Research focuses on supercritical carbon dioxide (S-CO 2 ) Brayton-cycle turbines, which typically would be used for bulk thermal and nuclear generation of electricity, including next-generation power reactors. The goal is eventually to replace steam-driven Rankine cycle turbines, which have lower efficiency, are corrosive at high temperature and occupy 30 times as much space because of the need for very large turbines and condensers to dispose of excess steam. The Brayton cycle could yield 20 MW of electricity from a package with a volume as small as four cubic meters.

Sandia National Laboratories (SNL) researchers are progressing to the demonstration phase of a supercritical CO 2 (S-CO 2 ) Brayton-cycle turbine system for power generation, with the promise that thermal-to-electric conversion efficiency will be increased as much as 50% for nuclear power stations equipped with steam turbines, or 40% for simple gas turbines. The S-CO 2 system is also very compact, meaning that capital costs would be relatively low.

The Brayton cycle, named after George Brayton, originally functioned by heating air in a confined space and then releasing it in a particular direction. The same principle is used to power jet engines today.

This machine is basically a jet engine running on a hot liquid. There is a tremendous amount of industrial and scientific interest in supercritical CO 2 systems for power generation using all potential heat sources including solar, geothermal, fossil fuel, biofuel and nuclear. —principal investigator Steve Wright of Sandia’s Advanced Nuclear Concepts group

The supercritical properties of carbon dioxide at temperatures above 500 °C and pressures above 7.6 MPa enable the system to operate with very high thermal efficiency, exceeding even those of a large coal-generated power plant and nearly twice as efficient as that of a gasoline engine (about 25%). As a result, there has been research interest in producing a commercially viable S-CO 2 Brayton-cycle turbine for power generation, especially nuclear; however, much of the work has been largely analytical. A number of DOE labs, including Idaho National and Argonne National, have contributed to the body of work.

Turbo-alternator-shaft design for the SNL S-CO 2 test loop. This configuration uses gas-foil bearings and includes a small turbine (red). Source: Wright et al. 2010. Click to enlarge.

Sandia’s effort is hardware-focused and requires the development of turbo-alternator-compressor technologies capable of operating with supercritical CO 2 at very high power densities, high speeds, high pressures, and high fluid densities.

Sandia currently has two supercritical CO 2 test loops. The key component of these loops is the turbo-alternator-compressor unit (TAC) and the technologies used in its design. In its final configuration, the TAC uses gas foil bearings, a high speed permanent magnet motor/alternator and labyrinth gas seals to reduce the rotor cavity pressure. Because of the extremely high power densities and fluid density, Sandia has filed a Technical Advance for the TAC design.

A power production loop is located at the Arvada, Colo., site of contractor Barber Nichols Inc., where it has been running and producing approximately 240 kW of electricity during the developmental phase that began in March 2010. It is now being upgraded and is expected to be shipped to Sandia this summer.

A second loop, located at Sandia in Albuquerque, is used to research the unusual issues of compression, bearings, seals, and friction that exist near the critical point, where the carbon dioxide has the density of liquid but otherwise has many of the properties of a gas.

Immediate plans call for Sandia to continue to develop and operate the small test loops to identify key features and technologies. Test results will illustrate the capability of the concept, particularly its compactness, efficiency and scalability to larger systems. Future plans call for commercialization of the technology and development of an industrial demonstration plant at 10 MW of electricity.

A competing system, also at Sandia and using Brayton cycles with helium as the working fluid, is designed to operate at about 925 °C and is expected to produce electrical power at 43% to 46% efficiency. By contrast, the supercritical CO 2 Brayton cycle provides the same efficiency as helium Brayton systems but at a considerably lower temperature (250-300 ° C). The S-CO 2 equipment is also more compact than that of the helium cycle, which in turn is more compact than the conventional steam cycle.

Sandia’s S-CO 2 Brayton cycle program is supported by DOE with funding from the Labs’ Laboratory Directed Research & Development (LDRD) program.

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