8 GEOTHERMAL ENERGY - GeoCom

1 PRE-PRINT Pre-Print from: Clauser, C., 2006. GEOTHERMAL ENERGY , In: K. Heinloth (ed), Landolt-B rnstein, Group VIII: Advanced Materials and Technologies, Vol. 3: ENERGY Technologies, Subvol. C: Renewable Energies, Springer Verlag, Heidelberg-Berlin, 493-604. 8 GEOTHERMAL THE EARTH S THERMAL 1 The Structure of the Earth .. 1 ENERGY Budget of the Earth .. 4 Heat Income .. 5 External Heat Sources .. 5 Internal Heat Sources .. 6 Heat 8 Heat 9 The Thermal Regime of the Earth s Crust .. 9 Heat Storage .. 10 Measuring Techniques .. 12 Calculated Heat Capacity .. 12 Heat Transport .. 23 Heat 23 Measuring Techniques ..23 Indirect 24 Thermal Conductivity of Minerals .. 28 Thermal Conductivity of Rocks .. 35 Thermal Conductivity of Sedimentary, Volcanic, Plutonic, and Metamorphic Rocks .. 36 Influence of Various Factors on Thermal Conductivity .. 46 Heat 52 Heat Radiation and Thermal Conductivity in the Earth s 56 GEOTHERMAL ENERGY 58 TYPES OF GEOTHERMAL ENERGY 60 Direct 60 Space 63 Earth Coupled Heat Extraction 64 Hydrothermal Heating 67 Commercial and Industrial Applications.

2 68 Power TECHNOLOGICAL AND ECONOMICAL ASPECTS OF GEOTHERMAL 74 Direct 75 Earth Coupled Heat Extraction 75 Hydrothermal Heating 81 Power Natural Steam Power Plants .. 83 Binary Power Plants .. 87 Power Plants for Hot Dry Rock or Enhanced GEOTHERMAL Systems .. 90 Technical, Economic, and Ecological Aspects of GEOTHERMAL Power 93 Efficiency .. 93 Cost and Life 95 Pollution .. 98 103 105 PRE-PRINT 1 Clauser, C., 2006. GEOTHERMAL ENERGY , In: K. Heinloth (Ed), Landolt-B rnstein, Group VIII "Advanced Materials and Technologies", Vol. 3 " ENERGY Technologies", Subvol. C "Renewable Energies", 480 595, Springer Verlag, Heidelberg-Berlin. 8 GEOTHERMAL ENERGY GEOTHERMAL ENERGY is the heat contained in the solid Earth and its internal fluids.

3 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?

4 (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. 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.

5 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, 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 ( S= Q/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 =( V/ T)P/V, and the isobaric specific heat capacity cP (at constant pressure): ()SPTgTzc= , ( )where g is gravity and subscripts P and S refer to isobaric and adiabatic conditions, respectively, constant pressure and constant entropy.

6 2 PRE-PRINT Clauser, C., 2006. GEOTHERMAL ENERGY , In: K. Heinloth (Ed), Landolt-B rnstein, Group VIII: Advanced Materials and Technologies , Vol. 3 ENERGY Technologies , Subvol. C Renewable Energies , 480 595, Springer Verlag, Heidelberg-Berlin. 50010001500cP (J kg-1 K-1)0100020003000400050006000 Radius (km)5 10152025 ( K-1)6000500040003000200010000 Depth (km)Gr neisenCubic Expansion050001000015000 (kg m-3)010002000300040005000600060005000400 03000200010000 Depth (km) ( )0100200300400P (GPa)0100020003000400050006000 Radius (km)024681012g (m s-2)6000500040003000200010000 Depth (km)PressureGravity050001000015000vs (m s-1)010002000300040005000600060005000400 03000200010000 Depth (km)050001000015000vp (m s-1)SonicShearHeat CapacityDensity Fig. Variation of selected properties versus depth in the Earth according to the Earth models PREM (gravity g, pressure P, coefficient of thermal volume expansion and isobaric specific heat capacity cP; Gr neisen parameter ) [1981 Dzi; data: 1992 Sta] and AK135-f (sonic and shear wave velocities vp and vs; density )[1995 Ken; 1995 Mon].

7 PRE-PRINT 3 Clauser, C., 2006. GEOTHERMAL ENERGY , In: K. Heinloth (Ed), Landolt-B rnstein, Group VIII "Advanced Materials and Technologies", Vol. 3 " ENERGY Technologies", Subvol. C "Renewable Energies", 480 595, Springer Verlag, Heidelberg-Berlin. Assuming lower mantle values (at about 1500 km depth) of T=2400 K, g= m s-2, cP=1200 J kg-1 K-1, and =14 K-1, yields an adiabatic temperature gradient of about K km-1; the corresponding values for the outer core (at about 3500 km depth) of T=4000 K, g= m s-2, cP=700 J kg-1 K-1, and =14 K-1 (Fig. ), yield an adiabatic temperature gradient of about K km-1 [1992 Sta; 1997 Low]. Approximate estimates for the adiabatic temperature inside the Earth can be obtained with the aid of the dimensionless thermodynamic Gr neisen parameter = KS /( cP), where KS is the adiabatic incom-pressibility or bulk modulus and is density (Fig.)

8 : 00Td, or: T TT = = . ( )From a known temperature T0 and density 0 at a given depth, eq. ( ) allows computing the adiabatic temperature from the density profile in a region where the Gr neisen parameter is known. Fortunately, the Gr neisen parameter does not vary too much within large regions of the Earth s interior (Fig. ). However, eq. ( ) cannot be applied across the boundaries between these domains, where is discon-tinuous. But if T0 and 0 are known at calibration points, the adiabatic temperature profile can be computed in an iterative fashion within these depth intervals. The currently accepted estimate of the temperature profile is characterized by steep gradients in the lithosphere, asthenosphere and in the lower mantle D layer (immediately above the core-mantle boundary). Neglecting large lateral variations in the crust and lithosphere it indicates, on average, temperatures of less than 1000 K in the lithosphere, close to 3750 K at the core-mantle boundary, and around 5100 K at the center of the Earth (Fig.)

9 [1992 Sta; 1997 Low;]. 7300 C010002000300040005000T (K)600050004000300020001000 Depth (km)100020003000400050006000 Radius (km)010002000300040005000T ( C)L: Lithosphere (0-80 km)A: Asthenosphere (80-220 km)TZ: Transition Zone (220-670 km)D'': D'' layer (2741-2891 km)400 km: Phase transition olivine-spinel670 km: Phase transition spinel-perovskiteD''InnerCoreOuterCoreLo werMantleTZAS olidusL63715150289167080 TemperatureUpperMantle8000 C220 Fig. Variation of estimated temperature and melting point in the Earth with depth; Data according to Stacey [1992 Sta] selected to be representative and consistent with the Preliminary Reference Earth Model (PREM) [1981 Dzi]. Temperature is poorly constrained in the deeper sections, indicated by large error bars; data: [1993 Bro]. 4 PRE-PRINT Clauser, C., 2006. GEOTHERMAL ENERGY , In: K.

10 Heinloth (Ed), Landolt-B rnstein, Group VIII: Advanced Materials and Technologies , Vol. 3 ENERGY Technologies , Subvol. C Renewable Energies , 480 595, Springer Verlag, Heidelberg-Berlin. However, there are large uncertainties, particularly in the mantle and core [1993 Bro; 2001 Bea], indicating ranges for conceivable minimum and maximum temperatures of 3000 C 4500 C at the core-mantle boundary, 4400 C 7300 C at the transition between outer and inner core, and a maximum temperature at the center of the Earth of less than 8000 C (Fig. ). From another one of Maxwell s thermodynamic relations, ( S/ P)T=-( V/ T)P, one can derive the fractional variation of the melting point temperature Tmp with depth within the Earth: ()solidliquidliquidmpmpdT1g,TdzL= ( )where L is the latent heat of fusion, and solid and liquid are the densities of the solid and liquid phases, respectively.