Geothermal energy systems rely on two basic components: the heat beneath the earth’s crust, and the subterranean waters that the earth’s heat will turn to steam. In most geothermal systems, accessing these components involves drilling as deep as two miles below the surface of the earth.
In direct-use geothermal systems, the earth’s natural steam is piped directly into buildings to warm them in winter and, perhaps surprisingly, to cool them in summer. While the temperature on the surface of the earth varies throughout the year, the temperature in the upper ten feet of the earth remains fairly constant, usually between 50 and 60 degrees F. The benefits of this constant temperature can be accessed by pumping the water from springs or reservoirs near the earth’s surface into buildings for interior climate control.
Direct-use of geothermal heat is often achieved through the use of a heat pump, which efficiently extracts the earth’s thermal energy. Besides maintaining indoor temperatures, geothermal heat can be used to heat greenhouses, heat water at fish farms, pasteurize milk, and provide the heat required for range of industrial processes. A large centralized geothermal pump can even provide heating for an entire community, known as “district heating.”
Geothermal energy is also used to drive electric generators in a number of ways:
Dry steam systems are the oldest and simplest application of geothermal power, in which the steam released from a geothermal reservoir is captured and used to rotate turbines which generate electricity.
Flash steam systems utilize a more technologically sophisticated method of electrical generation and are the most widely deployed. These systems use intense pressure to keep water in liquid form, even as it is heated to temperatures well above its boiling point at sea level. The water is then exposed to an abrupt drop in pressure, causing it to convert in a flash to steam, which more efficiently rotates the steam turbines to generate electricity.
Binary cycle systems direct the earth’s hot water upward to a heat exchanger above ground, where the heat is transferred to a pipe containing a fluid with a much lower boiling point than water (usually isobutane or isopentane gas). The transferred heat vaporizes the liquid, and that steam rotates turbines to produce electricity. The advantage of this system is that it can make use of geothermal reservoirs that have lower temperatures, increasing the places where geothermal systems can be located.
Enhanced geothermal systems (EGS) (or hot dry rock systems) may be yet another avenue into the earth’s deep power potential. Rather than harvesting the heated water already in the earth, this method involves manufacturing steam by piping surface water down into the hot but dry rocks in the earth’s crust. A main benefit of this system is that it does not require the high temperature geothermal resources of other geothermal electric technologies, and it can be used nearly anywhere on the planet. While the technology’s potential is great, further research and development is required before it can be deployed at scale.