GEOTHERMAL ENERGY

Geothermal energy is heat from within the earth. We can use the steam and hot water produced inside the earth to heat buildings or to generate electricity. Geothermal energy is a renewable energy source because the water is replenished by rainfall and the heat is continuously produced inside the earth.

It has been in use for centuries when natural hot springs were first used for cooking and bathing. Some innovative bathers in ancient Rome realized that the thermal waters welling up in their bathhouses could be used not only for bathing but to heat the bathhouses, as well.

Following the lead of their ancestors in the bathhouses, Italians in Tuscany were the first to generate electricity from geothermal water in 1904.

Most geothermal reservoirs are deep underground with no visible clues showing above ground. The most active geothermal resources are usually found along major plate boundaries where earthquakes and volcanoes are concentrated. Geologists use different methods to look for geothermal reservoirs. Drilling a well and testing the temperature deep underground is the only way to be sure a geothermal reservoir really exists.

Most of the geothermal reservoirs in the United States are located in the western states, Alaska, and Hawaii. California is the state that generates the most electricity from geothermal energy.

The geothermal heat pumps that use earth energy sources to supply direct heat to homes are the most efficient technology currently available for heating and cooling. They are actually net producers of energy, delivering 3 to 4 times more energy than they consume. They can reduce the peak generating capacity for residential installations by 1-5 kW and can be used effectively even with a wide range of ground temperatures.

Electricity generation usually requires higher temperature fluids, above 140 degrees Celsius, although in California electricity is currently being generated using geothermal water resources that are as low as 100 degrees Celsius.

Geothermal power plants use wells to draw water from depths of 1 to 3 kilometers and then produce electricity in one of two types of plants.

1. Steam turbine plants release the pressure on the water at the surface of the well in a flash tank where some of the water "flashes" or explosively boils to steam.  The steam then turns a turbine engine which drives a generator to produce electricity. The water that does not boil to steam is injected back into the ground to maintain the pressure of the reservoir.
2. The second type of plant is called a binary plant. Instead of being flashed to steam, the water actually heats a secondary working fluid such as isobutane or isopentane through a heat exchanger. This secondary fluid is then vaporized and sent through a turbine to turn a generator after which it is cooled and condensed into a liquid again. It is then sent back through the heat exchanger to be vaporized again - it is not consumed in the process. The water is injected back into the reservoir to recharge the system.

Because the working fluids vaporize at lower temperatures than water, binary plants can produce electricity from lower temperature geothermal resources. Although binary plants are more expensive to build than steam-turbine plants, they are becoming more common.

Globally, geothermal power plants supply about 8,000 megawatts of electricity and are operating in several countries, including China, Costa Rica, El Salvador, Iceland, Indonesia, Italy, Japan, Kenya, Mexico, New Zealand, the Philippines, Romania, Russia, Turkey, and the US.

Geothermal energy has many advantages over conventional energy sources. They are located throughout the world and are particularly abundant in many developing countries where there is growing demand for energy services.

Geothermal power plants are extremely reliable and flexible. According to the U.S. Environmental Protection Agency (EPA), geothermal heat pumps are the most energy-efficient, environmentally clean, and cost-effective systems for temperature control.
Geothermal power has environmental benefits, as well. Geothermal power plants have very low emissions of the sulfur oxide and nitrogen oxide that cause acid rain and the carbon dioxide that contributes to climate change.

Geothermal power plants use only a fraction of the land that is needed for plants that supply power from other energy sources - and that same land can be used simultaneously for other purposes (such as agriculture) with little interference or chance of accidents. In the Imperial Valley of Southern California, which is one of the most productive agricultural areas in the US, 15 individual geothermal plants currently produce 400 MW of electrical power.

While geothermal energy may not be able to supply all the world's clean energy needs, the large geothermal systems operating now are certainly considered "world-class" energy resources, and there is potential for significant expansion.

Even though geothermal energy is technically a finite resource, the typical lifetime for geothermal activity around magmatic centers - from 5,000 years to 1,000,000 years - is so long that it is considered a renewable resource.

Using a geothermal source for commercial purposes, however, does affect its lifetime. While geothermal reservoirs do recharge naturally at a rate of anywhere from a few to over 1,000 thermal megawatts, in order for it to be economically feasible to use the heat commercially, it must be drawn at a faster rate. However, after a particular system is no longer able to supply heat at temperatures hot enough for electric power, using it for direct-heat applications, which can use lower-temperature heat, can extend its lifetime as a useful source of energy.

While it is difficult initially to know how long a particular system will be productive as an energy source, it is easier to project as a production record is established. It is also possible to gauge how long a given reservoir might be tapped by estimating the thermal resource found in the rocks where most of the heat is concentrated.

Touted as the next big thing in the energy world by both environmental groups and industry, hydrogen fuel cells still have some major technological hurdles to clear. For example, there is currently no infrastructure for the delivery of hydrogen. Storage of such a volatile gas also presents problems.

Additional Resources

US Department of Energy: Geothermal Topics
List of websites and reports dealing with geothermal power.

Hawaii Geothermal Page
Hawaii has great geothermal potential, some of which is already utilized. The state maintains this site to disseminate information about the process.

International Geothermal Association
A group which seeks to encourage research, development and utilization of geothermal resources worldwide through the compilation, publication and dissemination of scientific and technical data and information, both within the community of geothermal specialists and between geothermal specialists and the general public.

Stanford Geothermal Program
Stanford University's program for research and development of geothermal technologies

International Ground Source Heat Pump Association
Non-profit based at Oklahoma State University. This site was designed to educate the public and connect it with professional heat pump installers. Ground Source Heat Pumps differ from conventional geothermal electricity generation in that they are used for heating and cooling, not power production.