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I have heard that geothermal heating is a way of generating energy from the temperature difference between the inner layers of the Earth and the Earth's crust. How is it possible to extract this energy? Must one be near a tectonic plate boundary to gain access to this temperature gradient?

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  • $\begingroup$ It goes better near a tectonic plate boundary, but it is possible elsewhere too. See a paper on a drill near my home. $\endgroup$ – Pavel V. Apr 16 '14 at 10:40
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    $\begingroup$ Can you clarify a bit? Geothermal heating is one thing, generating electricity is something else, 'generating energy' is vague and sounds physically impossible. What's more, 'inner layers of the Earth' vs 'Earth's crust' is a confusing way to put it — we certainly don't exploit any temperature gradients on that scale. $\endgroup$ – kwinkunks Apr 21 '14 at 16:28
  • $\begingroup$ Agree with @kwinkunks. The heat gradient is actually in the first few kilometers of the crust. Anything deeper than that and we wouldn't be able to dig for it. $\endgroup$ – Leo Uieda May 15 '14 at 18:50
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To explain the earth science bit (how a heat exchanger works is beyond the scope of the site). The Earth is essentially cooling, losing heat from the interior by conduction to space. this heat flux can be used to heat, for example water at depth since temperature increases with (significant) depth in the earth's crust. It is not necessary to be near a plate boundary although such locations may have anomalously large heat fluxes. A good example is Iceland. For an igneous plate, heat fluxes may average just above 40 milliwatts per square meter. In addition to pure heat flow out from the crust, heat is also generated within the bedrock due to radioactive decay. Measurements in Scandinavia shows that heat fluxes may vary between 30 and 82 miliwatt per square meter (see article).

Hence there is an from a human perspective inexhaustible energy source that can be tapped into. the point is to use the ground eat to warm some liquid, usually water and use a heat exchanger (essentially a revered refrigerator to extract, for example, heat for heating or hot water.

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There are currently two ways to extract geothermal energy: one mainstream, one still at the experimental / demonstration level.

Pollack et al (2010) estimate the global geoethermal heat loss at 44 TW (by comparison, civilisation's rate of energy use is about 20 TW).

As the USA Department of Energy notes, all methods rely on the combination of three factors:

  • heat underground,
  • fluid to carry the heat to the surface, and
  • permeability to allow the fluid to flow.

The mainstream geothermal method

This happens close to tectonic plate boundaries. Reservoirs of hot liquid are tapped, to bring the hot fluids to the surface. Typically the temperature of the fluids is well over 100 °C;, and can be used for electricity generation, as well as for heating.

The experimental method is Hot Dry Rock

aka Enhanced Geothermal System or EGS.

As Pavel V. commented, this is being piloted in the Czech Republic, amongst other places, and allows geothermal heat extraction from places that are not close to tectonic plate boundaries. From that link:

the projected depth of the ... borehole was between 2100 and 2500 m. The drilling diameter ranges from 393,7 mm for the starting upper section to 152 mm at the borehole bottom

One tube is placed inside another, inside the borehole. Water can be injected down the inner tube, and returned up through the gap between tubes, or the other way around.

That paper reproduces the following chart from Šafanda, J., Dědeček, P., Krešl, M., Čermák, V. [2007] Report from geothermal research for PVGT-LT1 (in Czech):

enter image description here

As you can see, these temperatures are much lower, so are unsuitable for electricity generation, because one must accept very low efficiencies (Carnot's Law). They are used for space- and water-heating.

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  • $\begingroup$ This answer seems incomplete. There are more than two ways to exploit the earth's heat. When I first read the question, I thought the OP was referring to a ground source heat pump, sometimes called a geothermal heat pump. You have also not mentioned direct use of geothermal fluids for heating or low-temperature power generation using volatile working fluids. Last thing: you can do conventional geothermal far from plate boundaries. $\endgroup$ – kwinkunks Apr 21 '14 at 16:38
  • $\begingroup$ @kwinkunks A GSHP is not geothermal: it pumps solar energy captured in the earth. $\endgroup$ – EnergyNumbers Apr 22 '14 at 7:51
  • $\begingroup$ I know what it is, my point was that it's often (usually, in my experience) called a geothermal heat pump and is therefore often confused with geothermal power. My first reading of the question made me think the OP might be talking about that. But you were quite right to leave it out of your answer. $\endgroup$ – kwinkunks Apr 22 '14 at 10:43

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