1 Geothermal energy

Although energy from the Earth's interior that flows though the surface is on average very low — about a thousand times less than the solar energy that falls on the surface — it is sufficiently abundant worldwide to make it locally worth exploiting. The top 3 km of the Earth's crust stores an estimated 4.3 × 10⁷ EJ of thermal energy by virtue of the temperature of rocks and their thermal capacity. Because global consumption of energy during 2002 was 451 EJ heat stored within the Earth might seem vastly more than sufficient for all humanity's needs. Stored energy is added to by heat flowing from the deeper Earth, but this heat is eventually lost by escape from the Earth's surface. In theory it would be possible to tap the stored heat, but in practice the principal geothermal potential is from heat that flows through the crust. As you will see, that is a far smaller potential resource.

Where geothermally heated water rises to the surface in hot springs it has sometimes been used directly for heating, recreational and horticultural purposes since Roman times. Geothermal energy was used indirectly for the first time when geothermally generated electricity was produced in 1904 at Larderello, Italy. In 2003 about 8 GW of geothermal electrical power capacity had been installed globally (Table 1), providing about 0.2 EJ of energy each year — a tiny fraction of global electricity consumption.

Technologically less taxing, direct use of geothermal heat contributed about the same to global energy supply annually (Table 2).

The Earth's interior becomes hotter with increasing depth for two reasons: heat is generated by the decay of the long-lived, natural radioactive isotopes (uranium-238 and uranium-235, thorium-232 and potassium-40) and heat flows from the hot interior by means of convection and conduction. The manner in which geothermal heat flows through the Earth is reflected by the geothermal gradient (the rate at which temperature increases with depth). Figure 1 shows how the geothermal gradient varies beneath the surface of a continent; note how temperature increases much less with depth in the deep mantle than it does in the lithosphere.

Figure 1 The geothermal gradient beneath a continent, showing how temperature increases more rapidly with depth in the lithosphere than it does in the deep mantle.

Although they are not molten, rocks that form the deep mantle are hot enough to behave in a ductile manner. Parts of the mantle that are hotter than their surroundings rise slowly because they are slightly less dense than cooler mantle, thereby physically transporting their heat content towards the surface. This process, called convection, is an efficient means of heat transport. As a result, the geothermal gradient in the deep mantle involves less increase in temperature with depth than occurs within the lithosphere (Figure 1).

Down to around 150 km below the surface, the Earth's lithosphere behaves in a more rigid fashion — it is this rigidity that creates the stability of tectonic plates. Except at plate boundaries and hotspots (Sheldon, 2005) convection does not transfer heat in the lithosphere. Heat is transferred instead by conduction, which is a far less efficient process than convection, so the lithosphere acts as an insulating blanket (trapping the heat rising from below). Heat transfer by conduction explains why temperature increases more rapidly with depth in the lithosphere than in the deeper mantle (Figure 1).

Harvesting geothermal energy depends on the relationship between heat flow through the lithosphere and the local geothermal gradient (Box 1).

Box 1 Heat conduction

The origin of a geothermal resource relates to the following equation for conductive heat flow:

where q is heat flow (in W m⁻²); k is thermal conductivity (in W m⁻¹ K⁻¹); and ΔT is the temperature difference (in kelvin or K = °C + 273 °C) through a depth z. So, the geothermal gradient (Δ T/z ) has units of K km⁻¹. Thermal conductivity expresses the ease with which a material transmits heat. Thus a metal pan has a high thermal conductivity whereas an oven glove is a poor conductor of heat. All rocks are poor conductors of heat in the everyday sense, but some rocks, such as sandstones and granites, are better conductors of heat than others, such as shales and many metamorphic rocks.

• If heat flow q in- Equation 2 is constant will the geothermal gradient (ΔT/z) be greater where the thermal conductivity (k) is high (good conductor) or where it is low (poor conductor)?
• For a higher value of ΔT/z, at constant q a low thermal conductivity is required.

For example, if heat flow (q) is 100 mW m⁻² (100 × 10⁻³ W m⁻²), as it could be in some volcanically active areas, and the geothermal gradient is 100 K km⁻¹, i.e. the temperature difference ΔT between the surface and at 1 km depth is 100 K, then thermal conductivity k can be calculated from Equation 2:

Rearranging gives:

If the geothermal gradient were less, say 50 K km⁻¹, then the same heat flow could only be produced if it passed through rocks with a thermal conductivity given by:

i.e. if the geothermal gradient is halved, for the same heat flow the thermal conductivity must be doubled.

The most important practical factors involved in assessing an area's potential are:

• the presence of magmas close to the surface in volcanically active regions;
• high heat flow in near-surface solid rocks;
• the ability to transfer geothermal heat to power plants and other energy consuming technologies efficiently, usually as hot water or steam.

The most favourable areas for geothermal exploitation might seem to be those in volcanically active regions, at destructive and constructive plate margins. The distribution of geothermal electricity generation plants shown in Figure 2 confirms this. What is less obvious is that the geothermal potential of some areas that are remote from volcanically active regions is also worth exploiting. The geothermal energy exploited directly by two of the major users, China and Turkey (Table 2), is not related to active volcanism, but to abnormally high heat flow.

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Figure 2 The main sites of geothermal electricity production in 2003 in relation to plate boundaries.

The local potential of geothermal energy is fundamentally a function of the enthalpy of the area, i.e. the total energy content of the geothermal system that lies below it. The concept of enthalpy is

As you will see shortly, both the heat content (U ) and the pressure (P ) of steam in a geothermal system play a role in converting geothermal energy into electricity.

Enthalpy cannot be measured directly for a geothermal energy source. However, it can be estimated from the heat flow through the crust, which is measurable at the surface. Heat flow is expressed as the geothermal power associated with an area of the surface, measured in mW m⁻². Figure 3 shows that large areas of the Earth's surface have much the same heat flow. However, the temperature at depth in the crust depends on how conductive different rocks are, and this in turn depends on their physical properties. This subsurface temperature is crucial in evaluating whether or not an area constitutes a geothermal resource (Box 1).

Figure 3 World map showing variation in surface heat flow in mW m⁻² in relation to continental and oceanic crust, and major plate boundaries. The colours relate to the scale in mW m⁻². Note that there are areas in continents where heat flow does rise to very high values, but they are too small to show on this map.

Figure 3 shows that on a global scale, the main areas where heat flow is much greater than the global average of 60 mW m⁻² are close to constructive plate margins beneath the oceans. In volcanically active continental areas that lie above subduction zones, as around the Pacific, regional heat flow can be above the average, but only up to 120 mW m⁻². However, values on the continents can be as high as 300 mW m⁻² where magma is being generated locally. Such continental occurrences and volcanically active oceanic islands are the most favourable for geothermal exploitation, for obvious reasons.

Specialists recognize three kinds of area with geothermal potential; those with high, medium and low enthalpy. High-enthalpy areas are those where subterranean water and steam are at temperatures greater than 180 °C, medium-enthalpy areas those where temperatures range between the boiling temperature of water (100 °C) and 180 °C, and low-enthalpy areas where temperatures are lower than 100 °C. A mass of pressurized steam has an energy content that includes the pressure-volume (PV) term in Equation 1, and also contains the heat involved in converting liquid water into gas (the latent heat of vaporization).

Using geothermal energy effectively speeds up heat flow and locally increases the heat lost by the Earth; it is akin to mining. But geothermal heat is continually supplied, so the 'mining' depletes hot water and steam rather than the energy resource itself. However, if hot fluids removed through one borehole are replaced by cold water pumped in through another to recharge the geothermal field, the cold water heats up again. Carefully managed recharge enables the enthalpy of an area to be exploited continuously.

To allow their successful exploitation, geothermal fields usually need three characteristics:

1. An energy source to heat the fluids;

2. An aquifer (see Smith, 2005), either natural or artificially created , to act as a reservoir for fluids that can transport heat energy to the surface;

3. A cap rock with low permeability to seal in these geothermal fluids — in a similar way to the trapping of petroleum.