Geothermal energy, which taps into reservoirs of steam and hot water beneath the earth's surface, is among the least explored sources of renewable energy in the United States. In 2005, geothermal energy produced just 0.36 percent of the electricity used in this country, about 3,000 megawatts of energy.(1)

In 2006, the National Renewable Energy Laboratory estimated that 26,000 megawatts of geothermal power could be developed by 2015, and that by 2025 more than 100,000 megawatts of geothermal power could be in production. Today more than 100 geothermal projects, with a total capacity of nearly 4,000 megawatts, are in various stages of development across the western United States, where most geothermal resources are concentrated.(2)

How It Works

When superhot magma from deep within the earth comes close to the surface, it heats underground water and traps it in cracks and porous rock, creating reservoirs of very hot water and steam. Deep wells can tap the high energy content of this water and steam to drive a myriad of energy services, including electricity, heating, cooling, industrial processes and even melting snow on roads.

Another way to use geothermal energy on a smaller scale is through a geothermal heat pump, which exploits the temperature difference between the earth's surface and the air. In most places, the temperature at 10 feet below ground level remains between 50 and 60 degrees Fahrenheit year-round. In winter, a geothermal heat pump pulls heat from the relatively warmer ground and pumps it into a building. In summer, the pump cools the same building by pulling the building's warmer air into the relatively cooler ground, where the excess energy can be used in turn to heat water.

Where It's Used

Most of the United States' geothermal resources are located in the West, and in Alaska and Hawaii.

The vast majority of geothermal power is produced in California.(3) In addition to fueling power plants, geothermal energy is used to heat spas, pools, homes, greenhouses and aquaculture ponds.(4) In Klamath Falls, Oregon, geothermal heat melts snow on sidewalks during the winter.(5) In Empire, Nevada, 15 million pounds of dried onions and garlic are produced each year at a dehydration plant powered by geothermal energy.(6)

How Much It Costs

At California's The Geysers, which has been operational since 1960, power is sold at $0.03 to $0.035 per kilowatt-hour. A new geothermal plant would probably charge about $0.05 per kilowatt-hour, though some plants can charge more during peak demand periods.(7) While the initial costs of drilling and installing geothermal power plants are high, operation and maintenance costs are low – and there are no fuel costs at all, which keeps the price of the energy from fluctuating.

Industry experts agree that geothermal energy could be cost-competitive with fossil fuel energy in the short term.

Advantages

Geothermal energy is a clean, green, renewable resource. The earth has been emitting heat from its center for 4.5 billion years and shows no signs of slowing down. A geothermal plant in Italy has been operation since 1913, demonstrating the sustainability of this power source. However, some underground aquifers can be affected by a lack of precipitation, in which case geothermal power plants can reinject fluids underground to replenish reservoirs. For example, the city of Santa Rosa, California, pipes its treated wastewater to the Geysers power plant to be used as reinjection fluid.(8)

Geothermal power plants produce almost no global warming pollution and emit very little air pollution – sometimes none at all. And unlike solar or wind energy, geothermal energy is available around the clock.

According to a report by the Western Governors Association (WGA), near-term development of 5,600 megawatts of geothermal energy would result in the creation of almost 100,000 jobs.

What's around the Corner

Enhanced Geothermal Systems, or EGS technology, is being developed to produce energy from hard-to-reach geothermal resources, such as those with less water or those in harder beds of rock. This advanced technology would expand the capabilities of geothermal power many times over. The Department of Energy estimates that EGS could produce at least 100,000 megawatts of electricity within 50 years.(9)

Google.org, the philanthropic arm of Google, is a major investor in EGS. The organization is funding research and developing informational tools to help advance the technology.(10)

Renewable Energy Meets Wildland and Wildlife Conservation

Certain sensitive lands -- such as parks, monuments and wildlife conservation areas -- and ecologically sensitive marine areas are not appropriate for energy development. In some of these places, energy development is prohibited or limited by law or policy, and in others it would be highly controversial. NRDC does not endorse locating energy facilities or transmission lines in such areas. Siting decisions must always be made extremely carefully, with impacts mitigated and operations conducted in an environmentally responsible manner.

For more information on the intersection between clean energy development and wildland and wildlife conservation in the American West, including locations of parks, wildlife refuges and other conservation areas, see this Google Earth-based feature.