Rising oil prices and fears of global warming are bringing new life to renewable energy technologies. Solar photovoltaics, wind turbines and fuel cells all offer great potential for lowering our fossil-fuel consumption, but one of the most promising sources of renewable energy may be right under our feet. Geothermal energy—energy generated by the heat of Earth’s interior—already is powering homes and businesses in the western United States and around the globe, and proponents say it could someday be a significant component of our overall renewable-energy portfolio.

Geothermal energy refers to energy derived from heat created in our planet’s deeper layers. In many areas of the world, geothermal energy is used to provide heating and cooling through heat pumps or direct piping of underground steam or hot water sources. Those steam and water sources also can be tapped to drive electricity-generating turbines in hydrothermal power plants. These are, essentially, the same types of turbines used in coal, natural gas and nuclear facilities, except hydrothermal versions produce minimal amounts of dangerous emissions or waste.

Generating electricity using geothermal energy is not a new concept. The first geothermal generating plant was developed in Lardarello, Italy, in 1904, and that plant still is operating today. Until recently, these plants—also called “hydrothermal”—were limited to locations where steam or hot water was close to the Earth’s surface and easily accessible. Today, researchers are considering more advanced enhanced geothermal systems (EGS), which could draw on a wider range of geological resources and provide a significant contribution to overall energy needs.

The United States currently generates approximately 2,800 megawatts (MW) of electricity using geothermal resources (for comparison, a single modern coal-fired plant typically produces between 700 MW and 1,000 MW). However, experts see the potential for tremendously higher geothermal contributions to the nation’s energy portfolio. Industry professionals gathered at a May 2006 workshop organized by the National Renewable Energy Laboratory estimated as much as 20,000 MW of geothermal-based energy capacity could be developed by 2025, based on existing hydrothermal technologies. With needed research, up to an additional 80,000 MW could be possible using yet-to-be-developed EGS approaches, workshop experts determined.

Staying in hot water

Currently, North America’s largest geothermal installation is a series of plants located at The Geysers in Northern California. Collectively, this facility now generates less than 1,000 MW of electricity, 725 MW of which is produced from plants owned by San Jose, Calif.-based Calpine. The operation’s future seemed uncertain just a decade ago, but an innovative engineering effort may have extended The Geysers’ functional life and provided valuable field data for other geothermal projects as well.

The Geysers’ steam fields initially were tapped in the early 1920s, but weren’t really commercially developed until the early 1980s. Expansion there continued through the 1990s, but production peaked at 1,500 MW in 1987. Aggressive exploitation posed a threat to the area’s finite steam supplies, and by the mid-1990s, plant managers began noticing a disturbing trend.

“We were seeing a steep decline across the fields in steam pressure and steam productivity,” said Karl Urbank, Calpine’s vice president of geothermal operations. Seeking new means to slow the loss of steam, Calpine engineers began first reinjecting condensate gathered during steam-turbine operations into the steam fields, and then capturing storm water for the same purpose. Tracers in these fluids enabled engineers to verify that reinjected liquid was, indeed, being recycled through the field, raising hopes that productivity declines could be reversed.

While seeking new sources for water to replenish their fields, Calpine officials learned nearby Lake County was having problems disposing of its treated wastewater. In 1997, a 21-mile underground pipeline began delivering 7 million gallons of treated wastewater to The Geysers daily, and a year later, a 41-mile pipeline began moving an additional 11 million gallons daily from Santa Rosa in Sonoma County.

“We’re essentially putting back 85 percent of the mass flow we’re taking out,” Urbank said. As a result, he adds, productivity has stopped declining, and it may even be rising again. He said Calpine officials believe generating output is now 115 MW higher than it would have been, had these wastewater-to-electricity projects not been initiated. Calpine now is discussing plans with Santa Rosa to boost wastewater delivery by another 5 million gallons a day, a process that would require upgrading existing pumps and other equipment.

Engineering a future

The Geysers’ success at reversing productivity declines addressed some researchers’ fears that adding new water to a steam field would cool down the superheated surfaces that create the steam in the first place. It also gives hope to scientists interested in exploring the opportunities enhanced geothermal systems (also called engineered geothermal systems) might offer.

One of the most promising engineered approaches would tap the hot water co-produced during oil and natural gas drilling operations. Oil and natural gas deposits exist side-by-side with enormous quantities of hot water, creating a large waste-treatment problem because that water often is saturated with minerals and chemicals that make it unsuitable for drinking or irrigation.

“To an oil and gas company, the water is nothing more than a nuisance,” said Karl Gawell, executive director of the Geothermal Energy Association, Washington, D.C. “Sometimes the water is so hot, they have to cool it before they can reinject it.”

Siting a geothermal generating operation alongside a drill field could offer multiple benefits, Gawell noted. Some of the drilling and site-development expenses could be shared, transmission lines built to serve drilling equipment could just as easily be used to transfer generated electricity back to the larger grid, and that geothermal power also could be used to provide an alternative to expensive grid-supplied electricity for ongoing drilling operations.

In fact, this last advantage is justifying a research effort getting underway at a U.S.-owned oil installation at the Teapot Dome field in Wyoming. Ormat Technologies Inc., Reno, Nev., a geothermal-energy developer, is partnering with the U.S. Department of Energy (DOE) to build a demonstration plant to test geothermal energy’s viability as a means of extending this uneconomical plant’s financial viability.

“Because they can now provide their own power, the government has estimated they can continue to operate that field for 10 more years,” Gawell said. “It’s a really new area with lots of interesting implications.”

Advanced approaches

Looking beyond co-produced systems in wellfields, geothermal advocates also see opportunities in more fully engineered applications. These approaches, called “deep geothermal” designs, would, essentially, use technology to develop the conditions nature has perfected for creating underground steam and hot-water fields, by drilling down to hot rock, using cold water and chemical etching to create fractures in that rock for water to flow through and then collecting the injected and heated water or steam through a second pipe to use in powering a turbine.

Gerry Nix, technology manager for the National Renewable Energy Laboratory’s geothermal and industrial programs, lists three basic requirements for such formations: heat, water and permeability—the ability to flow water through the earth.

“The major challenge is how to create that reservoir and how to manage that resource,” Nix said. “The real challenge is underground—it’s an extension of petroleum techniques.”

One potential problem is addressing the geochemistry of the underground rock, because coursing hot water can pull minerals out of the rock that can end up clogging the return piping and even the artificially created fractures themselves. Another possible concern is this activity could stimulate seismic activity. A trial project near Basel, Switzerland, was terminated after small tremors were recorded.

Funding hurdles

Seeing the possibilities geothermal technologies offer for clean, renewable electricity generation, many in the field are frustrated by the lack of DOE funding for advanced geothermal research. Because plants such as those at The Geysers are commercially successful, the DOE considers the technology to be mature, without need for added government support. Supporters, on the other hand, have great optimism for the technology’s potential, and are frustrated by what they view as a seesawing approach to DOE funding for U.S.-funded geothermal research.

“The fact they’ve kept trying to shut down that program really hurts,” Gawell said. “It’s not how much money you have but the consistency of that funding. If you want to start a project, you want to know you’ll be able to finish it.” EC

ROSS is a freelance writer located in Brewster, Mass. He can be reached at chuck@chuck-ross.com.