Geothermal

The word “geothermal” comes from the Greek geo, meaning “earth,” and therme, meaning “heat.” Geothermal energy systems make use of the heat that’s produced deep inside of the earth, at its core, 4,000 miles below the planet’s surface.

The earth’s core actually has two layers: an inner core of iron and an outer layer of very hot melted rock, called magma. On top of the magma layer comes the mantle, a layer of earth that’s about 1,800 miles thick, made of magma and rock. On top of the mantle is the earth’s crust, a layer that’s relatively thin—from three to five miles deep beneath the ocean and fifteen to thirty-five miles deep under landmasses. The earth’s crust is broken into pieces called plates, and it is at these broken places that heat manufactured in the magma layer by slowly decaying radioactive particles comes closest to the surface. The heat manifests itself as volcanoes, geysers, and hot springs—places where the Romans, Chinese, and Native Americans built their baths, and where today spas still draw enthusiastic patrons to their mineral-rich healing waters. It’s these same places that are, in general, the best locations for geothermal heating and electric-generating facilities.

All geothermal systems rely on two basic components: the heat beneath the earth’s crust and the subterranean waters that the earth’s heat will turn to steam. In most geothermal systems, accessing these components involves drilling up to two miles into the earth’s crust. In direct heating systems, the earth’s natural steam is piped directly into buildings to warm them in winter and—perhaps surprisingly—to cool them in summer.

How does that work? While the seasons change from cold to hot and back again out here on the surface of the planet, the temperature in the upper ten feet of the earth remains fairly constant, at between 50 and 60 F. The benefits of this constant temperature can be accessed by pumping the water of springs or reservoirs near the earth’s surface into buildings for interior climate control. In some cities in Iceland, a leader in using geothermal technology, the climate in nearly 95 percent of its buildings is managed in this manner.

Geothermal power can also be used to make electricity; it already supplies over twenty countries, including France, New Zealand, Russia, China, and the Philippines, with about 8 percent of the renewable energy generated globally. Though at the present time it costs between $4,000 and $5,000 to install 1 kilowatt, it has the potential to become a very cost-effective way to produce electricity, and its development potential is broad worldwide, so the technology deserves to be a little better understood.

There are four different ways to drive electric generators using geothermal energy. The first is called the dry steam method. First developed in 1904 by Prince Piero Ginori Conti at the Lardello field in Tuscany, this method uses the steam released directly from a geothermal reservoir to drive generator turbines.

A more technologically sophisticated method of geothermal electrical generation is called the flash steam system. This is the most common system in use today, and it works by taking advantage of the high pressure beneath the earth’s crust. Under this intense pressure, water remains liquid though it’s heated to what would be well over the boiling point were it at sea level. As the water is pumped from within the earth, an abrupt drop in pressure causes it to convert—in a flash—to steam, which more efficiently powers the turbines that energize the electrical generators.

Most geothermal facilities that are now in the planning stages incorporate a third, and even more efficient, technology to access geothermal power. Called the binary system, this method directs the earth’s hot water to a heat exchanger, where the heat is transferred to a second pipe containing a fluid with a much lower boiling point than water, usually either isobutane or isopentane gas, which is then vaporized to power the turbines. The advantage of this system is that it can make use of those geothermal reservoirs that have lower temperatures, which increases the places where geothermal systems can be located.

Finally, enhanced geothermal, or the hot dry rock system, may be yet another avenue into deep earth’s power potential. Rather than harvesting the heated water already in the earth, this method involves manufacturing steam by piping surface water into the hot but dry rocks in the earth’s crust. The benefit of this system is that it can be used anywhere on the planet simply by drilling a hole. The downside is that the hole has got to be dug deep—deeper than for any other geothermal system—and the environmental impacts of deep drilling aren’t yet fully understood.

Source: Green: Your Place in the New Energy Revolution, Palgrave Macmillan, 2008

• The first U.S. geothermal power plant, opened at The Geysers in California in
• 1960, continues to operate successfully.
• The United States, as the world’s largest producer of geothermal electricity, generates an average of 15 billion kilowatt hours of power per year, comparable to burning close to 25 million barrels of oil or 6 million shorttons of coal per year.
• A geothermal resource assessment shows that nine western states together have the potential to provide over 20 percent of national electricity needs.
• Although geothermal power plants, concentrated in the West, provide the third largest domestic source of renewable electricity after hydropower and biomass, they currently produce less than one percent of total U.S. electricity.
• Over 30 years, the period of time commonly used to compare the life cycle impacts from different power sources, a geothermal facility uses 404 square meters of land per gigawatt hour, while a coal facility uses 3632 square meters per gigawatt hour.

Source: The Geothermal Energy Association, A Guide to Geothermal Energy and the Environment, April 2007