Geothermal power

Geothermal power, a source derived from the earth’s internal heat, offers a form of energy used in parts of the United States and other countries. Although limited by technology, the earth’s heat as a power source offers immense resources, high versatility, and cost-effectiveness.

Geothermal Phenomena

Geothermal power, as evidenced by hot springs, steam vents, geysers, and volcanic activity, is the earth’s inner heat escaping through faults and weak spots in the crust. Temperatures below the surface vary by location, depth and temperature of the heat source, and the ability of the subsurface rocks to conduct heat. Generally, geothermal water is not sufficiently hot to produce electricity but can be used in a variety of industrial and agricultural applications.

Most of the heat energy escapes through the crust at lithospheric plate boundaries near young volcanic centers, such as those in Hawaii, Alaska, and the western United States. In the latter region, most of the geothermal hot spots are in Nevada, which has more than nine hundred hot springs and wells. This region is the Basin and Range geologic province, where a thin continental crust traversed by a system of north-south faults, allows an easy path for hot water to percolate to the surface. By way of comparison, most of the earth’s surface is nonthermal, having temperature gradients from 10 to 40 degrees Celsius per kilometer of depth. It is important to keep in mind, however, that even in nonthermal areas there is a great amount of heat several kilometers below the surface. The hot rocks at these depths, although not permeable, could be accessible except for the cost of extracting heat of this grade. The presence of a geothermal field is not always indicated by surface thermal activity. Excellent fields have been detected in places completely devoid of any surface thermal manifestations.

Geothermal Energy Resources

Geothermal heat can be commercially exploited in regions where hot pore fluids circulate within permeable formations and can be reached by cost-effective drilling. Geothermal energy resources include dry steam, hot water convection systems, geopressured systems, and hot dry rocks. For temperatures of 200 degrees Celsius or higher, the most economical and easily used source is dry steam—so called because the incredibly hot pure vapor lacks droplets—but this accounts for only a small fraction of all US geothermal resources. Hot water convection systems are heated by magma sources near the surface, thereby transferring heat to the surface rocks. Water that is confined within the rock layers is heated above the boiling point, but pressure from surrounding rocks prevents the water from becoming steam. Water heated by molten magma may reach temperatures of more than 300 degrees Celsius.

Geopressured fields do not fall within the general classification of geothermal fields, as they occur in nonthermal areas. These fields occur at depths up to 6,000 meters where temperatures range from less than 93 to more than 150 degrees Celsius. The hot water is pressurized in excess of the hydrostatic values encountered at that depth; that is, the water is sealed in the rocks and compressed. These very high pressures are thought to be caused by gradual subsidence along faults of rock that has trapped pockets of water below and between alternating layers of sandstone and shale. Geopressurized fields produce three types of energy—thermal, as a result of the fluid temperatures; hydraulic, as a result of the high excess fluid pressure; and chemical, as a result of the caloric value of the methane gas dissolved in the water. The immense energy contained in geopressurized fields has been difficult to attain because of their great depths below the surface and the need for advanced technology to exploit this source.

These geothermal systems utilize processes of groundwater circulation whereby drillers merely need to construct a well to reach the water table. If nature does not oblige, drillers attempt to inject water in the ground and create fractures in the rock strata using explosives. The fractures, in turn, provide pathways for the flowing water to pick up heat from the surrounding rock. Hot dry rocks are found at moderate depths by proven methods, but a far greater number lie at depths beyond which drilling may not be cost-effective. Many believe that the large-scale extraction of energy from hot dry rock could have significant long-range payoffs. Considering regions of thermal gradients of 40 degrees Celsius or more for each kilometer of depth, it is estimated that these heat sources could provide up to ten times the heat energy of all coal deposits in the United States.

A small-scale geothermal resource is heat stored near the surface, which can be extracted by a heat pump for heating a building during the winter. The same system usually can be reversed for cooling in the summer. Heat pumps are most suitable for climates with moderate winters. In this case, the source is mostly solar heat absorbed by the earth’s surface, not heat from the deep interior of the earth.

Geothermal Facilities

The direct flash system was used at the Geysers, the world’s largest geothermal facility, located 140 kilometers north of San Francisco. In 2021, the Calpine Corporation, which operates the Geysers facility, declared its total capacity to be 900 megawatts. Commercial geothermal power has been produced at the Geysers since 1960. Steam at these facilities is first allowed to pass through centrifugal separators, which remove rock particles that may damage turbine blades. The steam travels to the turbines by means of insulated pipes.

The United States’ largest geothermal resource may be located in the Imperial Valley of California. The valley itself is flat but covers at least six known geothermal resources that have thermal fluid temperatures ranging from 120 to 330 degrees Celsius, circulating in layers of porous rock between 1,200 and 6,000 meters below the surface. By th3 2010s, ten plants with a combined capacity of 327 megawatts were in operation in the Salton Sea Known Geothermal Resource Area of Imperial Valley. In 2022, eleven plants were contributing to production.

A 10-megawatt demonstration plant using the direct flash method operated in Brawley, California, from 1980 to 1985. In the direct flash method, water at temperatures above 180 degrees Celsius is pumped to the surface under pressure and routed through a series of pathways, which reduces the pressure and allows some of the liquid to vaporize or “flash boil.” The steam, constituting 20 percent of the fluid, is expanded through a standard steam turbine. A problem with the Brawley geothermal fluid was that it contained 15 percent total dissolved solids, which could choke a three-inch pipe diameter to half an inch after 100 hours of operation. Mineral buildup, or scaling, has been a major engineering challenge at the first hot-water power plants. Consequently, when the Brawley area underwent further geothermal development in 2008, a 50-megawatt binary system was installed instead.

Facilities Utilizing Binary Systems

In the binary system, pressure prevents the water from flashing to steam. Instead, the hot brine is allowed to flow through a heat exchanger that is surrounded by a heat exchanging fluid (commonly isobutane, which vaporizes at –11 degrees Celsius). The fluid expands and, under high pressure, drives a turbine. By keeping the brine under high pressure when it leaves the turbine at a temperature of about 105 to 180 degrees Celsius, engineers believe that they can avoid the scaling problems that clog pipes. The brine is pumped to a well injection station 4 kilometers away, on the edge of the Heber geothermal reservoir. Brine returned to the ground is geologically recycled and available for future use.

The Imperial Magma test facility in the East Mesa, California, geothermal field was a binary system that used brine at a low to moderate temperature with two hydrocarbon working fluids, each passing through a heat exchanger. Heat first moves from the hot brine to vaporize isobutane to run the turbine. The brine cools and moves on to a second-stage heat exchanger, which vaporizes the propane at a lower temperature and rotates a 2-megawatt turbine generator. The second turbine, operating at a considerably lower temperature, increases the efficiency of the system.

The appeal of the binary system is that it gains higher efficiency with moderately low-temperature brines. For high-temperature geothermal fields, using working fluids with lower boiling points may permit engineers to extract energy from the fluids as the brine temperature drops over time. This will help extend the production life of some fields, since most must produce for thirty to thirty-five years to be economical. Another appeal is that a well-designed binary power plant can operate with virtually no emissions. Geothermal systems at low temperatures operate at a lower efficiency than conventional power plants, which typically operate at high temperatures.

Geothermal Exploration

Geothermal exploration involves the teamwork of specialists versed in a variety of disciplines. Geothermal explorers in general will have several objectives in mind: They must find likely locations of geothermal fields and decide whether the field located has a sufficient source of heat and whether the field is steam- or water-dominated. Of the sources of exploitable geothermal energy, the high-temperature fields are the most promising commercially. These fields are almost always located in young orogenic regions or mountain belts where there has been recent volcanism.

The task of the team’s geologist is to construct as accurately as possible a model of the thermal region’s geological structure and to predict promising drilling sites. This is accomplished to a great extent by surface mapping and the study of the tilt of rock outcrops. Much of the model is deduced by direct observations, which are not generally possible below the surface. By studying hot-spring deposits, the geologist can estimate approximate subsurface temperatures.

The function of the hydrologist is to work closely with the geologist and to determine the paths that water will follow underground through geologic strata and within the boundaries of the geologist’s model. The hydrogeologist studies the gradients, porosities, and permeabilities of the various geological formations and may be able to offer a reasonable explanation for the thermal fluids reaching permeable zones in the field and, from that point, how they escape to the surface.

The geophysicist’s task in geothermal exploration is to determine as accurately as possible the physical properties of the subsurface and to detect anomalies. A geothermal field with large volumes of steam and hot water in permeable rocks will likely appear anomalous when compared to surrounding regions. The geophysicist will use the thermometer and its various forms, including thermocouples and thermistors, to deduce temperature gradients, heat-flow rates, and local hot spots. Electrical resistivity measurements may be taken by placing electrodes into the ground and measuring the voltages between them. Differences in resistance of rocks can be attributed not only to the physical differences in rocks but also to the presence of steam or electrically conductive thermal waters. Additional techniques employed may be gravity and seismic measurements, which can delineate variations in the densities of rock strata and the presence of faults for the migration of pore fluids.

The function of the geochemist is to analyze the chemistry of natural thermal discharges. If the fluids are hot, chemical equilibrium will be achieved rapidly; the chemical nature of the discharged fluids reflects the temperature achieved at equilibrium. The geochemist will look for the presence of silica and magnesium and at ratios of sodium to potassium as indicators of deep reservoir temperatures. The lower the sodium-potassium ratio, the higher the fluid temperature. The geochemist also can detect valuable minerals in the thermal fluids that are of interest to industrial geothermal developers.

The aim of this preliminary exploration is to choose promising locations for exploratory well drilling. If the exploration team believes that a useful field exists, then drilling is used to locate zones of permeable rocks saturated with hot thermal fluids. Drilling expense and time are saved if the exploration drilling bore samples are of small diameter; larger bores may follow once the potential of the field has been established.

Value as Energy Source

Geothermal energy offers significant savings over conventional thermal cost and nuclear fuel expense. The construction of geothermal power plants can yield significant savings, as the geothermal energy is already present within the earth and does not need to be generated. When used along with other forms of energy production, geothermal power can help reduce the overall cost of energy. The use of geothermal power is becoming increasingly important on a worldwide scale; in fact, as of 2015 twenty-five countries were actively producing geothermal power, with more than fifty countries involved in geothermal power development and anticipated to be generating geothermal power by 2020. The key lies in the extent to which humans can harness the earth’s heat, particularly the deep heat sources that have been at the limits of technology.

The ultimate goal is universal heat mining; that is, utilizing the earth’s heat wherever it is needed, even in the nonthermal areas. Meeting this goal could require drilling 6.5 to 9 kilometers for power generators operating at thermal efficiencies of 15 to 20 percent. Included in heat mining is the direct tapping of the magma pockets of active volcanoes; because of the very high temperatures encountered, this is a formidable task for conventional drilling even at shallow depths. Drilling into magma presents no danger of triggering a volcanic eruption because the drill hole is far smaller than the size of a volcanic vent. In fact, a geothermal project in Iceland briefly spurted lava in 2009.

Geothermal energy has several limitations. Such gases as methane and carbon dioxide do not contribute to energy production by flashing and must be removed in some geothermal fields. The thermodynamic efficiency is always low as well. Thermodynamic efficiency is the temperature drop in a process divided by the initial temperature, as measured in degrees kelvin, or degrees from absolute zero (–273 degrees Celsius). If steam has a temperature of 400 kelvins (127 degrees Celsius) and 300 kelvins (27 degrees Celsius) after flashing, the thermodynamic efficiency is 100/400 = 25 percent. The low efficiency is the inevitable result of using steam at relatively low temperatures. After engineering losses are considered, most geothermal plants achieve efficiencies of only a small percentage.

In addition, the fluids from hydrothermal fields accompanied by gases and certain water-soluble chemicals can be potentially hazardous to the machinery components of a geothermal facility as well as to the environment. Some deposits, such as silica and iron sulfide, build up in discharge channels on turbine blades and pipes, which reduces efficiency. Carbon dioxide and hydrogen sulfide in solution may form large corrosion pits on the pipe walls. The discharge of hydrogen sulfide into the atmosphere at geothermal power plants may have an adverse effect on crops, river life, and electrical equipment. Fortunately, scrubber systems can clean hydrogen sulfide and other gases from the effluent, and sometimes those gases can even be converted into useful products, such as fertilizer. Also, well-designed geothermal plants release only a few percent of the carbon dioxide and sulfur compounds produced by coal-fired plants and oil-fired plants. As stated above, closed binary cycle plants can operate with virtually no emissions.

By the end of the 1990s, the installed capacity of geothermal electrical power plants was about 6,000 megawatts worldwide, including 2,200 megawatts in the United States; as of 2015, the installed capacity of geothermal power plants was about 13,000 megawatts worldwide, according to International Energy Agency data. The United States had 3,794 megawatts of geothermal power capacity by 2022. That year geothermal energy consumption ranked fourth in the United States among renewable energy sources, following hydroelectricity, biomass, and wind power but ahead of solar power. The top producers of geothermal power in 2020 were the United States, Indonesia, the Philippines, Turkey, New Zealand, and Mexico. Though the use of hydroelectric power increased in the first two decades of the twenty first century, some issues arose. In 2021, one of California's largest plants, Oroville Dam, was closed because of droughts. It was restarted in 2022.

Principal Terms

binary cycle: the process whereby hot water in the primary cycle gives up heat in a heat exchanger; a fluid such as isobutane in the secondary cycle absorbs heat, is pressurized, and drives a turbine generator

direct or single flash cycle: the process whereby hot water under great pressure is brought to the surface and is allowed to turn, or “flash,” to steam, driving an electrical turbine generator

double flash cycle: the process whereby two flash vessels are employed in cascade, each operating a turbine to extract more power

hydraulic fracturing: also called "fracking"; the underground splitting of rocks by hydraulic or water pressure as a means of increasing the permeability of a formation

hydrostatic pressure: pressure within a fluid at rest, exerted at a specific point

hyperthermal field: a region having a thermal gradient many times greater than that found in nonthermal, or normal, areas

permeable formation: a rock formation that, through interconnected pore spaces or fractures, is capable of transmitting fluids

thermal gradient: the increase of temperature with depth below the earth’s surface, expressed as degrees Celsius per kilometer; the average is 25 to 30 degrees Celsius per kilometer

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