Organic Rankine Cycle: How ORC Turns Warm Water Into Power

An organic Rankine cycle, or ORC, is a heat engine used in binary-cycle geothermal power plants. It works like a conventional steam cycle — a fluid is vaporized, the vapour drives a turbine, and the turbine drives a generator — but with a different fluid. Turboden, a company that manufactures ORC systems, states the substitution in a sentence: "Instead of generating steam from water, the ORC system vaporizes an organic fluid."

That substitution is a central reason the cycle is useful on lower-temperature heat. Water has to reach a higher temperature than these fluids before it will vaporize at a given pressure. A fluid that boils at a lower temperature can be driven by a resource that is warm rather than hot, and that is what allows geothermal generation to use reservoirs the U.S. Energy Information Administration puts at 200°F to 330°F rather than only on the 400°F to 650°F reservoirs that dry steam and flash plants typically use.

This page covers what the cycle is, what the fluid substitution enables, which of the circulating temperature figures applies where, what the published efficiency data do and do not show, and where the ORC is used outside geothermal. It is the companion to our guide to the three types of geothermal power plant, which treats the binary-cycle plant as one design among three. It explains the cycle used within that design.

What an Organic Rankine Cycle Is

A Rankine cycle is the thermodynamic cycle behind steam power: a liquid is heated until it vaporizes, the vapour expands through a turbine, and the fluid is returned to be heated again. "Organic" refers to the fluid, not to the heat source. Turboden, which manufactures and sells ORC systems, describes its product as a "closed thermodynamic cycle" and states the difference from steam directly: "Instead of generating steam from water, the ORC system vaporizes an organic fluid." That is a vendor's description of its own equipment. The mechanism it describes matches the federal accounts, though the definitions are not identical: Turboden specifies an organic fluid, while the EIA describes a secondary fluid with a lower boiling point without requiring it to be organic — wording broad enough to cover the non-ORC binary plants named later on this page.

The EIA describes the same mechanism from the geothermal side, without using the ORC name. A binary-cycle plant, per the EIA, uses "a secondary working fluid with a lower boiling point than water." Then: "The working fluid vaporizes and passes through the turbine, generating electricity." The secondary fluid takes the supplied heat, vaporizes, and drives the turbine that drives the generator.

In Turboden's description, "closed" means the working fluid recirculates rather than being consumed. After the vapour has passed through the turbine, the fluid is returned to be heated and vaporized again. Turboden describes "low refilling of fluid required" rather than none, so the loop is closed in the sense that the charge is reused, not that it never needs topping up. In a geothermal plant the heat that vaporizes it comes from the reservoir fluid, transferred through a heat exchanger — the arrangement covered in the binary-cycle section below.

How It Differs From the Conventional Rankine Cycle

In an ORC the working fluid changes; the machine the vapour passes through is covered in our companion article on the geothermal turbine. The turbine receives vapour from the organic working fluid rather than steam.

In a conventional Rankine cycle, the fluid is water and the vapour is steam. In an ORC, the fluid is, in the EIA's words, "a secondary working fluid with a lower boiling point than water" — or in Turboden's, "an organic fluid." Because the fluid boils at a lower temperature, the cycle can use a lower-temperature heat source. A fluid that vaporizes at a lower temperature can be driven by a lower-temperature heat source, and the heat source is set by the resource rather than by the cycle.

The EIA quantifies the difference in resource temperature. Per the EIA, "Dry steam and flash plants typically have reservoir temperatures of 400°F to 650°F," while "Binary cycle plants are used for lower temperature reservoirs (200°F to 330°F)." The EIA credits both ranges to the Department of Energy's GeoVision analysis, so they are DOE figures republished by the EIA rather than the EIA's own measurements. The DOE itself separates its two descriptions at a single number: flash plants take "Fluids at temperatures greater than 182°C/360°F," and binary plants take "Low-temperature (below 182°C/360°F)" fluids. The bottom of the EIA's binary range, 200°F, is 160°F below that threshold. It is a resource the DOE assigns to the binary category rather than to a flash plant, and one a lower-boiling working fluid can be vaporized by.

The federal descriptions and the vendor description agree on the mechanism. They describe the fluid differently: the EIA by its property (a lower boiling point than water), Turboden by its class (organic), and the DOE-hosted report cited in the next section by name.

The Working Fluid

Two fluids are named in the sources cited on this page, both in a DOE-hosted technical report on energy conversion systems — Chapter 7 of a larger report on enhanced geothermal systems.

• R-134a. The report names R-134a as the working fluid in a 100°C basic-binary case that uses waters from oil-and-gas operations as the heat source. The 100°C is the temperature of the heat source in that case, not a property of the fluid.
• Isobutane. The report names isobutane as an ORC working fluid.

Those are the two ORC working fluids named in the specific cases cited here. The same report also names a water-ammonia mixture, as the working fluid of the Kalina-type plant in its table. The sources do not present any of these as the only fluids in use.

What the fluid choice governs is the temperature at which the cycle will vaporize and run. The EIA's description carries the requirement — "a lower boiling point than water" — and the DOE-hosted report's 100°C case shows what that requirement looks like in practice: a heat source at 100°C, and a fluid, R-134a, that the report models as vaporizing on it. The cited material does not compare the fluids against each other.

"Organic" in the cycle's name is a description of the fluid's chemical class. It says nothing about the heat source, which can be a geothermal reservoir, a stream of waste heat, or the other sources listed in the section on non-geothermal uses.

Where the ORC Fits: Binary-Cycle Geothermal Plants

A binary-cycle plant can use an ORC in its second-fluid loop. The terms are sometimes used interchangeably, but they describe different things. "Binary" describes the architecture — a second fluid, in a separate loop — while "organic Rankine cycle" names one implementation of it, distinguished by the fluid being organic. The DOE-hosted EGS report states the distinction directly when it introduces its own table of binary plants: "All of the plants are organic Rankine cycles (ORCs), with the Húsavík plant being a Kalina-type plant using a water-ammonia mixture as the working fluid." Húsavík is binary and not an ORC. The DOE describes the separation between the two fluid loops directly. Per the DOE, in a binary-cycle plant "the geothermal reservoir fluids never come into contact with the power plant's turbine units." The DOE gives that as the feature that separates binary plants from the two steam designs.

The two fluid loops are:

• The reservoir loop. Geothermal fluid — hot water from below ground — comes up the well and passes through a heat exchanger, where it gives up heat. Its role in generating power ends at the exchanger. It does not enter the turbine.
• The second loop. In an ORC plant the organic working fluid takes that heat on the other side of the exchanger, vaporizes, passes through the turbine, and circulates back to be heated again. This is the closed cycle Turboden describes, and it is the fluid the EIA describes as vaporizing and passing "through the turbine, generating electricity."

Where that second fluid is organic, the cycle it runs is an ORC. "Binary" names the two-fluid architecture; "organic Rankine cycle" names the cycle that second loop runs when the working fluid is organic. The generator that the turbine drives is a separate machine again, covered in our guide to the geothermal generator.

The DOE's temperature category for this arrangement is the 182°C/360°F figure from the previous section: binary plants are the DOE's answer for "Low-temperature (below 182°C/360°F)" fluids. That is a design classification rather than a statement of physical impossibility: whether water flashes depends on pressure as well as temperature, and the DOE frames the choice as one that "generally depends" on the fluid's state and temperature. The same report treats 200°C — which is 392°F, above the line rather than below it — as a temperature at which either a binary or a single-flash design may be considered. The report therefore considers both designs at a temperature above the DOE's general binary threshold. The second loop uses a fluid selected to vaporize at the temperature the resource delivers.

Three Kinds of Temperature Numbers

The cited sources report three temperature figures for ORC geothermal plants, and each measures a different thing.

1. A reservoir range: 200°F to 330°F

The EIA's figure — "Binary cycle plants are used for lower temperature reservoirs (200°F to 330°F)" — is a reservoir temperature. It describes the resource underground. It is not the temperature at which the working fluid enters the turbine. Between the reservoir and the turbine sit the well, the heat exchanger, and the working fluid itself; the fluid that reaches the turbine is the organic fluid, and its temperature is set by the exchanger, not read off the reservoir. None of the cited sources publishes an ORC turbine-inlet temperature. The EIA credits the 200°F to 330°F range, and the 400°F to 650°F range for dry steam and flash plants, to the DOE's GeoVision analysis.

2. A design threshold: 182°C / 360°F

The DOE's figure is a single temperature rather than a range, and it marks where the DOE's own descriptions divide rather than a hard rule about what can be built. Flash plants take "Fluids at temperatures greater than 182°C/360°F"; binary plants take "Low-temperature (below 182°C/360°F)" fluids. The DOE frames the choice as a tendency rather than a rule: "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." The EIA's binary range and the DOE's threshold are consistent — 330°F is below 360°F — but they answer different questions: the reservoir range the EIA cites for binary-cycle plants, versus where the design line falls.

3. A brine inlet temperature: 166°C, at one plant

The DOE-hosted EGS report publishes a table of brine inlet temperature, in °C, against cycle efficiency, in per cent. The column is labelled brine inlet temperature — the reported temperature of the geothermal fluid entering the conversion system, rather than a reservoir temperature measured underground. The report does not state where in the plant each value was taken. One entry in that table is Miravalles Unit 5 in Costa Rica, with a brine inlet temperature of 166°C. That figure sits below the DOE's 182°C threshold, consistent with its general classification of binary plants. It is a measurement at one plant and describes no other.

Efficiency: Ten Plants, With Their Conditions

Cycle efficiency is the share of the heat the cycle takes in that comes out as output. The DOE-hosted EGS report's Table 7.2, "Cycle thermal efficiencies for several binary power plants", publishes ten of them against the brine inlet temperature each plant was running:

Efficiency rises overall, with substantial plant-to-plant scatter. It rises with brine inlet temperature — 5.8% at 103°C, 13.8% at 166°C. But Steamboat SB-2 & SB-3 runs at 152°C for 8.2%, below Otake's 12.9% at 130°C. The report notes "considerable scatter in the efficiency data because of the variety of plant configurations represented by the data." It also names a specific gap in the record — the pinch-point temperature difference in the brine-working-fluid heat exchangers is, in the report's words, "an important factor in determining the plant thermal efficiency, and this value is not reported in the literature."

One of the ten is not an ORC. The report is explicit: "All of the plants are organic Rankine cycles (ORCs), with the Húsavík plant being a Kalina-type plant using a water-ammonia mixture as the working fluid." Húsavík is therefore a binary plant that does not use an ORC.

The ten-plant table does not establish a typical ORC efficiency. It reports a spread of 5.8% to 13.8% at brine inlet temperatures from 103°C to 166°C, and its own authors caution against reading that spread too closely.

The sources discuss cycle thermal efficiency and economic efficiency, which are different measures. The DOE-hosted report's 13.8% is a cycle efficiency — heat in against output out. The EIA's statement that dry steam and flash plants "have higher power output and are, therefore, more economically efficient than binary plants" is about economic efficiency, a term the EIA uses without defining and without publishing any comparative cost figure. The EIA does publish average generator capacity, and its own count gives the scale: 23 MW average per steam generator against 8 MW per binary-cycle generator. It does not publish a cycle efficiency for either family, and the DOE-hosted report's single entry does not convert into an economic comparison. The figures measure different quantities and are not directly comparable.

The report the 13.8% comes from is a chapter on energy conversion in a larger study of enhanced geothermal systems — engineered reservoirs, covered in enhanced geothermal systems explained. Its interest in binary conversion follows from that: if a reservoir has to be engineered, the resulting resource temperature affects the available conversion options, and a lower-temperature conversion cycle can use a broader range of engineered-reservoir outputs.

Generators, Not Plants

The EIA's published US count for binary-cycle generation uses a specific unit. The EIA reports that "the United States has 93 binary cycle generators, averaging 8 MW of capacity each, and 79 steam generators that average 23 MW each."

The figures count generators rather than plants. A plant can hold several generating units, so 93 binary-cycle generators does not mean 93 binary-cycle plants, and 79 steam generators does not mean 79 steam plants. The EIA's count cannot be restated as a plant count. The count is a snapshot from the EIA's analysis of July 30, 2020, not a live figure.

The EIA uses the figures to compare average generator capacity, and reports construction patterns alongside them. On unit size, a binary-cycle generator in the EIA's count averages 8 MW, a steam generator 23 MW. That gap is the basis for the EIA's statement that steam designs "have higher power output." On construction patterns, the EIA's analysis carries the title "Most U.S. utility-scale geothermal power plants built since 2000 are binary-cycle plants." That is a statement about the plant architecture in the US since 2000. What cycle those second loops run is a separate question, and the only cited sample that answers it is the DOE-hosted report's table of ten plants — a different and much smaller population, spanning several countries — in which nine are ORCs and the tenth is a Kalina-type. The EIA reports smaller average generator capacity for binary units, and says most US utility-scale plants built since 2000 are binary-cycle. The 79 steam generators are dry steam and flash together; the EIA's figure cannot be split between the two steam designs. How the generator count sits against installed capacity, where dry-steam and flash generation together still hold the larger share, is set out on the parent page on the three plant types.

ORC Beyond Geothermal: Waste Heat and Other Sources

The ORC needs a heat source warm enough to vaporize its working fluid. Nothing in the cycle requires that source to be a geothermal reservoir. Turboden, the ORC manufacturer cited above, lists the heat sources its systems are offered for, and that list includes "geothermal, solar, biomass" and "waste heat." That is a vendor's statement of the applications it sells into: a product list, not a survey of where ORC systems are deployed.

"Waste heat recovery" is the application in which the ORC is fed by heat that a process would otherwise discard. In Turboden's list, waste heat sits alongside the fuel-fed sources — biomass — and the resource-fed ones — geothermal and solar. The cited sources publish no figure for how much ORC capacity serves each application.

The DOE-hosted EGS report offers a case using hot water produced by oil-and-gas operations: the 100°C basic-binary case with R-134a as the working fluid, using waters from oil-and-gas operations as the heat source. Those waters are a by-product of a different industry, at a temperature the report treats as usable by a binary cycle. The report models the case; it does not, in the material cited here, report an operating plant built on it.

Limitations

The DOE associates binary generation with resources below its general flash threshold, and the EIA's published unit sizes for those generators are smaller. The limits in the cited sources are these.

Economic efficiency. The EIA states that dry steam and flash plants "have higher power output and are, therefore, more economically efficient than binary plants." That is the EIA's own conclusion about the family the ORC belongs to, and it rests on the output figures in the EIA's count.

Unit size. In the EIA's count, binary-cycle generators average 8 MW each against 23 MW for steam generators. Those are the two averages the EIA publishes; it does not publish a ratio between them.

Cycle efficiency. The cited table runs from 5.8% at a 103°C brine inlet to 13.8% at 166°C, across ten binary plants — and its authors warn of "considerable scatter" across plant configurations, with Steamboat SB-2 & SB-3 returning 8.2% at 152°C against Otake's 12.9% at 130°C. The table establishes neither a ceiling nor a floor, and it gives no basis for stating what an ORC plant "typically" achieves.

Resource dependence. The DOE describes binary plants as operating on "Low-temperature (below 182°C/360°F)" fluids, in the DOE's terms, and reservoirs of 200°F to 330°F in the EIA's. Plant output remains constrained by the resource temperature. Temperature is not the only term — the EGS report computes net power from the geofluid inlet temperature, the outlet temperature and the mass flow rate, and attributes the spread in its efficiency table to differing plant configurations — and it is the input the resource supplies rather than the plant. The hotter-resource requirement of the steam designs — the steam designs' need for the rarer, hotter resource — is set out on the parent page.

Manufacturers and the Market

Turboden, which manufactures ORC systems, is the one manufacturer cited on this page, and it is cited for its description of the cycle and its list of offered applications — both vendor statements about its own products. No other manufacturer is identified in the cited primary sources.

On market size: the cited sources publish no figure for the size, growth or installed base of the ORC market. The cited federal record includes the EIA's count of 93 binary-cycle generators averaging 8 MW — a U.S. count, of generators, as of the EIA's July 30, 2020 analysis — and it is not a market figure.

Frequently Asked Questions

What is an organic Rankine cycle?

A closed thermodynamic cycle that generates power the way a steam cycle does — vaporize a fluid, expand the vapour through a turbine, drive a generator — but with an organic fluid in place of water. Turboden, an ORC manufacturer, puts it as: "Instead of generating steam from water, the ORC system vaporizes an organic fluid." The EIA describes the same mechanism in a binary-cycle geothermal plant: "a secondary working fluid with a lower boiling point than water" that "vaporizes and passes through the turbine, generating electricity."

What is an organic Rankine cycle generator?

The cited sources do not define the phrase. The EIA instead reports a count: the EIA counts "binary cycle generators": in its July 30, 2020 analysis, 93 in the United States, averaging 8 MW of capacity each, against 79 steam generators averaging 23 MW. Those are counts of generators, not of plants; a plant can hold several. The generator itself is the machine covered in our guide to the geothermal generator.

What working fluid does an ORC use?

A fluid with a lower boiling point than water, in the EIA's description. The DOE-hosted EGS report names two: R-134a, as the working fluid in a 100°C basic-binary case using oil-and-gas waters, and isobutane, named as an ORC working fluid. Those are the two named in the sources cited on this page; the sources do not present them as the only fluids in use.

How efficient is an organic Rankine cycle?

A DOE-hosted report tabulates cycle thermal efficiencies for ten binary power plants, ranging from 5.8% at Amedee, California, on a 103°C brine inlet to 13.8% at Miravalles Unit 5, Costa Rica, on a 166°C brine inlet. Efficiency broadly rises with brine inlet temperature, but not uniformly — Steamboat SB-2 & SB-3 returns 8.2% at 152°C, below Otake's 12.9% at 130°C. The report warns that its data show "considerable scatter" attributable to differing plant configurations, and notes that the heat-exchanger pinch-point temperature difference, which it calls an important factor, "is not reported in the literature." The table reports only the plant-specific range and its conditions; the cited sources publish no typical ORC efficiency.

Is an organic Rankine cycle the same thing as a binary-cycle plant?

No. "Binary cycle" names the plant architecture — a second fluid in its own loop, taking heat from the geothermal fluid across a heat exchanger. "Organic Rankine cycle" names one way of running that second loop, distinguished by the working fluid being organic. In the cited ten-plant sample, nine of ten binary plants are ORCs — and the DOE-hosted report identifies Húsavík as the exception: "All of the plants are organic Rankine cycles (ORCs), with the Húsavík plant being a Kalina-type plant using a water-ammonia mixture as the working fluid." Húsavík is a binary plant that is not an ORC.

How is an ORC used in geothermal power?

It is one of the cycles a binary plant can run in its second loop, and the DOE-hosted table identifies it in nine of the ten plants. The DOE assigns binary plants to "Low-temperature (below 182°C/360°F)" fluids, and the EIA places them on reservoirs of 200°F to 330°F. The geothermal fluid heats the organic working fluid through a heat exchanger; per the DOE, "the geothermal reservoir fluids never come into contact with the power plant's turbine units." The organic fluid, not the geothermal water, passes through the turbine. The three plant types are compared on the parent page.

Is the ORC used for waste heat recovery?

Turboden, an ORC manufacturer, lists "waste heat" among the heat sources its systems are offered for, alongside "geothermal, solar, biomass." That vendor list is not evidence of deployment by application. The DOE-hosted EGS report models a related case — a 100°C binary cycle on R-134a using waters from oil-and-gas operations as the heat source. The cited sources give no figure for how much ORC capacity serves waste-heat applications.

Who manufactures organic Rankine cycle systems?

Turboden is the manufacturer cited on this page, for its own description of its ORC product. Turboden is the only manufacturer identified in the cited primary sources. No figure for the size or growth of the ORC market appears in the cited sources, and none is asserted.