A form of "clean energy," geothermal energy projects deliberately drill into the Earth to harness its inherent energy in the form of heat. But the projects, located in northern Carlifornia and Basel, Switzerland, were deemed too dangerous and capable of triggering earthquakes.
The California project, located 100 miles north of San Francisco and operated by AltaRock Energy, was put on hold in September after the company's drilling efforts met "physical difficulties" in the form of snapping drill bits. The project is located in The Geysers, the world's largest dry-steam geothermal steam field.
The Swiss project has been on hold since 2006, when it caused a 3.4 magnitude earthquake thousands of aftershocks and millions worth of damage in Basel, a town of about 167,000 people.
Both projects were based on an "enhanced geothermal" system, which fractures bedrock and circulates water through the cracks to heat it and produce steam that powered the turbines of a power plant. Naturally, both projects were also based in areas with a history of seismic activity.
The projects were originally supposed to force water underground to open new cracks, into which workers could then pump additional water, to be heated and turned into steam.
Geothermal energy is currently used to heat and cool homes by circulating fluids at shallower depths to be passively warmed by the Earth's heat. But scaling that technology requires companies to tap into heat at much deeper levels.
By name alone, it's no surprise that the "fracturing" process can cause earthquakes. Most are small enough to go unnoticed.
But a Swiss government study released last week showed that the $60 million geothermal project was likely to cause earthquakes that would do several millions of dollars in damage each year. Residents could expect anywhere from 14 to 170 more earthquakes, most minor, over the 30 years the project would be in service.
U.S. officials have not yet completed a study on the California project, but the company said it is now removing its drill rig due to its difficulties drilling into resident caprock. The project had won a $6 million grant from the Energy Department and $30 million in private investment.
The U.S. Department of Energy remains optimistic on geothermal power as a clean energy source. The department is working on drafting its National Geothermal Action Plan and has invested more than $400 million in geothermal projects in 2009 alone, including enhanced geothermal projects in Idaho, Nevada and California.