All of the following power plants use steam to drive the turbines to produce electricity except

There are three types of hydropower facilities: impoundment, diversion, and pumped storage. Some hydropower plants use dams and some do not.

Although not all dams were built for hydropower, they have proven useful for pumping tons of renewable energy to the grid. In the United States, there are more than 90,000 dams, of which less than 2,300 produce power as of 2020. The other dams are used for recreation, stock/farm ponds, flood control, water supply, and irrigation.

Hydropower plants range in size from small systems suitable for a single home or village to large projects producing electricity for utilities. Learn more about the sizes of hydropower plants.

IMPOUNDMENT

The most common type of hydroelectric power plant is an impoundment facility. An impoundment facility, typically a large hydropower system, uses a dam to store river water in a reservoir. Water released from the reservoir flows through a turbine, spinning it, which in turn activates a generator to produce electricity. The water may be released to meet changing electricity needs or other needs, such as flood control, recreation, fish passage, and other environmental and water quality needs.

A diversion, sometimes called a “run-of-river” facility, channels a portion of a river through a canal and/or a penstock to utilize the natural decline of the river bed elevation to produce energy. A penstock is a closed conduit that channels the flow of water to turbines with water flow regulated by gates, valves, and turbines. A diversion may not require the use of a dam.

Another type of hydropower, called pumped storage hydropower, or PSH, works like a giant battery. A PSH facility is able to store the electricity generated by other power sources, like solar, wind, and nuclear, for later use. These facilities store energy by pumping water from a reservoir at a lower elevation to a reservoir at a higher elevation.

When the demand for electricity is low, a PSH facility stores energy by pumping water from the lower reservoir to an upper reservoir. During periods of high electrical demand, the water is released back to the lower reservoir and turns a turbine, generating electricity. 

Hydropower facilities range in size from large power plants, which supply many consumers with electricity, to small and even ‘micro’ plants, which are operated by individuals for their own energy needs or to sell power to utilities.

Large Hydropower

Although definitions vary, DOE defines large hydropower plants as facilities that have a capacity of more than 30 megawatts (MW).

Small Hydropower

Although definitions vary, DOE defines small hydropower plants as projects that generate between 100 kilowatts and 10 MW.

Micro Hydropower

A micro hydropower plant has a capacity of up to 100 kilowatts. A small or micro hydroelectric power system can produce enough electricity for a single home, farm, ranch, or village.

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The United States of America continues to generate the most geothermal electricity in the world: more than 3.5 gigawatts, predominantly from the western United States. That's enough to power about three and half million homes! Pictured above, the Raft River geothermal plant is located in Idaho. Source: Geothermal Resources Council

A geothermal resource requires fluid, heat and permeability in order to generate electricity:

Fluid—Sufficient fluid must exist naturally or be pumped into the reservoir.

Heat—The earth's temperature naturally increases with depth and varies based on geographic location.

Permeability—In order to access heat, the fluid must come into contact with the heated rock, either via natural fractures or through stimulating the rock.

Conventional hydrothermal resources contain all three elements naturally. Increasingly, however, geothermal systems where subsurface fluid and permeability are lacking are being engineered or enhanced to access the earth's heat by adding fluid to these hot subsurface resources. Known as enhanced geothermal systems (EGS), this technology could be a game-changer in the geothermal sector, tapping 100+ gigawatts of geothermal energy, roughly ten percent of domestic energy demand.

In addition, low-temperature and coproduced technologies are being explored for near-term power solutions. .

POWER PLANTS

Power plants use steam produced from geothermal reservoirs to generate electricity. There are three geothermal power plant technologies being used to convert hydrothermal fluids to electricity—dry steam, flash steam and binary cycle. The type of conversion used (selected in development) depends on the state of the fluid (steam or water) and its temperature.

Dry Steam Power Plant

Dry steam plants use hydrothermal fluids that are primarily steam. The steam travels directly to a turbine, which drives a generator that produces electricity. The steam eliminates the need to burn fossil fuels to run the turbine (also eliminating the need to transport and store fuels). These plants emit only excess steam and very minor amounts of gases.

Dry steam power plants systems were the first type of geothermal power generation plants built (they were first used at Lardarello in Italy in 1904). Steam technology is still effective today at currently in use at The Geysers in northern California, the world's largest single source of geothermal power.

Flash steam plants are the most common type of geothermal power generation plants in operation today. Fluid at temperatures greater than 360°F (182°C) is pumped under high pressure into a tank at the surface held at a much lower pressure, causing some of the fluid to rapidly vaporize, or "flash." The vapor then drives a turbine, which drives a generator. If any liquid remains in the tank, it can be flashed again in a second tank to extract even more energy.

Binary cycle geothermal power generation plants differ from Dry Steam and Flash Steam systems in that the water or steam from the geothermal reservoir never comes in contact with the turbine/generator units. Low to moderately heated (below 400°F) geothermal fluid and a secondary (hence, "binary") fluid with a much lower boiling point that water pass through a heat exchanger. Heat from the geothermal fluid causes the secondary fluid to flash to vapor, which then drives the turbines and subsequently, the generators.

Binary cycle power plants are closed-loop systems, and virtually nothing (except water vapor) is emitted to the atmosphere. Because resources below 300°F represent the most common geothermal resource, a significant proportion of geothermal electricity in the future could come from binary-cycle plants.