The Heat Beneath Your Feet

** Editor’s Note: The staff of Scienceline is taking a short break to work on future stories. In the meantime, we will repost some of the site’s most popular articles from the last six months. We will return to publishing new content on May 19. This article originally appeared March 14. 

Whether you are cresting the peak of a majestic volcano or walking on a city street, there are immense amounts of heat beneath your feet – enough to provide all the energy the human population will ever need. In most places, though, that heat is trapped by solid rock – unavailable to our carbon-choked, energy-hungry populace.

But the power could now be unleashed. The 2009 Department of Energy (DOE) budget released in early February includes about $30 million for geothermal energy exploration – mostly for the construction of Enhanced Geothermal Systems (EGS) demonstration plants. This is the first step toward achieving enough clean, constant power from the earth to provide around 10 percent of our baseline energy needs – a goal put forward by a panel of experts in January 2007.

Before EGS could be implemented, scientists would need to calm concerns about insufficient technology and the possibility of earthquakes at EGS sites. The allotted $30 million would also have to increase in later years to reach the $1 billion the panel report calls for overall. Still, many scientists view the project as our best baseline energy option.

“We’re no longer limited by just discovering the Icelands of the world,” says Jefferson Tester, a professor of chemical engineering at MIT who chaired the EGS panel. The report estimates that by 2050, EGS could be implemented to a capacity of 100,000 new megawatts of power – more electricity capacity than all of the nuclear power plants in the United States combined.

An Enhanced Geothermal System is a drilled and blasted version of natural geothermal systems like geysers and hot springs. After making a well, engineers pump water down to an area of hot solid rock, where it causes the rock to break up and become porous. The water then trickles through the rock fractures, heats up, and is drawn back through an uptake well to the surface, where its heat can be used to drive turbines and generate electricity.

Because many areas in the United States have very hot rocks but not the water or pressure to drive that heat to the surface, EGS could dramatically expand the amount of geothermal heat we are able to harvest by forcing areas with heat potential to become productive.

“Environmentally, [EGS] should be very, very positive if done right,” says Gerald Nix, a researcher at the National Renewable Energy Laboratory in Golden, Colo. The power is endlessly renewable and doesn’t generate large amounts of carbon dioxide or other greenhouse pollutants.

Until late January, the United States’ main plan for clean baseline power was the development of clean coal technology, with the construction of a plant called FutureGen. But that project’s cancellation was just announced on Jan. 30, as was its possible price tag: $1.8 billion for 275 megawatts of capacity, or over 600 times more than EGS per unit of electricity.

In contrast, the EGS panel report called for only about $1 billion invested over 15 years. Next to that, FutureGen would have looked like a $300 roast beef sandwich. Yet the DOE was prepared to invest heavily in the clean coal plant, while geothermal development has received only a small budget for the next two years.

If 2009’s $30 million is only the beginning of the DOE’s geothermal support, as the EGS panel would hope, part of the $1 billion price tag would go to research and development. Without some advancement, as well as government support and subsidies, EGS may not be as inexpensive as anticipated. Still, Tester regards the panel’s budgeting and predictions as conservative.

Right now, the drilling technology used for EGS is borrowed from the petroleum industry. Drilling in hard solid rock is very different from drilling in loose sediments or gravels, so the process is not as efficient as it should be. But Tester and other researchers are working to create drill heads that better survive the harsh trip down to the hot solid rocks EGS aims to tap, as well as improved drill housings and ways to turn heat into electricity.

Scientists also need to ensure that underground sensors, which monitor the shape and path of fractures, are as efficient as possible. If the rock breaks one way, hot water will flow through well. If it breaks another way, it might not. Engineers need to “really learn how to enhance and manage the underground reservoir so it is a very effective and long-lived heat exchanger,” says Nix.

At this point there are some enhanced geothermal sites in the United States, most notably at The Geysers in California, where water pumping increases the yield of a natural geothermal system. But there are no projects yet at the level the EGS panel suggests.

One huge issue that has come to light at EGS plants in the United States, France, Australia and Switzerland is the possible side effect of induced earthquakes. Small earthquakes occur in all geothermal systems, says Mark Anders, a Columbia University geologist.

Some of these quakes are just vibrations of moving water and steam. But the kind of quake that shakes Californian cities, in which one side of a fault or crack slips against another, can also take place if the geothermal system is connected to an area with active faults, says geologist Charles Visser, who is collaborating with Nix at the federal laboratory in Colorado. Theoretically, a quake on a big fault could be dangerous to structures and human life.

But a major quake requires a several-kilometer-long fault, argues Ernest Majer, a seismologist at the Lawrence Berkeley National Laboratory. Engineers know not to put EGS sites near large or dangerous faults, and the small cracks created by the system itself are not dangerous. “We can’t make faults as big as Mother Nature does … and there has never been a damaging geothermal earthquake anywhere in the world,” he adds.

At Geysers in California, there are about 3,000 earthquakes per month, according to Majer. The largest, however, reached only 4.6 magnitude – big enough to be noticeable, but not dangerous.

Majer is enthusiastic about how education and community involvement can help to allay earthquake fears. The quakes at EGS plants can be controlled and monitored for safety, and better research will help scientists and engineers understand how to make EGS plants even safer, he says.

The most important next step, according to Tester, is to demonstrate that large-scale EGS is even possible and profitable.

“You can’t just calculate this,” he says. “You have to go do it.”

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