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Renewable Energy - Geothermal Energy

water power reservoirs hot

Since ancient times, humans have exploited the earth's natural hot water sources. Although bubbling hot springs became public baths as early as ancient Rome, using hot water and underground steam to produce power is a relatively recent development. Electricity was first generated from natural steam in Italy in 1904. The world's first steam power plant was built in 1958 in a volcanic region of New Zealand. A field of twenty-eight geothermal power plants covering thirty square miles in northern California was completed in 1960.

What Is Geothermal Energy?

Geothermal energy is the natural, internal heat of the earth trapped in rock formations deep underground. Only a fraction of this vast storehouse of energy can be extracted, usually through large fractures in the earth's crust. Hot springs, geysers, and fumaroles (holes in or near volcanoes from which vapor escapes) are the most easily exploitable sources of geothermal energy. (See Figure 6.3.) Geothermal reservoirs provide hot water or steam that can be used for heating buildings, processing food, and generating electricity.

To produce power from a geothermal energy source, pressurized steam or hot water is extracted from the earth and directed toward turbines. The electricity produced by the turbines is then fed into a utility grid and distributed to residential and commercial customers.

Types of Geothermal Energy

Like most natural energy sources, geothermal energy is usable only when it is concentrated in one spot, in this

FIGURE 6.3

case in what is called a "thermal reservoir." The four basic categories of thermal reservoirs are hydrothermal reservoirs, dry rock, geopressurized reservoirs, and magma resources. Most of the known reservoirs for geothermal power in the United States are located west of the Mississippi River, and the highest-temperature geothermal resources occur mostly west of the Rocky Mountains. According to the Energy Information Administration, in 2002 geothermal resources produced nearly 13.4 billion kilowatt hours, which is a little less than 4% of the energy generated by renewable sources.

HYDROTHERMAL RESERVOIRS. Hydrothermal reservoirs consist of a heat source covered by a permeable formation through which water circulates. Steam is produced when hot water boils underground and some of the steam escapes to the surface under pressure. Once at the surface, impurities and tiny rock particles are removed, and the steam is piped directly to the electrical generating station. These systems are the cheapest and simplest form of geothermal energy. The Geysers, ninety miles north of San Francisco, California, are the most famous example of this type. The Geysers Geothermal Field is the world's largest source of geothermal power, according to the Energy Information Administration.

DRY ROCK. Dry rock formations are the most common geothermal source, especially in the West. To tap this source of energy, water is injected into hot rock formations that have been fractured and the resulting steam or water is collected.

GEOPRESSURIZED RESERVOIRS. Geopressurized reservoirs are sedimentary formations containing hot water and methane gas. Supplies of geopressurized energy remain uncertain, and drilling is expensive. Scientists hope that advancing technology will eventually permit the commercial exploitation of the methane content in these reservoirs.

MAGMA RESOURCES. Magma resources are found from ten thousand to thirty-three thousand feet below the earth's surface, where molten or partially liquefied rock is located. Because magma is so hot, ranging from 1,650 to 2,200 degrees Fahrenheit, it is a good geothermal resource. The process for extracting energy from magma is still in the experimental stages.

Domestic Production of Geothermal Energy

Geothermal energy ranked third in renewable energy production in the United States in 2003, after biomass (wood, waste, alcohol) and hydroelectric power. (See Table 6.1.) According to the International Geothermal Association, in 2002 the United States had 28% of the installed geothermal generating capacity of the world, but most of the easily exploited geothermal reserves in the United States have already been developed. In addition, utility companies and independent power producers are arguing over who should build additional generating capacity and what prices should be paid for the power. Continued growth in the American market depends on the regulatory environment, oil price trends, and the success of unproven technologies for economically exploiting some of the presently inaccessible geothermal reserves.

International Production of Geothermal Energy

Since 1970, worldwide geothermal electrical generating capacity has more than tripled. According to the EIA's Annual Energy Review 2003, geothermal energy made up about 1.2% of world electrical production in 2002. Geothermal sources worldwide produce little more than the energy output of ten average-size coal-fired power plants.

World geothermal reserves are immense but unevenly distributed. They fall mostly in seismically active areas at the margins or borders of the earth's nine tectonic plates. Currently, exploited reserves represent only a small fraction of the overall potential—many countries are believed to have in excess of 100,000 megawatts of geothermal energy available.

The World Geothermal Congress (WGC), with representation by delegates from sixty countries, met in Kyushu and Tohoku, Japan in 2000. At that meeting the WGC noted that nearly 90% of homes and other buildings in Iceland are heated by geothermal waters, and approximately 26% of electrical power generation in the Philippines comes from geothermal steam. The WGC supports the use of geothermal energy; one of its goals is to replicate such high use of geothermal resources in other countries. The next meeting of WGC is in 2005.

Disadvantages of Geothermal Energy

Geothermal plants must be built near a geothermal source, are not very efficient, produce unpleasant odors from sulfur released in processing, generate noise, are inaccessible for most states, release potentially harmful pollutants (hydrogen sulfide, ammonia, and radon), and release poisonous arsenic or boron often found in geothermal waters. Serious environmental concerns have been raised over the release of chemical compounds, the potential contamination of water sources, the collapse of the land surface around the area from which the water is being drained, and potential water shortages resulting from massive withdrawals of water.

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