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Geothermal and Hot Rocks

September 20, 2010

Traditional geothermal units built over existing hydrothermal areas can produce electricity at reasonable cost, but there are too few such locations for a large number of these plants to be built in the United States.

Current U. S. installed geothermal generating capacity is around 3,000 MW, mostly in California: This produces less than 0.4% of total U.S. generated electricity. The EIA projection for 2030 is 6,500 MW of geothermal power. There is a potential for twice this much traditional geothermal power generation in the U.S.

Traditional geothermal uses three methods for generating electricity.

The direct steam method uses high temperature steam as it emerges from the earth to drive a turbune generator. These are the most cost effective plants, but sites with steam are rare. 

Moat conventional geothermal systems inject high-temperature brine (above 400 °F) from the earth into a low-pressure chamber where the super-heated water, often contaminated with salts and silica, flashes directly into steam, and where the steam then drives a turbine generator.

For low temperature resources, binary cycle systems are used. Moderate temperature geothermal fluid is passed through a heat exchanger where the heat is transferred to a fluid such as iso-butane which vaporizes: The vaporized fluid then drives a turbine generator.

Some of the existing installations have trouble sustaining the system because the super-heated water supply is gradually depleted. But now that most plants re-inject water, the major problem is long-term degradation because the reservoir heat supply is not being replenished as fast as it its being drawn off.

A few ingenious people have taken action to replace the dwindling supply of water, such as by using the water left over from treating sewage and piping it to geothermal plants to provide make-up water for the system.

Even though these geothermal plants can provide electricity at a reasonable cost, conventional geothermal cannot provide large amounts of electricity because there are too few suitable geothermal resources.

Many have heard about Enhanced Geothermal, commonly referred to as Hot Rock geothermal.

Theoretically Hot Rocks could produce huge amounts of electricity.

Hot Rocks entails drilling two wells to depths reaching over 10,000 feet where there are high temperature rock formations. Fracturing techniques are used to open fractures in the rocks between the two wells. Water is injected down one well where it would be converted to steam as it travels through the fractures in the hot rocks. The steam would rise to the surface through the second well and be used to drive a turbine generator.

Geodynamics Limited in Australia has been in the process of attempting to develop such an installation since before 2003. They have drilled wells to a depth of 14,500 feet and the project is still under development. Without massive subsidies, the chances of profitable operation are not high. There is a high risk that the fractured rock will cool faster than expected and continued drilling will be needed.

An effort to develop Hot Rock geothermal power in Basel, Switzerland was stopped in 2006 when the project caused seismic activity. Tremors exceeding 3 on the Richter scale were observed.

In spite of all the press time devoted to Enhanced Geothermal by the Sierra Club and others, it is a distraction that cannot supply the United States with low-cost electricity.

While Hot Rocks is a distraction, traditional geothermal can produce a small amount of electricity at reasonable cost.

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