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Geothermal Power
Chapter 12
Geothermal Power Geothermal energy is the heat contained within the body of the earth. The origins of this heat are found in the processes that led to the formation of the earth from the consolidation of stellar gas and dust some 4 billion years ago into a high-temperature ball of matter. Over time the outer regions of that ball cooled, but the core remains at a very high temperature. Radioactive decay within the body of the earth continually generates additional heat which augments that already present. The distance from the surface of the earth to its core is 6500 km. At the core the temperature may be close to 6000 C, creating a temperature gradient between the centre and the much cooler outer regions. As a consequence, heat flows continuously towards the surface. Most of this heat reaches the surface at a low temperature and cannot be exploited, but in some places a geothermal anomaly creates a region of high temperature close to the surface. In such regions it may be possible to extract and use the energy, either for heating or in some cases to generate electricity. The geothermal energy that is capable of being exploited at the surface is contained within the solid outer shell of the earth called its crust. The earth’s crust is generally around 56 km thick. Starting from the ambient surface temperature, the temperature within the crust increases on average by 17 C 30 C for each kilometre below the surface. Based on this, it has been estimated that the top 3 km of the crust contain around 4.3 3 107 EJ of energy, around 10,000 times more than annual global energy consumption. Below the crust is the mantle, a viscous semi-molten rock, which has a temperature of between 650 C and 1250 C. Inside the mantle is the core. The earth’s core consists of a liquid outer core and a solid inner core where the highest temperatures are found. The earth’s crust is not a shell of uniform thickness. Exploitable geothermal temperature anomalies occur where molten/semi-molten magma in the mantle comes closer than normal to the surface. In such regions the temperature gradient within the rock may be 100 C/km or more. Sometimes water can travel down through fractured rock to such anomalies and by convective flow carry the heat back to the surface. More dramatically, plumes of magma may rise to within 1 5 km of the surface, and at the sites of volcanoes, it actually reaches the surface from time to time. However, direct exploitation Power Generation Technologies. DOI: https://doi.org/10.1016/B978-0-08-102631-1.00012-2 Copyright © 2019 Paul Breeze. Published by Elsevier Ltd. All rights reserved.
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of this energy source is likely to be difficult. The magma also intrudes into the crust at the boundaries between the tectonic plates which make up the surface of the earth. These boundaries can be identified by earthquake regions such as the Pacific basin “ring of fire.” The most obvious surface signs of an exploitable geothermal resource are hot springs and geysers. These have been used by man for at least 10,000 years. Both the Romans and the ancient Chinese used hot springs for bathing and for therapeutic treatment. Such use continues in several parts of the world and is particularly popular in Iceland and Japan. In Europe, a district heating system based on geothermal heat was inaugurated in Chaude-Aigues, France, in the 14th century; this system is still in existence. Industrial exploitation of hot springs dates from the discovery of boric acid in spring waters at Larderello in Italy around 1770. This led to the development of a chemical industry based on the springs. It was here, too, that the first experimental electricity generation from geothermal heat took place in 1904. This led, in 1915, to a 250 kW power plant which exported power to the local region. Exploitation elsewhere had to wait until 1958 when a plant was built at Wairakei in New Zealand, and for The Geysers development in the United States which began operating in 1960. Global geothermal generating capacity has grown slowly since then. Geothermal energy is classified as a renewable resource by most international agencies because the energy extracted for power generation is constantly replenished from within the earth’s body. Local depletion of the heat has been recorded in some reservoirs but where they are properly managed the heat source will remain constant. The Larderello geothermal field has been operating continuously since 1946, according to the World Energy Council, without the heat supply in the reservoir failing.
GLOBAL GEOTHERMAL CAPACITY Table 12.1 shows the growth in global geothermal electricity generating capacity from 1990 until 2015. In 1990 the total global capacity was 5832 MW, and geothermal power made a relatively small contribution to overall global power production. Since then the global capacity has grown, reaching 7974 MW in 2000 and 10,717 MW in 2010. By the end of 2015, there was 12,636 MW of installed geothermal capacity worldwide. This is still relatively small by global power generation standards, accounting for perhaps 0.2% of total global installed capacity. This small contribution is a reflection of the limited global geothermal resource that is currently available to exploit. Around 40% of global geothermal power generation is found in the Asia Pacific region, where there are powerful tectonic plate boundaries that provide geothermal heat. A further 30% is found in North America. Exploitation is more limited in Europe but still significant. However, there is
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TABLE 12.1 Global Annual Geothermal Power Generating Capacity Year
Global Installed Capacity (MW)
1990
5832
1995
6887
2000
7974
2005
9064
2010
10,717
2013
11,772
2015
12,636
Source: International Geothermal Association.
little geothermal power generation in Africa, South and Central America or continental Asia.1 The principal 20 exploiters of geothermal power generation, by country, are shown in Table 12.2. The largest user is the United States where there was 3450 MW of generating capacity at the end of 2015. The Philippines had 1870 MW and Indonesia had 1340 MW. There were also large geothermal capacities in Mexico, Italy, New Zealand, Iceland, Japan and Kenya, and smaller capacities in several countries in Central America. According to the International Geothermal Association, from which these figures are derived, there were 26 nations exploiting geothermal electricity generation in 2015. However, several of these have less than 1 MW of installed generating capacity. As Table 12.2 suggests, easily accessible geothermal resources suitable for power generation are not widely distributed, neither are they large. Even so, geothermal energy is attractive for power generation because it is simple and relatively cheap to exploit. In the simplest case, steam can be extracted from a borehole and used directly to drive a steam turbine. Such easily exploited geothermal resources are rare, but others can be used with little more complexity. The virtual absence of atmospheric emissions (although geothermal wells can release carbon dioxide) means that geothermal energy is also clean compared to fossil fuel fired power. Although natural geothermal fields are relatively rare, there is a much larger geothermal potential linked to deep, hot underground rock within the crust. This heat is much more expensive to exploit but could potentially offer a far larger generating capacity. Today, however, only experimental exploitation of this resource has been carried out.
1. World Energy Resources 2016, World Energy Council.
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TABLE 12.2 National Geothermal Power Generating Capacities Country
Geothermal Generating Capacity (MW)
The United States
3450
Philippines
1870
Indonesia
1340
Mexico
1017
New Zealand
1005
Italy
916
Iceland
665
Kenya
594
Japan
519
Costa Rica
207
El Salvador
204
Kenya
594
Turkey
397
Nicaragua
159
Guatemala
52
Papua New Guinea
50
Portugal
28
China
27
Germany
27
France
16
Source: International Geothermal Association.
According to the World Energy Council, the total amount of power generation from geothermal sources in 2015 was 75 TWh. This can be compared to a global total generation in 2015 of 24,255 TWh base on International Energy Agency (IEA) data. Thus, geothermal production accounted for 0.3% of the global total. Under a high renewable growth scenario developed by the IEA for future electricity production, geothermal sources would produce around 1400 TWh of power every year from 200 GW of installed geothermal capacity.
THE GEOTHERMAL RESOURCE There are three principle categories of the geothermal resource. The simplest to exploit is a source of hot underground water (the geothermal reservoir)
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which either reaches the surface naturally or can be tapped by drilling boreholes. This is the geothermal source upon which all existing commercial geothermal power plants are based as well as geothermal heating systems. Where there are no underground water sources, anomalies in the crust can create regions where the rock close to the surface is much hotter than usual. This hot rock can be accessed by drilling and pumping a heat transfer fluid into the rock, then bringing it back to the surface. The process has been tested but never exploited commercially. The third, and potentially the richest source of geothermal energy, is the magma itself. The magma contains by far the greatest amount of accessible heat energy, but because of the temperatures and pressures found within it, this is also the most difficult geothermal energy source to exploit. Estimating the amount of energy in the crust of the earth that could be exploited for power generation is not easy. It has been suggested that there is between 10 and 100 times more heat energy available for power generation as there is energy recoverable from uranium and thorium in nuclear reactors. Certainly, the resource is enormous, if difficult to access.
GEOTHERMAL FIELDS Geothermal fields are formed when water from the earth’s surface is able to seep through faults and cracks within rock, sometimes to depths of several kilometres, to reach hot regions within the crust. As the water is heated, it rises naturally back towards the surface by a process of convection and may appear there again in the form of hot springs, geysers, fumaroles or hot mud holes. These are particularly common along tectonic plate boundaries. Sometimes the route of the ascending water is blocked by an impermeable layer of rock. Under these conditions the hot water collects underground within the porous rock beneath the impermeable barrier. This water can reach a much higher temperature than the water which emerges at the surface naturally. Temperatures as high as 350 C have been found in such reservoirs. This geothermal fluid can be accessed by boring through the impermeable rock. Steam and hot water will then flow upwards through the borehole under pressure and can be used at the surface. Most of the geothermal fields that are known today have been identified by the presence of hot springs. In California, Italy, New Zealand and many other countries, the springs led to prospecting using boreholes drilled deep into the earth to locate the underground reservoirs of hot water and steam that were feeding them. More recently, geological exploration techniques have been used to try and locate underground geothermal fields where no hot springs exist. Sites in Imperial Valley in southern California were found in this way. Some geothermal fields produce simply steam, but these are rare. Larderello in Italy and the Geysers in California, The United States, are the main fields of this type in use today though others exist in Mexico,
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Power to the grid
Heat exchanger
Pump
Turbine
Generator
Pump
Geothermal reservoir
FIGURE 12.1 Schematic of a geothermal power plant.
Indonesia and Japan. More often the field will produce either a mixture of steam and hot water or hot water alone, often under high pressure. All three can be used to generate electricity. Deep geothermal reservoirs, as much as 2 km or more below the surface, produce fluid at the highest temperature. Typically, they will produce water at a temperature of 120 C 350 C. High-temperature reservoirs of this type are the best for power generation and the higher the temperature, the more energy can be extracted by a turbine. Shallower reservoirs may be a little as 100 m below the surface. These are cheaper and easier to access but the water they produce is cooler, often less than 150 C. This can still be used to generate electricity but is more often used for heating. A schematic of a geothermal power plant for exploiting an underground geothermal reservoir is shown in Figure 12.1. The fluid emerging from a geothermal reservoir, at a high temperature and usually under high pressure, contains enormous quantities of dissolved minerals such a silica, boric acid and metallic salts. Quantities of hydrogen sulphide and some carbon dioxide are often present too. The concentrated brine from a geothermal borehole is often corrosive and if allowed to pollute local groundwater sources can become an environmental hazard. This problem can be avoided if the brine is reinjected into the geothermal reservoir after heat has been extracted from it. Geothermal reservoirs are all of the limited extent and contain a finite amount of water and energy. As a consequence, both water and heat can become depleted if overexploited. When this happens, either the pressure or the temperature or both of the fluid from the reservoir declines.
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In theory the heat within a subterranean reservoir will continuously be replenished by the heat flow from below. This rate of replenishment may be as high as 1000 MW although it is usually smaller. In practice, geothermal plants have often extracted the heat faster than it is replenished. Under these circumstances the temperature of the geothermal fluid falls and the practical life of the reservoir is limited. The reservoir also loses fluid that is extracted through the geothermal boreholes used to supply hot brine to the power plant. This fluid is not replaced naturally. Reinjection of the brine after it has passed through the power plant helps to maintain the fluid in a reservoir. However, reservoirs such as the Geysers in the United States, where fluid exiting the boreholes is steam, have proved more difficult to maintain because the steam is generally not returned after use. This has led to a marked decline in the quantity of heat from the Geysers. In an attempt to correct this, waste water from local towns has been reinjected into the reservoir. Some improvement has been noted since this practice began. Estimates for the practical lifetime of a geothermal reservoir vary. This is partly because it is extremely difficult to gauge the size of the reservoir and therefore easy to overexploit it. Although some reservoirs may become virtually exhausted over the lifetime of a power plant, around 30 years, others appear to continue to supply energy for 100 years or more. A better understanding of the nature of the reservoirs and improved management will potentially help to maintain geothermal reservoirs for longer in the future.
BRINE-METHANE RESERVOIRS In some rare cases the hot brine in an underwater reservoir is found to be saturated with methane too. Such reservoirs normally occur in regions rich in fossil fuel. Where such a reservoir is found, it is possible in principle to exploit both the heat in the brine and the dissolved methane gas to generate electricity. The only major reservoirs of this type known today are in the Gulf of Mexico.
HOT DRY ROCK Underground geothermal reservoirs are relatively rare. More normally hot underground rock is not permeated by water, and hence there is no medium naturally available to bring the heat energy to the surface. Where hot rock exists close to the surface, it is possible to create a manmade hydrothermal source. This is accomplished by drilling into the rock and them pumping water down through the borehole that has been created. If water is pumped under sufficiently high pressure, it will cause the rock to fracture in a process similar in concept to “fracking” used to release shale gas from underground rock, creating faults and cracks through which the liquid can flow. (In fact, underground rock often contains natural faults and
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Power to the grid
Heat exchanger
Pump
Injection well
Turbine
Generator
Pump
Production well
Fractured rock
FIGURE 12.2 Schematic of a hot dry rock power plant.
fractures through which the water will percolate too. This often helps with the fracturing.) If a second borehole is drilled adjacent to the first, then water which has become heated as it has percolated through the rock can be extracted and used to generate electricity. This is shown schematically in Figure 12.2. The first attempt at this hot dry rock technique was carried out by scientists from the Los Alamos laboratory in New Mexico in 1973. Since then experiments have been carried out in Japan, the United Kingdom, Germany and France. One such project, part of the European Hot Dry Rock Research project, is at Soultz-sous-Foreˆts in France. Here boreholes have been drilled to 5 km below the surface and temperature of 201 C found. The project is
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now producing a small amount (around 1.5 MW of generating capacity) of electric power. Based on pilot schemes such as this, estimates suggest that a commercial hot dry rock system will need to provide 10 100 MW of generating capacity over at least 20 years to be economical. The technology is still in an early stage of development, and it is likely to be 10 15 years before commercial exploitation is possible. Hot dry rock geothermal technology is often called an Enhanced Geothermal System (EGS) today.
EXPLOITING THE MAGMA Extracting energy from accessible magma plumes which have formed within the earth’s outer crust is the most difficult way of obtaining geothermal energy, but it is also most exciting because of the enormous quantities of heat available. A single plume can contain between 100,000 and 300,000 MW-centuries of energy. Drilling into or close to such hot regions is difficult because the equipment can easily fail at the elevated temperatures to which it will be subjected. As an additional hazard, if a drill causes a sudden release of pressure, the result can be explosive. Furthermore, ways have yet to be found to tap the heat. Research continues but exploiting magma for power generation is a long-term project with no immediate prospect of exploitation.
LOCATION OF GEOTHERMAL RESOURCES The easiest geothermal resources to exploit are those that can provide water or steam with a temperature above 200 C. Resources of this type are located almost exclusively along the boundaries between the earth’s crustal plates, in regions where there is significant plate movement. These areas are found around the Pacific Ocean in New Zealand, Japan, Indonesia, the Philippines and the western coasts of North and South America, in the central and eastern parts of the Mediterranean, East Africa, the Azores and Iceland. Geographically, 72% of the global geothermal electricity generating capacity resides at sites along tectonic plate boundaries or hotspot features. Lower temperature underground reservoirs exist in many other parts of the world, and although these contain less energy, they can be used to generate electricity too. A project installed in Austria in 2001, for example, generates electricity from 106 C water which is also used for district heating. However, these reservoirs can be more difficult to locate in the absence of hot surface springs. Nevertheless, there were around 60 countries using geothermal energy at the beginning of the 21st century for heating, generating electricity or both. Of these, more than 30 countries exploit the geothermal resource for heating alone. The countries that are the heaviest exploiters of geothermal energy for heating include China, Turkey, Iceland and Japan.
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THE SIZE OF THE RESOURCE Today it is difficult to estimate the size of this energy resource but as survey techniques improve more accurate data will become available. Based on the data available at the beginning of the 21st century, it has been estimated that reservoirs located in the United States, for example, might provide 10% of US electricity. According to figures published by World Energy Council in its 2010 Survey of Energy Resources, the global geothermal generating potential could be between 35 and 210 GW. However, if hot dry rock techniques could be exploited, the total potential could be 5 10 times higher than this. Another estimate, again from the World Energy Council, suggests that 8.3% of global electricity generation could be provided by geothermal sources. That would suggest generating capacity of between 300 and 400 GW, roughly 30 times larger than that exists today. As these figures show, estimates vary widely.
GEOTHERMAL ENERGY CONVERSION TECHNOLOGIES There are three principle ways that have been developed for converting geothermal energy into electricity. Each is designed to exploit a specific type of geothermal resource. The most straightforward of the three is only feasible when a geothermal reservoir produces high-temperature dry steam alone. Under these circumstances, it is possible to use a direct steam power plant which is analogous to the power train of a steam turbine power station, with the boiler of the plant replaced by the geothermal steam source. So long as the steam exiting the reservoir is of suitable quality, this provides an extremely cheap and effective means of generating electricity. Most high-temperature geothermal fields produce not dry steam but a mixture of steam and hot brine. This mixture cannot be utilised quite so simply and is most effectively exploited using a configuration called a flash steam geothermal plant. The flash process converts a part of the hot, highpressure liquid to steam, and this steam, together with any steam extracted directly from the borehole, is used to drive a steam turbine. Where the geothermal resource is of a relatively low temperature, a third system called a binary plant is more appropriate. This uses the lower temperature geothermal fluid to vaporise a second low-boiling point fluid contained in a separate, closed cycle system. The vapour then drives a turbine which turns a generator to produce electricity. Although the overall efficiency of such binary systems is low, the availability of a cheap heat source makes the system economical to operate.
DIRECT STEAM POWER PLANTS Dry steam geothermal reservoirs are extremely rare. Where they exist, the steam, with a temperature of 180 C 350 C, can be extracted from the
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reservoir through a borehole and fed directly into a steam turbine. Steam from several wells of this type will normally be fed to a single turbine to allow an economically large turbine to be used. The pipes which carry the steam from the wellheads to the turbine contain various filters to remove particles of rock and any steam which condenses as it passes through the pipework. This is the configuration which was used in the very first geothermal power plant in Larderello, Italy, in 1904. The steam turbine in a direct steam geothermal plant is usually a standard reaction steam turbine. The turbine size in a modern geothermal plant is typically between 20 and 120 MW. As a result of the relatively low steam temperature and the small size of the turbines, energy conversion efficiency is generally low at around 30%. In some cases the steam exiting the turbine may be released directly into the atmosphere. However, the steam usually contains between 2% and 10% of other gases such as carbon dioxide and hydrogen sulphide. Under these circumstances, the exhaust from the steam turbine must be condensed to remove the water and then treated to remove any pollutants such as hydrogen sulphide before release into the atmosphere. At the Geysers plant in the United States, the hydrogen sulphide from the steam is converted into sulphur which is sold, creating a small additional source of revenue. Condensing the steam also extracts more energy, hence increasing plant efficiency. A schematic of a direct steam geothermal power plant is shown in Figure 12.3.
Air and water vapour
Generator Turbine Condenser
Cooling tower
Steam Air Water
Air
Hot well
Condensate Ground level Geothermal zone
Production well FIGURE 12.3 A direct steam geothermal power plant.
Injection well
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Ideally the geothermal fluid should be returned to the underground reservoir but it is often more economical to release the gas and dispose of the water produced as a result of condensing the steam from the turbine at the surface. Carbon dioxide emissions from such plants could become an issue in the future and, depending upon the amounts involved, it may be feasible to capture and either reinject or sequester the carbon dioxide generated from such a plant. In general, however, carbon dioxide emissions are low compared to fossil fuel sources. In the United States, for example, typical emissions from a geothermal plant are equivalent to 91 gCO2/kWh. Typical fossil fuel power plants release between 600 gCO2/kWh and 1000 gCO2/kWh according to the World Energy Council. Continual removal of underground fluid without replenishment eventually leads to depletion in the quality of fluid available from the reservoir. At the Geysers geothermal field in southern California, urban waste water has been pumped into the underground reservoir in an attempt to maintain and eventually boost output from the resource. This has led to both a reduction of urban effluent in the local river system and more power from the geothermal reservoir.
FLASH STEAM PLANTS Most high-temperature geothermal reservoirs yield a fluid which is a mixture of steam and liquid brine, both under high pressure (typically up to 10 atmospheres). The steam content, by weight, is typically between 10% and 50%. The simplest method to exploit such a resource is to separate the steam from the liquid and use the steam alone to drive a steam turbine. This would be equivalent to the dry steam plant described earlier. However, using only the steam throws away much of the available energy, particularly where the proportion of steam in the fluid is small. A more productive alternative is to pass the combined fluid through a valve into a vessel maintained at a lower pressure than the geothermal fluid from the reservoir. The sudden reduction in pressure “flashes” a proportion of the hot liquid to steam, creating a larger quantity of steam than was available previously. All the steam can then be separated from the liquid and used to drive a steam turbine. The steam exiting the steam turbine must be treated in exactly the same way as the exhaust from a direct steam geothermal plant to prevent atmospheric pollution. The remaining liquid, which contains high levels of dissolved salts and presents a potential pollution problem, is usually injected back into the geothermal reservoir. A schematic of a flash steam geothermal power plant is shown in Figure 12.4. A further refinement to the flash steam plant is called double flash technology. This involves taking the fluid remaining after the first flash process and releasing it into a second vessel at even lower pressure. Double flashing results in the production of more steam which can be fed to a second, lowpressure turbine or injected into a later stage of the turbine powered by the
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Air and water vapour
Generator Turbine Condenser Steam
Cooling tower Air
Water
Steam
Brine
Air
Condensate Waste brine
Hot well Direct heat uses Ground level
Geothermal zone
Production well
Injection well
FIGURE 12.4 A flash steam geothermal power plant.
steam from the first flash. A double flash plant can produce up to 25% more power than a single flash plant. However, it is more expensive and may not always be cost-effective. Flash technology plants will generally return a much higher percentage of the geothermal fluid up to 85% for a single flash plant and somewhat less for the double flash plant to the geothermal reservoir. This will include most of the dissolved chemicals contained in the original fluid. However, some reservoir depletion will still take place, and without action this is likely to lead to a fall-off in output from the reservoir with time. Capacities for flash geothermal power plants are normally between 20 and 55 MW.
BINARY CYCLE POWER PLANTS Direct steam and flash geothermal power plants utilise geothermal fluid with a temperature of between 180 and 350 C. If the fluid is cooler than this, conventional steam technology will normally prove too inefficient to be economically viable. However, energy can still be extracted from the fluid to generate power using a binary cycle power plant. This type of plant has two fluid cycles: the first involves the low-temperature fluid from the geothermal resource and the second is a thermodynamic turbine cycle involving a lowboiling point fluid.
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Air and water vapour
Generator Turbine Condenser Iso-butane (vapour)
Cooling tower
Iso-butane
Air
Air Water
Heat exchanger Hot brine
Ground level Geothermal zone
Production well
Injection well
FIGURE 12.5 A binary cycle geothermal power plant.
In a binary power plant the geothermal fluid is extracted from the reservoir and immediately passed through a heat exchanger where the heat it contains is used to volatilise a secondary fluid. This secondary fluid is contained within a second, completely closed cycle system. The fluid may be an organic liquid which vaporises at a relatively low temperature or, in the case of the Kalina Cycle,2 a mixture of water and ammonia. The vaporised secondary fluid is used to drive a small turbine from which power can be extracted with a generator. Once it has exited the turbine, the secondary vapour is condensed and then pumped through the heat exchanger once more. Thus the cycle is repeated. The geothermal fluid exiting the heat exchanger is, meanwhile, reinjected into the geothermal reservoir. Because 100% of the fluid is returned underground, this type of geothermal power plant has the smallest environmental impact. A schematic of a binary cycle geothermal power plant is shown in Figure 12.5. Typical binary cycle plant unit size is 1 3 MW, much smaller than the other types of geothermal technology. However, the small units required for such plants often lend themselves to standardisation, reducing manufacturing costs. Several units can be placed in parallel to provide a plant with a larger power output. 2. The Kalina cycle is a special thermodynamic cycle designed to obtain maximum efficiency from low energy resources such as low-temperature geothermal fluids.
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Although the normal application for binary technology is to exploit a low-temperature geothermal resource, the technology can also be used to generate more power from a flash plant. In this case the fluid left after flashing is passed through a heat exchanger before reinjection, allowing extra energy to be taken to power a small binary unit. Adding a binary unit to a conventional flash plant increases the cost, but the resultant hybrid plant will have a larger power output.
ADVANCED GEOTHERMAL TECHNOLOGIES There are several areas in which current geothermal technology might be developed to provide more power. The main avenue in the immediate future is by the development of hot dry rock or EGS technology, with better drilling techniques and better prospecting techniques. There is also scope for identifying and exploiting undersea geothermal resources. There are underground geothermal reservoirs that contain very high temperature, high-pressure brine where the water has a temperature of more than 375 C and a pressure of 220 bar. Under these conditions the water in the reservoir is in a supercritical state. If this supercritical steam/water could be extracted, it could provide up to 10 times more energy than from a conventional reservoir, according to the IEA. However, extracting and exploiting the fluid in such reservoirs cannot be carried out today.
GEOTHERMAL POWER AND DISTRICT HEATING Although high-temperature geothermal resources are suitable for power generation and lower temperature resources are often exploited for district heating, it is possible to combine the two. This is typical, for example, of geothermal power plants in Iceland where there is high demand for both power and heat. Iceland’s geothermal reservoirs generally provide a hot brine and energy is extracted from this with a flash plant as described earlier. However, after the brine exits the flash plant, it still contains a significant amount of heat energy, and this can be exploited to provide heat for a district heating system. For this, the brine is passed through a series of heat exchangers so that heat can be extracted with fresh water. The hot water is then stored in a large tank from which it can be taken to supply the district heating system. The brine, upon exiting these heat exchangers, is reinjected into the reservoir. With this, as well as direct use of geothermal heat, geothermal sources account for around 66% of Iceland’s primary energy use and 25% of the electricity production according to Orkustofnun, the National Energy Authority of Iceland.
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FINDING AND EXPLOITING GEOTHERMAL SOURCES The exploitation of geothermal power is often considered to be riskier than the development of other renewable resources because of the uncertainty and high costs associated with finding suitable geothermal fields. Initial surveys of geothermal resources are often carried out by national institutions but for developers, these then need to be backed up with test wells to determine the exact nature of the resource available. Data from Pacific Rim volcanic regions suggest that the presence of single hot spring will provide a 50% change of an exploitable geothermal field. A boiling spring or fumarole increases the probability to 70%.3 Having identified a suitable surface site, prefeasibility studies are likely to cost around $1 million, with a 30% change of failure. Test drilling, usually three wells at up to $2 million per well, has a similar prospect of failure. This risk can be reduced by careful surface study followed by prioritisation of the available sites. Such an approach has led to success rates for well drilling in excess of 83% in countries such as Indonesia, Kenya and New Zealand which have high-temperature resources. However, success rates can be much lower where low-temperature resources are concerned. Once a usable underground reservoir has been located, its size must be determined. This involves fluid withdrawal over a long period; indeed it may not be until several years after production has started that a good picture of the resource can be obtained. Careful sizing of the geothermal plant to match the reservoir size will prolong the lifetime of a reservoir. However, this may not be possible if the plant has to be constructed before full data are available and, as noted, the data may not be available anyway until production has started. Oversized plants such as that installed at the Geysers in the United States have led to a premature fall in output, and this will have an impact on overall economics.
THE COST OF GEOTHERMAL POWER In common with many renewable resources, geothermal power generation involves a high-capital outlay to establish the facility but extremely low fuel and operating costs. In the case of a geothermal plant there are three initial areas of outlay, prospecting and exploration for the geothermal resource, development of the steam field and the cost of the power plant itself. As already noted, the cost of identifying a suitable geothermal reservoir is likely to be several million dollars and may not always be successful. If the resource that is found turns out to be small, this will create a much heavier burden on the project costs than if the resource is large. Steam field development will depend on plant size and the total cost will be determined 3. World Bank geothermal assessment.
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by the number of boreholes that are to be drilled. For example, the Hellisheidi geothermal power plant in Iceland, with a generating capacity of 300 MW, has 50 boreholes. The capital cost of building a power plant to exploit a geothermal resource will depend on the quality of the resource. A good resource will have a temperature of above 250 C and good permeability of the reservoir, so providing a good fluid flow. Ideally, it will provide either dry steam or steam and brine, the latter being relatively noncorrosive and with low gas content. For good-quality resources of this type today the lowest cost in the United States is around $2400/kW according to the US Energy Information Administration (EIA). Similar costs in Europe are h3000/kW h4500/kW. The lowest cost will be for a plant of more than 30 MW with smaller plants being relatively more expensive to build. A poor-quality resource will have a temperature of below 150 C, or it could provide fluid at a higher temperature but with some other defect such as corrosive brine or poor fluid flow. Resources of this type will generally only be capable of supplying heat for a small geothermal plant of less than 5 MW, and the capital cost could be up to twice that of a large facility based on a good resource. Further indirect costs will be incurred, depending on the location and ease of access to the site. These will vary from 5% for an easily accessible site and a local skilled workforce to 60% of the initial cost in remote regions where skilled labour is scarce. All these costs will be part of the initial investment required to construct a plant. The cost of actual projects built in recent years shows the cheapest to be $2200/kW in Indonesia and the most expensive to be $9910/kW in the United States.4 The cost of electricity from the geothermal plant will depend primarily on the capital cost of building the plant and the cost of financing the construction. There is also the potential for an additional cost resulting from a local rent for exploiting the resource but that is rare. European estimates for the levelized cost of geothermal electricity are h40/MWh h100/MWh. Meanwhile the International Renewable Energy Agency has put the levelized cost of electricity from geothermal plants at between $70/MWh and $120/ MWh depending upon the region. The cost was highest in Europe and lowest in Asia and Eurasia. In the United States the US EIA has estimated that the cost of energy from a new geothermal plant entering service in 2017 is $100/ MWh. These levelized costs, at the lower end, make geothermal power competitive with virtually all other forms of power generation.
4. World Energy Resources 2016, World Energy Council.
Geothermal Power Geothermal energy is the heat contained within the body of the earth. The origins of this heat are found in the processes that led to the formation of the earth from the consolidation of stellar gas and dust some 4 billion years ago into a high-temperature ball of matter. Over time the outer regions of that ball cooled, but the core remains at a very high temperature. Radioactive decay within the body of the earth continually generates additional heat which augments that already present. The distance from the surface of the earth to its core is 6500 km. At the core the temperature may be close to 6000 C, creating a temperature gradient between the centre and the much cooler outer regions. As a consequence, heat flows continuously towards the surface. Most of this heat reaches the surface at a low temperature and cannot be exploited, but in some places a geothermal anomaly creates a region of high temperature close to the surface. In such regions it may be possible to extract and use the energy, either for heating or in some cases to generate electricity. The geothermal energy that is capable of being exploited at the surface is contained within the solid outer shell of the earth called its crust. The earth’s crust is generally around 56 km thick. Starting from the ambient surface temperature, the temperature within the crust increases on average by 17 C 30 C for each kilometre below the surface. Based on this, it has been estimated that the top 3 km of the crust contain around 4.3 3 107 EJ of energy, around 10,000 times more than annual global energy consumption. Below the crust is the mantle, a viscous semi-molten rock, which has a temperature of between 650 C and 1250 C. Inside the mantle is the core. The earth’s core consists of a liquid outer core and a solid inner core where the highest temperatures are found. The earth’s crust is not a shell of uniform thickness. Exploitable geothermal temperature anomalies occur where molten/semi-molten magma in the mantle comes closer than normal to the surface. In such regions the temperature gradient within the rock may be 100 C/km or more. Sometimes water can travel down through fractured rock to such anomalies and by convective flow carry the heat back to the surface. More dramatically, plumes of magma may rise to within 1 5 km of the surface, and at the sites of volcanoes, it actually reaches the surface from time to time. However, direct exploitation Power Generation Technologies. DOI: https://doi.org/10.1016/B978-0-08-102631-1.00012-2 Copyright © 2019 Paul Breeze. Published by Elsevier Ltd. All rights reserved.
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of this energy source is likely to be difficult. The magma also intrudes into the crust at the boundaries between the tectonic plates which make up the surface of the earth. These boundaries can be identified by earthquake regions such as the Pacific basin “ring of fire.” The most obvious surface signs of an exploitable geothermal resource are hot springs and geysers. These have been used by man for at least 10,000 years. Both the Romans and the ancient Chinese used hot springs for bathing and for therapeutic treatment. Such use continues in several parts of the world and is particularly popular in Iceland and Japan. In Europe, a district heating system based on geothermal heat was inaugurated in Chaude-Aigues, France, in the 14th century; this system is still in existence. Industrial exploitation of hot springs dates from the discovery of boric acid in spring waters at Larderello in Italy around 1770. This led to the development of a chemical industry based on the springs. It was here, too, that the first experimental electricity generation from geothermal heat took place in 1904. This led, in 1915, to a 250 kW power plant which exported power to the local region. Exploitation elsewhere had to wait until 1958 when a plant was built at Wairakei in New Zealand, and for The Geysers development in the United States which began operating in 1960. Global geothermal generating capacity has grown slowly since then. Geothermal energy is classified as a renewable resource by most international agencies because the energy extracted for power generation is constantly replenished from within the earth’s body. Local depletion of the heat has been recorded in some reservoirs but where they are properly managed the heat source will remain constant. The Larderello geothermal field has been operating continuously since 1946, according to the World Energy Council, without the heat supply in the reservoir failing.
GLOBAL GEOTHERMAL CAPACITY Table 12.1 shows the growth in global geothermal electricity generating capacity from 1990 until 2015. In 1990 the total global capacity was 5832 MW, and geothermal power made a relatively small contribution to overall global power production. Since then the global capacity has grown, reaching 7974 MW in 2000 and 10,717 MW in 2010. By the end of 2015, there was 12,636 MW of installed geothermal capacity worldwide. This is still relatively small by global power generation standards, accounting for perhaps 0.2% of total global installed capacity. This small contribution is a reflection of the limited global geothermal resource that is currently available to exploit. Around 40% of global geothermal power generation is found in the Asia Pacific region, where there are powerful tectonic plate boundaries that provide geothermal heat. A further 30% is found in North America. Exploitation is more limited in Europe but still significant. However, there is
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TABLE 12.1 Global Annual Geothermal Power Generating Capacity Year
Global Installed Capacity (MW)
1990
5832
1995
6887
2000
7974
2005
9064
2010
10,717
2013
11,772
2015
12,636
Source: International Geothermal Association.
little geothermal power generation in Africa, South and Central America or continental Asia.1 The principal 20 exploiters of geothermal power generation, by country, are shown in Table 12.2. The largest user is the United States where there was 3450 MW of generating capacity at the end of 2015. The Philippines had 1870 MW and Indonesia had 1340 MW. There were also large geothermal capacities in Mexico, Italy, New Zealand, Iceland, Japan and Kenya, and smaller capacities in several countries in Central America. According to the International Geothermal Association, from which these figures are derived, there were 26 nations exploiting geothermal electricity generation in 2015. However, several of these have less than 1 MW of installed generating capacity. As Table 12.2 suggests, easily accessible geothermal resources suitable for power generation are not widely distributed, neither are they large. Even so, geothermal energy is attractive for power generation because it is simple and relatively cheap to exploit. In the simplest case, steam can be extracted from a borehole and used directly to drive a steam turbine. Such easily exploited geothermal resources are rare, but others can be used with little more complexity. The virtual absence of atmospheric emissions (although geothermal wells can release carbon dioxide) means that geothermal energy is also clean compared to fossil fuel fired power. Although natural geothermal fields are relatively rare, there is a much larger geothermal potential linked to deep, hot underground rock within the crust. This heat is much more expensive to exploit but could potentially offer a far larger generating capacity. Today, however, only experimental exploitation of this resource has been carried out.
1. World Energy Resources 2016, World Energy Council.
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TABLE 12.2 National Geothermal Power Generating Capacities Country
Geothermal Generating Capacity (MW)
The United States
3450
Philippines
1870
Indonesia
1340
Mexico
1017
New Zealand
1005
Italy
916
Iceland
665
Kenya
594
Japan
519
Costa Rica
207
El Salvador
204
Kenya
594
Turkey
397
Nicaragua
159
Guatemala
52
Papua New Guinea
50
Portugal
28
China
27
Germany
27
France
16
Source: International Geothermal Association.
According to the World Energy Council, the total amount of power generation from geothermal sources in 2015 was 75 TWh. This can be compared to a global total generation in 2015 of 24,255 TWh base on International Energy Agency (IEA) data. Thus, geothermal production accounted for 0.3% of the global total. Under a high renewable growth scenario developed by the IEA for future electricity production, geothermal sources would produce around 1400 TWh of power every year from 200 GW of installed geothermal capacity.
THE GEOTHERMAL RESOURCE There are three principle categories of the geothermal resource. The simplest to exploit is a source of hot underground water (the geothermal reservoir)
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which either reaches the surface naturally or can be tapped by drilling boreholes. This is the geothermal source upon which all existing commercial geothermal power plants are based as well as geothermal heating systems. Where there are no underground water sources, anomalies in the crust can create regions where the rock close to the surface is much hotter than usual. This hot rock can be accessed by drilling and pumping a heat transfer fluid into the rock, then bringing it back to the surface. The process has been tested but never exploited commercially. The third, and potentially the richest source of geothermal energy, is the magma itself. The magma contains by far the greatest amount of accessible heat energy, but because of the temperatures and pressures found within it, this is also the most difficult geothermal energy source to exploit. Estimating the amount of energy in the crust of the earth that could be exploited for power generation is not easy. It has been suggested that there is between 10 and 100 times more heat energy available for power generation as there is energy recoverable from uranium and thorium in nuclear reactors. Certainly, the resource is enormous, if difficult to access.
GEOTHERMAL FIELDS Geothermal fields are formed when water from the earth’s surface is able to seep through faults and cracks within rock, sometimes to depths of several kilometres, to reach hot regions within the crust. As the water is heated, it rises naturally back towards the surface by a process of convection and may appear there again in the form of hot springs, geysers, fumaroles or hot mud holes. These are particularly common along tectonic plate boundaries. Sometimes the route of the ascending water is blocked by an impermeable layer of rock. Under these conditions the hot water collects underground within the porous rock beneath the impermeable barrier. This water can reach a much higher temperature than the water which emerges at the surface naturally. Temperatures as high as 350 C have been found in such reservoirs. This geothermal fluid can be accessed by boring through the impermeable rock. Steam and hot water will then flow upwards through the borehole under pressure and can be used at the surface. Most of the geothermal fields that are known today have been identified by the presence of hot springs. In California, Italy, New Zealand and many other countries, the springs led to prospecting using boreholes drilled deep into the earth to locate the underground reservoirs of hot water and steam that were feeding them. More recently, geological exploration techniques have been used to try and locate underground geothermal fields where no hot springs exist. Sites in Imperial Valley in southern California were found in this way. Some geothermal fields produce simply steam, but these are rare. Larderello in Italy and the Geysers in California, The United States, are the main fields of this type in use today though others exist in Mexico,
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Power to the grid
Heat exchanger
Pump
Turbine
Generator
Pump
Geothermal reservoir
FIGURE 12.1 Schematic of a geothermal power plant.
Indonesia and Japan. More often the field will produce either a mixture of steam and hot water or hot water alone, often under high pressure. All three can be used to generate electricity. Deep geothermal reservoirs, as much as 2 km or more below the surface, produce fluid at the highest temperature. Typically, they will produce water at a temperature of 120 C 350 C. High-temperature reservoirs of this type are the best for power generation and the higher the temperature, the more energy can be extracted by a turbine. Shallower reservoirs may be a little as 100 m below the surface. These are cheaper and easier to access but the water they produce is cooler, often less than 150 C. This can still be used to generate electricity but is more often used for heating. A schematic of a geothermal power plant for exploiting an underground geothermal reservoir is shown in Figure 12.1. The fluid emerging from a geothermal reservoir, at a high temperature and usually under high pressure, contains enormous quantities of dissolved minerals such a silica, boric acid and metallic salts. Quantities of hydrogen sulphide and some carbon dioxide are often present too. The concentrated brine from a geothermal borehole is often corrosive and if allowed to pollute local groundwater sources can become an environmental hazard. This problem can be avoided if the brine is reinjected into the geothermal reservoir after heat has been extracted from it. Geothermal reservoirs are all of the limited extent and contain a finite amount of water and energy. As a consequence, both water and heat can become depleted if overexploited. When this happens, either the pressure or the temperature or both of the fluid from the reservoir declines.
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In theory the heat within a subterranean reservoir will continuously be replenished by the heat flow from below. This rate of replenishment may be as high as 1000 MW although it is usually smaller. In practice, geothermal plants have often extracted the heat faster than it is replenished. Under these circumstances the temperature of the geothermal fluid falls and the practical life of the reservoir is limited. The reservoir also loses fluid that is extracted through the geothermal boreholes used to supply hot brine to the power plant. This fluid is not replaced naturally. Reinjection of the brine after it has passed through the power plant helps to maintain the fluid in a reservoir. However, reservoirs such as the Geysers in the United States, where fluid exiting the boreholes is steam, have proved more difficult to maintain because the steam is generally not returned after use. This has led to a marked decline in the quantity of heat from the Geysers. In an attempt to correct this, waste water from local towns has been reinjected into the reservoir. Some improvement has been noted since this practice began. Estimates for the practical lifetime of a geothermal reservoir vary. This is partly because it is extremely difficult to gauge the size of the reservoir and therefore easy to overexploit it. Although some reservoirs may become virtually exhausted over the lifetime of a power plant, around 30 years, others appear to continue to supply energy for 100 years or more. A better understanding of the nature of the reservoirs and improved management will potentially help to maintain geothermal reservoirs for longer in the future.
BRINE-METHANE RESERVOIRS In some rare cases the hot brine in an underwater reservoir is found to be saturated with methane too. Such reservoirs normally occur in regions rich in fossil fuel. Where such a reservoir is found, it is possible in principle to exploit both the heat in the brine and the dissolved methane gas to generate electricity. The only major reservoirs of this type known today are in the Gulf of Mexico.
HOT DRY ROCK Underground geothermal reservoirs are relatively rare. More normally hot underground rock is not permeated by water, and hence there is no medium naturally available to bring the heat energy to the surface. Where hot rock exists close to the surface, it is possible to create a manmade hydrothermal source. This is accomplished by drilling into the rock and them pumping water down through the borehole that has been created. If water is pumped under sufficiently high pressure, it will cause the rock to fracture in a process similar in concept to “fracking” used to release shale gas from underground rock, creating faults and cracks through which the liquid can flow. (In fact, underground rock often contains natural faults and
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Power to the grid
Heat exchanger
Pump
Injection well
Turbine
Generator
Pump
Production well
Fractured rock
FIGURE 12.2 Schematic of a hot dry rock power plant.
fractures through which the water will percolate too. This often helps with the fracturing.) If a second borehole is drilled adjacent to the first, then water which has become heated as it has percolated through the rock can be extracted and used to generate electricity. This is shown schematically in Figure 12.2. The first attempt at this hot dry rock technique was carried out by scientists from the Los Alamos laboratory in New Mexico in 1973. Since then experiments have been carried out in Japan, the United Kingdom, Germany and France. One such project, part of the European Hot Dry Rock Research project, is at Soultz-sous-Foreˆts in France. Here boreholes have been drilled to 5 km below the surface and temperature of 201 C found. The project is
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now producing a small amount (around 1.5 MW of generating capacity) of electric power. Based on pilot schemes such as this, estimates suggest that a commercial hot dry rock system will need to provide 10 100 MW of generating capacity over at least 20 years to be economical. The technology is still in an early stage of development, and it is likely to be 10 15 years before commercial exploitation is possible. Hot dry rock geothermal technology is often called an Enhanced Geothermal System (EGS) today.
EXPLOITING THE MAGMA Extracting energy from accessible magma plumes which have formed within the earth’s outer crust is the most difficult way of obtaining geothermal energy, but it is also most exciting because of the enormous quantities of heat available. A single plume can contain between 100,000 and 300,000 MW-centuries of energy. Drilling into or close to such hot regions is difficult because the equipment can easily fail at the elevated temperatures to which it will be subjected. As an additional hazard, if a drill causes a sudden release of pressure, the result can be explosive. Furthermore, ways have yet to be found to tap the heat. Research continues but exploiting magma for power generation is a long-term project with no immediate prospect of exploitation.
LOCATION OF GEOTHERMAL RESOURCES The easiest geothermal resources to exploit are those that can provide water or steam with a temperature above 200 C. Resources of this type are located almost exclusively along the boundaries between the earth’s crustal plates, in regions where there is significant plate movement. These areas are found around the Pacific Ocean in New Zealand, Japan, Indonesia, the Philippines and the western coasts of North and South America, in the central and eastern parts of the Mediterranean, East Africa, the Azores and Iceland. Geographically, 72% of the global geothermal electricity generating capacity resides at sites along tectonic plate boundaries or hotspot features. Lower temperature underground reservoirs exist in many other parts of the world, and although these contain less energy, they can be used to generate electricity too. A project installed in Austria in 2001, for example, generates electricity from 106 C water which is also used for district heating. However, these reservoirs can be more difficult to locate in the absence of hot surface springs. Nevertheless, there were around 60 countries using geothermal energy at the beginning of the 21st century for heating, generating electricity or both. Of these, more than 30 countries exploit the geothermal resource for heating alone. The countries that are the heaviest exploiters of geothermal energy for heating include China, Turkey, Iceland and Japan.
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THE SIZE OF THE RESOURCE Today it is difficult to estimate the size of this energy resource but as survey techniques improve more accurate data will become available. Based on the data available at the beginning of the 21st century, it has been estimated that reservoirs located in the United States, for example, might provide 10% of US electricity. According to figures published by World Energy Council in its 2010 Survey of Energy Resources, the global geothermal generating potential could be between 35 and 210 GW. However, if hot dry rock techniques could be exploited, the total potential could be 5 10 times higher than this. Another estimate, again from the World Energy Council, suggests that 8.3% of global electricity generation could be provided by geothermal sources. That would suggest generating capacity of between 300 and 400 GW, roughly 30 times larger than that exists today. As these figures show, estimates vary widely.
GEOTHERMAL ENERGY CONVERSION TECHNOLOGIES There are three principle ways that have been developed for converting geothermal energy into electricity. Each is designed to exploit a specific type of geothermal resource. The most straightforward of the three is only feasible when a geothermal reservoir produces high-temperature dry steam alone. Under these circumstances, it is possible to use a direct steam power plant which is analogous to the power train of a steam turbine power station, with the boiler of the plant replaced by the geothermal steam source. So long as the steam exiting the reservoir is of suitable quality, this provides an extremely cheap and effective means of generating electricity. Most high-temperature geothermal fields produce not dry steam but a mixture of steam and hot brine. This mixture cannot be utilised quite so simply and is most effectively exploited using a configuration called a flash steam geothermal plant. The flash process converts a part of the hot, highpressure liquid to steam, and this steam, together with any steam extracted directly from the borehole, is used to drive a steam turbine. Where the geothermal resource is of a relatively low temperature, a third system called a binary plant is more appropriate. This uses the lower temperature geothermal fluid to vaporise a second low-boiling point fluid contained in a separate, closed cycle system. The vapour then drives a turbine which turns a generator to produce electricity. Although the overall efficiency of such binary systems is low, the availability of a cheap heat source makes the system economical to operate.
DIRECT STEAM POWER PLANTS Dry steam geothermal reservoirs are extremely rare. Where they exist, the steam, with a temperature of 180 C 350 C, can be extracted from the
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reservoir through a borehole and fed directly into a steam turbine. Steam from several wells of this type will normally be fed to a single turbine to allow an economically large turbine to be used. The pipes which carry the steam from the wellheads to the turbine contain various filters to remove particles of rock and any steam which condenses as it passes through the pipework. This is the configuration which was used in the very first geothermal power plant in Larderello, Italy, in 1904. The steam turbine in a direct steam geothermal plant is usually a standard reaction steam turbine. The turbine size in a modern geothermal plant is typically between 20 and 120 MW. As a result of the relatively low steam temperature and the small size of the turbines, energy conversion efficiency is generally low at around 30%. In some cases the steam exiting the turbine may be released directly into the atmosphere. However, the steam usually contains between 2% and 10% of other gases such as carbon dioxide and hydrogen sulphide. Under these circumstances, the exhaust from the steam turbine must be condensed to remove the water and then treated to remove any pollutants such as hydrogen sulphide before release into the atmosphere. At the Geysers plant in the United States, the hydrogen sulphide from the steam is converted into sulphur which is sold, creating a small additional source of revenue. Condensing the steam also extracts more energy, hence increasing plant efficiency. A schematic of a direct steam geothermal power plant is shown in Figure 12.3.
Air and water vapour
Generator Turbine Condenser
Cooling tower
Steam Air Water
Air
Hot well
Condensate Ground level Geothermal zone
Production well FIGURE 12.3 A direct steam geothermal power plant.
Injection well
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Ideally the geothermal fluid should be returned to the underground reservoir but it is often more economical to release the gas and dispose of the water produced as a result of condensing the steam from the turbine at the surface. Carbon dioxide emissions from such plants could become an issue in the future and, depending upon the amounts involved, it may be feasible to capture and either reinject or sequester the carbon dioxide generated from such a plant. In general, however, carbon dioxide emissions are low compared to fossil fuel sources. In the United States, for example, typical emissions from a geothermal plant are equivalent to 91 gCO2/kWh. Typical fossil fuel power plants release between 600 gCO2/kWh and 1000 gCO2/kWh according to the World Energy Council. Continual removal of underground fluid without replenishment eventually leads to depletion in the quality of fluid available from the reservoir. At the Geysers geothermal field in southern California, urban waste water has been pumped into the underground reservoir in an attempt to maintain and eventually boost output from the resource. This has led to both a reduction of urban effluent in the local river system and more power from the geothermal reservoir.
FLASH STEAM PLANTS Most high-temperature geothermal reservoirs yield a fluid which is a mixture of steam and liquid brine, both under high pressure (typically up to 10 atmospheres). The steam content, by weight, is typically between 10% and 50%. The simplest method to exploit such a resource is to separate the steam from the liquid and use the steam alone to drive a steam turbine. This would be equivalent to the dry steam plant described earlier. However, using only the steam throws away much of the available energy, particularly where the proportion of steam in the fluid is small. A more productive alternative is to pass the combined fluid through a valve into a vessel maintained at a lower pressure than the geothermal fluid from the reservoir. The sudden reduction in pressure “flashes” a proportion of the hot liquid to steam, creating a larger quantity of steam than was available previously. All the steam can then be separated from the liquid and used to drive a steam turbine. The steam exiting the steam turbine must be treated in exactly the same way as the exhaust from a direct steam geothermal plant to prevent atmospheric pollution. The remaining liquid, which contains high levels of dissolved salts and presents a potential pollution problem, is usually injected back into the geothermal reservoir. A schematic of a flash steam geothermal power plant is shown in Figure 12.4. A further refinement to the flash steam plant is called double flash technology. This involves taking the fluid remaining after the first flash process and releasing it into a second vessel at even lower pressure. Double flashing results in the production of more steam which can be fed to a second, lowpressure turbine or injected into a later stage of the turbine powered by the
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Air and water vapour
Generator Turbine Condenser Steam
Cooling tower Air
Water
Steam
Brine
Air
Condensate Waste brine
Hot well Direct heat uses Ground level
Geothermal zone
Production well
Injection well
FIGURE 12.4 A flash steam geothermal power plant.
steam from the first flash. A double flash plant can produce up to 25% more power than a single flash plant. However, it is more expensive and may not always be cost-effective. Flash technology plants will generally return a much higher percentage of the geothermal fluid up to 85% for a single flash plant and somewhat less for the double flash plant to the geothermal reservoir. This will include most of the dissolved chemicals contained in the original fluid. However, some reservoir depletion will still take place, and without action this is likely to lead to a fall-off in output from the reservoir with time. Capacities for flash geothermal power plants are normally between 20 and 55 MW.
BINARY CYCLE POWER PLANTS Direct steam and flash geothermal power plants utilise geothermal fluid with a temperature of between 180 and 350 C. If the fluid is cooler than this, conventional steam technology will normally prove too inefficient to be economically viable. However, energy can still be extracted from the fluid to generate power using a binary cycle power plant. This type of plant has two fluid cycles: the first involves the low-temperature fluid from the geothermal resource and the second is a thermodynamic turbine cycle involving a lowboiling point fluid.
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Air and water vapour
Generator Turbine Condenser Iso-butane (vapour)
Cooling tower
Iso-butane
Air
Air Water
Heat exchanger Hot brine
Ground level Geothermal zone
Production well
Injection well
FIGURE 12.5 A binary cycle geothermal power plant.
In a binary power plant the geothermal fluid is extracted from the reservoir and immediately passed through a heat exchanger where the heat it contains is used to volatilise a secondary fluid. This secondary fluid is contained within a second, completely closed cycle system. The fluid may be an organic liquid which vaporises at a relatively low temperature or, in the case of the Kalina Cycle,2 a mixture of water and ammonia. The vaporised secondary fluid is used to drive a small turbine from which power can be extracted with a generator. Once it has exited the turbine, the secondary vapour is condensed and then pumped through the heat exchanger once more. Thus the cycle is repeated. The geothermal fluid exiting the heat exchanger is, meanwhile, reinjected into the geothermal reservoir. Because 100% of the fluid is returned underground, this type of geothermal power plant has the smallest environmental impact. A schematic of a binary cycle geothermal power plant is shown in Figure 12.5. Typical binary cycle plant unit size is 1 3 MW, much smaller than the other types of geothermal technology. However, the small units required for such plants often lend themselves to standardisation, reducing manufacturing costs. Several units can be placed in parallel to provide a plant with a larger power output. 2. The Kalina cycle is a special thermodynamic cycle designed to obtain maximum efficiency from low energy resources such as low-temperature geothermal fluids.
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Although the normal application for binary technology is to exploit a low-temperature geothermal resource, the technology can also be used to generate more power from a flash plant. In this case the fluid left after flashing is passed through a heat exchanger before reinjection, allowing extra energy to be taken to power a small binary unit. Adding a binary unit to a conventional flash plant increases the cost, but the resultant hybrid plant will have a larger power output.
ADVANCED GEOTHERMAL TECHNOLOGIES There are several areas in which current geothermal technology might be developed to provide more power. The main avenue in the immediate future is by the development of hot dry rock or EGS technology, with better drilling techniques and better prospecting techniques. There is also scope for identifying and exploiting undersea geothermal resources. There are underground geothermal reservoirs that contain very high temperature, high-pressure brine where the water has a temperature of more than 375 C and a pressure of 220 bar. Under these conditions the water in the reservoir is in a supercritical state. If this supercritical steam/water could be extracted, it could provide up to 10 times more energy than from a conventional reservoir, according to the IEA. However, extracting and exploiting the fluid in such reservoirs cannot be carried out today.
GEOTHERMAL POWER AND DISTRICT HEATING Although high-temperature geothermal resources are suitable for power generation and lower temperature resources are often exploited for district heating, it is possible to combine the two. This is typical, for example, of geothermal power plants in Iceland where there is high demand for both power and heat. Iceland’s geothermal reservoirs generally provide a hot brine and energy is extracted from this with a flash plant as described earlier. However, after the brine exits the flash plant, it still contains a significant amount of heat energy, and this can be exploited to provide heat for a district heating system. For this, the brine is passed through a series of heat exchangers so that heat can be extracted with fresh water. The hot water is then stored in a large tank from which it can be taken to supply the district heating system. The brine, upon exiting these heat exchangers, is reinjected into the reservoir. With this, as well as direct use of geothermal heat, geothermal sources account for around 66% of Iceland’s primary energy use and 25% of the electricity production according to Orkustofnun, the National Energy Authority of Iceland.
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FINDING AND EXPLOITING GEOTHERMAL SOURCES The exploitation of geothermal power is often considered to be riskier than the development of other renewable resources because of the uncertainty and high costs associated with finding suitable geothermal fields. Initial surveys of geothermal resources are often carried out by national institutions but for developers, these then need to be backed up with test wells to determine the exact nature of the resource available. Data from Pacific Rim volcanic regions suggest that the presence of single hot spring will provide a 50% change of an exploitable geothermal field. A boiling spring or fumarole increases the probability to 70%.3 Having identified a suitable surface site, prefeasibility studies are likely to cost around $1 million, with a 30% change of failure. Test drilling, usually three wells at up to $2 million per well, has a similar prospect of failure. This risk can be reduced by careful surface study followed by prioritisation of the available sites. Such an approach has led to success rates for well drilling in excess of 83% in countries such as Indonesia, Kenya and New Zealand which have high-temperature resources. However, success rates can be much lower where low-temperature resources are concerned. Once a usable underground reservoir has been located, its size must be determined. This involves fluid withdrawal over a long period; indeed it may not be until several years after production has started that a good picture of the resource can be obtained. Careful sizing of the geothermal plant to match the reservoir size will prolong the lifetime of a reservoir. However, this may not be possible if the plant has to be constructed before full data are available and, as noted, the data may not be available anyway until production has started. Oversized plants such as that installed at the Geysers in the United States have led to a premature fall in output, and this will have an impact on overall economics.
THE COST OF GEOTHERMAL POWER In common with many renewable resources, geothermal power generation involves a high-capital outlay to establish the facility but extremely low fuel and operating costs. In the case of a geothermal plant there are three initial areas of outlay, prospecting and exploration for the geothermal resource, development of the steam field and the cost of the power plant itself. As already noted, the cost of identifying a suitable geothermal reservoir is likely to be several million dollars and may not always be successful. If the resource that is found turns out to be small, this will create a much heavier burden on the project costs than if the resource is large. Steam field development will depend on plant size and the total cost will be determined 3. World Bank geothermal assessment.
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by the number of boreholes that are to be drilled. For example, the Hellisheidi geothermal power plant in Iceland, with a generating capacity of 300 MW, has 50 boreholes. The capital cost of building a power plant to exploit a geothermal resource will depend on the quality of the resource. A good resource will have a temperature of above 250 C and good permeability of the reservoir, so providing a good fluid flow. Ideally, it will provide either dry steam or steam and brine, the latter being relatively noncorrosive and with low gas content. For good-quality resources of this type today the lowest cost in the United States is around $2400/kW according to the US Energy Information Administration (EIA). Similar costs in Europe are h3000/kW h4500/kW. The lowest cost will be for a plant of more than 30 MW with smaller plants being relatively more expensive to build. A poor-quality resource will have a temperature of below 150 C, or it could provide fluid at a higher temperature but with some other defect such as corrosive brine or poor fluid flow. Resources of this type will generally only be capable of supplying heat for a small geothermal plant of less than 5 MW, and the capital cost could be up to twice that of a large facility based on a good resource. Further indirect costs will be incurred, depending on the location and ease of access to the site. These will vary from 5% for an easily accessible site and a local skilled workforce to 60% of the initial cost in remote regions where skilled labour is scarce. All these costs will be part of the initial investment required to construct a plant. The cost of actual projects built in recent years shows the cheapest to be $2200/kW in Indonesia and the most expensive to be $9910/kW in the United States.4 The cost of electricity from the geothermal plant will depend primarily on the capital cost of building the plant and the cost of financing the construction. There is also the potential for an additional cost resulting from a local rent for exploiting the resource but that is rare. European estimates for the levelized cost of geothermal electricity are h40/MWh h100/MWh. Meanwhile the International Renewable Energy Agency has put the levelized cost of electricity from geothermal plants at between $70/MWh and $120/ MWh depending upon the region. The cost was highest in Europe and lowest in Asia and Eurasia. In the United States the US EIA has estimated that the cost of energy from a new geothermal plant entering service in 2017 is $100/ MWh. These levelized costs, at the lower end, make geothermal power competitive with virtually all other forms of power generation.
4. World Energy Resources 2016, World Energy Council.