Researchers Develop Supercritical CO2 Brayton Cycle Turbines

Researchers at Sandia National Laboratories say a project that focuses on supercritical carbon dioxide (CO2) Brayton cycle turbines is moving to the demonstration stage. Project developers are ultimately seeking to modify and use the centuries-old thermodynamic cycle to replace steam-driven Rankine cycle turbines—which need very large turbines and condensers, operate at lower efficiencies, and are corrosive at high temperatures—with simple gas turbine systems that could yield 20 MW from a package with a volume as small as 4 cubic meters (Figure 6).

6. Gassing up an old idea. Researchers at Sandia National Laboratories are developing supercritical carbon dioxide Brayton cycle turbines, which they say could increase thermal-to-electric conversion efficiencies by 50% for nuclear power stations equipped with steam turbines and by 40% for simple gas turbines. The researchers add that the supercritical carbon dioxide system provides the same 43% to 46% efficiency as a competing Brayton cycle system that uses helium as a working fluid, but it operates at much lower temperatures (250C to 300C, compared with 925C for the helium system). This image shows an early test loop of a Brayton cycle turbine. Source: Sandia National Laboratories

“This machine is basically a jet engine running on a hot liquid,” says principal investigator Steve Wright of Advanced Nuclear Concepts Dept. 6221. “There is a tremendous amount of industrial and scientific interest in supercritical CO2 systems for power generation using all potential heat sources including solar, geothermal, fossil fuel, biofuel, and nuclear.”

The lab’s research is centered around two test loops. The power production loop at the Arvada, Colo., site of contractor Barber Nichols Inc. has been running and producing approximately 240 kWe during the developmental phase that began in March 2010. The second loop, at Sandia in Albuquerque, N.M., has been focused on understanding issues of compression, bearings, seals, and friction that exist near the critical point, where the CO2 has the density of liquid but otherwise has many of the properties of a gas.

“The supercritical properties of carbon dioxide at temperatures above 500C and pressures above 7.6 megapascals enable the system to operate with very high thermal efficiency, exceeding even those of a large coal-fired power plant and nearly twice as efficient as that of a gasoline engine (about 25%),” researchers say. The combination of low temperatures, high efficiency, and high power density allows for the development of very compact, transportable systems that are more affordable because only standard engineering materials (stainless steel) are required, less material is needed, and the small size allows for advanced, modular manufacturing processes.

Sandia admits that it is not alone in the field but says it is “in the lead.” The lab will test 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 MWe.

—Sonal Patel is POWER’s senior writer.