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Geothermal Energy All these reckonings of the history of underground heat, the details of which I am sure you do not wish me to put before you at present, are founded on the very sure assumption that the material of our present solid earth all round its surface was at one time a white-hot liquid. ~ Lord Kelvin (1824-1907) CHAPTER 9 Geothermal energy is heat energy from magma (molten rock) deep within the earth, brought to the surface naturally by geysers and springs, by drilling into hot reservoirs or magma, or by exploiting the thermal mass of soil and ground water to drive a heat pump. Geothermal energy has been used since ancient times when the Romans, Greeks, and Japanese among others - used naturally heated mineral water for bathing, cooking, heating, and medical applications. Today, hot springs and spas1 are still some of the most attractive spots for recreation and therapeutic bathing. In addition, geothermal energy is used for power generation, space heating, and in greenhouses, swimming pools, and steam processing for industrial applications. Geothermal energy can be considered renewable as long as the rate of heat extraction from a reservoir does not exceed the rate it is recharged by the earth’s heat, which, depending on its temperature, may take tens to hundreds of years. Although only 10% of the earth’s surface are at temperatures hot enough to be used as a practical energy source, the earth’s geothermal resources are large enough to provide many times the energy needs of the entire population.2 There are areas where a substantial amount of geothermal energy diffuses through the crust and reaches the surface of the earth. Low- to moderate-temperature geothermal reservoirs are present on most continents; the largest resources in the world are believed to be in Iceland. High-temperature geothermal resources are predominately found in volcanic ranges and island chains. Other especially attractive regions for geothermal energy are in Central America, Indonesia, East Africa, and the Philippines. Worldwide, geothermal energy provides about 8,000 megawatts of electricity and another 8,000 megawatts of direct-use thermal energy. The largest geothermal electricity generation plant in the world is located in the Geyser area in Northern California, generating about 1,000 MW – about 1/3 of the total US geothermal output and enough electricity to supply a city of over a million homes (Figure 9-1).3 In the past few decades, researchers have found new ways to trap natural 1 2 3 Named after town of Spa in Belgium, famed for healing hot mineral springs since 14th century. Norton, G. A., and Goat, C. G., “Geothermal Energy – Clean Power from the Earth’s Heat,” US Geological Survey, USGS, Circular 1249, 2003. K utscher, C.F., “The Status and Future of Geothermal Electric Power,” National Renewable Energy laboratory ( Figure 9-1 Dry steam plants at the Geysers Source: National Renewable Laboratory. thermal gradients in dry rocks. When this technology matures, geothermal energy promises to become an important source of renewable energy available virtually anywhere on the planet. The Earth The earth is the third planet from the sun and has an average radius of 6,400 kilometers. Its internal structure includes an inner core, an outer core, lower and upper mantles, and a crust (Figure 9-2). The inner core – the center of the earth – is a sphere that is 2,400 kilometers in diameter. Because of the extreme pressures in the center, the earth’s inner core remains solid and becomes molten only in the outer core, allowing rocks to “flow” and form the convection cells within the mantles. Near the top of the mantle, rock cools sufficiently by conduction, causing it to solidify and form the earth’s crust. Heat transfer causes warming of the rocks and fluid, providing what we call the geothermal energy. The inner core consists mainly of iron, nickel, uranium, and various heavy metals. The ultimate source of geothermal energy is the radioactive decay of the isotopes of these materials and the original heat produced from the formation of the earth due to gravitational collapse. The outer core is 2,300 kilometers thick and composed of liquid molten metals, iron, nickel, and lighter elements. The outer core along with earth’s rotation is believed to be responsible for the earth’s magnetic field. The temperature of the outer core is relatively uniform and varies from 5000oC to about 4,000oC at the outer edges. Surrounding the core are the lower and upper mantles. The combined mantles constitute 80% of the earth’s volume and about 67% of its mass. The lower mantle is 2,300 kilometers thick and is made of highly viscous fluids. The upper mantle is solid, has a thickness of about 600 kilometers, and may contain pockets of high-temperature volcanic reservoirs. The top layer is the earth’s crust, which is not a continuous sheet of rock, but is broken into solid plates that float independently on top of the hot mantle. The crust is only about 40 kilometers thick and is comprised of all the earth’s water and landmasses. The temperature drops rapidly in the upper mantle and crust at a rate of 20-30oC/km until it reaches surface temperatures. Magmas reside at the 206 The Earth Crust Mantle Outer core Inner core Figure 9-2 Earth’s internal structure. Chapter 9 - Geothermal Energy Ring of Fire FYI ... T he horseshoe region encircling the Pacific Ocean covering western coasts of southern, central, and northern Americas, the Alaskan southern coast, Japan, Philippine, Indonesia, and New Zealand houses many hundreds of active and dormant volcanos and thus is called the “Ring of Fire.” This region is also the area most frequented by earthquakes and volcanic eruptions and is suitable for exploiting geothermal energy. Volcanoes are formed when convection currents originating deep inside the earth cause lateral forces that push one oceanic plate beneath the other, causing it to melt in a process called subduction. Some of the molten rock or magma rise find their way toward the surface and flow as lava. The “Ring of Fire.” The solid lines are locations of deep-sea trenches formed when oceanic plates collided (Image Courtesy of USGS). depths of 5-10 kilometers beneath the crust and range in temperature from about 650 to 1,300oC, depending on chemical composition. Moving toward the surface, the temperature decreases; heat is transferred to nonporous, hot, dry rocks, which become increasingly porous and permeable near the surface. The capacity of a porous rock to transmit water is called permeability. As the rock becomes saturated, it will form reservoirs of water, steam, or both, which will eventually find their way to the earth’s surface through faults and cracks, making geothermal energy available for exploitation.4 Question: Considering that our best drills can barely cut through a very small depth of the earth’s surface, how can we know anything about the earth’s interior structure? Answer: We know that the average density of the earth (as measured by dividing its volume by mass determined from its gravitational force field) is higher than the surface density, so it can be concluded that the earth’s interior must be denser than its surface. Furthermore, detailed analyses of the seismic waves following an earthquake give us information about the internal structure. The speed of propagation of the waves is a function of the density of materials through which they travel. Geothermal Resources The earth is a geologically active planet with distinct features such as volcanic eruptions and earthquakes. The energy required to drive these activities is the internal heat stored during earth’s formation. Because of its relatively smaller size and lower internal heat, the moon is devoid of these activities. K ious, W. J., and Tilling, R. I., “ The Dynamic Earth: The Story of Plate Tectonics,” US Geological Survey, 2001. The electronic version of this book can be downloaded from http:// . 4 207 Geothermal resources are of four types: hydrothermal, pressurized, hot dry rock, and magma. Hydrothermal resources are the most common source of geothermal energy and are referred to as underground reservoirs containing hot water or steam. Pressurized resources are high-temperature, high-pressure brines trapped in porous rocks. Hot dry rocks (HDR) refer to solid slabs of rock that can be up to several kilometers thick. Finally, magma is the molten rock in volcanic formations rising up from deep within the mantle. Almost all geothermal sources currently being utilized are hydrothermal. Depending on how the geothermal eruption appears, a hydrothermal source can be classified as a hot spring, a warm spring, a fumarole, or a geyser. Hot springs refer to upwelling of ground water with temperatures above that of the human body. It is called a warm spring when the temperature is lower body temperature but higher than the surrounding atmosphere. When a reservoir does not contain adequate water to seep through and is eventually converted to steam, the geothermal resource is referred to as a fumarole. Geysers are intermittent hot springs that, depending on geological conditions, erupt in regular intervals ranging from a few minutes to hours or months apart (See box “The Old Faithful”). Sixty percent of all geysers in the world are in Yellowstone National Park in Wyoming; others are scattered elsewhere, mainly in California, Italy, New Zealand, and Iceland. Geothermal resources typically lie from between a few hundred to a few thousand meters below the earth’s surface. Deeper reservoirs are generally higher in temperature and therefore yield higher efficiencies. The cost of the power plant itself is less, but drilling costs increase exponentially with depth, increasing the overall cost of extracting energy from deeper mines. With current technologies, only reservoirs within roughly four kilometers are considered to be economically viable sources of geothermal energy. Question: Can geothermal energy be considered a renewable source of energy? Old Faithful Digging Deeper ... A geyser is like a periodically erupting pressure cooker. The one known as “Old Faithful” has been a popular attraction in Wyoming’s Yellowstone National Park for many years. Geysers work the same way a coffee percolator works; a long narrow column of water is heated from below. In the case of geysers, the source of this heat is volcanic activity and magma. Because of its weight, water reaches temperatures well above its boiling temperature under atmospheric conditions. As the lower layer of water turns into steam, it expands, pushing out the upper layer lifting the column of water above it. As the eruption continues, the weight of the column of water, and with it the pressure at the bottom, decreases and water turns liquid again. After the water begins to reenter the channel, the whole process repeats in 50-70 minutes. 208 Chapter 9 - Geothermal Energy Answer: The earth, like the sun, has an immense amount of stored energy which will by all accounts last billions of years into the future. Unlike solar energy, which is readily available, we must tap geothermal resources. With the current technology, only hydrothermal resources are economically suitable and these have a limited lifetime before they are exhausted. (For example, the power-generating capacity of Geysers has dropped by 40% since only a decade ago). These resources can be recharged in the future as they are gradually reheated by the internal heat of the earth and, depending on their locations, can take several decades to hundreds of years to become operational again. It has therefore been suggested that geothermal energy be considered as a sustainable resource, one whose usefulness can be prolonged or sustained by optimum production strategies and methods.5 In the case of the Geysers, wastewater from a nearby community has increased the efficiency; it is estimated that at the rate of 1000 megawatts, geysers will remain sustainable for a few decades. Finding Resources Short of drilling directly into the hydrothermal reservoir, chemical geothermometry is perhaps the best tool for surface exploration of deep geothermal resources. The concentrations of the many minerals dissolved in underground reservoirs are highly temperature dependent. The temperature of an underground geothermal reservoir can therefore be estimated by measuring of chemical composition of various minerals in the hot spring water. The ratios of concentration of sodium, potassium, calcium, magnesium, and lithium, as well as the isotopes of individual atoms are commonly used as indicators for viability of the subsurface reservoir for geothermal exploitation. Applications Depending on the source and temperature, geothermal heat can be used either directly or by converting it to electricity. Geothermal resources can be classified as low-temperature (less than 100°C), moderate-temperature (100-200°C), and high-temperature (greater than 200°C). In addition to hot water and steam reservoirs, hot dry rocks and magma have enormous potential. In principal, the natural gradients present in these rocks can be used to extract an unlimited amount of energy - if the technology to exploit them is developed. Magma cannot be economically developed in the near future and will not be discussed further. Hot Water and Steam Geothermal Power Plants The highest temperature resources are generally best for electric power generation. When dry (superheated) steam is available, it can be directly expanded through a turbine to generate electricity. For hot water reservoirs under high pressure, the geothermal fluid is brought to the surface and sprayed into a tank. As the pressure drops, some of the water flashes into 209 steam. The steam is subsequently cleaned and piped directly into steam turbines, which drive electric generators. The remaining water is injected back into the reservoir to help maintain its high temperature and pressure. Dry steam reservoirs (fumaroles) are highly efficient, but rare. The largest plant of this type in the world is at the Geysers in northern California. Moderate steam temperatures are not practical for use in turbine facilities because the steam condenses before it has appreciably expanded through the turbines. Under such circumstances, binary-cycle power plants are more efficient. In a binary plant, the geothermal water is pumped at high pressure through a heat exchanger where it passes its internal heat to a secondary fluid with a lower vapor pressure (alcohol, isopropane, or other refrigerants), causing it to boil and turn into high-pressure steam. The steam is then used in a closed-loop cycle to drive a turbine/generator assembly. Because binary plants do not use the steam or hot water directly, the fluids can be reinjected back into the reservoir. This maintains the pressure and prevents any toxic or noxious gases from entering the atmosphere. To optimize efficiency, flash and binary cycles are often combined in a hybrid design. One such plant is the binary power plant near Mammoth Lakes in California; this system transfers heat from steam at 170oC to isobutene, which vaporizes and drives the turbines for a net generating capacity of 37 megawatts. Warm Water Systems: Direct Use At temperatures below 100°C, geothermal sources are available in the form of liquid water. Hot water can be pumped through pipes and used directly for residential, office, and greenhouse heating, domestic water heaters, swimming pools and spas, or can be passed underground to boost agricultural and aqua-cultural production in colder climates. Worldwide, around 12,000 megawatts of thermal energy is available from warm water systems for direct use. Iceland boasts the title of the largest consumer of hot water thermals; 87% of all homes are heated with geothermal water.5 Hot Dry Rocks Another method for using geothermal energy is to utilize the heat from aquifers and hot dry rocks (HDR). Hot dry rocks are impermeable solid slabs of hot rocks found a few kilometers below the surface. Granite usually contains trace amounts of radioactive uranium and thorium and therefore is substantially hotter than surrounding nonradioactive rocks. Cold water is injected through a bore hole to a bed of hot dry underground rock under pressures high enough to fracture the surrounding rock. Water is heated through conduction as it moves toward one or more nearby production wells where water can be extracted and, depending on the temperature, used either directly or for the generation of electricity. Since 5 Gawell, K., and Greenberg, G., “2007 Interim Report: Update on World Geothermal Development,” US Geothermal Energy Association, May 2007. 210 Chapter 9 - Geothermal Energy the pressure is high, water is still liquid even at temperatures of 200°C or more. When electricity generation is of interest, the hot water gives off its heat to a secondary fluid such as a refrigerant and turns it into vapor before returning to the well to become reheated -- closing the loop. The refrigerant steam drives a turbine-generator to produce electricity. A plant operated in such a closed-loop fashion is virtually pollution free and sustainable.6 An experimental plant of this type was constructed by the Los Alamos National Laboratory in New Mexico in early 1980s, which operated for 20 years before it was shut down.7 Many technological issues must be resolved before this type of plant is suitable for commercialization. Enhanced Geothermal Systems (EGS) Hydrothermal reservoirs are useful only when they are in the geographical areas where rocks are sufficiently permeable to allow easy flow of fluid. Enhanced Geothermal Systems essentially use the same technology developed for creating reservoirs in HDR. The fractures induced in surrounding rocks increase the permeability, which allows geothermal exploration in areas that otherwise were deemed inoperable. Many geothermal plants in Europe and Japan employ EGS techniques, thus increasing the productivity of existing hydrothermal reservoirs rather than creating new ones. Magma Magma is hot molten rock in the earth’s crust, much of it resides within the 5-km of the surface. Thermal energy from magma can be recovered by drilling holes and injecting cold water through magma. The magma heats the water into steam which will rise buoyantly through a second pipe. Magma is, however, highly corrosive and when solidified, creates a layer of insulation that limits the operability of such systems. Finding materials that can withstand hot corrosive magma for an extended time is a major obstacle to commercial development of this technology. Geothermal Heat Pumps Heat pumps, as the name implies, are devices that move heat from one place to another up the temperature gradient, i.e. removing heat from cold outside air in the winter and deliver heat to hot outside air in the summer. Geothermal heat pumps (GHPs) take advantage of the relatively constant temperature of water underground from one season to another. Because rocks and soil are good insulators, they are rather insensitive to variations in the ambient air temperature. They are warmer than the ambient air temperature above ground in the winter, and colder during the summer. Below a depth of approximately two meters, the temperature of the soil in Hooper, G., and Duchane, D., “Hot Dry Rock: An Untapped Sustainable Energy Resource,” US Department of Energy Website ( documents/HDREnergy.pdf). 7 Dateline Los Alamos, Monthly publication NO. W-7405-ENG-36, Los Alamos National Laboratory, 1995. 6 211 most of the world’s regions remains stable between 7°C and 20°C. Geothermal heat pumps exploit the earth or groundwater as heat sources by transferring heat from the soil to the house in winter and as a heat sink by transferring heat from the house to the soil in summer. In the simplest form, a mixture of water and ethylene glycol solution (antifreeze) circulating in underground pipes or loops is used as a medium for transferring heat into or out of a building. In the heating mode (during winters), the mixture is colder than the surrounding ground; thus it absorbs heat from the ground and warms up. The heated liquid is pumped into the building where it transfers its heat through a heat exchanger into the room. In the cooling mode (during summers), the process is reversed. Here, the water/glycol mixture is hotter than the surrounding soil, and thus it releases its energy to the ground and cools. The cold water exchanges heat with a refrigerant, circulating through a heat exchanger located inside the heat pump. A blower cools the air by forcing it across the refrigerant coil. Just like conventional heat pumps, geothermal heat pumps can operate in a closed- or an open-loop cycle. In closed-loop systems, a small pump is used to circulate the fluid. In open-loop systems, water from an underground aquifer is piped directly from a well to a building, where it transfers its heat to a heat pump. The water is then returned to the aquifer through a second well some distance away. Open-loop systems are simpler and less costly, but can be used only when there is an abundant supply of ground water. Depending on the location and the availability of ground space, loops can be installed either horizontally or vertically and in linear or loop configurations (See Figure 9-3). Horizontal installation is the most costeffective for residential sites and in new construction where sufficient land is available. It requires trenches between one and two meters deep. Vertical installations are used in large commercial buildings, in places where land area is limited, where land is too rocky to dig trenches, or where digging causes major disturbances to the existing landscape. To save space, the pipe may be coiled or looped into a spiral. When there is an adequate body of water, submerging the loop into the water may be the most cost effective strategy. In such instances, the supply line is run underground from the building to the water and coiled into circles a few meters below the surface to assure that it is not susceptible to winter freezing. Geothermal heat pumps are rated in two ways: the coefficient of performance (COP)8 and the energy efficiency rating (EER)9. Although COP is defined for either heating or cooling, EER is used to indicate the cooling efficiency. Geothermal heat pumps can be installed practically anywhere and, when compared to conventional cooling and heating systems, use 30-50% less electricity. If a geothermal system is planned before a building is 8 9 Figure 9-3 Different loop configurations used with geothermal heat pumps – (a) horizontal, (b) vertical, (c) coil. COP is the ratio of heating or cooling achieved per unit of work supplied (See Chapter 5). E ER = COP(cooling) x 3.413 212 Chapter 9 - Geothermal Energy constructed, installation is easy, and cost is relatively small. Furthermore, domestic hot water production is essentially free during summers. Unlike conventional rooftop models, geothermal heat pumps are small and can be installed indoors, typically in a basement or attic. Worldwide, there are currently more than a half million geothermal heat pumps installed, totaling a thermal output of 7,000 megawatts; nearly 70% of them are in the United States. Advantages and Disadvantages Unlike wind, solar, or tidal plants, geothermal power plants can deliver power continuously and thus provide base-load electricity.10 Furthermore, geothermal plants are not vulnerable to weather changes, no storage is needed, and distribution is not an issue. Geothermal reservoirs are, however, limited to specific geographical areas. Depending on the resources and on power demand, geothermal plants can be constructed in any size-- as small as 100 kW (convenient for local grid applications and rural electrification) to many hundreds of megawatts for base-load and load-demand applications and for national grids. Modular plants can be built so that capacity can be added as the need for power increases. Like most renewable energies, direct-use systems require a larger capital investment as compared to traditional systems, but the lack of fuel cost and lower operating expenses offset the initial investment in only a few years. The cost of power production varies greatly from 2.5 to 10 cents per kilowatt-hour, depending on such factors as the size, depth, location, and temperature of the reservoir. As the price of petroleum and other fossil fuels increase, geothermal sources become more competitively priced or even cheaper than fossil plants. The currently installed US capacity of direct-use systems totals to 470 MW, enough energy to heat 40,000 average-sized houses. Environmental Concerns Compared to fossil fuels, geothermal energy produces much less pollution. Current geothermal fields produce 60% less carbon dioxide that a natural gas fueled electricity-generating power plant produces,11 with none of the nitric oxide (NOx) gases and much lower sulfur compounds than coal and oil power plants. Geothermal plants do, however, produce trace amounts of hydrogen sulfide, a gas that smells like rotten eggs-- usually associated with hot mineral springs-- and which can be harmful at high concentrations. Some arsenic, mercuric and other toxic gases are also released by the plant. Newer binary- and combined-cycle geothermal plants inject these gases back into the geothermal wells and produce 10 11 Base-load refers to minimum electricity needs, independent of the time of the day. Combustion of bituminous coal emits about 900 kg of carbon dioxide per MWh; the corresponding numbers for natural gas and geothermal plants are 300 and 120 kg per MWh. 213 virtually no air emissions. Geothermal production can adversely affect the environment by degrading of geothermal features, promoting ground subsidence, and increasing seismic activities, all of which occur as a result of declining reservoir pressures. Another concern is the release of hot waste water containing significant amounts of toxic substances such as lithium, boron, mercury, and arsenic into existing waterways. This impact can be mitigated by controlling reservoir pressure through adjusting the discharge rate and reinjecting the geothermal fluid back into the ground. Binary plants work in this fashion and typically have no releases. Summary Geothermal energy has been used for many thousands of years by people who enjoyed natural hot water springs for bathing and therapeutic reasons. Today, geothermal resources are not only used for direct applications, but also to generate electricity. Currently, most geothermal resources are limited to steam and hot water reservoirs. Innovative technologies are being developed that enable us to exploit the temperature gradients in hot dry rocks, allowing geothermal energy to be used for electricity generation anywhere in the world. This requires the ability to drill approximately 5 to 20 kilometers into the earth’s surface and at reasonable cost, a technology that is not currently available. The exploitation of energy from volcanic eruptions and magma flow remains in the distant future. Additional Information Books 1. Dipippo, R., Geothermal Power Plants: Principals, Applications and Case Histories, Elsevier, 2005. 2. Dickson, M. H., Fanelli, M., Geothermal Energy: Utilization and Technology, Stylus Pub., 2005. 3. Ochsner, K, Geothermal Heat Pumps: A Guide for Planning and Installing , Earthscan Ltd, 2007. 4. Gupta, H. , and Roy, S., Geothermal Energy: An Alternative Resource for the 21st Century, Elsevier, 2007. Periodicals 1. Geothermics, Direct Science Elsevier Publish. Company, publishes articles on geothermal energy resources and technologies. 2. Geotimes, Journal of the American Geological Institute. 3. Geo-heat Center Quarterly Bulletin, covers how-to articles on various geothermal applications and equipment, progress in research and development activities of direct heat utilization 4. Journal of Volcanology and Geothermal Research, an international journal on the geophysical, geochemical, petrological, economic, and environmental aspects of volcanology and geothermal research. 214 Chapter 9 - Geothermal Energy Government Agencies and Websites 1. National Renewable Energy Laboratory Geothermal Energy Program ( 2. Idaho National Laboratory Geothermal Program (http://geothermal. 3. US Department of Energy Geothermal Technology Program ( 4. California Energy Commission (( geothermal). Non-Government Agencies and Websites 1. Geothermal Resources Council ( 2. Geothermal Energy Association ( 215 Exercises I. Essay Questions 1. What countries have the most potential for using geothermal energy? 2. Describe the principal of operation of geothermal heat pumps. Where can they be used? 3. Can geothermal energy be considered a renewable energy? Explain. 4. What are the uses for low-temperature geothermal resources? 5. How can we use the earth’s temperature gradient to drive a heat engine? 6. What are the factors that constitute a good geothermal location? Where are such sites located? 7. Explain how chemical geothermometry works and how it is used to explore the ideal sites for geothermal excavation? 8. With current technologies, which types of geothermal sources can be exploited? 9. Is geothermal renewable? Sustainable? Explain. 10. What are the environmental considerations of geothermal energy? II. Multiple Choice Questions 1. How do temperature and pressure vary with depth inside the earth? a. Both temperature and pressure increase with depth. b. Temperature increases with depth, but pressure decreases. c. Pressure increases with depth, but temperature decreases. d. Temperature increases with depth, but pressure remains relatively constant. e. Both temperature and pressure decrease with depth. 216 2. What distinguishes earth’s inner and outer cores? a. Because of its extreme temperatures, the inner core is a gas. The outer core is cooler and remains liquid. b. Both the inner and outer cores are liquid. The only difference is their temperatures. c. Both the inner and outer cores are solid. The only difference is their temperatures. d. The inner core is liquid. It solidifies gradually as temperature drops toward the surface. e. The inner core is solid. The outer core is liquid. 3. The inner core is composed mostly of a. Iron b. Silicon c. Sulfur d. Oxygen e. Steam 4. The average thickness of the crust is a. Less than 1 km b. 30-50 km c. 100-200 km d. 200-1000 km e. More than 1000 km 5. Where is the source of earth’s magnetic field? a. Crust b. Mantle c. Inner core d. Outer core e. Electromagnetic radiation from the sun 6. Today, which renewable energy sources provide the US with the most energy? a. Solar b. Wind c. Hydropower d. Geothermal e. Waves and Tides 7. On average, the energy available from geothermal resources a. Is much greater than the solar influx b. Is about the same as the solar influx c. Is much less than the solar influx Chapter 9 - Geothermal Energy 8. The source of geothermal energy is a. Radioactive decay of elements below the earth’s crust b. Heat still left from the time when the earth was formed c. Chemical reactions among gases trapped below the earth d. Underground nuclear explosions e. Both a and b 9. The temperature in the upper mantle and crust falls at the rate of a. 1°C/m b. 1°C/km c. 5-10°C/km d. 20-30°C/km e. More than 100 °C/km 10. Which layer of the earth’s interior is liquid? a. Crust b. Mantle c. Inner core d. Outer core e. None is liquid 11. Which layer of the earth’s interior has the greatest density? a. Crust b. Mantle c. Inner core d. Outer core e. All about equal 12. Which layer of the earth’s interior has the greatest volume? a. Crust b. Mantle c. Inner core d. Outer core e. About the same 13. Which layer of the earth’s interior has the least thickness? a. Crust b. Lower mantle c. Upper mantle d. Inner core e. Outer core 14. The major obstacle(s) associated with the use of geothermal energy is/are a. That many sites are difficult to access b. That they produce hydrogen sulfide which is an environmental pollutant c. That no technology exists for exploiting temperature gradients in hot solid rocks d. High initial cost e. All of the above 15. Most of the geothermal energy available today is from a. Volcanoes b. Hot water springs and geysers c. Hot dry rocks d. Desert surfaces e. All of the above 16. Binary-cycle power plants are most often used a. When the geothermal source is at a temperature below 100°C b. When the geothermal source is at a moderate temperature, between 100-200°C c. Only when steam is available at temperatures above 300°C d. Only when they are in association with hybrid design plants e. Only when underground sources contain large fractions of alcohol and other refrigerants 17. Depending on their size, geothermal plants can be a. Used for base load electricity production b. Designed to follow daily load demands c. Used for mini-grid and rural electrification d. Used for direct thermal use e. Used for all of the above 18. The largest geothermal power plant in the United States is located near a. San Francisco b. The Rocky Mountains c. Hawaii d. Los Angeles e. New York 217 19. Geothermal heat pumps can be used for a. Space heating b. Space cooling c. Both space heating and cooling d. Processing steam e. All of the above 20. Geothermal energy a. Is essentially a zero emission source of energy b. Produces some pollution, but not as much as fossil fuels c. Produces some radioactive waste, but with a short half-life d. Produces more pollution than fossil fuels, but with no radioactive waste e. None of the above III. True or False? 1. Boron, mercury, and arsenic are all types of pollution that come from geothermal plants. 2. Earth receives most of its life-supporting energy from geothermal sources. 3. Temperatures within the earth’s inner core could reach millions of degrees. 4. The outer core is composed of liquid molten metals, mostly iron and nickel. 5. Most geothermal resources lie only a few meters below the earth’s surface. 6. Binary geothermal plants usually use geothermal water at high pressures and moderate temperatures. 7. HDR is an innovative technology that can possibly meet many of the energy needs of the 21st century. 8. Geysers are considered renewable only if they are used at a sustained rate. 9. Binary plants are advantageous over non-binary plants, because they can be designed virtually pollution-free. 10. It is best to decide on the use of geothermal plants before a building is constructed. IV. Fill-in the Blanks 1. Geothermal resources can be categorized as hydrothermal, ___________, ___________, and ____________. 2. The largest concentration of hydrothermal activities lie in ____________________. 3. __________ are vents in the earth’s surface that release only steam. 4. The fluid often used in geothermal heat pumps is a mixture of water and __________solution. 5. Geothermal plants produce trace amounts of _____________, a gas that smells like rotten egg. 6. _________ installation of heat pumps is most cost-effective for residential sites and new construction where sufficient land is available. 7. Geothermal plants produce little carbon dioxide and practically none of the _______________ commonly produced in power plants using fossil fuels. 8. Upwelling of ground water with temperatures above surrounding atmospheric temperature but lower than body temperature is called ___________. 9. The water spewing out of hot springs have temperatures _______ that of human body. 10. The performance of a geothermal heat pump is measured by its ________________. 218