Geothermal Legacy Collection

[Image: OSTI] Geothermal Technologies Legacy Collection Logo
[Image: OSTI]

The Geothermal Legacy Collection is a searchable online database maintained by the Office of Scientific and Technical Information under the U.S. Department of Energy.

The web site allows users to search for technical and programmatic reports published in the last forty years. The reports included in the database are intended for a variety of users interested in factual information about geothermal energy.

What are Greenhouse Gases?

The Greenhouse Effect [Image: NOAA Paleoclimatology]
The Greenhouse Effect
[Image: NOAA Paleoclimatology]

Gases in the Earth's atmosphere that trap heat are referred to as "greenhouse gases". Some of these greenhouse gases occur naturally in the atmosphere but are augmented by inputs from human activities. Others are unnatural compounds that are only created and emitted into the atmosphere from human activities. The principal greenhouse gases released into the atmosphere from human activities are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and fluorinated gases. The U.S. Environmental Protection Agency inventories greenhouse gas emissions and sinks and provides information about greenhouse gases and climate change.

Geothermal Energy

Geothermal power plant near Santa Rosa, California. <br>[Photo: U.S. Geological Survey]
Geothermal power plant near Santa Rosa, California.
[Photo: U.S. Geological Survey]

Geothermal energy is a renewable and sustainable form of energy that supplies a small part of the world's electricity, heat, and other energy needs. The extraction process for geothermal energy is similar to that of drilling for oil, except the drilled wells extract water and steam rather than oil. Geothermal energy sources tap the abundant heat available from within the earth, at depths ranging from less than a mile to several miles underground. Geothermal energy development is focused in areas where high heat can be found relatively close to the surface of the earth; these areas are often located near the edges of tectonic plates. Geothermal energy can be used directly for heating purposes or indirectly, using steam to turn turbines and generate electricity in a power plant. There are three types of geothermal power plants operating today in the US: dry steam plants, flash steam plants, and binary-cycle plants.

Advantages: clean and domestically available
As a renewable energy source, geothermal energy is advantageous because it is "clean", meaning the processing and usage of the resource emits few or no greenhouse gases. Once developed, geothermal power plants can operate nearly continuously, at availabilities of 90% or higher. In addition, extraction of geothermal energy can take place within the borders of the United States, thus reducing the need for reliance on imported energy.

Disadvantages: economic feasibility and water use
Although geothermal energy is abundant, extraction is not always economically feasible. The costs of developing and maintaining geothermal power plants must be evaluated against the price of electricity generated. In addition, salts and dissolved minerals associated with the extraction of geothermal fluids must be disposed of properly, either by reinjection below ground, or by disposal in approved sites. Depending on the specific characteristics of the geothermal resource and power plant, plant operations may require from several hundred to several thousand gallons per minute of water use per megawatt of energy produced.

U.S. Department of Energy Geothermal Technologies Program

Environmental Impacts of Geothermal Energy

Geothermal system [Image courtesy of U.S. Department of Energy, Energy Efficiency and Renewable Energy, Geothermal Technologies Program]

The U.S. Environmental Protection Agency lists the following as environmental impacts of geothermal power:

Air Emissions   Emissions associated with generating electricity from geothermal technologies are negligible because no fuels are combusted.
Water Resource Use   Geothermal power plants usually re-inject the hot water that they remove from the ground back into wells. However, a small amount of water used by geothermal plants in the process of creating electricity may evaporate and therefore not be returned to the ground. Also, for those geothermal plants that rely on hot, dry rocks for energy, water from local resources is needed to extract the energy from the dry rocks.
Water Discharges   Geothermal power plants can possibly cause groundwater contamination when drilling wells and extracting hot water or steam. However, this type of contamination can be prevented with proper management techniques. In addition, geothermal power plants often re-inject used water back into the ground (through separate wells) instead of discharging the used water into surface waters. This prevents underground minerals or pollutants from being introduced into surface waters.
Solid Waste Generation   Geothermal technologies do not produce a substantial amount of solid waste while creating electricity.

Land Resource Use   Geothermal power plants typically require the use of less land than fossil fuel power plants. However, if water is not re-injected into the ground after use to maintain pressure underground, it may cause sinking of land at the surface.

Geothermal Energy Resources
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Featured Geothermal Site

The Geysers geothermal power plant; near Santa Rosa, California. [Photo: U.S. Geological Survey]
The Geysers geothermal power plant; near Santa Rosa, California.
[Photo: U.S. Geological Survey]

Located in northern California, west of the Great Central Valley in the Mayacamas Mountains, lies the world's largest geothermal power plant complex: The Geysers.

In 1987, its production topped out at 1,600 megawatts (MW) of electricity, but this level could not be sustained because of pressure loss inside the reservoir. To mitigate this, in 1997, the world’s first wastewater to electricity system was created, and 7.8 million gallons of water per day are piped and injected back into the ground. Currently, The Geysers currently produce around 750 MW of electricity, which is used across Northern California.

Geothermal Resource Council

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