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Geothermal Energy System Descriptions

Utility-scale geothermal energy developments typically involve a moderate commitment of land and consist of primary and ancillary facilities, including a grid connection.

Geothermal Energy System Types

The three basic types of utility-scale geothermal power plants are flash steam, binary cycle, and dry steam. The type of power plant used to generate electricity depends on the temperature, depth, and quality of the water and steam in the geothermal reservoir. These power plants can also be hybridized by including elements of the different power plant types at a single location. All three types of power plants inject the used geothermal fluid back into the ground to replenish the reservoir and recycle the hot water.

A cooling system is essential for the operation of a geothermal power plant. Cooling towers prevent turbines from overheating and prolong the life of the facility. The two types of cooling systems are water cooling and air cooling. Most power plants, including geothermal power plants, use water cooling (evaporative) systems.

Flash Steam Power Plant Schematic
Flash Steam Power Plant Schematic
Source: BLM and USFS
Flash Steam Power Plant Schematic
Source: BLM and USFS
Click to enlarge

Flash Steam Power Plants

The most common type of geothermal power plant to date is the flash power plant with a water cooling system. This system uses geothermal reservoirs of water with temperatures greater than 360°F (182°C). In this system, very hot water (above water's boiling point at standard atmospheric pressure) flows up through wells under its own pressure. As the water is pumped from the reservoir to the power plant, the drop in pressure causes the water to convert or "flash" into steam that is separated in a surface vessel (the steam separator) and delivered to a turbine that powers a generator. Leftover water and condensed steam are injected back into the reservoir for reuse. Flash plants emit small amounts of steam and gases.

Binary Cycle Power Plant Schematic
Binary Cycle Power Plant Schematic
Source: BLM and USFS
Binary Cycle Power Plant Schematic
Source: BLM and USFS
Click to enlarge

Binary Cycle Power Plants

Recent advances in geothermal technology have made it possible to produce electricity economically from lower-temperature geothermal resources, at 212° (100°C) to 302°F (150°C). Known as "binary" geothermal plants, these facilities reduce the emission rate to zero. In the binary process, geothermal water is used to heat another liquid, such as isobutene, that boils at a lower temperature than water. The two liquids are kept completely separate through the use of a heat exchanger used to transfer the heat energy from the geothermal water to a secondary fluid (also known as the "working fluid"). The secondary fluid vaporizes into gaseous vapor (like steam) and the force of the expanding vapor turns the turbines that power the generators. If the power plant uses air cooling, the geothermal fluids never make contact with the atmosphere before they are pumped back into the geothermal reservoir — effectively making the power plant emission-free.

Dry Steam Power Plant Schematic
Dry Steam Power Plant Schematic
Source: BLM and USFS
Dry Steam Power Plant Schematic
Source: BLM and USFS
Click to enlarge

Dry Steam Power Plants

Geothermal sources with dry steam generation capacity are very rare. In a dry steam plant, like those at The Geysers in northern California, steam (at temperatures greater than 455°F [235°C]) is piped directly from the geothermal reservoir to run the turbines that power a generator. No separation is necessary since wells produce only steam. Dry steam power plants emit excess steam and very small amounts of gases.

Flash/Binary Combined Cycle Power Plants

A combination of flash and binary technology, known as the flash/binary combined cycle, has been used effectively to take advantage of the benefits of both technologies. In this type of power plant, the flashed steam is first converted to electricity with a backpressure steam turbine, and the low-pressure steam exiting the backpressure turbine is condensed in a binary system. This allows for the effective use of air cooling towers with flash applications and takes advantage of the binary process. The flash/binary system has a higher efficiency where the well-field produces high-pressure steam, while the elimination of vacuum pumping of noncondensable gases allows for 100% injection. The Puna Geo Venture Power Plant in Hawaii is a flash/binary combined cycle power plant, where the geothermal fluid is over 600°F (316°C).

Emerging Technologies

Geothermal Energy from Oil and Gas Production

Oil and gas wells are typically thousands of feet deep and often produce very hot fluid. Along with the oil and gas, wells produce water that must be separated from the oil and gas and is usually reinjected deep below domestic aquifers. A new technology is currently being demonstrated at the Rocky Mountain Oilfield Testing Center (also known as the Teapot Dome Oilfield) near Casper, Wyoming in which the energy derived from the hot fluid during oil and production is used to produce electricity that can power the oil and gas pumps, eliminating the need for to purchase additional electricity (or construct power lines) to run the oil and gas facility.

Enhanced Geothermal System

Many deep wells in the United States do not encounter adequate fracture permeability to deliver a continuous source of fluid. An Enhanced Geothermal System (EGS) uses a combination of hydraulic, thermal, and chemical processes that improve the natural permeability of a geothermal reservoir by creating a conductive fracture network to which water can be added through injected wells. Injected water is heated by contact with the rock and returns to the surface through production wells, as in natural hydrothermal systems. This process extends the margins of existing geothermal systems and can be used to create entirely new ones. EGS technology was proven feasible through a demonstration project in Soultz-sous Forệts, France.

Hot Dry Rock

Most of the world's accessible geothermal energy is found in rock that is hot, but essentially dry. The potential for hot dry rock (HDR) technology is high in areas where the geothermal gradient is high, typically in tectonically active areas and in areas of high volcanic activity. Some scientists estimate that the energy content of HDR is 800 times greater than all hydrothermal resources and 300 times greater than fossil fuel resources, including petroleum, natural gas, and coal. The general concept of HDR technology involves the drilling of a well into hot crystalline rock and using water under pressure to create a large vertical fracture in the hot rock. A second well would access the fracture at some distance from the first well. Pressurized cold water would be injected into the deeper part of the fracture via the first well and, after passing across the hot surface of the fracture, would return to the surface as a superheated fluid through the second well. After extracting its energy, the same water would be recirculated to the HDR to mine more heat. HDR technology was proven feasible through a demonstration project at Fenton Hill, on the western rim of the Valles Caldera near Los Alamos, New Mexico, where the local geothermal gradient is about 3.6°F/100 feet (65°C/km). HDR programs have also been demonstrated in France, Japan, and Australia.

Geothermal Energy Facility Size

Geothermal energy facilities can range from small (300 kW) to medium (10 to 50 MW) to large (50 MW and greater). A 50-MW plant is estimated to use an area of up to 20 to 25 acres; a smaller 20 MW plant would require about five to 10 acres. However, the land area encompassing all on-site and off-site facility components (well pads, pipelines, power plants, access roads, and transmission lines) could be as high as 370 acres for a 50-MW plant. Depending on the cooling system used, the power plant itself would occupy from about 25% (water cooled plant) to 50% (air cooled plant) of this area.

Geothermal Energy Facility Components

Production and Injection Wells

Multiple production and injection wells (with depths of over 10,000 ft) may be drilled at a geothermal facility. The number of wells is dependent on the characteristics of the geothermal reservoir and its power generation capacity. For example, a 50-MW plant could require up to 25 production wells and 10 injection wells. Wells would be installed on a well pad, typically with an area of about one to five acres.

Power Plants and Cooling Systems

The types of geothermal power plants include flash steam, binary cycle, and dry steam, as described above. The operation of a geothermal power plant requires cooling towers to prevent turbines from overheating and prolong the life of the facility. Most geothermal power plants use water cooling (evaporative) systems. Water-cooled systems generally require less land than air-cooled systems and are considered overall to be stable, effective, and efficient. The drawback of water cooling systems, however, is that they require a continuous supply of cooling water and create vapor plumes. Air-cooled systems are efficient in the winter months (but are less efficient in hotter seasons) and in arid regions where water resources are limited. Air-cooled systems are preferred in areas where the viewshed is particularly sensitive to the effects of vapor plumes.

Pipeline System

Geothermal power plants are typically supported by a pipeline system in the plant's vicinity. A pipeline system includes a gathering system for produced geothermal fluids and an injection system for the reinjection of geothermal fluids after heat extraction takes place. Pipelines would be constructed on supports above ground and covered with insulation.

On-Site Ancillary Facilities and Components

Geothermal energy facilities would require a number of ancillary facilities and components. These would include fencing, buildings (other than the power plant), drilling rigs or derricks, water use, storage, and discharge facilities, and off-site components such as access roads and a transmission line.

Fencing: Some geothermal energy facilities may require fencing. Fencing would be about 6 to 10 feet high and have electric, barbed, or razor wire on top. Some fences would be modified to exclude some wildlife (e.g., desert tortoises) or allow access for other wildlife (e.g., kit foxes).

Buildings: Buildings other than the power plant itself could be required at a geothermal energy facility. These would include an administration/control building, fire control, and a maintenance building.

Drilling rigs or derricks and temporary support facilities: Drilling of wells at a geothermal energy facility would require a large drilling rig or the erection of a derrick. Various support facilities would also be needed on-site temporarily. These include generators, mud tanks, cement tanks, trailers for the drillers and mud loggers, housing trailers, and equipment storage sheds.

Water use, storage, and discharge facilities: Geothermal energy facilities would require water to meet cooling (if water-cooled) and potable needs. Water needs could be met by on-site wells, if water rights can be obtained. On-site tanks would be used to store raw water, fire water, and demineralized water. On-site evaporation ponds may be required to contain circulating water, blowdown, or other discharges. An on-site wastewater treatment system (e.g., septic system and leach field to treat sanitary wastes) would also be required. These could be sized to retain all solids that would be generated during the lifetime of the project.

Off-site facilities: Off-site facilities required for a geothermal energy plant would include access roads (to accommodate the larger equipment associated with the development phase and to access remote sites) and a transmission line. Environmental impacts from construction and operation of transmission lines are discussed separately in the Energy Transmission section.