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

Australian “hot rocks” offer 26,000 yrs of power

 By Michael Perry

SYDNEY (Reuters) – Barely one percent of Australia’s untapped geothermal energy could produce 26,000 years worth of clean electricity, scientists said, as the government announced a A$50 million (US$43 million) project to help develop the technology.

AustraliaFerguson said the first commercially viable geothermal power plants could be in place within four to five years. is the world’s biggest coal exporter with coal used to generate about 77 percent of its electricity. Its reliance on coal for generating electricity makes it the world’s biggest per-person polluter, with five times more emissions per head than China.

“Geothermal energy which is sometimes known as hot rocks has got a huge potential for Australia, both as a solution to climate change and in terms of national energy security,” said Resource Minister Martin Ferguson.

To produce power from geothermal energy, water is pumped below ground where it is heated and the heat energy used to generate power.

Geoscience Australia has mapped the nation’s geothermal energy, using temperature recordings from decades of drilling by energy and exploration firms, sometimes to a depth of five kms (three miles).

A total of 5,722 petroleum and mineral boreholes across Australia were used to generate the map.

“One percent of reserves would produce 26,000 years of energy supplies,” Geoscience’s Anthony Budd told Reuters on Wednesday.

Budd said “hot rocks” needed to be 150 degrees Celsius to produce electricity, which was achievable at a depth of one to five kms, noting temperature rose deeper into the earth’s crust.

An Australian Geothermal Energy Association report this week forecast it could potentially produce 2,200 megawatts of baseload power by 2020, adding that represented up to 40 percent of Australia’s 2020 renewable energy target.

The association estimated A$12 billion would need to be invested to develop the 2,200 megawatts of power, but added the cost of generating electricity would fall to acceptable levels by the time commercial projects were up and running.

It estimated it would cost A$120 per megawatt hour from a small pilot plant producing 10 to 50 megawatts of power, and A$80 per megawatt hour for a large scale plant of 300 megawatts or greater.

“The upper cost boundary will decline over time because the level of uncertainty is expected to narrow as the industry grows. This cost is expected to be lowest cost of any form of renewable or low emissions energy,” it said.

The government’s geothermal drilling project will see different technologies used at various locations around Australia to try and determine the best technology for converting “hot rock” energy into electricity.


“Geothermal energy provides clean base-load power and is potentially a very important contributor to Australia’s energy mix in a carbon-constrained world,” he said.

Prime Minister Kevin Rudd won power last November in part by promising to sign the Kyoto Protocol, which sets binding limits on emissions from developed countries, and to cut emissions by 60 percent of 2000 levels by 2050.


(Editing by Sanjeev Miglani)

Source: Reuters