supercritical carbon dioxide turbine

supercritical carbon dioxide turbine, type of heat engine that uses supercritical carbon dioxide (CO₂)—a state in which carbon dioxide is as dense as a liquid but flows as easily as a gas—as a working fluid. The high density of sCO₂ allows compact designs and improved performance of the turbine compared with conventional steam turbines, making the technology promising for modern power generation and waste-heat recovery.

Supercritical CO₂ can be used with Rankine and Allam-Fetvedt heat cycles (processes in which a fluid gains and loses heat as it moves through a system), but large-scale installations typically use a closed Brayton cycle (or Joule cycle), in which a pressurized fluid is heated, expanded through a turbine, and recycled.

The science of supercritical fluids

Supercritical CO₂ turbines depend on the unique properties of a substance that is neither entirely liquid nor gas. Under normal pressure conditions, supplying enough heat to a liquid turns it into a gas. The heat provides energy to the molecules within the liquid and increases their motion. At the liquid’s boiling point, the molecules move vigorously enough to break the attractive forces holding them together, turning the substance into a gas. But if both temperature and pressure increase simultaneously to a sufficient degree, a substance reaches a point at which the distinction between liquid and gas disappears and the densities of the liquid and the gas become equal. The set of conditions under which a substance exists in this state, or phase, is called its critical point. The critical point for carbon dioxide is a pressure of 73.8 bar (1,070 pounds per square inch) and a temperature of 31.1 °C (88 °F).

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Operation

Supercritical CO₂ is very dense—nearly twice as dense as steam. This high density is important to the operation of an sCO₂ turbine, as it allows more energy to be extracted per unit volume of working fluid.

An sCO₂ engine follows the Brayton cycle, but unlike traditional Brayton cycles that use gases as working fluids, the carbon dioxide is typically maintained in a supercritical state throughout the cycle. The sCO₂ first enters a compressor, where it is pressurized, preparing it for expansion later in the cycle. The compressed fluid then passes through a heat source, often after moving through a recuperator, which recovers heat from the exhaust to increase the fluid’s temperature. The hot, high-pressure fluid then flows through a turbine, where it expands and turns the turbine blades, producing mechanical work. Afterward, the fluid passes through a cooler to prepare it for the next cycle in closed-cycle turbines. In semi-closed-cycle turbines, the sCO₂ is intended to recirculate as in the closed model. The turbine drives both a generator, which produces electricity, and the compressor. Part of the energy generated is used to sustain the cycle.

Because the working fluid remains in a single phase, sCO₂ systems avoid the energy losses associated with boiling and condensation processes that are seen in traditional steam turbines (in which molecules absorb energy to boil and become a gas and lose energy to their surroundings to condense into a liquid). Since sCO₂ is also much denser than steam and can be compressed more easily, it transfers more energy to the turbine blades. This makes it possible to generate power comparable to a steam turbine with smaller equipment, resulting in more compact systems. The overall efficiency gains over steam turbines are up to 5 percent.

Despite its advantages over a conventional steam turbine, however, sCO₂ engines face significant engineering and economic challenges. Although the overall size of an sCO₂ installation is smaller than a comparable steam turbine, the smaller machinery needs to be made of specialized and costly alloys to prevent the metal from cracking over time from reactions with carbon. These alloys are also more difficult to craft into parts because of their tighter molecular buildup. The smaller size of the turbines can also be a disadvantage because smaller rotors reach lower top speeds than conventional turbines. This necessitates greater flow of the high-temperature, high-pressure working fluid for faster rotation, which leads to quick material degradation and is another factor that demands more durable and expensive materials.

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Large-scale installations

A demonstration project employing sCO₂ in a turbine was launched in Texas in 2017. Called the Supercritical Transformational Electric Power (STEP) Demo, the project’s turbine and other mechanical components were fully installed by 2023. In May 2024 it generated electricity for the first time at a temperature of 260 °C (500 °F) and a peak speed of 27,000 revolutions per minute, marking one of the first large-scale tests of sCO₂ power cycles. By the end of its development phase, STEP is expected to produce up to 10 megawatts of power, enough to run 10,000 homes. It is a pilot facility and not for commercial use.

China launched Chaotan One, its own sCO₂ turbine, on December 20, 2025, in Liupanshui, Guizhou province. It is the world’s first commercial use of an sCO₂ turbine. The facility uses two 15-megawatt turbines powered by industrial waste heat. The system is said to reduce machinery size by half and is projected to produce more than 70 million kilowatt-hours of electricity annually, generating approximately $4.3 million in revenue.