Chapter 8:
Geothermal

Credit: Unsplash

Abstract

Contents

Abstract

Background

Geothermal Systems

Natural Geothermal Systems

Enhanced Geothermal Systems

Status

Heat Pumps

Technology

Status

Conclusions

Credit: U.S. Geological Service

Figure 9.4

Enhanced Geothermal System Process Sketch

Status

The process shown in Figure 9.4 works as follows.

 

  1. An exploration injection well (the right hand side of the sketch) is drilled. It is used to gather data to do with rock porosity and the temperature gradient.

  2. If the exploration results are positive, water is pumped down the injection well.

  3. The water enters a zone where the rocks have high permeability. It fills the spaces in the rock formation. The high pressure of the water causes the rocks to fracture, thus opening up more space for the water to be stored. (This process is analogous to the hydraulic fracturing (“fracking”) and horizontal drilling that is used to extract tight oil. However, there is no need to use a chemical cocktail to break the rock formation.)

  4. The water fills the newly created reservoir. It is prevented from migrating upwards due to the presence of an impermeable rock layer.

  5. The temperature of the water increases as it absorbs heat from the surrounding rocks.

  6. A second well is drilled (the red line in the sketch). The distance between the cold and hot water bores will vary depending on the rock formation, but a value of 1 km is representative. (Two or three production wells may be drilled so as to capture the hot water from different locations.)

  7. The hot water flows up to the surface, where it heats up a circulating liquid such as butane or isopentane. The butane is vaporized across an expansion valve and drives a generator that makes electricity. (This is known as the Rankine or binary cycle.)

  8. The warm water that leaves the heat exchanger is used for space heating.

  9. It is then reinjected into the well.

  10. Makeup water is added as needed.

 

Technical difficulties that may need to be overcome include the following.

 

  • The newly opened fractures may close as the pressure in the well declines.

  • The water may dissolve salts present in the rock. These sales could precipitate in the piping and valves of the water circulation system. They may also be corrosive. Bacteria formation could create a similar set of difficulties.

  • The creation of underground fractures could lead to seismic events that could be felt at the surface.

  • Water may short circuit between the injection and production wells, and so not pick up the desired amount of heat.

  • Attractive geothermal sites are often located a long distance away from the power that they generate.

In other words, the development of geothermal well is expensive and risky, just like the development of an oil well, but the final product is much less energy dense.

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