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Clauser, C., 2006. Geothermal Energy, In: K PRE-PRINT 8 GEOTHERMAL ENERGY...............................................................................................................1 8.1 THE EARTH’S THERMAL REGIME ..................................................................................................1 8.1.1 The Structure of the Earth ....................................................................................................1 8.1.2 Energy Budget of the Earth ..................................................................................................4 8.1.2.1 Heat Income .................................................................................................................................... 5 8.1.2.1.1 External Heat Sources ............................................................................................................... 5 8.1.2.1.2 Internal Heat Sources ................................................................................................................ 6 8.1.2.2 Heat Expenditure............................................................................................................................. 8 8.1.2.3 Heat Budget..................................................................................................................................... 9 8.1.3 The Thermal Regime of the Earth’s Crust............................................................................9 8.1.4 Heat Storage.......................................................................................................................10 8.1.4.1 Measuring Techniques .................................................................................................................. 12 8.1.4.2 Calculated Heat Capacity .............................................................................................................. 12 8.1.5 Heat Transport ...................................................................................................................23 8.1.5.1 Heat Conduction............................................................................................................................ 23 8.1.5.1.1 Measuring Techniques ............................................................................................................23 8.1.5.1.2 Indirect Methods...................................................................................................................... 24 8.1.5.1.3 Thermal Conductivity of Minerals .......................................................................................... 28 8.1.5.2 Thermal Conductivity of Rocks .................................................................................................... 35 8.1.5.2.1 Thermal Conductivity of Sedimentary, Volcanic, Plutonic, and Metamorphic Rocks ............ 36 8.1.5.2.2 Influence of Various Factors on Thermal Conductivity .......................................................... 46 8.1.5.3 Heat Advection.............................................................................................................................. 52 8.1.5.4 Heat Radiation and Thermal Conductivity in the Earth’s Mantle.................................................. 56 8.2 GEOTHERMAL ENERGY RESOURCES............................................................................................58 8.3 TYPES OF GEOTHERMAL ENERGY USE ........................................................................................60 8.3.1 Direct Use...........................................................................................................................60 8.3.1.1 Space Heating................................................................................................................................ 63 8.3.1.1.1 Earth Coupled Heat Extraction Systems.................................................................................. 64 8.3.1.1.2 Hydrothermal Heating Systems............................................................................................... 67 8.3.1.2 Commercial and Industrial Applications....................................................................................... 68 8.3.2 Power Generation...............................................................................................................70 8.4 TECHNOLOGICAL AND ECONOMICAL ASPECTS OF GEOTHERMAL ENERGY .................................74 8.4.1 Direct Use...........................................................................................................................75 8.4.1.1 Earth Coupled Heat Extraction Systems........................................................................................ 75 8.4.1.2 Hydrothermal Heating Systems..................................................................................................... 81 8.4.2 Power Generation...............................................................................................................83 8.4.2.1 Natural Steam Power Plants .......................................................................................................... 83 8.4.2.2 Binary Power Plants...................................................................................................................... 87 8.4.2.3 Power Plants for Hot Dry Rock or Enhanced Geothermal Systems .............................................. 90 8.4.2.4 Technical, Economic, and Ecological Aspects of Geothermal Power Production......................... 93 8.4.2.4.1 Efficiency ................................................................................................................................ 93 8.4.2.4.2 Cost and Life Time.................................................................................................................. 95 8.4.2.4.3 Pollution .................................................................................................................................. 98 8.5 SUMMARY .................................................................................................................................103 REFERENCES .........................................................................................................................................105 Pre-Print from: Clauser, C., 2006. Geothermal Energy, In: K. Heinloth (ed), Landolt-Börnstein, Group VIII: Advanced Materials and Technologies, Vol. 3: Energy Technologies, Subvol. C: Renewable Energies, Springer Verlag, Heidelberg-Berlin, 493-604. PRE-PRINT 1 8 Geothermal Energy Geothermal energy is the heat contained in the solid Earth and its internal fluids. This sets it apart from other terrestrial energy sources such as − fossil or fissional fuels in the subsurface; − biomass, solar energy, and hydropower on the surface of the solid Earth and in its rivers and seas; − wind energy in the atmosphere. Geothermal energy is stored as sensible or latent heat. Supplied by both internal and external sources, it represents a vast supply which is only started to be tapped by mankind for space heating, process heat, and generation of electric power. The options and challenges involved in turning this promising potential into operational, efficient, and economic technologies are the topic of this assessment. The major topics associated with an enhanced future use of geothermal energy are reviewed in four main chapters: (1) The Earth’s thermal regime: where on Earth is heat, how much is there, where does it come from, and how is it transferred? (2) Geothermal energy resources: what kinds of resources are available in which reservoirs and how big are they? (3) Types of geothermal energy use: how can geothermal heat be used directly or converted into electricity and what is the present use of geothermal energy?; (4) Technological and economical aspects of geothermal energy use: which technologies are available to produce geothermal energy, and how much does it cost? A summary and outlook concludes this review. 8.1 The Earth’s Thermal Regime Since the conditions under which geothermal energy can be exploited strongly depend on both the origin of geothermal heat and the environment in which it is stored, we first need to examine briefly the internal structure of the Earth. Next we analyze the energy budget of the Earth and quantify the contributions of the various external and internal sources and sinks of heat. Then we examine the thermal regime of the Earth’ crust and the magnitudes of heat storage and transport and the associated physical properties specific heat capacity, thermal conductivity, and diffusivity. 8.1.1 The Structure of the Earth Our information on the internal structure of the Earth and the variation of its physical properties (pressure, temperature, density, seismic velocities) and chemical composition are derived from seismology, i.e. the interpretation of travel time curves of earthquakes which passed through the Earth. The variation with depth of the observed seismic velocities and elastic constants combined with Maxwell’s four thermo- dynamic relations between pressure P, volume V, entropy S (ΔS=ΔQ/T; Q: heat), and temperature T yield the predominantly radial structure of the Earth. From Maxwell’s relation (∂T/∂P)S=(∂V/∂S)P one obtains an expression for the adiabatic temperature gradient in terms of temperature, the volume coefficient of thermal expansion α=(∂V/∂T)P/V, and the isobaric specific heat capacity cP (at constant pressure): ∂αTg ()= T , ∂ (8.1) zcS P where g is gravity and subscripts P and S refer to isobaric and adiabatic conditions, respectively, i.e. constant pressure and constant
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