The pros and cons of geothermal energy.

Illuminating answers to environmental questions.
Oct. 26 2010 7:00 AM

Could Yellowstone Power My Home?

The pros and cons of geothermal energy.

Old Faithful. Click image to expand.
Old Faithful

I recently took my family to Yellowstone, and the shooting geysers and supervolcano got me wondering—can't we tap all that energy to produce electricity?

Theoretically, we could. Super-hot water is the primary ingredient in geothermal energy production and the earth beneath Yellowstone has it in spades, thanks to a massive, shallow body of magma and plenty of snow and rain. However, the Geothermal Steam Act of 1970 puts national park lands off-limits to geothermal energy developers.

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Even if it weren't prohibited, there are compelling reasons to leave Yellowstone alone—like its famous geysers, which could be damaged or even quenched by geothermal energy development in and around the park. (Areas outside the park's borders, however, aren't protected as strongly.) Yellowstone contains half of the world's remaining geysers, making them a precious scientific commodity. The park's hydrothermal features host one of the planet's greatest concentrations of extremophiles, organisms that live in environments at the extremes of heat, acidity, pressure, and so on. An estimated 99 percent of Yellowstone's extremophiles remain undiscovered, but the ones we've found have been quite useful—one bacterium, for example, made modern DNA analysis possible, while others help scientists theorize about life on other planets.

While drilling wells in Yellowstone might not be a great idea, geothermal energy in general is very promising. According to recent life-cycle analyses by Argonne National Laboratory, geothermal power plants emit between 18.7 grams to 103 grams of CO2-equivalent per kilowatt-hour—polite little hiccups compared with the 1,234.9 g/kWh belched out by coal or the 487 g/kWh by natural gas. (Those figures include building and running the power plants as well as extracting the fuel.) Unlike conventional coal-fired plants, geothermal plants emit very little sulfur dioxide and no nitrogen oxides, which are the precursors of acid rain. And unlike wind or solar power installations, geothermal power doesn't fluctuate with the weather. Last year, the United States' 77 geothermal power plants produced 15.2 billion kilowatt-hours of electricity, or about 0.4 percent of the U.S. total—more than any other nation in the world. (Wind machines generated 70.8 billion kWh of electricity, and solar 0.8 billion kWh.)

But as with any big industrial project, geothermal energy production also carries some environmental risks. The biggest issues revolve around water. Brackish waters drawn from deep underground are sometimes laced with toxic substances like mercury, so power producers have to be very careful with how they store and dispose it.To cool their working fluid, some geothermal power plants withdraw large amounts of surface water. In areas where fresh water is scarce, these plants may compete with farms and homes that need water for irrigation, bathing, and the like—but that's a problem for other kinds of power plants, as well.

The potential for geothermal projects to cause earthquakes has received a lot of attention in recent years. Most of the concern has been focused on projects known as enhanced geothermal systems, or EGS. There are plenty of underground zones that get scorching hot but remain dry, because the rock there is so dense. Without water to carry that thermal energy  to the Earth's surface, you can't generate electricity. (Not yet, at least.) In EGS applications, high-pressure water is injected into those impermeable, rocky areas to create a network of small fractures. Pumping surface water into the now-porous rock creates a brand-new hydrothermal reservoir. That fracturing process produces microearthquakes—small tremors that can be detected with a seismometer but generally aren't felt at the Earth's surface. To avoid creating more damaging earthquakes, EGS projects must steer clear of active fault lines and monitor seismic activity very closely. In the United States, EGS projects are still in the research and development phase, with none yet online.

Conventional geothermal plants have also been associated with earthquakes, in part because they tend to be located near active tectonic regions, where hydrothermal reservoirs are usually found. Additional tremors may arise, however, when extracting or injecting steam and water creates pressure or temperature changes underground. (Geothermal energy isn't the only industrial culprit when it comes to earthquakes: Reservoirs and injection wells have caused tremors, as well.)

If tapped, Yellowstone would likely be the world's single largest geothermal production field. But even with the park out of the picture, underground heat can still play a big role in America's energy future. According to a 2008 assessment by the U.S. Geological Survey, we could increase our geothermal power generation by 260 percent just by tapping the hot water and steam reservoirs we've already discovered—and that's not counting any located beneath public lands, such as national parks.

When you include all the undiscovered reservoirs that the U.S.G.S. estimates are out there, or all the new reservoirs that could be created using EGS technology, the power potential skyrockets. All told, we could theoretically produce nearly 4.9 million gigawatt-hours of electricity annually—more than was produced last year from all the country's energy sources combined. Of course, it's unlikely that all that power capacity would be online at any one time: For one thing, a tapped reservoir will cool over a period of several decades, and must be left fallow for another several decades to recover its heat. Still, those numbers indicate that we can leave Old Faithful to the tourists without compromising our clean energy future.

Is there an environmental quandary that's been keeping you up at night? Send it to ask.the.lantern@gmail.com, and check this space every Tuesday.

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