Geothermal energy is heat contained below the earth's surface. Hydrothermal energy (trapped hot water or steam) is the only type of geothermal energy that has been widely developed. New technologies are being explored to use hot dry rock (accessed by drilling deep into rock), geopressured resources (pressurized brine mixed with methane) and magma. Commercial electricity is produced by using geothermally-heated fluid to turn a turbine connected to a generator. The fluid may be the naturally occurring steam or water or another fluid which has had geothermal heat transferred to it through a heat exchange system. Closed-loop systems directly convert geothermal steam or hot water into electricity. Gases or fluids removed from the well are not exposed to the atmosphere and are usually injected back into the ground after releasing their heat. Open-loop systems expel waste steam and gases into the atmosphere and generally result in greater environmental impacts than closed-loop systems.

In a hot dry rock geothermal plant, water under high pressure is pumped through a specially drilled well into a deep body of hot compact rock, causing its hydraulic fracturing. The water is heated by passing through the fractures and drawing heat from the surrounding rock. The heated water is removed through a second well which was drilled for that purpose. The entire system consists of the borehole used to inject the cold water and a second borehole used to extract the geothermally heated water.

Fish and Wildlife Considerations

Air and water pollution are two leading environmental issues associated with geothermal energy technologies. Additional concerns are the safe disposal of hazardous waste, siting and land subsidence. Most geothermal power plants require a large amount of water for cooling or other purposes. This need could raise conflicts with other users or uses such as fish spawning and rearing in areas where water is in short supply. Steam vented at the surface may contain hydrogen sulfide, ammonia, methane and carbon dioxide. Dissolved solids discharged from geothermal systems include sulfur, chlorides, silica compounds, vanadium, arsenic, mercury, nickel and other toxic heavy metals. All of these releases, if concentrated, can create localized fish and wildlife kills. Geothermal resource development is often highly centralized, so reducing their environmental impacts to an acceptable level is achievable.