Geothermal Power Plants: The 3 Types and How Each Works

A geothermal power plant is a machine for turning underground heat into electricity: hot fluid comes up a well, the heat spins a turbine, and the turbine drives a generator. The U.S. Department of Energy names three: "There are three main types of geothermal power plant technologies: dry steam, flash steam, and binary cycle."

Which of the three gets built is not a designer's preference. Per the DOE, "the type of conversion is part of the power plant design and generally depends on the state of the subsurface fluid (steam or water) and its temperature." Find out what the reservoir holds — steam or hot water — and how hot it runs, and you know which plant belongs on top of it. The sections below take each design in turn — the conditions it needs, how it converts heat to electricity — and then what the federal data does and does not settle about how the three compare.

A geothermal power plant and a home geothermal system are not the same machine. A power plant is utility-scale generating infrastructure sitting on a hot underground reservoir. The system attached to a house is a heat pump exchanging heat with mild, shallow ground — a different technology entirely, covered in our guide to how geothermal heating works. Everything below is about the power plants.

How a Geothermal Power Plant Works

Before the three designs split apart, they share a starting point. The DOE lays out the entry requirements plainly: "To generate power from geothermal systems, three elements are needed." The three:

• Heat — "abundant heat found in rocks deep underground, varying by depth, geology, and geographic location."
• Fluid — "sufficient fluid to carry heat from the rocks to the earth's surface."
• Permeability — "small pathways that facilitate fluid movement through the hot rocks."

Hot rock alone generates nothing. There has to be water or steam down there to soak up the heat, and there have to be fractures and pathways for that fluid to move through. Where all three coincide, a natural geothermal system exists, and a power plant can be built on it. Where the heat itself originates — the radioactive decay and planetary structure underneath all of this — is its own subject, covered in where geothermal energy comes from.

From there, every geothermal plant does the same job. Fluid is drawn from the underground reservoir through wells to the surface, the steam or hot liquid drives turbines that generate electricity, and the fluid is reinjected back into the reservoir. Well, turbine, generator, and — in the DOE's descriptions of all three designs — the fluid going back down.

The fork comes at the wellhead, and it is the DOE sentence from the top of this page: the plant design "generally depends on the state of the subsurface fluid (steam or water) and its temperature." A reservoir that delivers steam gets one kind of plant. A reservoir of very hot water gets a second kind. A reservoir of merely hot water gets a third — and the DOE draws the line between those last two at a specific number: 182°C, or 360°F.

Dry Steam Plants: The Original Design

Dry steam is the simplest case: the reservoir does most of the work before the plant touches anything. Per the DOE, "dry steam plants use hydrothermal fluids that are already mostly steam, which is a relatively rare natural occurrence. The steam is drawn directly to a turbine, which drives a generator that produces electricity. After the steam condenses, it is frequently reinjected into the reservoir."

There is almost nothing in that mechanism. No boiling step at the surface, no pressure trick, no heat exchanger. The earth sends up steam; the steam goes straight into the turbine. The DOE dates the design precisely: "Dry steam power plant systems are the oldest type of geothermal power plants, first used in Italy, in 1904." That same site is still the technology's reference point — "the Larderello geothermal power plant in Tuscany is the oldest dry steam power plant in the world."

The constraint is in the DOE's own phrasing: "a relatively rare natural occurrence." Reservoirs that naturally yield mostly steam, rather than hot water, are uncommon — so the design that asks least of the engineering asks the most of the geology.

Rare does not mean obsolete. Per the DOE, "steam technology is still relevant today and is currently in use in northern California at The Geysers, the world's largest single source of geothermal power." The Geysers gets its own section below.

Flash Steam Plants: A Pressure Drop Does the Work

Where a reservoir holds hot water rather than ready-made steam, and that water is hot enough, the flash steam design applies. The DOE calls the all-steam case "a relatively rare natural occurrence"; it publishes no figure for how common the hot-water case is. The DOE calls flash steam "a common type of geothermal power plant in operation today".

Here is the DOE's description of the machine: "Fluids at temperatures greater than 182°C/360°F, pumped from deep underground, travel under high pressures to a low-pressure tank at the earth's surface. The change in pressure causes some of the fluid to rapidly transform, or 'flash,' into vapor. The vapor then drives a turbine, which drives a generator. If any liquid remains in the low-pressure tank, it can be 'flashed' again in a second tank to extract even more energy."

Nothing heats the water at the surface. What changes is the pressure. Deep underground, the water is far above 360°F but held liquid under high pressure; released into a low-pressure tank at the surface, part of it converts to vapor almost instantly — that is the "flash." The reservoir supplies the heat; the plant supplies a pressure drop, and a second tank flashes whatever liquid remains.

The design's entry ticket is that 182°C/360°F figure — below it, the DOE's taxonomy hands the resource to the third design.

Binary-Cycle Plants: The Heat Exchanger Design

The binary-cycle plant is built around one structural decision the other two designs never make. Per the DOE, "binary-cycle geothermal power plants differ from dry steam and flash steam systems in that the geothermal reservoir fluids never come into contact with the power plant's turbine units."

Instead, the heat changes hands. The DOE again: "Low-temperature (below 182°C/360°F) geothermal fluids pass through a heat exchanger with a secondary, or 'binary,' fluid. This binary fluid has a much lower boiling point than water, and the modest heat from the geothermal fluid causes it to flash to vapor, which then drives the turbines, spins the generators, and creates electricity."

So the geothermal water, too cool to flash on its own, gives up its heat to a second fluid that vaporizes at a much lower temperature — and it is that second fluid, not the reservoir water, that spins the turbine. The reservoir fluid never reaches the turbine; "binary" just means there are two working fluids in the design. As for what the secondary fluid actually is: the DOE and EIA pages cited here describe it only by the property that matters — a much lower boiling point than water — and name no specific substance, so neither does this page.

The Energy Information Administration adds the environmental detail, and its scope is worth quoting exactly: "In this type of plant, the geothermal water never touches the turbine and is returned to the earth. This closed system produces no air emissions." "No air emissions" is its description of the binary design's closed loop — this type of plant — not a blanket statement about geothermal power plants in general.

Why does a design for cooler water matter so much? Because cooler water is where the growth is. Per the DOE, "binary-cycle geothermal power plants can use lower temperature geothermal resources, making them an important technology for deploying geothermal electricity production in more locations." The EIA's July 2020 analysis states the trade-off between the families directly: "Dry steam and flash plants, which require rarer high-temperature, shallow reservoirs, have higher power output and are, therefore, more economically efficient than binary plants." However, the same analysis notes, because binary plants can operate at reservoirs with lower temperatures, they have more options for suitable locations.

That is the shape of the "which type is better" answer: steam designs win on output wherever the rare, very hot resource exists; binary wins on how many places it can be built at all.

Which Type Is Most Common? The Sources Disagree

The federal sources do not agree on which of the three types is most common in the United States.

The EIA's Energy Explained page says it outright: flash steam plants are "the most common type." Note that the page does not say most common where — it attaches no geography to the claim. The DOE's page on the same three technologies says only that flash steam is "a common type of geothermal power plant in operation today" — and ranks nothing.

Then there is the EIA's own count. In its July 30, 2020 analysis, the EIA reported: "Currently, the United States has 93 binary cycle generators, averaging 8 MW of capacity each, and 79 steam generators that average 23 MW each." That count has two limits. The 79 "steam generators" lump dry steam and flash together — the published figure cannot be split into the two steam designs. Second, the units do not match the claim: the count is of generators, while the "most common type" statement is about plants, and those are different things.

The same July 2020 snapshot carried the capacity side: "Of the 2,558 megawatts (MW) of geothermal power plant capacity currently operating in the United States, 1,826 MW of capacity is from steam-powered plants and 731 MW of capacity is from binary-cycle powered plants." So as of mid-2020, binary led the generator count while steam-powered plants held the larger share of installed capacity — steam generators simply ran much larger, 23 MW average against 8 MW.

Lay the pieces side by side and the situation is this: one EIA page calls flash the most common type; the EIA's own 2020 generator count puts binary units ahead of all steam units combined; the steam figure cannot be split; and the units of measurement differ. No authority publishes a type-by-type count of U.S. geothermal plants — not the DOE, not the EIA — so "which type is most common" has, as of this writing, no verifiable U.S. answer.

Two Kinds of Temperature Numbers

Temperature figures for geothermal plants come in two kinds, and keeping them apart matters more than memorizing either one.

The first kind is a design threshold. The DOE's 182°C/360°F figure is the boundary line in the plant taxonomy: fluids "at temperatures greater than 182°C/360°F" get flash plants; "low-temperature (below 182°C/360°F)" fluids get binary plants. It is a line on the drawing board — the number that decides which machine gets designed for a given resource.

The second kind is a typical operating range. The EIA's July 2020 analysis reports that "dry steam and flash plants typically have reservoir temperatures of 400°F to 650°F, and they range from 3,000 feet to 10,000 feet deep," while "binary cycle plants are used for lower temperature reservoirs (200°F to 330°F)." One sourcing note: the EIA credits those figures to the DOE's GeoVision 2019 analysis — so they are DOE GeoVision numbers republished by the EIA, not independent EIA measurements. The EIA's Energy Explained page offers a third, broader framing for the resource overall: "These power plants need very hot water or steam—from 300 degrees Fahrenheit (°F) to 700° F."

The threshold and the ranges do not contradict each other — reservoirs feeding steam-family plants typically run 400°F to 650°F, comfortably above the 360°F design boundary. But they answer different questions: "where is the line between flash and binary" versus "how hot are the reservoirs these plants typically use." Blended into one all-purpose range, both numbers lose their meaning.

The Geysers: 18 Plants and No Geysers

The largest operating example of steam technology sits in northern California, and the U.S. Geological Survey opens its account of it with a correction: "Despite the name of the steam field, no natural geysers exist anywhere in the Geysers-Clear Lake area." The steam field carries a name its own geology does not support.

What the site lacks in geysers it holds in scale. Per the USGS: "Commercial geothermal power has been continuously generated at The Geysers since 1960, and it is the largest complex of geothermal power plants in the world. There are 18 geothermal plants which use heat from the earth's interior to produce electricity around the clock. The plants produce about 835 megawatts of electricity." Those figures come from a USGS page dated September 2023.

Eighteen plants on one steam field, producing around the clock, on a field generating commercially since 1960. Note the USGS's own wording: a complex of plants, not a single plant. The DOE's superlative is phrased differently but points at the same place: The Geysers is "the world's largest single source of geothermal power," running on steam technology whose basic design predates the First World War.

Geographically, the USGS places the field near Clear Lake, California, with numerous geothermal wells located in the Mayacamas Mountains, north of San Francisco.

Where the Plants Are

Because these plants must sit on top of hydrothermal resources — the heat, fluid, and permeability from the first section — their map is short. Per the EIA, in 2025 the United States had geothermal power plants in seven states, and they produced about 0.4%, or 16 billion kilowatthours, of total U.S. utility-scale electricity generation. (The EIA's definition, for precision: "Utility-scale power plants have at least 1,000 kilowatts (1 megawatt) of electricity generation capacity.")

The seven states are California, Nevada, Utah, Hawaii, Oregon, Idaho, and New Mexico — and the distribution is lopsided. In the EIA's 2025 data, California accounted for 68.6% of U.S. geothermal generation and Nevada 24.7%, leaving the other five states with small single-digit shares. What that 16 BkWh means in context, and how geothermal's output compares over time, is the territory of our guide to geothermal electricity usage in the U.S. — this page stays with the machines.

On capacity, the DOE states that "the United States leads the world in geothermal electricity-generating capacity—just over 4 gigawatts." The DOE attaches no data year to that figure on its page, so it is reported here as published — undated.

The international picture runs on older data — the EIA's figures are for 2022 — and it rearranges the leaderboard depending on what you measure. In 2022, per the EIA, 25 countries including the United States generated about 92 BkWh of electricity from geothermal energy. Indonesia was the top producer at about 17 BkWh, which was about 5% of Indonesia's total electricity generation. Kenya, the seventh-highest producer at about 5 BkWh, got about 43% of its annual electricity generation from geothermal. Same technology, opposite roles: a modest slice of a large grid in one country, a large share of the grid in another.

The 2050 Projections, Read Carefully

Two large numbers circulate about American geothermal's future, and both carry a condition.

Per the DOE: "The 2019 GeoVision analysis concluded that, with advancements in EGS, geothermal electricity-generating capacity could reach at least 60 gigawatts by 2050. Updated U.S. Department of Energy (DOE) analysis found that the potential was even higher, with technical advances enabling at least 90 gigawatts of geothermal electricity-generating capacity across the United States by 2050."

The verbs are "could reach," "potential," "enabling." These are projections conditional on advances in enhanced geothermal systems — the engineered-reservoir approach that creates the heat-fluid-permeability conditions where nature only supplies the heat. They are not forecasts of what will happen. Set against the DOE's figure of just over 4 gigawatts — which the DOE's page carries without a data year — they mark out how much of geothermal's projected future rests on engineering reservoirs rather than finding them — a subject with its own machinery and its own open questions, covered in enhanced geothermal systems explained.

Frequently Asked Questions

What are the three types of geothermal power plants?

Per the DOE: dry steam, flash steam, and binary cycle. Dry steam plants use reservoirs that are already mostly steam, sent directly to the turbine. Flash steam plants take water hotter than 182°C/360°F and "flash" part of it to vapor with a pressure drop. Binary-cycle plants use cooler water (below 182°C/360°F) to vaporize a secondary fluid with a much lower boiling point, and that second fluid drives the turbine. The choice depends on the state of the reservoir fluid and its temperature.

What is the difference between a flash steam and a binary cycle plant?

Temperature and contact. A flash plant needs fluid above the DOE's 182°C/360°F threshold, and the reservoir fluid itself becomes the vapor that drives the turbine. A binary plant works below that threshold, and per the DOE its defining feature is that "the geothermal reservoir fluids never come into contact with the power plant's turbine units" — heat passes through an exchanger into a secondary fluid, and the secondary fluid does the work.

Do geothermal power plants produce air emissions?

The EIA's statement on this is specific to one design: in a binary-cycle plant, "the geothermal water never touches the turbine and is returned to the earth. This closed system produces no air emissions." That is a description of the binary closed loop. The sources cited on this page make no equivalent blanket claim for dry steam or flash plants, so no blanket claim appears here.

What is the largest geothermal power plant in the world?

The cited sources rank geothermal fields, not individual plants. The place people are usually reaching for is The Geysers in northern California — which is not one plant but 18. The USGS calls it "the largest complex of geothermal power plants in the world," producing about 835 megawatts continuously since commercial generation began in 1960. The DOE describes the same site as "the world's largest single source of geothermal power." It runs on steam technology.

How many types of geothermal energy systems are there?

For generating electricity, the DOE counts three plant technologies — dry steam, flash steam, and binary cycle — and that taxonomy is what this page covers. A home geothermal system is a different machine entirely: a heat pump that exchanges heat with shallow ground rather than a power plant sitting on a hot reservoir. If the system you are researching heats a building instead of feeding the grid, it belongs to that separate technology.