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Geothermal Energy Chapter 4 Given sufficient, accurate calibration with field data, geothermal reser- voir evolution can be adequately modelled and proactively managed. Field operators monitor the chemical and thermodynamic properties of geothermal fluids, and map their flow and movement in the reservoir. This information, combined with other geophysical data, is fed back to recalibrate models for better predictions of future production (Grant et al., 1982). 4.3.3 Powerplants The basic types of geothermal power plants in use today are steam condensing turbines and binary cycle units. Steam condensing tur- bines6 can be used in flash or dry-steam plants operating at sites with intermediate- and high-temperature resources (≥150°C). The power plant generally consists of pipelines, water-steam separators, vaporiz- ers, de-misters, heat exchangers, turbine generators, cooling systems, and a step-up transformer for transmission into the electrical grid (see Figure 4.3, top). The power unit size usually ranges from 20 to 110 MWe (DiPippo, 2008), and may utilize a multiple flash system, flashing the fluid in a series of vessels at successively lower pressures, to maximize the extraction of energy from the geothermal fluid. The only difference between a flash plant and a dry-steam plant is that the latter does not require brine separation, resulting in a simpler and cheaper design. Binary-cycle plants, typically organic Rankine cycle (ORC) units, are com- monly installed to extract heat from low- and intermediate-temperature geothermal fluids (generally from 70 to 170°C), from hydrothermal- and EGS-type reservoirs. Binary plants (Figure 4.3, bottom) are more com- plex than condensing ones since the geothermal fluid (water, steam or both) passes through a heat exchanger heating another working fluid. This working fluid, such as isopentane or isobutene with a low boiling point, vaporizes, drives a turbine, and then is air cooled or condensed with water. Binary plants are often constructed as linked modular units of a few MWe in capacity. There are also combined or hybrid plants, which comprise two or more of the above basic types, such as using a binary plant as a bottoming cycle with a flash steam plant, to improve versatility, increase overall thermal efficiency, improve load-following capability, and efficiently cover a wide resource temperature range. Cogeneration plants, or combined or cascaded heat and power plants (CHP), produce both electricity and hot water for direct use. Relatively small industries and communities of a few thousand people provide sufficient markets for CHP applications. Iceland has three geothermal cogeneration plants with a combined capacity of 580 MWth in operation (Hjartarson and Einarsson, 2010). At the Oregon Institute of Technology, 6 A condensing turbine will expand steam to below atmospheric pressure to maximize power production. Vacuum conditions are usually maintained by a direct contact condenser. Back-pressure turbines, much less common and less efficient than condensing turbines, let steam down to atmospheric pressure and avoid the need for condensers and cooling towers. a CHP plant provides most of the electricity needs and all the heat demand (Lund and Boyd, 2009). 4.3.4 Enhanced Geothermal Systems (EGS) EGS require stimulation of subsurface regions where temperatures are high enough for effective utilization. A reservoir consisting of a fracture network is created or enhanced to provide well-connected fluid path- ways between injection and production wells (see Figure 4.1). Heat is extracted by circulating water through the reservoir in a closed loop and can be used for power generation with binary-cycle plants and for industrial or residential heating (Armstead and Tester, 1987; Tester et al., 2006). Knowledge of temperature at drillable depth is a prerequisite for site selection for any EGS development. The thermo-mechanical signature of the lithosphere and crust are equally important as they provide critical constraints affecting the crustal stress field, heat flow and temperature gradients. Recently developed analogue and numerical models provide insights useful for geothermal exploration and production, including improved understanding of fundamental mechanisms for predicting crustal stress and basin and basement heat flow (Cloetingh et al., 2010). EGS projects are currently at a demonstration and experimental stage in a number of countries. The key challenge for EGS is to stimulate and maintain multiple reservoirs with sufficient volumes to sustain long-term production at acceptable rates, and flow impedances, while managing water losses and risk from induced seismicity (Tester et al., 2006). 4.3.5 Direct use Direct use provides heating and cooling for buildings7 including district heating, fish ponds, greenhouses, bathing, wellness and swimming pools, water purification/desalination, and industrial and process heat for agricultural products and mineral extraction and drying. For space heating, two basic types of systems are used: open or closed loop. Open loop (single pipe) systems utilize directly the geothermal water extracted from a well to circulate through radiators (Figure 4.4, top). Closed loop (double pipe) systems use heat exchangers to transfer heat from the geothermal water to a closed loop that circulates heated freshwater through the radiators (Figure 4.4, bottom). This system is commonly used because of the chemical composition of the geother- mal water. In both cases the spent geothermal water is disposed of into injection wells and a conventional backup boiler may be provided to meet peak demand. 7 Space and water heating are significant parts of the energy budget in large parts of the world. In Europe, 30% of energy use is for space and water heating alone, representing 75% of total building energy use (Lund et al., 2010a). 412PDF Image | Geothermal Energy 4
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