18-7 Geothermal Energy

What Is Geothermal Energy? Tapping the Earth’s Internal Heat

Geothermal energy consists of heat stored in soil, underground rocks, and fluids in the Earth’s mantle. Examples are volcanic rock, geysers, and hot springs. Scientists have developed several ways to tap into this stored energy to heat and cool buildings and to produce electricity.

A related way to heat or cool a building is geothermal exchange or geoexchange. It involves using buried pipes filled with a fluid to move heat in or out of the ground depending on the season and the heating or cooling requirements. In the winter, for example, heat is removed from fluid in pipes buried in the ground and blown through house ducts. In the summer, this process is reversed.

We have also learned to tap into deeper and more concentrated underground reservoirs of geothermal energy:

  • One type of reservoir contains dry steam with water vapour but no water droplets.

  • Another consists of wet steam, a mixture of water vapour and water droplets.

  • The third is hot water trapped in fractured or porous rock at various places in the Earth’s crust.

There are three other nearly nondepletable sources of geothermal energy:

  • One is molten rock (magma).

  • Another is hot dry-rock zones, where molten rock that has penetrated the Earth’s crust heats subsurface rock to high temperatures.

  • A third source is low- to moderate-temperature warm-rock reservoir deposits.

But geothermal energy has two main problems:

  1. One is that the cost of tapping large-scale reservoirs of geothermal energy is too high for all but the most concentrated and accessible sources. New technologies may bring these costs down.

  2. The other is that some dry or wet-steam geothermal reservoirs can be depleted if heat is removed faster than natural processes renew it. Thus geothermal resources can be nonrenewable on a human time scale, but the potential supply is so vast that it is usually classified as a renewable energy resource.