GEOTHERMAL ENERGY APPENDIX B B.1 INTRODUCTION

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1 APPENDIX B GEOTHERMAL ENERGY B.1 INTRODUCTION The Earth s crust is formed from enormous slabs the tectonic plates from the original ball of liquid and gas it was billions of years ago. The tectonic plates are actually moving very slowly over a massive layer of very hot rock, separating from, crushing into, or sliding under one another. The tectonic plate movement is also the way the geothermal resources were formed in our planet. The manifestations of this movement are the volcanoes that result from the high levels of heat energy found in the Earth s core. The Earth core is a very hot mixture of incandescent matter under its thin, crustlike mantle, and together with the tectonic plate movement is responsible for hot springs, steam vents, and geysers. Geo means Earth, and thermal means heat; so geothermal represents Earth heat. Once available, the geothermal energy can be used directly or indirectly, as discussed in this chapter. Geothermal energy is advantageous because it is renewable, reliable, and efficient, and as a group, geothermal power plants can generate power more than 95% of the time. These plants are seldom off-line for maintenance or repairs and are the highest-capacity factors of all power plants. The disadvantages of geothermal energy include its limited use due to site availability (requiring 1 to 8 acres of Integration of Alternative Sources of Energy, by Felix A. Farret and M. Godoy Sim~oes Copyright # 2006 John Wiley & Sons, Inc. 431

2 432 APPENDIX B: GEOTHERMAL ENERGY land per megawatt) and its environmental impact due to drilling. Its current prices also range from 5 to 8 cents per kilowatthour [1]. For every 100 m (about 328 ft) below ground, the temperature of the rock increases about 3 C (about 5.4 F). In other words, at about 3 km below ground, the temperature of the rock would be hot enough to boil water. Deep beneath the surface, water sometimes makes its way close to the hot rock and turns into boiling water or steam. The hot water can reach temperatures of more than 150 C. This is actually hotter than boiling water (100 C). It does not turn into steam because it is not in contact with the air. When this hot water comes up through a crack in the Earth, it is called a hot spring (e.g., Emerald Pool at Yellowstone National Park) or sometimes explodes into the air as a geyser (e.g., Old Faithful Geyser). Although the upper layer of the Earth, close to the surface, is not very hot, it gets much hotter with depth below ground, working like an isolation layer. Almost everywhere across the planet, the upper 3 m below ground level stays at the same temperature, between 10 and 16 C. This is the case in a basement of a building or in a cavern below ground, where the temperature of the area is almost always cool. About 10,000 years ago, Paleo-Indians used hot springs in North America for cooking. Areas around hot springs were neutral zones where warriors of fighting tribes would bathe together in peace. Every major hot spring in the United States can be associated with Native American tribes. California hot springs, such as the Geysers in the Napa area, were vital and sacred areas to tribes from that area. In other places around the world, people used hot springs for rest and relaxation. Hot springs in Japan are plentiful, attracting crowds of tourists throughout the year, and the Japanese have enjoyed natural hot springs for centuries. The ancient Romans built elaborate buildings to enjoy hot baths, such as those in the town of Bath, England. At present, areas with hot springs are being looked at as possible sources of geothermal energy. South Fork, Colorado, a town in the San Luis Valley that lags behind the rest of the state in economic development, recently conducted a preliminary feasibility study that indicated the presence of a source of geothermal energy. B.2 GEOTHERMAL AS A SOURCE OF ENERGY Heat flows outward from the Earth s interior, causing a convective motion in the mantle, which, in turn, drives plate tectonics. Plate tectonics is the movement of plates within the Earth, plates that collide periodically usually causing other events to take place. When two plates collide, one plate is subducted beneath the other and the subducted plate slides downward, reaching pressures and temperatures that cause the plate to melt, forming magma. The magma ascends on buoyant forces, pushing plates into the crust of the Earth and causing large amounts of heat to be produced. At the surface, magma forms volcanoes; below the surface, magma creates subterranean regions of hot rock. Faults and cracks at the Earth s surface

3 GEOTHERMALASASOURCEOFENERGY 433 allow water seepage into the ground. The water is then heated by the hot rock and can naturally circulate back to the surface, forming hot springs, geysers, and mud pots. Assuming that the water is not allowed to recirculate but is trapped by impermeable rock, the water fills the rock pores and cracks, forming a geothermal reservoir. The direct use of geothermal energy is achieved primarily by using shallower reservoirs of lower temperatures. Hot springs and spas are very popular around the world and are utilized when the sulfur content is relatively low. Other applications involve use of the reservoirs for agricultural production, aquacultures, industrial applications, and heating. Geothermal resources can be used either for power generation or direct consumption. There are four methods of power generation: direct-steam power generation, flash-steam power generation, binary cycle power generation, and combined/hybrid power generation. Direct-steam power generation is the least common type of generation and is characterized by vapor-dominated and dry steam reservoirs. Most common uses of shallower reservoirs of lower temperatures are for bathing and spas, agriculture, aquaculture, industrial use, and district heating. The industrial use is related to the drying of fish, fruits, vegetables, and timber; wool washing; cloth dying; paper manufacture; milk pasteurization; and piping under sidewalks and roads to keep them from icing over in freezing weather. Flash-steam generation, the most common type of generation, is used in hightemperature applications; an example is shown in Figure B.1. It is characterized by liquid-dominated reservoirs, flashing the water into steam to turn turbines. Figure B.1 Flash-steam electrical power generation.

4 434 APPENDIX B: GEOTHERMAL ENERGY Binary-cycle power generation is also very common, but is the most expensive, as it utilizes a working fluid. The binary cycle can be used with lower-temperature reservoirs, as the water heats the working fluid, and the working fluid steam actually turns the turbine. Finally, combined or hybrid generation is not used very widely and is currently being researched to combine the foregoing techniques to obtain the most efficient geothermal system. B.2.1 Geothermal Economics Today, it is common to use geothermally heated water in swimming pools and health spas. The hot water from below ground can also warm buildings for growing plants. In San Bernardino in southern California, hot water from below ground is used to heat buildings during the winter; the hot water runs through miles of insulated pipes to dozens of public buildings. City Hall, animal shelters, retirement homes, state agencies, a hotel, and a convention center are some of the San Bernardino buildings heated this way. In Iceland, many of the buildings and swimming pools in the capital of Reykjavik and elsewhere are heated with geothermal hot water. Iceland has at least 25 active volcanoes and many hot springs and geysers. There are many advantages of geothermal energy, but there are also some disadvantages. To determine whether or not the advantages outweigh the disadvantages, it is important to conduct a feasibility study for the area in question. The implementation cost of geothermal power depends on the depth and temperature of the resource, well productivity, environmental compliance, project infrastructure, and economic factors such as the scale of development and project financing costs. Tables B.1, B.2, and B.3 illustrate the costs associated with geothermal applications. TABLE B.1 Geothermal Resources Cost ($/tonne) Steam Hot water High temperature (>150 C) n/a Medium temperature ( C) Low temperature (<100 C) n/a TABLE B.2 Geothermal Plant Unit Cost (cents/kwh) High-Quality Medium-Quality Low-Quality Resource Resource Resource Small plants, <5 MW Medium plants, 5 30 MW Normally not suitable Large plants, >30 MW Normally not suitable

5 GEOTHERMALASASOURCEOFENERGY 435 TABLE B.3 Direct Capital Costs of Large Plant Exploration ($/kw Installed Capacity) Large Plants (>30 MW) Steam field $ $ Power plant $ $ Total $ $ As an example, the overall cost of a geothermal plant in South Fork, Colorado, with a capacity of about 300 MW, is approximately $39 million [2]. This cost does not include the exploration costs, as the initial feasibility study/exploration has already been completed. This cost would remain virtually unchanged for 25 to 30 years, as there is little maintenance and repair associated with geothermal systems. B.2.2 Geothermal Electricity As noted earlier, hot water or steam from below ground can also be used to make electricity in a geothermal power plant. Steam carries noncondensable gases of variable concentration and composition. In California, there are 14 areas where geothermal energy is used to make electricity. Some are not yet used because the resource is too small, too isolated, or the water temperatures are not hot enough to make electricity [2]. California s main sources are: The Geysers area north of San Francisco The northwest corner of the state near Lassen Volcanic National Park The Mammoth Lakes area, the site of a huge ancient volcano The Coso Hot Springs area in Inyo County The Imperial Valley in the south Some areas have enough steam and hot water to generate electricity. Holes are drilled into the ground and pipes lowered into the hot water, like a drinking straw in a soda. The hot steam or water comes up through these pipes from below ground, like the Geysers Unit 18 located in the Geysers geothermal area of California. California s geothermal power plants produce about one-half of the world s geothermally generated electricity. These geothermal power plants produce enough electricity for about 2 million homes. A geothermal power plant is like a regular electrical power plant except that no fuel is burned to heat water into steam. The Earth heats the steam or hot water in a geothermal power plant. It goes into a special turbine whose blades spin, and the shaft from the turbine is connected to a generator to make electricity. The steam is then cooled in a cooling tower. The white smoke rising from the plants is steam

6 436 APPENDIX B: GEOTHERMAL ENERGY Steam turbine Electrical generator Steam or hot water Lower-temperature steam or water Crust Cap rock 150 C Reservoir rock sealing hot water and steam Magma heated rock Magma Figure B.2 Geothermal electricity power plant given off in the cooling process. The cooled water can then be pumped back below ground to be reheated by the Earth. Figure B.2 gives a glimpse into the inside of a power plant using geothermalenergy.hotwaterflows into and out of the turbine, and this hot water circulation turns the generator. The electricity goes out to the transformer and then to huge transmission lines that link the power plants to homes, schools, and businesses. B.2.3 Geothermal/Ground Source Heat Pumps A geothermal or ground source heat pump system can use the constant temperature under the ground to heat or cool a building or drying crops. Pipes are buried in the ground near the building as depicted in Figure B.3. Inside these pipes, a fluid is circulated to harness the heat and move it to the interior of the ambient to be warmed. In winter, heat from the warmer ground goes through the heat exchanger of a heat pump, which sends warm air into the home or business. During hot weather, the process is reversed. Hot air from inside the building goes through the heat exchanger and the heat is passed into the relatively cooler ground. Heat removed during the summer can also be used to preheat water [3].

7 REFERENCES 437 Figure B.3 Direct extraction of geothermal heat. REFERENCES [1] [2] Renewable Energy Potential in South Fork, Internal Report for Colorado School of Mines, Engineering Division, Golden, CO, [3]