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Geothermal Energy Chapter 4 Table 4.1 | Types of geothermal resources, temperatures and uses. Convective systems (hydrothermal) Conductive systems Deep aquifer systems Power, direct use Power Direct use (GHP) Power, direct use Power, direct use Power, direct use Power, direct use Type In-situ fluids Subtype Temperature Range Current Future Yes Continental Hot rock (EGS) H, I & L Submarine H None Utilization No Shallow (<400 m) L H,I Prototypes Magma bodies H None Yes Hydrostatic aquifers H, I & L Direct use Geo-pressured Direct use Note: Temperature range: H: High (>180°C), I: Intermediate (100-180°C), L: Low (ambient to 100°C). EGS: Enhanced (or engineered) geothermal systems. GHP: Geothermal heat pumps. Geothermal energy is classified as a renewable resource (see Chapter 1) because the tapped heat from an active reservoir is continuously restored by natural heat production, conduction and convection from surrounding hotter regions, and the extracted geothermal fluids are replenished by natural recharge and by injection of the depleted (cooled) fluids. Geothermal fields are typically operated at production rates that cause local declines in pressure and/or in temperature within the reser- voir over the economic lifetime of the installed facilities. These cooler and lower-pressure zones are subsequently recharged from surrounding regions when extraction ceases. There are many examples where for economical reasons high extraction rates from hydrothermal reservoirs have resulted in local fluid depletion that exceeded the rate of its recharge, but detailed modelling studies (Pritchett, 1998; Mégel and Rybach, 2000; O’Sullivan and Mannington, 2005) have shown that resource exploitation can be economically fea- sible in practical situations, and still be renewable on a time scale of the order of 100 years or less, when non-productive recovery periods are considered. Models predict that replenishment will occur in hydro- thermal systems on time scales of the same order as the lifetime of the geothermal production cycle where the extraction rate is designed to be sustainable over a 20 to 30 year period (Axelsson et al., 2005, 2010). This chapter includes a brief discussion of the theoretical potential of geothermal resources, the global and regional technical potential, and the possible impacts of climate change on the resource (Section 4.2), the current technology and applications (Section 4.3) and the expected technological developments (Section 4.6), the present market status (Section 4.4) and its probable future evolution (Section 4.8), environ- mental and social impacts (Section 4.5) and cost trends (Section 4.7) in using geothermal energy to contribute to reduced GHG emissions. 4.2 ResourcePotential The total thermal energy contained in the Earth is of the order of 12.6 x 1012 EJ and that of the crust of the order of 5.4 x 109 EJ to depths of up to 50 km (Dickson and Fanelli, 2003). The main sources of this energy are due to the heat flow from the Earth’s core and mantle, and that generated by the continuous decay of radioactive isotopes in the crust itself. Heat is transferred from the interior towards the surface, mostly by conduction, at an average of 65 mW/m2 on continents and 101 mW/m2 through the ocean floor. The result is a global terrestrial heat flow rate of around 1,400 EJ/yr. Continents cover ~30% of the Earth’s surface and their terrestrial heat flow has been estimated at 315 EJ/yr (Stefansson, 2005). Stored thermal energy down to 3 km depth on continents was esti- mated to be 42.67 x 106 EJ by EPRI (1978), consisting of 34.14 x 106 EJ (80%) from hot dry rocks (or EGS resources) and 8.53 x 106 EJ (20%) from hydrothermal resources. Within 10 km depth, Rowley (1982) estimated the continental stored heat to be 403 x 106 EJ with no dis- tinction between hot dry rock and hydrothermal resources, and Tester et al. (2005) estimated it to be 110.4 x 106 EJ from hot dry rocks and only 0.14 x 106 EJ from hydrothermal resources. A linear interpolation between the EPRI (1978) values for 3 km depth and the values from Rowley (1982) results in 139.5 x 106 EJ down to 5 km depth, while linear interpolation between the EPRI (1978) values and those from Tester et al. (2005) only for EGS resources results in 55.9 x 106 EJ down to 5 km depth (see second column of Table 4.2). Based on these estimates, the theoretical potential is clearly not a limiting factor for global geothermal deployment. In practice geothermal plants can only utilize a portion of the stored thermal energy due to limitations in drilling technology and rock per- meability. Commercial utilization to date has concentrated on areas in which geological conditions create convective hydrothermal reservoirs where drilling to depths up to 4 km can access fluids at temperatures of 180°C to more than 350°C. 4.2.1 Global technical potential Regarding geothermal technical potentials,1 one recent and comprehen- sive estimate for conventional hydrothermal resources in the world was presented by Stefansson (2005). For electric generation, he calculated the global geothermal technical potential for identified hydrothermal 1 Definition of technical potential is included in the Glossary (Annex I). 408PDF Image | Geothermal Energy 4
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