IV.

Geothermal Power Plants

Geothermal reservoirs within about 5 km (about 3 mi) of Earth’s surface can be reached by drilling a well. The hot water or steam from wells can be used to turn turbine generators to produce electricity. A power plant that uses this natural source of hot water or steam is called a geothermal power plant.

At the beginning of the 21st century, there were some 380 geothermal power plants in 22 countries around the world with a combined installed capacity of about 8,000 megawatts. Geothermal energy provided 1.6 percent of the world’s total electricity, serving the electricity needs of about 60 million people, mostly in developing countries. About 2.5 percent of the electricity produced in the United States came from geothermal power plants. The electricity produced from geothermal power in the United States represented about 37 percent of the world’s output of electricity from geothermal power. The United States, the Philippines, Italy, Mexico, Indonesia, Japan, New Zealand, and Iceland are the largest producers of geothermal energy.

There are three types of geothermal power plants: flash steam plants, dry steam plants, and binary plants.

A.

Flash Steam Plants

Most operating geothermal power plants are flash steam plants. In a flash steam plant, hot water from wells is piped into the plant, where, released from the high pressure of its underground location, some of the hot water boils (flashes) to steam. The force of the expanding steam is used to spin a turbine generator, which produces electricity. After turning the turbine, the geothermal water, along with the condensed steam, is piped back down into the reservoir to be reheated so it can be used again.

B.

Dry Steam Plants

While most geothermal reservoirs produce hot water, a small number produce mostly steam. Steam from such a reservoir is used in a dry steam plant. In such a plant, the steam is piped directly through a turbine generator.

The first geothermal power plant, built at Larderello, Italy, in 1904, was a dry steam plant. The Larderello steam field is still producing electricity today. The largest producing dry steam geothermal reservoir in the world is located at The Geysers Geothermal Field in northern California. This dry steam geothermal reservoir, which supports 20 operating power plants, produces about 1,000 megawatts of electricity, enough to supply power to nearly 2 million people.

C.

Binary Power Plants

In a binary power plant, heat from geothermal water is transferred through heat exchangers to a second liquid (called a working fluid, usually isobutane) contained in adjacent but separate pipes. Heat transferred from the geothermal water converts this low-boiling point working fluid into vapor, which powers a turbine generator. After turning the turbine generator, the working fluid is condensed back into liquid, which is repeatedly vaporized by the geothermal heat. After heating the working liquid, the geothermal water is piped back into the reservoir.

The binary working fluid vaporizes at temperatures lower than is necessary to vaporize water. As a result, binary power plants can generate electricity using geothermal reservoirs of lower temperature, increasing the number of geothermal reservoirs in the world that can potentially be used for generating electricity. Binary power plants are generally more expensive to build and operate than flashed steam plants. However, binary power plants use geothermal heat and water more efficiently and have no emissions.