A geothermal system requires heat, permeability, and water. The heat from the Earth's core continuously flows outward. Sometimes the heat, as magma, reaches the surface as lava, but it usually remains below the Earth's crust, heating nearby rock and water, sometimes to levels as hot as 700 degrees F. When water is heated by the earth's heat, hot water or steam can be trapped in permeable and porous rocks under a layer of impermeable rock and a geothermal reservoir can form. This hot geothermal water can manifest itself on the surface as hot springs or geysers, but most of it stays deep underground, trapped in cracks and porous rock. This natural collection of hot water is called a geothermal reservoir.
To develop electricity from geothermal resources, wells are drilled into a geothermal reservoir. The wells bring the geothermal water to the surface, where its heat energy is converted into electricity at a geothermal power plant.
There are four commercial types of geothermal power plants: flash power plants; dry steam power plants; binary power plants; and flash/binary combined power plants.
Flash Power Plant. Geothermally heated water under pressure is separated in a surface vessel (called a steam separator) into steam and hot water. The steam is delivered to the turbine, and the turbine powers a generator. The liquid is injected back into the reservoir.
Dry Steam Power Plant. Steam is produced directly from the geothermal reservoir to run the turbines that power the generator, and no separation is necessary because wells only produce steam.
Binary Power Plant. Recent advances in geothermal technology have made possible the economic production of electricity from geothermal resources lower than 150 degrees C (302 degrees F). Known as binary geothermal plants, the facilities that make this possible reduce geothermal energy's already low emission rate to zero. Binary plants typically use an Organic Rankine Cycle system. The geothermal water (called geothermal fluid) heats another liquid, such as isobutane or other organic fluids such as pentafluoropropane, which boils at a lower temperature than water. The two liquids are kept completely separate through the use of a heat exchanger, which transfers the heat energy from the geothermal water to the working fluid. The secondary fluid expands into gaseous vapor. The force of the expanding vapor, like steam, turns the turbines that power the generators. All of the produced geothermal water is injected back into the reservoir.
Flash/Binary Combined Cycle. This type of plant, which uses a combination of flash and binary technology, has been used effectively to take advantage of the benefits of both technologies. In this type of plant, the portion of the geothermal water that flashes to steam under reduced pressure is first converted to electricity with a backpressure steam turbine and the low-pressure steam exiting the backpressure turbine is condensed in a binary system.
The geothermal power production in the U.S. today provides enough electricity to meet the electricity needs of about 2.4 million California households. In 2007, geothermal was the fourth largest source of renewable energy in the U.S. Today the U.S. has about 3,000 MW of geothermal electricity connected to the grid. Geothermal energy generated 14,885 gigawatt-hours (GWh) of electricity in 2007, which accounted for 4 percent of renewable energy-based electricity consumption in the U.S. (including large hydropower).
The U.S. continues to produce more geothermal electricity than any other country, comprising approximately 30 percent of the world total. In California, the state with the largest amount of geothermal power on line, electricity from geothermal resources accounted for 5 percent of the state's electricity generation in 2003 on a per kilowatt-hour basis. Geothermal is the largest non-hydro renewable energy source in the state, significantly exceeding the contribution of wind and solar combined.
As of August 2008, almost 4,000 MW of new geothermal power plant capacity was under development in the U.S. (this includes projects in the initial development phases). Those states with projects currently under consideration or development are: Alaska, Arizona, California, Colorado, Florida, Hawaii, Idaho, Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming. Combined, these states have approximately 103 projects in development ranging from initial to advanced stages.
Direct use applications of geothermal energy occur today in 26 states, almost as many states as produce coal. New direct use projects are encouraged by the provisions of the Geothermal Steam Act Amendments passed by Congress in 2005. There is interest in new direct use projects in numerous states and on various Indian reservations within several states.