Geothermal Power

Geothermal energy is clean, renewable energy from heat simmering within the earth’s bedrock.

Geothermal’s potential as a clean energy source has raised huge hopes, and its advocates believe it could put a significant dent in American dependence on fossil fuels.

The earth’s heat is always there waiting to be tapped, unlike wind and solar power, which are intermittent and thus more fickle. According to a 2007 geothermal report financed by the Energy Department, advanced geothermal power could in theory produce as much as 60,000 times the nation’s annual energy usage. President Obama has cited geothermal power as part of the “clean energy transformation” that a climate bill now before Congress could bring about.

Power companies have long produced limited amounts of geothermal energy by tapping shallow steam beds, often beneath geysers or vents called fumaroles. Those projects can induce earthquakes, although most are small. But for geothermal energy to be used more widely, engineers need to find a way to draw on the heat at deeper levels percolating in the earth’s core.

But because large earthquakes tend to originate at great depths, breaking rock that far down carries more serious risk, seismologists say. Seismologists have long known that human activities can trigger quakes, but they say the science is not developed enough to say for certain what will or will not set off a major temblor.

The technique to tap geothermal energy creates earthquakes because it requires injecting water at great pressure down drilled holes to fracture the deep bedrock. The opening of each fracture is, literally, a tiny earthquake in which subterranean stresses rip apart a weak vein, crack or fault in the rock. The high-pressure water can be thought of loosely as a lubricant that makes it easier for those forces to slide the earth along the weak points, creating a web or network of fractures.

On Dec. 8, 2006, Markus O. Häring’s geothermal project in Basel, Switzerland, was suspended when it set off an earthquake, shaking and damaging buildings and terrifying many.

The project was shut down permanently on Dec. 10, 2009, after a government study determined that earthquakes generated by the project were likely to do millions of dollars in damage each year.

In late June 2009, an American start-up company, AltaRock Energy, was set to begin using nearly the same method to drill deep into ground laced with fault lines in an area two hours’ drive north of San Francisco.

Residents of the region, which straddles Lake and Sonoma Counties, have already been protesting swarms of smaller earthquakes set off by a less geologically invasive set of energy projects there. AltaRock officials said that they chose the spot in part because the history of mostly small quakes reassured them that the risks were limited.

Like the effort in Basel, the new project will tap geothermal energy by fracturing hard rock more than two miles deep to extract its heat. AltaRock, founded by Susan Petty, a veteran geothermal researcher, has secured more than $36 million from the Energy Department and several large venture-capital firms, including Kleiner Perkins Caufield & Byers, and Google.

The California project is the first of dozens that could be operating in the United States in the next several years, driven by a push to cut emissions of heat-trapping gases and the Obama administration’s support for renewable energy.