Geothermal Electricity Production

What is it?

Most power plants need steam to generate electricity. The steam rotates a turbine that activates a generator, which produces electricity. Many power plants still use fossil fuels to boil water for steam. Geothermal power is a form of renewable energy utilizing subsurface hot water or steam created by the heat beneath the earth's surface. Heat from the earth's molten core in areas of volcanic activity or at the juncture of the earth's tectonic plates, flows naturally toward the cooler surface to form hot springs, geysers, steam vents (fumaroles) and boiling mud pots. 

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

Dry steam power plants draw from underground resources of steam. The steam is piped directly from underground wells to the power plant, where it is directed into a turbine/generator unit. There are only two known underground resources of steam in the United States: The Geysers in northern California and Yellowstone National Park in Wyoming, where there's a well-known geyser called Old Faithful. Since Yellowstone is protected from development, the only dry steam plants in the country are at The Geysers.

Flash steam power plants are the most common. They use geothermal reservoirs of water with temperatures greater than 360°F (182°C). This very hot water flows up through wells in the ground under its own pressure. As it flows upward, the pressure decreases and some of the hot water boils into steam. The steam is then separated from the water and used to power a turbine/generator. Any leftover water and condensed steam are injected back into the reservoir, making this a sustainable resource.

Binary cycle power plants operate on water at lower temperatures of about 225°—360°F (107°—182°C). These plants use the heat from the hot water to boil a working fluid, usually an organic compound with a low boiling point. The working fluid is vaporized in a heat exchanger and used to turn a turbine. The water is then injected back into the ground to be reheated. The water and the working fluid are kept separated during the whole process, so there are little or no air emissions.

In the United States, most geothermal reservoirs are located in the western states, Alaska, and Hawaii.

Why Geothermal?

• Geothermal energy offers a number of environmental and economic advantages over traditional fossil fuel sources.
• Geothermal power plants are both “clean” and “green.” Clean electricity refers to alternative energy technologies such as geothermal that result in a net environmental improvement relative to existing energy production (e.g. from fossil fuels). Green energy is energy generated from renewable resources through environmentally and socially responsible developments.
• Geothermal power plants also have the distinction of being “baseload” power sources. That is, they can operate continuously at up to 98% capacity because they have a constant source of “fuel” and require little downtime for maintenance. And, geothermal plants are not affected by changing weather conditions.
• From an economic view, geothermal energy is price competitive. While initial investment is high, lifetime costs are low because the fuel source is free, it is located at the generation plant site (no transportation costs) and it is renewable.
• Land impacts also are minimal. Geothermal power plants typically are constructed at or near the geothermal reservoir – there is no need to transport ‘fuel’ to the plant – and require only a few acres for the plant buildings. Geothermal plants generally have a low profile. Geothermal wells and pipelines may cover a considerable area but do not prohibit other uses such as farming, livestock or wildlife grazing and recreational activities.
• Small-scale geothermal power plants (under 5 megawatts) have the potential for widespread application in rural areas, possibly even as distributed energy resources. Distributed energy resources refer to a variety of small, modular power-generating technologies that can be combined to improve the operation of the electricity delivery system.

The Technology

• The physical facilities required for a geothermal power plant include production and injection wells, a gathering and injection system, a power generation plant and a transmission line.
• Production wells are constructed by directional drilling from a small number of drill pads (as few as three depending on the size of the reservoir), thus reducing both project costs and potential environmental impacts. Typical well depths are 3,000 meters or less. 
• Wells are drilled using established technology similar to that employed in the oil and gas industry. 
• The gathering system consists of pipelines that transport the steam or hot water from the wellheads to the generating plant which uses standard turbine technology. An injection system uses non-productive wells to return process water to the underground reservoir.