Sandia National Laboratories researchers are moving from theory and the lab bench to the demonstration phase of a new gas circulation system using supercritical CO2 for driving turbines for power generation. The theory and supporting lab work suggest that thermal-to-electric conversion efficiency will be increased to as much as 50 percent – an improvement of 50 percent for nuclear power stations equipped with steam turbines, or a 40 percent improvement for simple gas turbines. The press release from Scandia offers that the system is also very compact, meaning that capital costs would be relatively low.

Supercritical CO2 Engineering Test Rig. Click image for more info.

The most interesting point of the research is it’s not a binary system – one that takes the heat off the heat source to heat something else for driving the turbine – instead it’s a direct to turbine blade energy delivery.

The Sandia guys are really confident – Steve Wright of Sandia’s Advanced Nuclear Concepts group says, “Sandia is not alone in this field, but we are in the lead. We’re past the point of wondering if these power systems are going to be developed; the question remains of who will be first to market. Sandia and DOE have a wonderful opportunity in the commercialization effort.”

This will sell if priced right – for example as the Sandia group is focusing on nuclear heat sources – a full retrofit in the U.S. nuclear fleet would increase U.S. electrical output to nearly 30% from the fleet, a 10% total increase. That would be like building 51 more nuclear powers stations.

Moreover, Sandia’s supercritical CO2 Brayton cycle is expected to produce electrical power at a considerably lower temperature (250-300º C or 480-572º F).  That alone opens up a lot of alternative sources including all potential heat sources including solar, geothermal, fossil fuel, biofuel, as well as nuclear and including next-generation power reactors – perhaps even the Rossi / Focardi reactor.

Wright explains, “This machine is basically a jet engine running on a hot liquid. There is a tremendous amount of industrial and scientific interest in supercritical CO2 systems for power generation. . .”  This is a link to an analysis pdf that describes the testing taken place to prove up the hardware.

The technology is based on the Brayton cycle, named after George Brayton that in the original design functions simply by heating air in a confined space and then releasing it in a particular direction. The same principle is at work powering jet engines today.

The Sandia group has focused on supercritical carbon dioxide Brayton-cycle turbines converting heat energy from bulk thermal and nuclear power sources to the generation of electricity. The goal is eventually to replace steam-driven Rankine cycle turbines because they have lower efficiency, are corrosive at the high operating temperatures and occupy 30 times as much space due to the need for physically very large turbines and condenser sets to dispose of excess steam. The Sandia supercritical CO2 Brayton cycle is said it could yield 20 megawatts of electricity from a package with a volume as small as four cubic meters.  That’s a bit more than a large desk.

A competing system, also in research at Sandia and using the Brayton cycle with helium as the working fluid, is designed to operate at about 925º C and is expected to produce electrical power at same 43 to 46% efficiency range.  The supercritical CO2 equipment is more compact than that of the helium cycle, which is far more compact than the conventional water to steam cycle.

Understanding the technology is the realization or knowledge that under normal conditions materials behave in the classical or ideal way that’s predictable as conditions cause them to change phase, such as when heated water turns to steam. However the classical model tends not to work at lower temperatures or higher pressures than those that exist at the “critical” points.

In the case of carbon dioxide, it becomes an unusually dense or a “supercritical” liquid at the point where it is held between the gas phase and liquid phase. The supercritical properties of carbon dioxide at temperatures above 500 C and pressures above 7.6 megapascals (1102 psi) enable the system to operate with very high thermal efficiency.

Those efficiencies exceed those of large coal fueled power plants and compare as nearly twice as efficient as that of a good gasoline engine running at about 25% efficiency.

The Sandia team currently has two supercritical CO2 test loops.  A power production loop is located in Arvada, Colorado at contractor Barber Nichols Inc., where it has been running and producing approximately 240 kilowatts (equivalent to twenty 100 amp U.S. homes) of electricity during the developmental phase that began a year ago in March 2010. It is now being upgraded and its expected to ship to Sandia this summer.  When this unit runs the first electrical production will take place.

A second loop that’s driven rather than powering, located at Sandia in Albuquerque, is used to research the mechanics of 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.  That’s where the engineering issues for these matters are being worked out.

The close range publicly released 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. Longer-range plans call for an industrial demonstration plant at 10 MW of electricity that would lead to commercialization of the technology into the power generation market.

One gets the feeling from the press release information that the progress at Sandia might have resolutions at hand for the engineering issues sized to 250 kW and very good expectations as the design scales up.

The other matter of great interest is going to be in the delay from firing a heat source to getting to a fully heated working fluid.  In a water to steam system the starting delay is expensive in fuel costs, thus keeping the steam plants in a base load configuration and requiring considerable costs as running water to steam is less stable than supercritical CO2 because the water’s oxygen reacts more with the system’s components.

Sandia might just have a major contribution for power generation at hand.


2 Comments so far

  1. Musson on March 7, 2011 9:38 AM

    This is not your grandfather’s power turbine!

  2. Mike Swift on August 25, 2011 11:27 PM

    The CO2 Brayton cycle turbine is an ideal match for LFTR, (Liquid Fluoride Thorium Reactor). Without the need for Zirconium or H2O there is no problem with hydrogen during an accident. LFTRs ability to go from idle to full power in seconds combined with a turbine could eliminate the need for gas peaking plants.

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