Geothermal resources refer to the heat energy generated in Earth. Though it is more a potential than a reality as of 2008, geothermal resources can also refer to heat energy that can be gathered from the atmosphere and ocean. The energy stored in the hot water present at sites where steam vents out from the subsurface is an example of a geothermal resource.
As of 2008, global geothermal energy production is approximately 8,000 megawatts (MW), of which about 2,800 MW is produced in the United States, and which represents the third-largest renewable energy source in the United States after hydroelectricity and the burning of biofuels. Still, this represents less than 1% of the total energy production in the world. Interest in geothermal energy is growing because, in contrast to the energy produced by the burning of fossil fuels, it is a renewable resource and can be a source of much less carbon dioxide (CO2) than conventional sources of energy.
The reduction in carbon dioxide is beneficial in helping curb global warming, which is the warming of Earth’s atmosphere and oceans that has been occurring since the mid-nineteenth century and accelerating since the mid-twentieth century.
Historical Background and Scientific Foundations
The use of geothermal energy by humans dates back millennia. For example, the water in Roman bathhouses was heated in this way. In ancient Rome and in other societies, the soothing effects of hot springs were known over 2,000 years ago. Indeed, the term “spa” comes from the location of a hot spring in the community of Spa, Belgium.
In New Zealand and Iceland, where regions having many steam vents (steam fields) are found, geothermal heat has been used for centuries to cook food. As well, archeological evidence indicates that aboriginal North Americans who lived in what is now Yellowstone National Park (Wyoming) also used geothermal energy for cooking and as heat for their personal comfort.
Greater awareness of the higher temperature beneath Earth’s surface came about in the sixteenth century, when mining began. Then as now, the air in mines dug several hundred feet deep grew warmer with depth. By the early nineteenth century, geothermal energy was being used for more than cooking and personal comfort. For example, in Italy, the chemical extraction of boric acid (H3BO3) from the heated water was done in specially designed facilities. At about the same time, steam energy from vents was harnessed to power pumps and winches.
The first geothermal power plant in the United States began operation in 1892 in Boise, Idaho. By the first decade of the twentieth century, electrical generation from geothermal steam was being explored. Over the next 50 years, geothermal power plants were established in Italy, Japan, New Zealand, Mexico, Iceland, and the United States, as only a few examples.
As of 2008, the United States is the world’s leader in geothermal energy production, and The Geysers, a steam field located north of San Francisco, California, is the largest steam field in the world.
Geothermal energy is created by heat from the core of Earth. The core is located almost 3,700 mi (6,000 km) below the surface. Temperatures in the core reach 9,000°F (5,000°C), which is high enough to melt the rock. The melted rock is called magma and is familiar as the molten material ejected from erupting volcanoes. Volcanoes are one example of regions of Earth where the magma can contact the surface, through cracks in the overlying surface crust (the mantle).
In other areas, the magma comes close to the surface and can heat the underground water. The steaming hot water finds routes to the surface through cracks in the rock; these are the vents that can be harnessed to provide geothermal power. Areas where many vents are found are called steam fields. Another approach is to drill wells down to reservoirs of the hot water, which can then be pumped up, or, if the underground pressure is sufficient, spontaneously pushed upward to the surface.
A geothermal facility such as The Geysers is an example of a steam plant. The extremely hot water (over 572°F, or 300°C) is used to turn turbines, which in turn drive generators to create electricity. The main byproduct of the process is water vapor; the amounts of undesirable carbon dioxide, nitric oxide (NO), and sulfur (S) are nearly 50 times less than the amounts emitted by conventional electricity generation involving the burning of fossil fuels such as coal.
A second geothermal process uses facilities called binary plants. The binary (or secondary) process uses water that is not nearly as hot, typically 100-300°F (40-150°C). The water is passed through a heat exchanger in one pipe. An adjacent pipe contains a fluid that has a lower boiling point (typically a hydrocarbon such as isobutane or isopentane) and is flowing in the opposite direction. Some of the heat given off by the hot water is transferred to the other fluid, which forms a vapor. The vapor is what powers the turbine, leading to
WORDS TO KNOW
FOSSIL FUEL: Hydrocarbon fuel that has been obtained from the death and decay of living matter millions of years ago.
HOT SPRING: A spring produced by superheated water emerging from Earth’s crust.
MAGMA: Molten rock formed in the interior of Earth.
RENEWABLE ENERGY: Energy that can be naturally replenished. In contrast, fossil fuel energy is nonrenewable.
the generation of electricity. The geo-heated water is returned underground and is eventually cycled through the system again. Because a binary system is a closed system, there are no emissions.
Although geothermal plants offer the advantages of renewable energy with few emissions, such larger-scale plants do have challenges. The facility needs to be designed so as not to deplete the available water underground, especially if the water is not cycled back underground. As well, the design of some operations involves the injection of water to heat the water on underground hot rocks; this can destabilize the ground and needs to be done carefully.
Impacts and Issues
Geothermal energy can be an efficient and economical way to heat a house in winter and cool it in the summer. Pipes that are laid underground circulate water, which warms to the temperature of the surrounding ground. Since the temperature 4 to 6 ft (1.2 to 1.8 m) underground tends to remain constant through the year (about 55°F or 13°C in cooler regions of North America, for example), the water will be relatively warmer than the outside air in the winter and relatively cooler in the summer.
In houses and other applications, the economy of geothermal energy has long been the main attraction. Now, with the recognition that emission of compounds including carbon dioxide is driving atmospheric warming, geothermal resources are being increasingly appreciated for their environmental value.
In 2000 the U.S. government announced a plan to develop geothermal resources in the western states. Already, the states of California, Utah, and Nevada (along with Hawaii) generate over $1 billion of electricity each year, equivalent to 60 million barrels of oil. By 2020 it is anticipated that 30% of the energy needs of the western United States will be met by geothermal energy.
DiPippo, Ronald. Geothermal Power Plants: Principles, Applications, Case Studies and Environmental Impact. Portsmouth, NH: Butterworth-Heinemann, 2007.
Gupta, Harsh, and Roy Sukanta. Geothermal Energy: An Alternative Resource for the 21st Century. Boston: Elsevier Science, 2006.
Kruger, Paul. Alternative Energy Resources: The Quest for Sustainable Energy. New York: Wiley, 2006.
Brian D. Hoyle