Ground Source Geothermal
Few people realize that a form of geothermal energy is available almost everywhere, through use of the ground's massive heat capacity as a way to effectively store energy through the summer, and retrieve it during the winter. Ground source geothermal or Geo-exchange systems use heat pumps to provide hot water and air conditioning or heating throughout the year, using typically only one quarter to one fifth of the electricity required for resistive water heating.
Switching residential and commercial heating systems to geo-exchange systems provides an efficient and almost uniformly applicable means of replacing a larger fraction of current oil use with a much more efficient use of electricity.
Magma which protrudes closely to the earth's surface heats up the surrounding rock and water. Engineers have designed power plants that exploit this heat source and convert it to other forms of energy. Geothermal energy is a mainstay of the Icelandic economy, and is also heavily exploited in the United States, with 43 geothermal plants. Most American geothermal plants are located in California and Nevada. The largest site is The Geysers, located in the wine country of Northern California.
Geothermal energy plants produce no significant additional emissions. It is generally considered renewable energy, though hot spots can cool if the plants attempt to extract energy faster than it is renewed from deeper down.
The single biggest disadvantage of geothermal energy is that it is simply not available in areas which are not geologically "hot". Most of the United States has no capacity to generate geothermal power -- geothermal hotspots are basically baby volcanos and tend to be found in geologically active areas. Geothermal energy is also not without environmental impacts.
Residents of Lake County, California, one of the largest geothermal energy production areas in the United States, report that geothermal energy extraction by Calpine Energy in their area has resulted increasingly in low-level earthquakes (magnitude 1-4). These residents claim the earthquakes, which are now occuring twice a day, have damaged their homes and foundations. Calpine disputes the claim, but residents are pursuing the creation of a trust account from geothermal royalties that would compensate homeowners for damage.
Another disadvantage, present in most energy production, is the "footprint" of equipment on the ground and disruption of wildlife habitat.
In addition, lower-temperature water is injected back into the ground to acquire geothermal energy, potentially affecting geysers and other natural formations. For this reason (and others) geothermal exploitation is prohibited in Yellowstone National Park.
Finally, like nuclear power, coal produced power, and hydropower, geothermal energy is not easily transported. It can be used to produce electricity and, on a more localized basis, heat, but can't be used as a substitute for oil in the transporation industry except by producing hydrogen fuel.