostatic pressure.
Table 9.1 Types of geothermal resources
| Resource type | Temperature range (°C) |
| Convective hydrothermal resources | |
| Vapour-dominated | 240 |
| Hot-water-dominated | 20–350 |
| Geopressured (aquifer) | 90–200 |
| Hot rock resources (conductive) | |
| Solidified (hot dry rock) | 90–650 |
| Part still molten (magma) | >600 |
Ambient temperatures in the 5–30°C range can be used with geothermal (ground-source) heat pumps to provide both heating and cooling.
9.3 Hydrothermal Resources
Hydrothermal means that the transfer of heat involves water, either in liquid or in vapour state (hence the name “hydro”). Hot springs and geysers, for example, are hydrothermal features. All geothermal electricity produced today derives from the hydrothermal resource base. Hydrothermal systems occur where the Earth's heat is carried upwards by convective circulation of naturally occurring hot water or steam. Some high-temperature convective hydrothermal resources result from deep circulation of water along fractures. These systems are dominated by liquid and vapour. The presence of water facilitates not only vertical hydrothermal energy flows (convection) but also horizontal hydrothermal energy flows (through convection, advection and diffusion). These are high-enthalpy systems and found in areas of magmatic intrusions where temperatures above 1000°C can occur at less than 10 km of depth.
These geothermal reservoirs of steam or hot water occur naturally where magma comes close enough to the surface to heat the groundwater trapped in fractured or porous rocks or where water circulates at great depths along faults. The water in these resources is trapped in the underground reservoir (acquifers), and the Earth's heat is carried upwards by convective circulation of naturally occurring hot water or steam. Naturally occurring large areas of hydrothermal resources are called geothermal reservoirs, Fig. 9.2. Hydrothermal resources occur at a location, perhaps in conjunction with other hydrothermal features. They emit heat and discharge hydrothermal water. The water contains chemicals and gases. So, location, area, heat, water and chemistry become critical vital signs to monitor. Although these vital signs are observable at the surface, they also provide information about the sub-surface hydrothermal system.
Figure 9.2 Geothermal reservoir.
Source: Nasruddin et al. (2016) [8]. Reproduced with permission from Elsevier.
Hydrothermal resources are used for different energy purposes depending on their temperature and the depth. In places where the pressure is high, the temperature may reach as high as 350°C. As the water flows through the porous medium that constitutes the aquifer, its static pressure drops and large volume of steam is produced. When this steam escapes through cracks in the surface, it is called fumaroles. In areas where the water table rises near the surface, fumaroles can become hot springs or geysers.
Hydrothermal resources are further classified as (i) vapour-dominated, (ii) liquid-dominated and (iii) hot-water resource. Three geothermal power generation technologies are used to convert hydrothermal fluids to electricity: dry steam, flashy and binary cycle. The type of conversion technology used depends on whether the fluid is present as steam or water and its temperature.
9.3.1 Vapour-Dominated Systems
Vapour-dominated geothermal fields are located in the regions of recent volcanism, near the borders of tectonic plates. These reservoirs are few in number, with the largest sources located as geysers in northern California. Larderello in Italy and Matsukawa in Japan are also places where the steam is exploited to produce electric energy. In order to form a heat reservoir, the anomalous magmatic intrusion should encounter porous and permeable water-filled rock strata. In hard compact rocks, faulting may provide a channel for the magma to reach the surface. Soft or plastic rocks, when present, can flow and block the fault space, causing the magma to spread at the contact between the soft and the hard rocks. A vapour-dominated geothermal system is shown in Fig. 9.3.
Figure 9.3 Vapour-dominated system.
In these resources, the water boils underground and generates steam at about 165°C. These systems are the best and most productive geothermal resources because the steam is largely dry and is of very high enthalpy. Currently, hydrothermal systems are the only commercially exploited geothermal systems for power generation. In a steam-dominated reservoir, the fluid distribution is controlled by the flow of steam moving up and the water moving down. If the mass fluxes of water moving down and steam moving up are roughly similar, and the vertical pressure gradient is near steam-static, the relative permeability to water must be low. The flowing steam occupies most of the fracture space, and water occupies the remaining pore space. The mass of water in the reservoir is much greater than the mass of steam. Due to the decrease of steam production, it is necessary to inject water through wells into the reservoir.
When the geothermal resource produces a saturated or superheated vapour, the steam is collected from the production wells, and appropriate measures are taken to remove any solid debris from the steam flow, as well as corrosive substances contained in the process stream. After processing, the steam is sent to a conventional steam turbine. If the steam at the wellhead is saturated, steps are taken to remove any liquid that is present or forms prior to the steam entering the turbine. Normally, a condensing turbine is used; however, in some instances, a backpressure turbine is used that exhausts steam directly to the ambient temperature.
9.3.2 Water-Dominated Systems
Water-dominated systems are produced by groundwater circulating to depth and ascending from buoyancy in permeable reservoirs that are at a uniform temperature over large volumes. Typically, the temperature of hot-water reservoirs varies from 60° to 100°, and they occur at depths ranging from 1500 to 3000 m. The geology of hot-water geothermal fields is quite similar to that of an ordinary groundwater system. They differ from the vapour-dominated geothermal fields in the fact that the hot-water geothermal fields are characterized by liquid water being the continuous pressure-controlling fluid phase. As the water rises in the well, its static pressure is reduced due to gravitational and frictional forces. The temperature of the water also decreases because of heat loss to the surroundings. However, because rocks have high insulating property, the decrease in temperature is not much. Figure 9.4 presents a typical hot-water field. As shown in Fig. 9.4, a hot-water geothermal field could develop in the absence of a cap rock, if the thermal gradients and the depth of the aquifer are adequate to maintain a convective circulation.
Figure 9.4 Hot-water geothermal system.
In liquid-dominated plants, geothermal plants are built upon liquid reservoirs within the Earth's surface. This liquid is sent through one or more separators in order to lower the pressure of the water, creating steam. This steam then propels a turbine generator, causing it to produce electricity. This steam is then condensed back into a liquid and placed back into the liquid reservoir it originated from. This type of geothermal plant is very common and provides a sustainable, reusable form of energy.
9.4 The Geopressured Resources
Geopressured resources are reservoirs of naturally high-pressured hot water. Geopressured resources are formed when there is an impermeable layer of sedimentary cap rock that traps a geothermal reservoir. In these instances, the weight of the sediment layer and the lack of permeability increase the pressure inside the reservoir. Geopressured resources typically range from 90 to 200+°C, and the increase in pressure reduces the energy required to pump the resource, making geopressured resources desirable. These sources consist of deeply buried reservoirs of hot brine under abnormally high pressure that contain dissolved methane. Thermal waters under high pressure in sand aquifers are the target for drilling, mainly as they contain dissolved methane. Geopressured geothermal systems were first identified in the deep sedimentary layers underneath the Gulf of Mexico at a depth between 6 and 8 km with pore pressures of up to 130 MPa and temperatures in the range of 150–180°C.
In geopressured systems shown in Fig. 9.5, the water temperature can range from 90 to 200°C which correspond to low saturation pressures. The pressure in the liquid is significantly higher than saturation pressure at all levels in the well.
Figure 9.5 Geopressured geothermal system.
Three forms of energy are useable in geopressured wells: (i) thermal from the high temperatures, (ii) hydraulic from the high fluid flow pressure and (iii) chemical from the dissolved methane in the fluids. It is possible to exploit each form singly or in combination to satisfy a variety of energy needs. Production of electrical energy from geopressured sources is discussed later.
9.5 Hard Rock Resources
Hot dry rock geothermal reservoirs contrast with conventional geothermal reservoirs in that the rocks are generally deep, impermeable and with little or no porosity/fluid content to transport the heat. Hence, a geothermal reservoir must be engineered by pumping cool water down an injection well and recovering hot water from a nearby production well(s), with flow between the two wells stimulated by the pressure of injected fluids.
These are further classified as (i) solidified (hard dry rock) and (ii) magma. Hot dry rock resources are much more common than hydrothermal resources and are more accessible.
9.5.1 Solidified (Hot Dry Rock Resources)
The hot dry rock (HDR) resource is larger and more widespread than the conventional geothermal resource at around temperatures of 200°C. These sources are accessible under a significant proportion of the world's landmass. Since there is no water which can be transported to the surface of the Earth, recovery of heat involves creating a fracture system at depth (that acts as a heat exchanger) and circulating water from an injection borehole towards a production borehole. The water is pumped into the cracks from the surface and withdrawn from another well at a distance. Thus, a number of wells are required to be drilled through hard rocks. HDR geothermal resource represented by the vast regions of hot rock at accessible depths in the Earth's crust far exceeds that of the combined total of the world's fossil energy resources. Recently, it has been recognized that very few rocks are actually completely dry, and hence, these sources are now termed as enhanced or engineered geothermal systems (EGSs).
9.5.2 Part Still Molten (Magma)
Another source of geothermal energy is magma, which is partially molten rock. Molten rock is the largest global geothermal resource and is found at depths below 3–10 km. Magma is the ultimate source of all high-temperature geothermal resources. Plate boundaries are the most common sites of volcanic eruptions. At several volcanic locales, magma is present within the top 5 km of the crust. Its great depth and high temperature (between 700 and 1200°C) make the resource difficult to access and harness. Thus, technology to use magma resources is not well developed.
Magma (molten rock) may come quite close to the surface where the crust has been thinned, faulted or fractured by plate tectonics. When this near-surface heat is transferred to water, a usable form of geothermal energy is created. Magma, the naturally occurring molten rock material is a hot viscous liquid, which retains fluidity till solidification. It may contain gases and particles of solid materials such as crystals or fragments of solid rocks. However, the mobility of magma is not much affected until the content of solid material is too large.
The heat energy available from such sources, if harvested, would constitute very large additions to the global renewable energy. Extraction of thermal energy from magma was tested during the 1980s by drilling into the still-molten core of a lava lake in Hawaii.
9.6 Energy Contents of Geothermal Resources
9.6.1 Hard Dry Rock Resources
In order to evaluate the amount of energy available in a hot dry rock resource, it is assumed that the thermal gradient G is constant. Under this assumption, the temperature T at a depth of h will be
where T0 is the surface temperature.
Suppose that 
is the minimum depth needed to reach temperatures of T1 = 150° and 
is the maximum depth to which current technology allows wells to be drilled. Recall that by the definition of the specific heat of a substance, c, the stored thermal energy in a mass m whose temperature is higher by ΔT above some reference temperature can be expressed as
where mass 
and A is the area of rock. Thus, the amount of stored thermal energy below a surface area A between depths H and h + dh can be expressed as
The total useful energy of the rock between depths 
and 
is given by
Since 
9.7 Exploration of Geothermal Resources
Since geothermal resources are underground, exploration methods, geological, geochemical and geophysical, have been developed to locate and assess them. The objectives of geothermal exploration are to identify and rank prospective geothermal reservoirs prior to drilling and to provide methods of characterizing reservoirs (including the properties of the fluids) that enable estimates of geothermal reservoir performance. Exploration typically begins with gathering data from existing nearby wells and other surface manifestation. Most countries have existing databases of geological and hydrological data. These have usually been gathered for other purposes but may very well be useful for guiding early geothermal surveying and exploration. Every effort should be made to collect and analyze these data prior to designing and planning an additional exploration program. Remote sensing, in particular, is now playing a more significant role in preliminary surveying for geothermal resources.
In order to make the exploration program cost-effective while reducing risk, the survey design initially focuses on simpler (cheaper) methods and becomes progressively more complex and costly as early results show promise for more detailed efforts. Geophysical methods are among the three main disciplines applied on the surface to explore geothermal resources, including geology and the chemistry of thermal fluids.
The important physical parameters in a geothermal system are
- temperature;
- porosity;
- permeability;
- chemical content of fluid (salinity); and
- (pressure).
Most of these parameters cannot be measured directly through conventional geophysical methods applied on the surface of the Earth. However, there are other parameters that can be measured which are linked with these parameters and may give important information on the geothermal system. Among these parameters are
- temperature (°C);
- electrical resistivity (m);
- magnetization.
- density;
- seismic velocity;
- seismic activity;
- thermal conductivity; and
- streaming potential.
The decision to go for extracting energy from geothermal source depends upon exploration of surface features to determine first whether or not a resource is worth the large amount of investment required to drill a geothermal well. Once surface features have been investigated, several techniques are used to help identify drill targets without having to put a drill bit into the ground. These exploration technologies not only need to better locate geothermal resources but they must be able to provide more accurate imaging of the structure of the sub-surface reservoir and provide accurate reservoir temperatures at specified depths.
9.8 Geophysical Methods in Geothermal Exploration
The geothermal reservoirs and their immediate surroundings have certain specific physical characteristics that are susceptible to detection and mapping by geophysical methods. The temperature within the reservoir is the most important physical characteristic of a geothermal system. Electrical and magnetic properties of the rock also provide information about the location of geothermal resources.
The most important methods used in geophysical exploration of geothermal fields are as follows:
- Thermal methods
- Electrical methods
- Magnetic measurements
9.8.1 Thermal Methods
The base temperature constitutes the most important physical characteristic of a geothermal system; therefore, thermal exploration techniques provide the most direct method for making a first estimate of the size and potential of a geothermal system with surface geophysical exploration. Measurements can be made in holes as shallow as a few metres, but it is preferred to conduct temperature surveys in wells that are at least 100 m deep. Temperatures measured a short distance beneath the surface of the Earth are strongly affected by cyclic changes in temperature on the surface of the Earth. Drilling is usually fairly expensive and puts practical limits to the use of the method. Furthermore, shallow wells are not always adequate to get reliable values on the thermal gradient. In addition, determination of temperature in a test hole is not an easy task. In deep test holes, which must be drilled with a circulating fluid such as mud, a considerable disturbance of the normal-temperature environment will take place during drilling. This is particularly true if the gradient is relatively high and the temperature change over the well interval is relatively large. Temperature at depth can be sensed directly in boreholes or estimated by extrapolation of heat-flow measurements in both shallow and deep holes. Heat-flow measurements combine observed temperature gradients and thermal conductivity measurements to determine the vertical heat transport in areas where conduction is the primary mechanism of heat transport.
Normally, one must wait for a period of time comparable to that involved in drilling the well before the well temperatures return to within 10% of their undisturbed state. Drilling a well to several hundred metres depth may take a few days. In order to measure temperatures accurately for thermal gradient purposes, the temperatures are recorded for periods that are several times longer than the duration of drilling. However, there are methods which provide the relationship between the temperature and the time after the drilling.
9.8.2 Electrical Methods
Electrical methods or resistivity methods are the most important geophysical methods in the surface exploration of geothermal areas. Various methods used for measuring electrical resistivity are based on the proposition that temperature affects the electrical properties of rocks. Direct current resistivity methods measure Earth's resistivity using a direct or low-frequency alternating current source. High temperatures and hot thermal fluid circulation observed in a geothermal system have a severe impact on the electrical properties of the geological formations encountered in the geothermal field area. A large decrease in resistivity of the rocks is observed due to saline, brine hot waters that circulate in the permeable paths of it. Rocks are electrically conductive as consequences of ionic migration in pore space water and, more rarely, electronic conduction through metallic lustre minerals. The electrical conductivity of ionic conductors increases largely with temperature. Conductivity of the host rock of the geothermal field increases due to wall rock hydrothermal alteration and hydrothermal mineral deposition in fracture zones.
The maximum increase in conductivity for most electrolytes is about seven times between 20 and 350°C. Different methods of electrical measurements and setups are as follows:
- dc Methods, In this method, dc or low-frequency ac current (I) is passed through the ground. The developed electrical potential field is measured. Two current electrodes are used for passing current into the ground, while the developed potential difference, along the potential dipole length, is measured by two other potential electrodes.
- TEM, where current is induced by a time-varying magnetic field from a controlled source. The monitored signal is the decaying magnetic field at the surface from the secondary magnetic field.
- MT, magnetotelluric (MT) and the audio-magnetotelluric (AMT) use natural electromagnetic fields for energizing the ground. Since the origin of these fields is located at a large distance from the study area, the plane-wave assumption for the EM field is valid, thus simplifying the interpretational techniques.
Raw electrical field data have to be interpreted in order to derive models of the electrical structure of the sub-surface. Interpretational schemes exist for almost all of the electrical methods. Most modern interpretation methods involve either or both forward modelling and inversion. In forward modelling, as proposed resistivity model of the sub-surface is constructed and the response of this model to a particular electrical method being used is computed for comparison with the observed data. Adjustment in the model is made till the data closely matches the observed data.
9.8.3 Magnetic Measurements
The largest component (80–90%) of the Earth's field is believed to originate from convection of liquid iron in the Earth's outer core, which is monitored and studied using a global network of magnetic observatories and various satellite magnetic surveys. This field in first approximation is dipolar and has a strength of approximately 50,000 nT.
Magnetic surveys are an effective method to locate a prospective geothermal reservoir. It is found that all igneous and metamorphic rocks generally have a higher magnetic susceptibility than sedimentary rocks. In the magnetic method, the intensity of the natural magnetic field is measured. This includes contribution from the Earth's core and crust, as well as any secondary magnetic field induced in magnetic geological bodies, which locally creates positive and negative magnetic field anomalies activity. Ferromagnetic materials exhibit a phenomenon characterized by a loss of nearly all magnetic susceptibility at a temperature called the Curie temperature. At this temperature, ferromagnetic rocks become paramagnetic, and there are no detectable magnetic anomalies. The Curie temperature for titanomagnetic material, which is the most common magnetic mineral in igneous rocks, is less than approximately 570°C. It has been established that an increase in titanium content of titanomagnetite causes a reduction in Curie temperature, and the titanium content generally increases in the more mafic igneous rocks. The greatest potential for the magnetic method lies in its ability to detect the depth at which the Curie temperature is reached.
9.9 Geochemical Techniques
The basic philosophy behind using geochemical methods in geothermal exploration is that fluids on the surface (aqueous solutions or gas mixtures) reflect the physical, chemical and thermal conditions in the geothermal reservoir inside the Earth. Chemical data on hot water and steam discharges in a virgin area serve as useful indicators of the feasibility of further exploration in the area including preliminary drilling locations. Together with structural information from geological, hydrological and geophysical methods, they can guide decision-making on sub-surface exploration by drilling.
Chemical geothermometers, which relate the fluid chemistry and reservoir temperature, are routinely used in assessing the energy potential.
Geothermometers have been classified into three groups:
- 1. Water or solute geothermometers
- 2. Steam or gas geothermometers
- 3. Isotope geothermometers
9.9.1 Water or Solute Geothermometers
The most important water geothermometers are silica (quartz and chalcedony), Na/K ratio and Na-K-Ca geothermometers. Others are based on cation ratio and any uncharged aqueous species as long as equilibrium prevails. Silica geothermometers are one of the oldest and most commonly used types. Geochemical studies have shown that quartz is an important secondary mineral phase present, and therefore, it is common to compare the silica value of the thermal waters with the quartz solubility versus temperature curve for deducing the reservoir temperature. In systems above about 180°C, the silica concentration is controlled by the equilibrium with quartz. At lower temperatures, the equilibrium with chalcedony becomes important.
9.9.1.1 Na-K Geothermometer
A commonly used geothermometer where geothermal waters are known to come from high-temperature environments (>180°C, up to about 200°C) is the atomic ratio of sodium to potassium (Na/K). The ratio decreases with an increase in temperature. The cation concentrations (Na+, K+) are controlled by temperature-dependent equilibrium reactions with feldspar and mica. The main advantage of this thermometer is that it is less affected by dilution and steam separation than other geothrmometers, provided there is little Na+ and K+ in the diluting water compared to the reservoir water. Na/K geothermometer fails at temperatures lower than 100–120°C and gives high temperatures for solutions with high calcium contents.
9.9.1.2 Na-K-Ca Geothermometer
The temperatures obtained from Na-K geothermometers are generally higher than the actual values, for high calcium contents. To overcome this, Na-K-Ca thermometers are used.
A temperature equation for a geothermometer is a temperature equation for a specific equilibrium constant referring to a specific mineral–solution reaction.
9.9.2 Gas Thermometers
The gas geothermometers are useful for predicting sub-surface temperatures in high-temperature geothermal systems. They are applicable to systems in basaltic to acidic rocks and in sediments with similar composition, but should be used with reservation for systems located in rocks which differ much in composition from the basaltic to acidic ones. The geothermometry results may be used to obtain information on steam condensation in up flow zones or phase separation at elevated pressures.
9.9.3 Isotopes
Chemical elements with the same atomic number (protons) and different atomic mass are defined as isotopes. Isotopes have identical chemical behaviour but different physical properties. Hydrogen has three isotopes, 1H, 2H, 3H, respectively, and oxygen also has three isotopes, 16O, 17O and 18O. Isotope geothermometers involving oxygen- or hydrogen-exchange reactions with water. Several isotope-exchange reactions can be used as sub-surface temperature indicators. The exchange of 18O between dissolved sulfate and water is the most useful. An isotopic fractionation occurs when steam separates from hot water. The isotopic compositions of both the steam and water in a well sample may be determined from the total discharge, whose steam and water fraction is known.
Different parts of the Earth are composed of a variety of elements in varying amounts. The Earth's crust contains a variety of noble gases, one of those being helium. Natural helium occurs as two isotopes, helium-4 (4He) and helium-3 (3He). Typically, helium-4 is more abundant in the Earth's crust, whereas helium-3 is more abundant in the mantle below. Thus, the helium-3/helium-4 ratio of the gas found in groundwater can provide an indication of the extent to which the water has interacted with volcanic rocks derived from the mantle.
Almost all the geothermometers are empirical and involve many assumptions. Different temperatures can be given by using different ones with the same set of geochemical data. A particular geothermometer may be suitable to one place but not another. This deficiency prevents geothermometers from more effective application and sometimes causes considerable problems Geochemists will gather and interpret data points from multiple geothermometers in order to make the most reliable sub-surface temperature estimates.
9.9.4 Drilling
Drilling represents about 30–50% of the cost of a hydrothermal geothermal electricity project. The drilling of the first production well at a geothermal resource is therefore considered to be an exploration-phase activity. A prospect is usually not considered as being in the drilling phase until after at least one production well has been drilled successfully. The drilling of the first “wildcat” well has a success rate probability of approximately 25%. The success rate of further wells approaches 60–80% in construction stages.
Drilling for geothermal energy is quite similar to rotary drilling for oil and gas. The main differences are due to the high temperatures associated with geothermal wells, which affect the circulation system and the cementing procedures as well as the design of the drill string and casing. The established deep drilling technique is the rotary drilling whereby a string of drill pipe is hung from a derrick and turned by a diesel engine. The top section of the pipe, called the “Kelly”, is square in cross section to allow it to be rotated by the action of a rotary table through which it passes. The bit is a tri-cone roller bit that applies very concentrated loads on the rock face, causing it to crack and spall. The rock fragments or chips must be removed for the bit to proceed. Broken rock chips are removed by scraping or hydraulic cleaning. The drilling fluid, or “mud”, is a critical element in the removal of chips.
Rotary bits drill the formation using primarily two techniques:
- 1. rock removal by exceeding its shear strength and
- 2. removal by exceeding the compressive strength.
First, a small-diameter “temperature gradient hole” is drilled (some only 200 ft deep, some over 4000 ft deep) with a truck-mounted rig to determine the temperatures and underground rock types. Either rock fragments or long cores of rock are brought up from deep down the hole, and temperatures are measured at depth. Depending on the temperature gradient, the drilling is decided.
Typically, wells are deviated from vertical to about 30–50° inclination from a “kick-off point” at depths between 200 and 2000 m. Several wells can be drilled from the same pad, heading in different directions to access larger resource volumes, targeting permeable structures and minimizing the surface impact.
While rotary cone bits have been sufficient to drill production wells to date, other conventional drilling technologies as well as potential “breakthrough” technologies could potentially be implemented into geothermal drilling to increase returns. The diamond PDC drill bit commonly employed in oil and gas operations with success is viewed as inherently more efficient than rotary cone bits.
Recently, a company called Hyper Sciences has suggested to harness geothermal energy with a new kind of drilling technology, using projectiles fired into the ground. It completely eliminates the use drilling bit which by slowly grinding away at rocks perpetually wears them out. Instead, it just shoots holes in the ground with bullets which can bore a hole 10 times faster than traditional drill.
9.10 Utilization of Geothermal Resource
Geothermal resources are suitable for many different types of uses but are commonly divided into two categories, high and low enthalpy, according to their energy content [6, 7, 9]. Utilization of geothermal fluid depends heavily on its thermodynamic characteristics and chemistry. High-enthalpy resources, such as dry steam and hot fluids that are found in volcanic regions and island chains, are utilized to generate electric power with conventional cycles. Low-enthalpy resources (<150°C) are employed for direct heat uses and electricity generation using a binary fluid cycle. In direct heat use, the geothermal sources are used for space heating/cooling, water heating for domestic and industrial use and in hybrid steam power plants.
9.10.11 Electricity Generation from Geothermal Resources
The present-day annual electrical energy produced from geothermal resources is about 57,000 GWh, which is less than 0.4% of the total worldwide electricity generation which is very modest. Power from high-enthalpy geothermal sources is generated in many places, especially those that are located on plate boundaries or tectonically active regions. A vapour-dominated (dry steam) resource can be used directly, whereas a hot-water resource needs to be flashed by reducing the pressure to produce steam. In places where low-temperature resource is available (below 150°C), a secondary low-boiling-point fluid (hydrocarbon) is required to generate the vapour, in a binary or organic Rankine cycle plant.
Presently, geothermal power plants in operation are essentially of three types: dry steam, flash steam and binary cycle. A combination of flash and binary technology, known as the flash/binary combined cycle, has also been used. It takes advantage of both technologies.
In general, geothermal power plants have lower efficiency when compared with fossil-fuelled plants primarily due to (i) low temperature of the steam, which is usually much below 250°C; and (ii) presence of non-condensable gases such as CO2, H2S, NH3 and so on. in the steam, which have to be removed from the condensers of power plants. However, the capacity factor of geothermal power plants can be quite large as compared to other renewable sources such as wind and solar which are intermittent in nature. The global average was 73% in 2005 and can be as high as 96% in some cases.
The total generation from geothermal sources is growing by 3% annually because of a growing number of plants and improvements in their capacity factors, development of binary cycle power plants and improvements in drilling and extraction technology.
The total installed capacity from worldwide geothermal power plants is given in Table 9.2.
Table 9.2 Total worldwide installed capacity from 1950 up to the end of 2010
| Year | Installed capacity (MW) | Energy produced (GWh) |
| 1950 | 200 | |
| 1955 | 270 | |
| 1960 | 386 | |
| 1965 | 520 | |
| 1970 | 720 | |
| 1975 | 1,180 | |
| 1980 | 2,110 | |
| 1985 | 4,764 | |
| 1990 | 5,834 | |
| 1995 | 6,883 | 38,035 |
| 2000 | 7,972 | 48,261 |
| 2005 | 8,933 | 55,709 |
| 2010 (projected) | 10,715 | 67,246 |
Source: World Geothermal congress (2010) [10]. Reproduced with permission from Ruggero Bertani.
9.10.2 Dry Steam Power Plants
Dry steam plants were the first type of geothermal power plants that were developed for commercial use. These plants have been operating for more than hundred years longer than any other geothermal conversion technology. In a dry steam plant, the steam produced in dry steam fields runs the turbines and the generator. The availability of dry steam eliminates the need to burn fossil fuels to generate steam, eliminating the need to transport and store fuels. These plants emit only excess steam and very minor amounts of gases.
These power plants are most suitable for vapour-dominated resources where steam production is not contaminated with liquid. The reservoirs produce superheated steam at 180–225° and 4–8 MPa. This steam is piped directly from underground wells to the power plant where it is directed into a turbine/unit. Conventional dry steam turbines require fluids of at least 150°C and are available with either non-condensing (backpressure) or condensing exhausts. The non-condensing steam turbine uses high-pressure steam for the rotation of blades. This steam then leaves the turbine at the atmospheric pressure or lower pressure. Therefore, this turbine is also known as the backpressure steam turbine. This low-pressure steam is used for processing, and no steam is used for condensation.
In the backpressure system, steam is passed through the turbine and vented to atmosphere. This cycle consumes twice more steam per produced kilowatt-hour (kWh), at identical turbine inlet pressure, compared to a condensing cycle.
Condensing units are more complex in design, requiring more ancillary equipment and space. The condensing turbine contains two outlets. The first outlet extracts the steam with intermediate pressure for the feeding of the heating process while the second outlet extracts the remaining steam with low-pressure steam for the condensation. The condensed water then goes back to the boiler for the regeneration of the electricity.
The steam after passing through the turbine rotates its shaft which in turn drives the generator producing electricity. A dry steam power plant is shown in Fig. 9.6.
Figure 9.6 Dry steam power plant.
Source: Nasruddin et al. (2016) [8]. Reproduced with permission from Elsevier.
9.10.3 Single-Flash Steam Power Plant
Flash steam power plants are the most common form of geothermal power plants. Flash power plants typically require resource temperatures in the range of 170–260°C. A single-flash power plant is shown in Fig. 9.7. When the geothermal wells produce a mixture of steam and liquid, the single-flash plant is a relatively simple way to convert the geothermal energy into electricity. First, the mixture is separated into distinct steam and liquid phases with a minimum loss of pressure. This is done in a cylindrical cyclonic pressure vessel, usually oriented with its axis vertical, where the two phases disengage owing to their inherently large density difference. The fluid at temperatures greater than 182°C is pumped under high pressure into a tank at the surface held at a much lower pressure, causing some of the fluid to rapidly vaporize or “flash”. The vapour then drives a turbine, which drives a generator. The liquid if not separated from the steam can cause scaling and/or erosion of piping and turbine components. The cooled water is returned to the reservoir to be heated by geothermal rocks again. Normally, a conventional condensing steam turbine is used in single-flash system, but lower steam pressures and temperatures are common. The plant therefore requires more steam per kWh than in dry steam power plant. Moreover, the bulk of the water produced may contain as unflashed hot brine which is then reinjected unless it can be used for direct heating.
Figure 9.7 Single-flash steam.
Source: Nasruddin et al. (2016) [8]. Reproduced with permission from Elsevier.
The single-flash steam plant is the mainstay of the geothermal power industry. Single-flash plants account for about 32% of all geothermal plants. The turbines used must be made of corrosion-resistant materials owing to the presence of gases such as hydrogen sulfide that can corrode the ordinary steel.
9.10.4 Double-Flash Power Plant
The schematic diagram for a double-flash plant is shown in Fig. 9.8. The double-flash steam plant is an improvement on the single-flash design, and it can produce 15–25% more power output for similar geothermal fluid. It is a simple extension of the single-flash cycle which makes use of the energy remaining in the separated brine. By directing this brine to a low-pressure separator, additional steam at a lower pressure than the primary steam can be generated which can increase the total power generated by more than 50%. This additional power generation is limited by the low flash separation pressure, which is generally maintained above atmospheric. The plant is more complex, more costly and requires more maintenance. The turbine used may be a dual-admission, single-flow machine as shown in Fig. 9.8, where the low-pressure steam is admitted to the steam path at an appropriate stage so as to merge smoothly with the partially expanded high-pressure steam. Other designs are possible; for example, two separate turbines could be used, one for the high-pressure steam and one for the low-pressure steam. Because of the additional flash process, and the hotter resource, the waste brine becomes more highly concentrated in silica than in a single-flash plant. Thus, silica scaling problem is more serious in this case.
Figure 9.8 Double-flash steam.
Source: Dipippo (2008) [6]. Reproduced with permission from Elsevier.
9.10.5 Binary Cycle Power Plant
Among geothermal energy resources, the medium- and low-temperature water-dominated systems, with temperatures between 110 and 160°C, are the most abundant. Binary cycle geothermal power plants are similar to conventional fossil or nuclear plants in that the working fluid undergoes an actual closed cycle. Binary power plants (or Organic Rankine Cycle units, ORC) are the best energy conversion systems to exploit them, both from a technical and an environmental point of view. Binary cycle geothermal power generation plants differ from dry steam and flash steam systems in that the water or steam from the geothermal reservoir never comes in contact with the turbine/generator units. In these plants, the heat is recovered from the geothermal fluid, via a heat exchanger, to vaporize a low-boiling-point organic fluid and drive an organic vapour turbine. The heat-depleted geothermal brine is pumped back into the source reservoir, thus securing sustainable resource exploitation. Because the geothermal water never flashes in air-cooled binary plants, the total water can be injected back into the system through a closed loop. This serves the dual purpose of reducing already low emissions to near zero and maintaining the reservoir pressure, thereby extending the project lifetime.
A simple binary cycle power plant is shown in Fig. 9.9. In the binary process, the geothermal fluid, which can be either hot water, steam or a mixture of the two, heats another liquid such as isopentane or isobutane (known as the “working fluid”) that boils at a lower temperature than water. The two liquids are kept completely separate through the use of a heat exchanger used to transfer heat energy from the geothermal water to the working fluid. When heated, the working fluid vaporizes into gas, and (as steam) the force of the expanding gas turns the turbines that power the generator.
Figure 9.9 Binary cycle. Image credit: Idaho national laboratory, DOE.
Source: Dipippo (2008) [6]. Reproduced with permission from Elsevier.
9.11 Enhanced Geothermal Systems
At places where no natural geothermal resources in the form of steam or hot water exist, the heat of the rock can be used by creating artificial permeability for fluids extracting that heat. Known as “hot dry rock” technology (HDR), this method is under development since the 1970s. In an EGS, fluid is injected into the sub-surface under carefully controlled conditions, which cause pre-existing fractures to re-open, creating permeability. The principle of EGS is simple: in the deep sub-surface where temperatures are high enough for power generation (150–200°C), an extended fracture network is created and/or enlarged to act as a new pathway. Water from the deep wells and/or cold water from the surface is transported through this deep reservoir using injection and production wells and recovered as steam/hot water. Injection and production wells as well as further surface installations complete the circulation system. The extracted heat can be used for district heating and/or for power generation.
EGS plants, once operational, can be expected to have great environmental benefits (CO2 emission is zero).
9.11.1 Combined or Hybrid Plants
Because of relative simplicity and reliability of single-flash plants, they are often the first type of plants installed at a newly developed field. However, their utilization efficiency is lower than that of a double-flash plant. Since single-flash plants have a significant amount of waste liquid from their separators that is still fairly hot, typically 150–170°C, the possibility of using it to generate more power can be an option instead of directly reinjecting. In many places, combined single- and double-flash plants have been built to increase the efficiency. The single- and double-flash combined system shown in Fig. 9.10 consists of two single-flash units, Units 1 and 2, and a third unit, Unit 3. Unit 3 appears to be simply another single-flash unit, but the power plant as a whole is an integrated single- and double-flash facility since the original geofluid experiences two stages of flashing.
Figure 9.10 Combined single and double flash.
Combined flash and binary types, where a binary plant is used as a bottoming cycle with a flash steam plant, increase the overall thermal efficiency, improve load-following capability and efficiently cover a wide resource temperature range. A combination of flash and binary technologies, known as the flash/binary combined cycle, has been used effectively to take advantage of the benefits of both technologies. In this type of plant, the flashed steam is first converted to electricity with a steam turbine, and the low-pressure steam exiting the backpressure turbine is condensed in a binary system. This allows for the effective use of air-cooling towers with flash applications and takes advantage of the binary process. The flash/binary system has a higher efficiency where the well fields produce high-pressure steam.
9.11.2 Combined Heat and Power (CHP) Plants
In many places, it is common to combine both power generation and direct heat usage in a single geothermal plant. By capturing some of the waste heat in the leftover brine before it is reinjected, the overall utilization efficiency of the resource is enhanced. CHP essentially takes the “waste” heat produced by geothermal electric plants and uses co-generation plants, or combined or cascaded heat and power plants (CHP) produce both electricity and hot water for direct use. Relatively small industries and communities of a few thousand people provide sufficient markets for CHP applications.
In this case, each unit operates with common controls, fluid collection and reinjection systems. The plant requires close monitoring of the injection water temperature in combined cycle systems, as declines could occur that lead to scaling. Iceland has three geothermal co-generation plants with a combined capacity of 580 MW in operation.
9.12 Direct Use of Geothermal Energy
Direct use of geothermal energy is one of the oldest, most versatile and also the most common form of utilization of geothermal energy [11, 12]. Direct use of geothermal resources is primarily for direct heating and cooling. Direct or non-electric utilization of geothermal energy refers to the immediate use of the heat energy rather than to its conversion to some other form such as electrical energy. The primary forms of direct use include swimming, bathing, space heating and cooling including district heating, agriculture (mainly greenhouse heating, crop drying and some animal husbandry), aquaculture (mainly fish pond and raceway heating), industrial processes and heat pumps (for both heating and cooling). In general, the geothermal fluid temperatures required for direct heat use are lower than those for economic electric power generation.
According to the present estimate, district heating represents 88% of the installed capacity and 89% of the annual energy use. Geothermal (ground-source) heat pumps (GHPs) are used for both heating and cooling. Direct use of geothermal resources normally involves temperatures below 150°C.
For space heating, two basic types of systems are used: open or closed loop. Open-loop (single-pipe) systems directly utilize the geothermal water extracted from a well to circulate through radiators. Closed-loop (double-pipe) systems use heat exchangers to transfer heat from the geothermal water to a closed loop that circulates heated freshwater through the radiators. This system is commonly used because of the chemical composition of the geothermal water. In both cases, the spent geothermal water is disposed of into injection wells and a conventional backup boiler may be provided to meet peak demand. It is also used for greenhouse and covered ground heating as well as for agriculture crop drying.
Worldwide growth of installed geothermal direct-use capacity is shown in Figs 9.11 and 9.12. The main growth in the direct-use sector has during the last decade been the use of geothermal (ground-source) heat pumps. This is due, in part, to the ability of geothermal heat pumps to utilize groundwater or ground-coupled temperatures anywhere in the world. The annual energy use for these units grew 2.29 times at a compound annual rate of 18.0%. The installed capacity grew 2.15 times at a compound annual rate of 16.6%. This is due to better reporting and to the ability of geothermal heat pumps to utilize groundwater or ground-coupled temperatures anywhere in the world. Geothermal (ground-source) heat pumps have the largest energy use and installed capacity, accounting for 68.3% and 47.2% of the worldwide capacity and use. The installed capacity is 33,134 MW, and the annual energy use of 200,149 TJ/year, with a capacity factor of 0.19 (in the heating mode). Almost all of the installations occur in North American, Europe and China, increasing from 26 countries in 2000 to 33 countries in 2005, to the present 43 countries. The equivalent number of installed 12 kW units (typical of the US and Western European homes) is approximately 2.76 million, over double the number of units report for 2005 and four times the number for 2000. Summary of the various categories of direct-use worldwide for the period 1995–2015 is shown in Table 9.3.
Figure 9.11 Worldwide growth of installed geothermal direct-use capacity.
Figure 9.12 Breakdown of geothermal energy production in different countries.
Source: U.S. Energy Information Administration [13], Monthly Energy Review, March, 2012.
Table 9.3 Summary of the various categories of direct-use worldwide for the period 1995–2015
| Capacity (MW) | |||||
| 2015 | 2010 | 2005 | 2000 | 1995 | |
| Geothermal heat pumps | 49,898 | 33,134 | 15,384 | 5,275 | 1,854 |
| Space heating | 7,556 | 5,394 | 4,366 | 3,263 | 2,579 |
| Greenhouse heating | 1,830 | 1,544 | 1,404 | 1,246 | 1,085 |
| Aqua pond heating | 695 | 653 | 616 | 605 | 1,097 |
| Industrial uses | 610 | 533 | 484 | 474 | 544 |
| Bathing and swimming | 9,140 | 6,700 | 5,401 | 3,957 | 1,085 |
| Agriculture drying | 161 | 125 | 157 | 74 | 67 |
Source: World geothermal congress (2015) [14]. Reproduced with permission from John W. Lund.
9.13 Environmental Impact
Certain environmental impacts associated with the development of geothermal sites and the operation of plants are inevitable. However, under normal conditions, they are generally confined to the immediate vicinity of the plant and are of lesser impact than those of other electric power generation technologies, particularly those using carbon-based fossil fuels and nuclear fuels. There have now been more than one hundred years of experience in developing geothermal fields and in building, operating, upgrading and even decommissioning geothermal plants of various types. In the earliest days, drilling of wells could be a hazardous undertaking and the behaviour of geothermal reservoirs was mysterious. Early developers and operators learned by doing so, and eventually a scientific understanding of the nature of the resource evolved. Along with this came the technology of how best to exploit geothermal energy and how to deal with the potential environmental impacts.
9.14 Summary
Geothermal energy has the potential to provide long-term, secure base-load energy and greenhouse gas (GHG) emission reductions. Accessible geothermal energy from the Earth's interior supplies heat for direct use and to generate electric energy. History of geothermal energy and types of geothermal resources are described in this chapter.
Until recently, the main interest in geothermal resources has been on the availability of steam or high-temperature hot water in porous rock formations and use of extracted steam to generate electricity. The future use of geothermal energy from advanced technologies such as the exploitation of hot dry rock/hot wet rock systems, magma bodies and geopressured reservoirs is briefly discussed. While the viability of hot dry rock technology has been proven, research and development are still necessary for the other two sources. Direct use of geothermal energy is also discussed briefly.
References
- 1 Armstead, H.C.H. (1983) Geothermal Energy, E. & F. N. Spon, London.
- 2 Fridleifsson, I.B. (2001) Geothermal energy for the benefit of the people. Renewable and Sustainable Energy Reviews, 5, 299–312.
- 3 Stober, I. and Bucher, K. (2013) Geothermal Energy: From Theoretical Models to Exploration and Development, Springer.
- 4 Gupta, H. and Roy, S. (2007) Geothermal Energy-An Alternative Resource for 21st Century, 1st edn, Springer.
- 5 Lund, J.W. (2007) Characteristics, Development and Utilization of Geothermal Resources, GHC Bulletin.
- 6 Dipippo, R. (2008) Geothermal Power Plants: Principles, Applications, Case Studies and Environmental Impact, 2nd edn, Elsevier.
- 7 Bertani, R. (2007) World Geothermal generation in 2007, Proceedings European Geothermal Congress, Unterhaching, Germany, April-May, 2007
- 8 Nasruddin et al. (2016) Potential of geothermal energy for electricity generation in Indonesia-a review. Renewable and Sustainable Energy Reviews, 53 (2016), 733–740.
- 9 Ackerman, E., Digging for Geothermal Energy with Hypersonic Projectiles, IEEE Spectrum.
- 10 Bertani, R. (2010) Geothermal power generation in the world 2005-2010 update report Proceedings World geothermal congress Bali Indonesia.
- 11 Dickson, M.H. and Fanelli, M. (2003) Geothermal Energy Utilization and Technology, Unesco Publishing Renewable energy series.
- 12 Bloomquist, R.G. (2002) Direct use geothermal resources. IEEE Power Engineering Society Summer Meeting, 1, 15–16.
- 13 U.S. Energy Information Administration (2012), Monthly Energy Review, March, 2012
- 14 Lund, J.W. and Boyd, T.L. (2015) Direct Utilization of Geothermal Energy 2015 Worldwide Review, Proceedings World Geothermal Congress 2015 Melbourne, Australia.