8 November 2011—Geothermal energy is touted as one of the few renewable resources that could be used for base-load (round-the-clock) power generation: Earth’s heat is always on, and it’s not dependent on the vagaries of wind or sun. New research from Southern Methodist University—sponsored by Google’s philanthropic arm—suggests massive potential for geothermal power in the United States. But exploiting that resource will be slowed by the cost of the technology—and the fact that it can cause small earthquakes.
Researchers led by David Blackwell at SMU’s Geothermal Laboratory set out to update existing maps of the heat beneath our feet, maps that Blackwell says had significant gaps. The researchers doubled the number of locations measured from previous efforts, and by sampling more than 35 000 sites, they found a "technical potential" of almost 3 million megawatts.
"The technical potential is our best estimate of what actually might be extracted," says Blackwell. He says that depths greater than 6.5 kilometers are impractical to access, so his calculation does not take into account a supply of power that’s more than 10 times as much at depths up to 10 km.
To put this in perspective, there are only about 3000 MW of installed geothermal capacity in the United States today, and no other country has more. The total installed electricity capacity from all energy sources in the country is right around 1 million megawatts. So if all that geothermal energy were harnessed, it could power the country three times over.
According to Colin Williams, a scientist with the U.S. Geological Survey who has worked on similar geothermal resource estimates, the SMU work is less about new discoveries than about technological optimism. "It’s not like they discovered more thermal energy down there, but they’re pushing the scenario that you could get more of it out," he says. His own calculations from a 2008 study showed that even the most easily developed geothermal resources could bring 6500 MW online and that more "unconventional" resources represented more than half a million megawatts of potential. That assessment differed from the SMU work in several ways, including stopping at a depth of 6 km instead of 6.5, as well as focusing almost entirely on the western United States.
Blackwell also says the most notable improvements over previous estimates are in the East—especially under coal-rich West Virginia. The energy is there, he says, but "the question is, Do we have the will to go ahead and try to really develop it?"
The answer to that question is still up in the air and depends on some ongoing debates about the cost and risk associated with geothermal technology. Most existing geothermal projects come from hydrothermal reservoirs where hot water is brought up from below the surface to produce electricity. And such projects have been multiplying: Geothermal power was present in only four U.S. states little more than five years ago; now it is in nine, with plans or projects in another half dozen. But the new 3 million MW would almost all require what is known as enhanced geothermal systems (EGS). That technique allows the use of lower-temperature areas by fracturing the rock with high-pressure water, similar to the controversial "fracking" process in the natural gas industry. However, it’s worth noting that at this point, EGS uses only water and none of the toxic chemicals that have raised water-quality and health issues with natural-gas fracking. There is ample evidence, though, that EGS produces small earthquakes.
"We know that creating these EGS reservoirs involves making earthquakes. That’s just going to happen," Williams says. "The question then becomes, Are we going to be able to control the process of generating the microseismicity so that we don’t generate earthquakes that are magnitude 3.5 or 4.0 or something like that?" There will likely need to be geographic restrictions on development so that such a potential quake doesn’t occur near a large fault and possibly cause an even bigger quake. The U.S. National Academy of Sciences and the National Academy of Engineering have launched an investigation, looking across many energy technologies; their report is expected in 2012.
After EGS was blamed for a 3.4-magnitude earthquake in Basel, Switzerland, projects in Europe and the United States have struggled to get off the ground. Karl Gawell, the executive director of an industry group called the Geothermal Energy Association, says that the scrutiny now placed on the issue suggests that projects won’t move forward without strong indications of safety. "You won’t see another Basel, Switzerland, at least not in the United States," he says.
For the moment, cost is also a primary barrier to widespread adoption. USGS’s Williams says traditional geothermal electricity is "in the ballpark" in terms of cost with other electricity sources. A 2009 report by the investment bank Credit Suisse quoted a conventional geothermal cost of 3.6 U.S. cents per kilowatt-hour, below the 5.5 cents for coal. EGS is costlier. A 2007 report by consulting firm GeothermEx estimated the best possible cost for EGS systems in the future at 5.4 cents per kilowatt-hour and suggested that the technology won’t be truly cost competitive until 2050. "Until EGS is developed on a wide scale, initially it probably wouldn’t be competitive," Williams says. "Right now we’re looking at sort of slow but steady development."
A version of this article appeared in the December 2011 print edition of IEEE Spectrum.
About the Author
Dave Levitan is a science journalist who contributes regularly to IEEE Spectrum’s Energywise blog. He recently wrote about how biology is inspiring more efficient wind power.