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Geothermal Energy Chapter 4 4.1 Introduction Geothermal resources consist of thermal energy from the Earth’s interior stored in both rock and trapped steam or liquid water. As presented in this chapter, climate change has no major impacts on the effectiveness of geothermal energy utilization, but its widespread deployment could play a significant role in mitigating climate change by reducing green- house gas (GHG) emissions as an alternative for capacity addition and/ or replacement of existing base load fossil fuel-fired power and heating plants. Geothermal systems as they are currently exploited occur in a num- ber of geological environments where the temperatures and depths of the reservoirs vary accordingly. Many high-temperature (>180°C) hydrothermal systems are associated with recent volcanic activity and are found near plate tectonic boundaries (subduction, rifting, spread- ing or transform faulting), or at crustal and mantle hot spot anomalies. Intermediate- (100 to 180°C) and low-temperature (<100°C) systems are also found in continental settings, where above-normal heat produc- tion through radioactive isotope decay increases terrestrial heat flow or where aquifers are charged by water heated through circulation along deeply penetrating fault zones. Under appropriate conditions, high-, intermediate- and low-temperature geothermal fields can be utilized for both power generation and the direct use of heat (Tester et al., 2005). Geothermal resources can be classified as convective (hydrothermal) systems, conductive systems and deep aquifers. Hydrothermal systems include liquid- and vapour-dominated types. Conductive systems include hot rock and magma over a wide range of temperatures (Mock et al., 1997) (Figure 4.1). Deep aquifers contain circulating fluids in porous media or fracture zones at depths typically greater than 3 km, but lack a localized magmatic heat source. They are further subdivided into systems at hydrostatic pressure and systems at pressure higher than hydrostatic (geo-pressured). Enhanced or engineered geothermal sys- tem (EGS) technologies enable the utilization of low permeability and low porosity conductive (hot dry rock) and low productivity convective and aquifer systems by creating fluid connectivity through hydraulic stimulation and advanced well configurations. In general, the main types of geothermal systems are hydrothermal and EGS. Resource utilization technologies for geothermal energy can be grouped under types for electrical power generation, for direct use of the heat, or for combined heat and power in cogeneration applications. Geothermal heat pump (GHP) technologies are a subset of direct use. Currently, the only commercially exploited geothermal systems for power generation and direct use are hydrothermal (of continental subtype). Table 4.1 sum- marizes the resources and utilization technologies. Hydrothermal, convective systems are typically found in areas of mag- matic intrusions, where temperatures above 1,000°C can occur at less than 10 km depth. Magma typically emits mineralized liquids and gases, which then mix with deeply circulating groundwater. Such systems can last hundreds of thousands of years, and the gradually cooling magmatic heat sources can be replenished periodically with fresh intrusions from a deeper magma chamber. Heat energy is also transferred by conduction, but convection is the most important process in magmatic systems. Vapour Dominated Geothermal System Liquid Dominated Geothermal System Geyser Hot Spring Impermeable Rocks Permeable Rocks Natural Fracture or Joint Confined Permeable Reservoir Impermeable Rocks 406 Heat Source Figure 4.1a | Scheme showing convective (hydrothermal) resources. Adapted from Mock et al. (1997) and from US DOE publications. Subsurface temperatures increase with depth and if hot rocks within drillable depth can be stimulated to improve permeability, using hydraulic fracturing, chemical or thermal stimulation methods, they form a potential EGS resource that can be used for power generation and direct heat applications. EGS resources include hot dry rock (HDR), hot fractured rock (HFR) and hot wet rock (HWR), among other terms. They occur in all geothermal environments, but are likely to be eco- nomic in geological settings where the thermal gradient is high enough to permit exploitation at depths of less than 5 km. In the future, given average geothermal gradients of 25 to 30°C/km, EGS resources at rela- tively high temperature (≥180°C) may be exploitable in broad areas at depths as shallow as 7 km, which is well within the range of existing drilling technology (~10 km depth). Geothermal resources of different types may occur at different depths below the same surface location. For example, fractured and water-saturated hot-rock EGS resources lie below deep-aquifer resources in the Australian Cooper Basin (Goldstein et al., 2009). Direct use of geothermal energy has been practised at least since the Middle Palaeolithic when hot springs were used for ritual or routine bathing (Cataldi, 1999), and industrial utilization began in Italy by exploiting boric acid from the geothermal zone of Larderello, where in 1904 the first kilowatts of geothermal electric energy were generated and in 1913 the first 250-kWe commercial geothermal power unit was installed (Burgassi, 1999). Larderello is still active today.PDF Image | Geothermal Energy 4
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