The interior of the Earth is hot, mainly from energy released by decay of radioactive elements. In some places on the planet, the crust is either thin enough or fractured enough that some of that heat comes close to the surface. Geothermal electricity is created by using this heat to run turbines.
On paper, geothermal would seem to be an ideal non-carbon source of energy. The earth's heat is nearly inexhaustible from our perspective. Geothermal facilities require very little land compared to other renewable energy generating plants. Also unlike most renewable sources of energy, geothermal offers the possibility of base load power production: a geothermal plant could in theory run nearly non-stop.
That's not to say there are no environmental downsides to geothermal. For one thing, it's location dependent: in order to be economical, geothermal facilities must be sited where the earth's interior heat rises to within striking distance from the surface. For the moment that rules out places like Nebraska, where the crust is thick. Geothermal power plants are, at present, limited to volcanically and tectonically active parts of the planet. Iceland, which is thought to sit atop a volcanic plume or "hot spot," derives a third of its total energy consumption from geothermal.
There are, however, some experimental plants in parts of the US that aren't particularly geologically active, such as the lower Mississippi basin. Depending on the success of these experiments, we may someday be able to develop cost-effective geothermal power in states without earthquakes or volcanoes.
The United States has the largest geothermal infrastructure of any nation, though it contributes only a small amount of out total electrical consumption. Nevada and California lead the pack among the 15 states with geothermal installations, with installations totaling just under 4,000 megawatts and 2,000 megawatts, respectively. Northern California's cluster of geothermal plants at The Geysers is the largest in the world, at 725 megawatts of capacity.
There are three basic kinds of geothermal power plants:
- Dry steam, in which steam created naturally deep underground is harnessed to drive turbines. (The plants at The Geysers are of this type.)
- Flash steam, in which subterranean water that is still in liquid form due to high pressure is pumped into lower-pressure tanks, where it "flashes" into steam that then drives turbines. The geothermal plants in the Imperial Valley are of this type.
- Binary cycle plants, which take advantage of geothermal water that's not quite hot enough to drive turbines on its own. In these plants, the geothermal water is pumped out of the ground (rather than emerging due to its own pressure) into a heat exchanger, where it heats a secondary fluid with a lower boiling point than water. This secondary fluid, usually a light hydrocarbon like butane, then drives the turbines. There's one of these in Mammoth Lakes.
The environmental effects of geothermal generally stem from taking geothermal water out of the ground.
Most straightforward is the threat of altering the water table, drying up hot springs and possibly even causing land subsidence, as has happened at one geothermal field in New Zealand.
Geothermal water also carries with it a significant amount of dissolved material, and releasing that into the surface environment can cause problems. This material can include heavy metals and other toxic elements such as mercury, arsenic, and boron; dissolved gases like methane, hydrogen sulfide and carbon dioxide, and even radioactive elements. When flash steam plants in the Imperial Valley condense their used geothermal water back into "production brine," it may contain as much as 30% of its weight in these dissolved substances. Solids are separated out of production brine and the remaining liquid is reinjected into the geothermal field. The remaining solid waste -- called "filter cake" -- is sent to a hazardous waste facility. In the Imperial Valley's geothermal fields the total annual production of this hazardous solid waste runs between 40,000 and 60,000 tons.
One other technique often referred to as geothermal energy is the use of ground source heat pumps to heat and cool buildings. This is probably better referred to with some other terminology, as it doesn't generally involve taking advantage of the heat generated within the earth, but actually relies on subsoil and bedrock functioning as a thermal mass. A ground source heat pump moves air from a building through a series of pipes buried 20 feet or so beneath the surface. That deep, the earth tends to maintain a constant temperature somewhere between 50° and 60° F: a heat exchanger at that depth can provide room air that's moderately warm in winter, and comfortably cool in summer. A drawback of the technique is that electrical power is generally needed to operate the air pump, which can reduce the heat pump's carbon benefit if the power comes from conventional sources.