Geothermal Energy

Published: November 25, 2010 12:00 pm

Updated: October 30, 2016 10:14 am

Author: Arnold Bloom

Author: C. Michael Hogan

Topic Editor: Margaret Swisher

Topics: Geothermal (main)

Geothermal energy is any of several techniques used to capture the thermal energy beneath the Earth surface and that energy for power generation or direct heat transfer to serve human use. The two most common forms of geothermal energy are: (1) geothermal energy production and (2) direct use of warm water beneath the Earth crust to heat buildings.

Temperatures from the surface to the center of the Earth warm by 17° to 30°Celsius for every kilometre in depth, reaching 5100°C in the inner core, almost the temperature of the sun.(Alfe et al, 2007) The energy that heats Earth’s interior derives from several sources. First is the decay of radioactive isotopes, especially ²³⁸uranium, ²³⁵uranium, ²³²thorium, and ⁴⁰potassium. Second, because the Earth first formed regardless of density, heavy metals (like iron, nickel, and copper) are continually sinking towards the center of the Earth, displacing lighter elements (like aluminum, sulfur, and silicon), creating friction and heat. Third, the Earth’s interior still retains some of the energy from its formation 4.5 billion years ago. (Bloom, 2010)

The Earth crust varies in thickness from about 5 kilometres to 75 kilometres. Locations with thinner crust, such as at boundaries of tectonic plates, fault lines, and volcanoes, receive more heat from the interior and are promising sites for geothermal power. For example, California, which lies at the intersect between the Pacific and North American tectonic plates, has the world’s highest number geothermal power plants which supply seven percent of the California’s electricity.

Geothermal Electricity Generation

A geothermal power plant pumps water down a deep injection well. This water flows through fractures in the fiery brimstone and heats up until it escapes up a second borehole, the production well, as steam or superheated water (depending on temperature and pressure). If the production well outflow is steam, it may directly drive a turbine in the power plant (dry steam type). If the outflow is super-heated water, a release of pressure generates steam that may drive a turbine (flash-steam type). Alternatively, superheated water from the production well may pass through a heat exchanger that vaporizes a fluid with a lower boiling point such as butane, which then drives a turbine and returns to the heat exchanger (closed-loop binary type). A condenser and cooling tower lower the temperature of the water before it returns back down the injection well. Boreholes for the wells are lined with metal or concrete near the top to prevent water from penetrating through porous rock. A makeup tank replenishes the water lost during its passage through the system.

Most commercial geothermal wells are shallower than three kilometres. (Bertani, 2005) The petroleum industry is developing more economical methods for drilling oil or gas wells 6 kilometres to 10 kilometres deep. (MIT Study Group, 2006) Transfer of this technology can reduce the cost of constructing deeper, hotter wells.

A geothermal power plant works by pumping water down a deep injection well. The water heats up and escapes into the production well as steam or super-heated water.

Direct Use of Deep Warm Water

This renewable energy technology can be divided into two branches. The simplest manner is, in some locations such as Iceland, hot water emerges into the atmosphere due to very hot water under pressure quite near the Earth surface. In Iceland, for example much of this heat can be used to create hot water and home heating quite directly.

The more universal version of this technology is to drill a casing roughly 200 to 1000 metres into the earth, allowing for two way flow of water. Water pumped upward produces usable water warmer than temperatures at the Earth surface (or building into which the water is pumped). Return downward flow is the same water stream, but after the warm upward stream has been used to heat the subject building. Where subsurface conditions are favourable, a simple one directional loop can be employed with a lateral subsurface geometry.

The success of direct use of deep warm water is dependent upon suitable subsurface geology and also the magnitude of the vertical temperature gradient. Considerable research and pilot projects are taking place in the United Kingdom, (Scottish Government, 2013), USA and Australia. Deep water exchange systems have been in use since the 1940s, and generally have payback periods of six to nine years.

Comparison with other Energy Sources

Nuclear Energy

Nuclear energy is generally about the same safety regime as geothermal, although radioactive waste is produced with nuclear; fusion technology, developing to commercial scale use, will generally be superior to geothermal in overall safety, and fusion nuclear has zero radioactive waste. Both types of nuclear have a smaller footprint of environmental damage compared to geothermal. Nuclear energy can also be classified as more reliable, since two adjacent properties may have to compete for subsurface warm water when in situ geothermal is developed. However, nuclear power suffers from wheeling losses in power transmission compared with in situ geothermal that uses deep warm water.

Solar Energy

Solar energy is much less reliable, since its efficacy is low or non-existence in marginal sunlight or night conditions. Furthermore, solar panels include considerable amounts of arsenic and other toxins, and most countries do not have clear recycling standards to preclude arsenic from reaching groundwater and soil. In fact, solar energy produces about 300 times the amount of toxic waste per kilowatt hour delivered compared to nuclear. Maintenance costs and lifetime of solar systems is inferior to geothermal. Habitat impacts of solar farms are quite high, although rooftop solar eliminates these impacts; habitat impacts are particularly significant in desert habitats, due to the fragile nature of the desert soil crust. Solar energy also suffers from power wheeling losses compared to in situ geothermal.

Wind Energy

Wind energy is less reliable than geothermal, since it depends on achievement of threshold winds. The surface environmental footprint of windfarms is much higher than geothermal, and wind energy kills at least five million birds per year in North America alone. (Smallwood, 2007) Payback time of commercial installations is generally similar to geothermal. Wind energy also suffers from wheeling losses compared to in situ geothermal.

Natural Gas

Natural gas has a similar safety record to geothermal, and is also extremely reliable. Natural gas produces moderate amounts of carbon dioxide emissions, but much lower than coal or oil generation. Natural gas plants also suffer from wheeling losses to deliver power to end users.

Hydroelectric

Hydroelectric has a much poorer safety record than all of the above energy generation sources, chiefly due to the large average number of deaths worldwide from dam failures. Hydroelectric has a slightly poorer dependability due to some reservoirs having seasonal volume variation. The habitat damage of hydroelectric is generally one of the highest impact classes of all energy generation. Hydroelectric also suffers from wheeling losses to get power to end users.

References

• Alfe, D., M. J. Gillan, and G. D. Price (2007) Temperature and composition of the Earth's core. Contemporary Physics 48:63-80 doi:Doi 10.1080/00107510701529653.

• Bertani, R. (2005) World geothermal power generation in the period 2001-2005. Geothermics 34:651-690.
• Arnold J. Bloom. (2010) Global Climate Change: Convergence of Disciplines. from UCVerse of the University of California.
• R. Harrison, ‎N. D. Mortimer, ‎O. B. Smarason (2013) Geothermal Heating: A Handbook of Engineering Economics. Elsevier Science. ISBN:9781483287454, 1483287459. 572 pages

• MIT Study Group (2006) The Future of Geothermal Energy, Massachusetts Institute of Technology, Cambridge, MA, http://www1.eere.energy.gov/geothermal/pdfs/future_geo_energy.pdf.
• Scottish Government (2013) Potential for deep geothermal energy in Scotland. Study volume 1. ISBN: 9781782569855
• K. Shawn Smallwood. 2007. Estimating Wind Turbine-Caused Bird Mortality. The Journal of Wildlife Management. Volume 71, Issue 8, pages 2781–2791

Citation

Arnold Bloom & C. Michael Hogan (2010, updated 2016). Geothermal Energy. ed. Margaret Swisher. Encyclopedia of Earth. National Council for Science and Environment. Washington DC