Geothermal Energy

Geothermal energy is a renewable resource that is generated by natural processes within the Earth. Since prehistoric times, humans have enjoyed the recreational and perceived therapeutic properties of surface geysers and thermal hot springs. It has also been used to produce electricity on an industrial scale for more than 100 years. Geothermal energy provides a viable, sustainable, lowcarbon alternative to fossil fuels. Furthermore, the potential global energy production capacity of the Earth’s exploitable geothermal resources far exceeds both current and predicted future primary energy demand. However, in 2010, geothermal energy production comprised less than 1 percent of total global primary energy consumption.

The concept of geothermal energy raises some important questions: What is geothermal energy and under what conditions can it be exploited? What are some direct and indirect strategies for harnessing geothermal energy? What are some of the benefits and limitations of geothermal energy? And finally, what are the prospects for future development of this abundant and valuable energy source?

Geothermal Energy Basics

Over 99 percent of the Earth’s mass is extremely hot—above 1,830 degrees F (1,000 degrees C). In general, temperature increases with depth. The core of the planet, which is about 2,174 mi. (3,500 km) in radius, is composed of a solid iron inner core and a superheated liquid outer core that generates a magnetic field. The inner and outer cores are enveloped by another region called the mantle, which is about 1,800 mi. (2,900 km) thick. The mantle, in turn, is enclosed by an extremely thin and largely impervious outer crust (12–40 mi. or 20–65 km thick in continental areas, less in ocean regions), which in relative terms is analogous to the skin of an apple. In most places, the outer crust blocks a significant amount of heat from rising to the surface. However, there are some locations, such as a space between tectonic plates or active volcano systems, where the outer crust is unusually thin, permeable, or cracked, and heat can rise close enough to the planet’s exterior to allow human exploitation. Commercially exploitable resources are generally 1.8 mi. (3,000 m) or less below the Earth’s surface.

Besides heat and close proximity to the Earth’s surface, exploitable geothermal systems also require water and permeability. Hot water or steam is sometimes trapped in permeable rock formations, forming geothermal reservoirs below layers of impermeable crust. In some cases, these reservoirs can be accessed via geothermal wells; in other situations, the hot water or steam may seep to the surface naturally. It is also possible to artificially introduce water into a field of fractured hot rock where it is heated and then extracted by a production well.